Title of Invention	A PROCESS FOR THE PREPARATION OF CARBODIIMIDES
Abstract	A cost-effective, eco-friendly and energy efficient process for the preparation of high purity alkyl and aryl carbodiimides by the direct treatment of 1, 3-disubstituted thioureas with thiophilic halogens in the presence of base in organic solvent demonstrating good yields and high purity of the products. The process involves safe and inexpensive reagents and is energy efficient and is therefore favoured for wide scale industrial application and use.

Title of Invention

A PROCESS FOR THE PREPARATION OF CARBODIIMIDES

Abstract

A cost-effective, eco-friendly and energy efficient process for the preparation of high purity alkyl and aryl carbodiimides by the direct treatment of 1, 3-disubstituted thioureas with thiophilic halogens in the presence of base in organic solvent demonstrating good yields and high purity of the products. The process involves safe and inexpensive reagents and is energy efficient and is therefore favoured for wide scale industrial application and use.

Full Text	FIELD OF THE INVENTION The invention relates to the synthesis of alkyl and aryl carbodiimide and more particularly an eco-friendly and energy efficient process to produce the same involving very simple and cost-effective process steps. More specifically the alkyl and aryl carbodiimide with high purity, stability and yield are obtained involving direct treatment of the 1, 3-disubstituted alkyl and aryl thiourea with thiophilic halogen in the presence of base and in organic solvent. The reagents are easily available, safe, non-corrosive, environmentally benign and inexpensive. The process is further directed to a simple and faster production of the alkyl and aryl carbodiimide without the requirement of any extraneous heating making it further energy efficient. BACKGROUND ART Carbodiimides rank as one of the most important class of reagents in organic synthesis due to their easy accessibility, interesting chemical reactivity and utility in coupling reactions. Carbodiimides are widely used reagent for the synthesis of heterocycles, polysubstituted guanidines and various other compounds. Carbodiimides are an important class of molecules in synthetic organic chemistry which are widely used as synthetic intermediates and in several industrial applications such as synthesis of nucleotides and peptides, heterocycles, oxidation with dimethyl sulfoxide, permease inhibitors, polymer stabilizer, cycloaddition etc. Thus for its known wide use and application in various branches of chemistry, it is therefore not very surprising that there are several reports for the synthesis of carbodiimides exist in literature. Reference is drawn to arts, Chemical Review. 1953, 53, 145; Chem. Rev. 1967, 67, 107; Chem. Rev. 1981, 81, 689 and Tetrahedron 1981, 37, 233 where various methods of preparation of carbodiimides have been extensively discussed. The general methods involve the removal of hydrogen sulfide from thiourea by metal oxides (HgO, PbO, Ag2O), complexes of mercury, alkylcholoroformates, (CICOOEt), arylsulfonyl chlorides. The disadvantage of such a process resides in the use of polluting heavy metal ions. Carbodiimides can also be prepared by the oxidation of thioureas using various oxidizing agents and catalysts. Apart from the prior arts existing in literature there are numerous prior arts in the form of patents that are discussed below. Japanese patent JP06234725 relates to a process wherein carbodiimides are prepared by the treatment of 1,3-disubstituted thiourea with cyclic haloiminium salts such as l-choloro-l,3-dimethylimidazolium chloride and Et3N in CH2CI2 at room temperature for 4 hrs. The drawbacks of the method lie in the use of expensive reagents and halogenated solvent. Reference is drawn to EP307361 wherein the carbodiimide intermediate for the preparation of cyanoguanidine an epoxide hardener is prepared by photochemical oxidation of thiourea, NaOH, Rose Bengal in CH3CN/H2O under photochemical conditions. Specialized reaction conditions and the use of expensive Rose Bengal are the associated shortcomings of the method. Spanish patent ES481581, Britain patent GB1252707, US patent 3635947 and German patent DE1927529 are all directed to the synthesis of carbodiimides from RIMHC(Z)NHRj (Z= 0, S) with phosphorous based reagents such as Ph3PX2.Et3N (X=CI, Br), PhPO(NH2)2 in CH2CI2, under alkaline conditions at 0-5 °C. Use of expensive phosphorous reagents and halogenated solvents are the main disadvantages of the method. Yet again reference is drawn to US patents US4068065, US4068055, US3157662, US3056835, US3442947, US2853518, US2853473 and US3301895, German patent DE 2556760 and French patent FR1469946 that discloses the synthesis of carbodiimides from isocyanates using polymer supported catalyst such as 1- diethanolaminomethyl-3-methyl-3-phospholene-l-oxide-methyl-methacrylate copolymer and 2-methyl-l, 2-oxaphospholan-5-one 2-oxide, organometallic catalysts selected from the group consisting of (iso-Pro)4Ti (I), (BuCHEtCH2O)4Ti, (C8Hi7O)4Zr (II), (iso-PrO)4Zr and (EtO)5Nb, phosphorous based catalyst such as 1,2,3-trimethyl- 2-oxo-l,3,2-diazaphospholidine (I), or analogs, Ba[Bu2B(OBu)2]2 and (Bu3Sn)2O. Involvement of high reaction temperatures expensive catalysts are the main disadvantages associated with the above mentioned catalysts. US3301895 again discloses the synthesis of carbodiimides by mixing equi-molar amounts of 1, 3-disubstitutes thiourea with COCI2 in an inert atmosphere in anhydrous condition in the absence of acid acceptor at 0-6 °C. On completion the reaction is heated to yield the carbodiimides. Highly toxic reagents, requirement of inert atmosphere techniques damages the wide applicability of the process. Reference is drawn to Japanese patent JP 41011865 wherein carbodiimide is manufactured by treating 1, 3-disubstituted thioureas with NaOCI under alkaline NaOH conditions. The drawbacks of the method rest in the use of strong oxidizing agent, alkaline reaction conditions and elevated temperature. German patent DEI 149712 teaches the preparation of carbodiimides by heating isocyanide dichlorides with the HCI salt of primary amines. The reaction is carried out in an inert atmosphere at 180 °C for 18 hrs. Longer reaction time, expensive reagents and requirement of high temperature are the drawbacks of the method. Details of the various methods of preparation of carbodiimides are recited in Chem. Rev. 1953, 53, 145; Chem. Rev 1967, 67, 107; Chem. Rev. 1981, 81, 689 and Tetrahedron 1981, 37, 233. Some of the other exiting literature methodologies are described below. Tetrahedron Letters 2007, 48, 6002 illustrates a process wherein isocyanates are catalytically converted to the corresponding carbodiimides in the presence of low valent transition metal complexes. The best conversions and yields were accomplished using [(C5R5)Fe(CO)2]2 (R=H or Me) or MeCpMn(CO)3 as catalysts. Again use of expensive catalysts restricts the applicability of the stated process. Reference is again drawn to Organometallics 1993, 12, 1023 that is directed to the use of vanadium oxo and imido complexes as catalysts for the condensation of aryl isocyanates to N, N'-diarylcarbodiimides. Complexes such as V(O)(OtBu)3 (I), V(O)(OiPr)3 (II); V(NPhMe)(OtBu)3 (III); and V(O)(acac)2 (IV) were found to be more effective in this regard. Use of expensive vanadium based reagents is the main drawback of the method. Eur. 3. Org. Chem. 2006, 4170 and J. Org. Chem. 1994, 59, 4931 is directed to a method wherein iminophosphorane or P(MeNCH2CH2)3N when reacted with phenylisocyante yields a mixture of carbodiimides through both normal and abnormal aza-Wittig reactions. The reagent 3-aminoimidazolone was obtained from the reaction of hydrazine hydrate with carbodiimide. Further reaction of 3- aminoimidazolone with triphenylphosphane, hexacholoroethane and triethylamine produced iminophosphorane in good yield. Low yield and expensive phosphorous based reagents are the associated drawback of the method. Reference is made to J. Org. Chem. 2005, 70, 6362 wherein a thiophilic promoted synthesis of disubstituted 4H-[1, 2, 4] triazol-3-ylamines as urea mimetic from the corresponding 1,3-disubstituted thioureas were studied and found that the reaction proceeded through the formation of a carbodiimide. Use of expensive above mentioned thiophilic reagent is the major disadvantage of the method. J. Org. Chem. 1999, 64, 6984 and Bulletin of Korean Chemical Society, substantiates a process wherein 2-chloro-l,3-dimethylimidazolinium chloride (DMC) and N-methyl- 2-pyridinecarbamoyl chloride have been used as dehydrating agents, for the synthesis of carbodiimide in near neutral conditions from 1,3-disubstituted thiourea Use of expensive dehydrating agents are the limitations of this prior art. Reference is made to Synth Commun. 1995, 43 where in carbodiimides were prepared by treating thiourea with methanesulfonylchloride in the presence of triethylamine in DMAP. Moisture sensitive and lachrymatic methanesulfonyl chloride and the use of toxic DMAP are the stated disadvantages of the above prior art. Tetrahedron Lett. 1986, 27, 6099; Tetrahedron Lett. 1986, 27, 1925; Tetrahedron Lett. 1985, 26, 1661 and 3. Org. Chem. 1986, 51, 2613; Synth Commun. 1985, 171; Makromolecular Chemie, 1981, 182, 3411; teaches the use of desulfurylating agent utilized for the synthesis of carbodiimide starting from thiourea; such as p- methoxybenzenetellurinic acid anhydride, di-2-pyridyl sulfite and l,l'-thiocarbonyldi- 2,2'-pyridine and 2,4-dichloro-5-nitropyrimidine. All these methods use expensive reagents for carbodiimide synthesis. Reference is drawn to Tetrahedron Letters 1968, 29, 2473 and Tetrahedron 1970, 26, 5731 wherein carbodiimides were prepared from 1, 3-disubstituted thioureas, diethyl azodicarboxylate and triphenylphosphine refluxed in benzene or toluene. Uses of carcinogenic solvent and expensive reagents are the major detraction of the processes. Chem. Commun. 1997, 347 is directed to a method wherein the synthesis of N, N'- dialkyl carbodiimide was achieved by catalytic condensation of amines and isonitrile at 100 °C using a palladium complex catalyst, oxygen and molecular iodine. The use of expensive palladium catalyst and requirement of elevated temperature is the drawback of the method. Inspite of the huge number of teachings flowing from the background art, such above said processes for the synthesis of alkyl and aryl carbodiimides suffer from either one or many of the drawbacks which include: use of expensive/ toxic/ lachrymatic reagents, expensive dehydrating agents and catalysts; need of strongly basic and harsh reaction conditions; use of hazardous halo solvent and carcinogenic solvents such as benzene; low yield alongwith long reaction times and thus less efficient; moisture sensitive reagent needing inert atmosphere techniques thus difficult in handling. In light of the above said there is a continuous need in the art to develop a process for the manufacture of alkyl and aryl carbodiimides in an industrial scale which would circumvent all the aforementioned difficulties associated with the process. More particularly, it would be clearly apparent from the discussion above that there is a strong need in the art to provide for a process for the manufacture of alkyl and aryl carbodiimides in an industrial scale that would be: energy efficient; yield high purity product in good yields; economic with regard to the kind of reagents used and eco-friendly. More importantly, the method employed for product isolation in pure from is also an important tool to assess the feasibility of the said process. OBJECTS OF THE INVENTION It is thus the basic object of the present invention to provide for a process for the manufacture of alkyl and aryl carbodiimides in an industrial scale that would be efficient, economic, environment friendly and importantly would yield highly pure products in good yield. Another object of the invention is to provide for a clean simple process for a large scale manufacture of alkyl and aryl carbodiimides of general formula RN=C=NR'. Yet another object of the invention is to provide for a process for manufacturing alkyl and aryl carbodiimides of general formula RN=C=NR' that would be economically viable, eco-friendly and energy efficient for large scale and faster production of the said carbodiimides under mild conditions. Yet further object of the present invention is directed to the manufacture of alkyl and aryl carbodiimides involving inexpensive, non-toxic and easily available starting materials and non-corrosive reagents. A further object of the invention is to provide for a process for the manufacture of alkyl and aryl carbodiimides that would be simple and easily operable under mild reaction conditions. Yet another object of the invention is to provide for a process for the manufacture of alkyl and aryl carbodiimides that would be energy efficient with no requirement for any external heating. It is a further object of the invention to provide for a process for the manufacture of alkyl and aryl carbodiimides that would be economical and can be accomplished within a short time. Still another object of the invention is to provide for a process for the manufacture of alkyl and aryl carbodiimides that would results in highly pure product specifications. Yet another object of the invention is to provide for a process for the manufacture of alkyl and aryl carbodiimides wherein the desired products of alkyl and aryl carbodiimides would be obtained in quantitative yields. SUMMARY OF THE INVENTION This according to the basic aspect of the present invention there is provided a process for the preparation of carbodiimides of general formula RN=C=NR' wherein R, R' is an arkyl or an aryl group and -NCN- is the carbodiimide group comprising of (a) reacting or decomposing 1, 3-disubstituted thioureas with thiophilic halogen in the presence of a base in organic solvent and (b) obtaining therefrom the said carbodiimides. In accordance with a preferred aspect of the process of the present invention the said step of obtaining the carbodiimides by decomposing the 1,3-disubstituted thioureas in organic solvent comprises of (i) separating the precipitated sulphur and evaporating the solvent (ii) redissolving the residue in a water immiscible solvent, washing with water, (iii) drying the water immiscible layer with anhydrous Na2SO4 followed by (iv) evaporation of the solvent layer and (v) purifying over a short silica gel column (eluted with Hexane) to thereby obtain the said alkyl and aryl carbodiimides of general formula RN=C=NR'. In accordance with another preferred aspect of the process of the present invention the said step of obtaining carbodiimides of the said general formula RN=C=NR' wherein R, R' is an alkyl and aryl group selected from the group of aliphatic and aromatic amines comprising of cyclohexyl, phenyl, 4-methyl phenyl, 4-bromophenyl, 4-chloro phenyl, 2-methoxy phenyl, 4-methoxy phenyl, 2,4-dimethyl phenyl, 2- chloro phenyl, 3-nitrophenyl, 2,6-dimethylphenyl, 2-(3,4-dimethyl-phenyl)- ethylamine group and wherein -N=C=N- is the carbodiimide group. Advantageously, the process in accordance with the invention comprises providing carbodimides wherein the said carbodiimide of general formula RN=C=NR' comprises R=R'or R*R'. The said thiophiiic halogen is selected from I2, Br2 or its equivalents such as tribromides selected from the tribromides of tetramethylammonium, tetraethylammonium, tetrabutylamonium, cetyltriethylammonium, pyridinehydrobromideperbromide, phenyltrimethylammonium, benzyltriethylamonium, DABCO, DBU, bmim, MPHT and l,l'-(ethane-l,2-diyl) dipyridinium bis-tribromide preferably I2. The said base is preferably selected from triethylamine, pyridine, DBU, NH3, NaH, NaOH, KOH, NaHCO3, Na2CO3, K2CO3 and most preferably Et3N. The said organic solvent comprise aprotic solvent preferably CH2CI2, CHCI3, toluene, THF, hexane, CH3CN and the like and most preferably ethylacetate. In accordance with another aspect of the invention in the above process, the molar ratio of the said 1, 3-disubstituted thiourea to thiophilic halogen is in the range of 1: 1 to 1: 1.1 preferably 1: 1 more preferably 1: 1.05 and the molar ratio of the said 1, 3-disubstituted thioureas to base is in the range of 1:1.5 to 1: 2.5 preferably 1:2. Importantly also, in the above process the said thiophilic halogen is added in a controlled manner over a period of 15-20 minutes to an ice-cooled solution of the said 1, 3-disubstituted thiourea and said base in said solvent. In accordance with an aspect of the above process the 1, 3-disubstituted thiourea is suspended in organic solvent preferably ethylacetate and cooled in an ice bath. Advantageously in the above process the product obtained comprise a yield% of 58- 93% and purity of > 98%. The details of the invention, its objects and advantages are explained here under in greater detail in relation to non-limiting exemplary illustrations as per the following examples: DETAILED DESCRIPTION OF THE INVENTION As already disclosed herein before the present invention comprises of a simple and cost-effective and environment friendly process for manufacture of alkyl and aryl carbodiimides of the general formula RN=C=NR', wherein R is an alkyl and aryl group selected from the group of aliphatic and aromatic amines comprising of cyclohexyl, phenyl, 4-methyl phenyl, 4-bromophenyl, 4-chloro phenyl, 2-methoxy phenyl, 4-methoxy phenyl, 2,4-dimethyl phenyl, 2-chloro phenyl, 3-nitrophenyl, 2,6- dimethylphenyl, 2-(3,4-dimethyl-phenyl)-ethylamine group and wherein -N=C=N- is the carbodiimide group. The above mentioned alkyl and aryl carbodiimides are obtained in almost quantitative yields from the reaction involving a thiophilic halogen preferably iodine mediated decomposition of 1, 3-disubstituted thiourea in aprotic or polar protic solvent preferably ethylacetate (aprotic) at room temperature. The 1, 3-disubstituted thiourea are easily prepared by an established procedure by reacting aryl or alkyl isothiocyanate with aryl or alkyl amine. Once the synthesis of 1, 3-disubstituted thiourea is accomplished, a thiophilic reagent preferably iodine proved to be an effective reagent for the decomposition it to the desired carbodiimide in good yield within 15-20 minutes in the presence of triethylamine or NaHCO3. Some of the previously reported methods although giving better yields are quite expensive and environmentally unsafe and hence not economically viable and environmentally acceptable for large scale production. The reagent iodine for the decomposition of thiourea is non-corrosive, inexpensive and environmentally safe. Thus the method of the present invention provides for an economically viable process for the preparation of carbodiimide. The reaction is rapid and facile and accomplished under mild conditions whereby the product obtained is in high yields and purity. The reagent iodine for the decomposition of 1, 3-disubstituted thioureas is non corrosive, inexpensive and environmentally safe. Thus the method provides an economically viable process for the preparation of carbodiimides. The reaction is rapid and facile and accomplished under mild conditions. The product obtained is in high yields and high purity after chromatographic purification. Example I: Preparation of 1,3-disubstituted thiourea Phenyl isothiocyanate (675 mg, 5 mmol) in ethanol 5 mL was added aniline (505 mg, 5 mmol) and the reaction mixture was stirred for 30 minutes. After completion of the reaction solvent ethanol was evaporated and the crude product as obtained was used as such for the subsequent reaction. Synthesis for the preparation of the 1, 3-disubstituted thiourea was carried out as depicted below in Scheme 1 Scheme 1 Example II: Manufacture of alkyl and aryl carbodiimide from 1. 3- disubstituted thiourea in organic solvent The 1, 3-diphenyl thiourea (456 mg, 2 mmol) obtained as above following Example I, a base preferably triethylamine (558 µL, 4 mmol) was suspended in a solvent, ethylacetate (10 mL), which is cooled prior to reaction in an ice bath for 5 minutes. The reaction or decomposition of 1, 3-disubstituted thiourea was effected by the addition of a thiophilic halogen, iodine (558 mg, 2.2 mmol) by adding it in a controlled manner pinch wise. The reaction being exothermic addition of iodine in pinches was necessary while cooling for the reaction to proceed and to minimize the side products. Iodine was added in pinches over a period of 15-20 minutes. When the iodine addition was complete and after 10 minutes a TLC experiment on the reaction mixture revealed that the reaction was complete and overall time taken for the completion of the reaction was ~30 minutes. The precipitated sulphur was filtered and ethylacetate was evaporated in rotary evaporator under reduced pressure. The residue was redissolved in a water immiscible solvent, hexane (b. p. 60-70 °C). The hexane layer was washed with water and dried over anhydrous Na2SO4 and subsequently evaporated on a rotary evaporator. The crude product so obtained was further purified over a short silica gel column (eluted with 100 % hexane) to give 1, 3-diphenylcarbodiimide in 86 % isolated yield. The above synthetic procedure followed is further briefly outlined below in Scheme 2 hereunder. Where R = alkyl or aryl; R=R' or ≠ R' Scheme 2 The above examples are given by way of illustration of the present invention and therefore should not be construed to limit the scope of the present invention. All percentages are by weight. The above exemplary illustration of the process of the invention clearly reveals the efficient, economic and environment friendly method for the preparation of alkyl and carbodiimides directed to overcome the difficulties associated with what is already known in the art in its preparative process. Importantly, it is found by way of the invention that Iodine being highly thiophilic (electrophilic) in nature can interact with sulphur (nucleophile) thus assisting the desulphurization process of 1, 3- disubstituted thiourea. This is facilitated by the abstraction of NH protons by triethylamine / NaHCO3 present as a base in the reaction medium. Formation of elemental sulfur as the sole byproduct of the reaction supports the mechanism as proposed in Scheme 3. Performing the reaction under ice-cold conditions minimizes the side products. Thus the preferred use of iodine as a thiophilic reagent favours decomposition of pre-synthesized 1, 3-disubstituted thioureas to carbodiimides. Controlled addition of iodine in pinches over a time span of 15-20 minutes in the reaction flask is a prerequisite to prevent side reactions. The mechanism for the formation of alkyl and aryl carbodiimides is further illustrated in the following Scheme 3. Scheme 3 Thus the examples clearly illustrate an overall simple, fast and efficient process for the manufacture of alkyl and aryl carbodiimides. The above process is quick in yielding the said carbodiimides in a short time span of ~30 minutes, more preferably within 15 minutes and in > 98% pure form in good yields of 58-93 %. It is also noteworthy that the process is very economical for the inexpensive nature and easy commercial availability of the reagents employed. Advantageously the process provided in this invention is environmentally safe for the non corrosive, non toxic nature of the reagents and mild basic conditions needed for the process. Accordingly the process provided in this invention is favourable with respect to easy handling of the reagents which are moisture insensitive and non lachrymators. The advantage associated with the above said process also resides in the easily isolable and harmless byproduct such as sulphur which can be separated by simple filtration. The above process is advantageously also energy efficient since no external heating is required. It is thus possible by way of the present invention to provide for a process for the manufacture of alkyl and aryl carbodiimides which is efficient in terms of energy needed for activation for the formation of products from reactants, time required and high purity associated with the isolated yields of the product. The process is environment friendly especially in terms of non toxic nature of the reagents needed, mild reaction conditions employed and the harmless by product obtained. It is easy to operate in terms of the moisture insensitive and non lachrymator reagents needed for the purpose. WE CLAIM: 1. A process for the preparation of carbodiimides of general formula RN=C=NR' wherein R, R' is an arkyl or an aryl group and -NCN- is the carbodiimide group comprising of (a) reacting or decomposing 1, 3-disubstituted thioureas with thiophilic halogen in the presence of a base in organic solvent and (b) obtaining therefrom the said carbodiimide. 2. A process as claimed in claim 1 wherein said step of obtaining the carbodiimides by decomposing the 1, 3-disubstituted thioureas in organic solvent comprises of (i) separating the precipitated sulphur and evaporating the solvent (ii) redissolving the residue in a water immiscible solvent, washing with water, (iii) drying the water immiscible layer with anhydrous Na2SO4 followed by (iv) evaporation of the solvent layer and (v) purifying over a short silica gel column (eluted with hexane) to thereby obtain the said alkyl and aryl carbodiimides of general formula RN=C=NR'. 3. A process as claimed in anyone of the preceding claims comprising providing carbodiimides of the said general formula RN=C=NR' wherein R, R' is an alkyl and aryl group selected from the group of aliphatic and aromatic amines comprising of cyclohexyl, phenyl, 4-methyl phenyl, 4-bromophenyl, 4-chloro phenyl, 2-methoxy phenyl, 4-methoxy phenyl, 2, 4-dimethyl phenyl, 2-chloro phenyl, 3-nitrophenyl, 2,6-dimethylphenyl, 2-(3,4-dimethyl-phenyl)- ethylamine group and wherein -N=C=N- is the carbodiimide group. 4. A process as claimed in any one of the preceding claims wherein the said carbodiimide of general formula RN=C=NR' comprises R=R' or R≠R'. 5. A process as claimed in any one of the preceding claims wherein the said thiophilic halogen is selected from I2, Br2 or its equivalents such as tribromides selected from the tribromides of tetramethylammonium, tetraethylammonium, tetrabutylamonium, cetyltriethylammonium, pyridinehydrobromideperbromide, phenyltrimethylammonium, benzyltriethylamonium, DABCO, DBU, bmim, MPHT and l,l'-(ethane-l,2-diyl) dipyridinium bis-tribromide preferably I2. 6. A process as claimed in any one of the preceding claims wherein the said base is preferably selected from triethylamine, pyridine, DBU, MH3, NaH, NaOH, KOH, NaHCO3, Na2CO3, K2CO3 and most preferably Et3N. 7. A process as claimed in any one of the preceding claims wherein the said organic solvent comprise aprotic solvent preferably CH2CI2, CHCI3, toluene, THF, hexane, CH3CN and the like and most preferably ethylacetate. 8. A process as claimed in any one of the preceding claims wherein the molar ratio of the said 1, 3-disubstituted thiourea to thiophilic halogen is in the range of 1: 1 to 1: 1.1 preferably 1: 1 more preferably 1: 1.05 and the molar ratio of the said 1, 3-disubstituted thioureas to base is in the range of 1:1.5 to 1: 2.5 preferably 1: 2. 9. A process as claimed in any one of the preceding claims wherein the said thiophilic halogen is added in a controlled manner over a period of 15-20 minutes to an ice-cooled solution of the said 1, 3-disubstituted thiourea and said base in said solvent. 10. A process as claimed in anyone of the preceding claims wherein the 1, 3- disubstituted thiourea is suspended in organic solvent preferably ethylacetate and cooled in an ice bath. 11. A process as claimed in anyone of the preceding claims wherein the product obtained comprise a yield% of 58-93% and purity of >98 %. 12. A process for the preparation of carbodiimides of general formula RN=C=NR' wherein R, R' is an alkyl or an aryl group and -N=C=N- is the carbodiimide group substantially as herein described and illustrated with reference to the accompanying examples. A cost-effective, eco-friendly and energy efficient process for the preparation of high purity alkyl and aryl carbodiimides by the direct treatment of 1, 3-disubstituted thioureas with thiophilic halogens in the presence of base in organic solvent demonstrating good yields and high purity of the products. The process involves safe and inexpensive reagents and is energy efficient and is therefore favoured for wide scale industrial application and use.

Full Text

FIELD OF THE INVENTION
The invention relates to the synthesis of alkyl and aryl carbodiimide and more
particularly an eco-friendly and energy efficient process to produce the same
involving very simple and cost-effective process steps. More specifically the alkyl and
aryl carbodiimide with high purity, stability and yield are obtained involving direct
treatment of the 1, 3-disubstituted alkyl and aryl thiourea with thiophilic halogen in
the presence of base and in organic solvent. The reagents are easily available, safe,
non-corrosive, environmentally benign and inexpensive. The process is further
directed to a simple and faster production of the alkyl and aryl carbodiimide without
the requirement of any extraneous heating making it further energy efficient.
BACKGROUND ART
Carbodiimides rank as one of the most important class of reagents in organic
synthesis due to their easy accessibility, interesting chemical reactivity and utility in
coupling reactions. Carbodiimides are widely used reagent for the synthesis of
heterocycles, polysubstituted guanidines and various other compounds.
Carbodiimides are an important class of molecules in synthetic organic chemistry
which are widely used as synthetic intermediates and in several industrial
applications such as synthesis of nucleotides and peptides, heterocycles, oxidation
with dimethyl sulfoxide, permease inhibitors, polymer stabilizer, cycloaddition etc.
Thus for its known wide use and application in various branches of chemistry, it is
therefore not very surprising that there are several reports for the synthesis of
carbodiimides exist in literature.
Reference is drawn to arts, Chemical Review. 1953, 53, 145; Chem. Rev. 1967, 67,
107; Chem. Rev. 1981, 81, 689 and Tetrahedron 1981, 37, 233 where various
methods of preparation of carbodiimides have been extensively discussed. The
general methods involve the removal of hydrogen sulfide from thiourea by metal
oxides (HgO, PbO, Ag2O), complexes of mercury, alkylcholoroformates, (CICOOEt),
arylsulfonyl chlorides. The disadvantage of such a process resides in the use of
polluting heavy metal ions.

Carbodiimides can also be prepared by the oxidation of thioureas using various
oxidizing agents and catalysts.
Apart from the prior arts existing in literature there are numerous prior arts in the
form of patents that are discussed below.
Japanese patent JP06234725 relates to a process wherein carbodiimides are
prepared by the treatment of 1,3-disubstituted thiourea with cyclic haloiminium salts
such as l-choloro-l,3-dimethylimidazolium chloride and Et3N in CH2CI2 at room
temperature for 4 hrs. The drawbacks of the method lie in the use of expensive
reagents and halogenated solvent.
Reference is drawn to EP307361 wherein the carbodiimide intermediate for the
preparation of cyanoguanidine an epoxide hardener is prepared by photochemical
oxidation of thiourea, NaOH, Rose Bengal in CH3CN/H2O under photochemical
conditions. Specialized reaction conditions and the use of expensive Rose Bengal are
the associated shortcomings of the method.
Spanish patent ES481581, Britain patent GB1252707, US patent 3635947 and
German patent DE1927529 are all directed to the synthesis of carbodiimides from
RIMHC(Z)NHRj (Z= 0, S) with phosphorous based reagents such as Ph3PX2.Et3N
(X=CI, Br), PhPO(NH2)2 in CH2CI2, under alkaline conditions at 0-5 °C. Use of
expensive phosphorous reagents and halogenated solvents are the main
disadvantages of the method.
Yet again reference is drawn to US patents US4068065, US4068055, US3157662,
US3056835, US3442947, US2853518, US2853473 and US3301895, German patent
DE 2556760 and French patent FR1469946 that discloses the synthesis of
carbodiimides from isocyanates using polymer supported catalyst such as 1-
diethanolaminomethyl-3-methyl-3-phospholene-l-oxide-methyl-methacrylate
copolymer and 2-methyl-l, 2-oxaphospholan-5-one 2-oxide, organometallic catalysts
selected from the group consisting of (iso-Pro)4Ti (I), (BuCHEtCH2O)4Ti, (C8Hi7O)4Zr
(II), (iso-PrO)4Zr and (EtO)5Nb, phosphorous based catalyst such as 1,2,3-trimethyl-
2-oxo-l,3,2-diazaphospholidine (I), or analogs, Ba[Bu2B(OBu)2]2 and (Bu3Sn)2O.

Involvement of high reaction temperatures expensive catalysts are the main
disadvantages associated with the above mentioned catalysts.
US3301895 again discloses the synthesis of carbodiimides by mixing equi-molar
amounts of 1, 3-disubstitutes thiourea with COCI2 in an inert atmosphere in
anhydrous condition in the absence of acid acceptor at 0-6 °C. On completion the
reaction is heated to yield the carbodiimides. Highly toxic reagents, requirement of
inert atmosphere techniques damages the wide applicability of the process.
Reference is drawn to Japanese patent JP 41011865 wherein carbodiimide is
manufactured by treating 1, 3-disubstituted thioureas with NaOCI under alkaline
NaOH conditions. The drawbacks of the method rest in the use of strong oxidizing
agent, alkaline reaction conditions and elevated temperature.
German patent DEI 149712 teaches the preparation of carbodiimides by heating
isocyanide dichlorides with the HCI salt of primary amines. The reaction is carried out
in an inert atmosphere at 180 °C for 18 hrs. Longer reaction time, expensive
reagents and requirement of high temperature are the drawbacks of the method.
Details of the various methods of preparation of carbodiimides are recited in Chem.
Rev. 1953, 53, 145; Chem. Rev 1967, 67, 107; Chem. Rev. 1981, 81, 689 and
Tetrahedron 1981, 37, 233. Some of the other exiting literature methodologies are
described below.
Tetrahedron Letters 2007, 48, 6002 illustrates a process wherein isocyanates are
catalytically converted to the corresponding carbodiimides in the presence of low
valent transition metal complexes. The best conversions and yields were
accomplished using [(C5R5)Fe(CO)2]2 (R=H or Me) or MeCpMn(CO)3 as catalysts.
Again use of expensive catalysts restricts the applicability of the stated process.
Reference is again drawn to Organometallics 1993, 12, 1023 that is directed to the
use of vanadium oxo and imido complexes as catalysts for the condensation of aryl
isocyanates to N, N'-diarylcarbodiimides. Complexes such as V(O)(OtBu)3 (I),
V(O)(OiPr)3 (II); V(NPhMe)(OtBu)3 (III); and V(O)(acac)2 (IV) were found to be more

effective in this regard. Use of expensive vanadium based reagents is the main
drawback of the method.
Eur. 3. Org. Chem. 2006, 4170 and J. Org. Chem. 1994, 59, 4931 is directed to a
method wherein iminophosphorane or P(MeNCH2CH2)3N when reacted with
phenylisocyante yields a mixture of carbodiimides through both normal and abnormal
aza-Wittig reactions. The reagent 3-aminoimidazolone was obtained from the
reaction of hydrazine hydrate with carbodiimide. Further reaction of 3-
aminoimidazolone with triphenylphosphane, hexacholoroethane and triethylamine
produced iminophosphorane in good yield. Low yield and expensive phosphorous
based reagents are the associated drawback of the method.
Reference is made to J. Org. Chem. 2005, 70, 6362 wherein a thiophilic promoted
synthesis of disubstituted 4H-[1, 2, 4] triazol-3-ylamines as urea mimetic from the
corresponding 1,3-disubstituted thioureas were studied and found that the reaction
proceeded through the formation of a carbodiimide. Use of expensive above
mentioned thiophilic reagent is the major disadvantage of the method.
J. Org. Chem. 1999, 64, 6984 and Bulletin of Korean Chemical Society, substantiates
a process wherein 2-chloro-l,3-dimethylimidazolinium chloride (DMC) and N-methyl-
2-pyridinecarbamoyl chloride have been used as dehydrating agents, for the
synthesis of carbodiimide in near neutral conditions from 1,3-disubstituted thiourea
Use of expensive dehydrating agents are the limitations of this prior art.
Reference is made to Synth Commun. 1995, 43 where in carbodiimides were
prepared by treating thiourea with methanesulfonylchloride in the presence of
triethylamine in DMAP. Moisture sensitive and lachrymatic methanesulfonyl chloride
and the use of toxic DMAP are the stated disadvantages of the above prior art.
Tetrahedron Lett. 1986, 27, 6099; Tetrahedron Lett. 1986, 27, 1925; Tetrahedron
Lett. 1985, 26, 1661 and 3. Org. Chem. 1986, 51, 2613; Synth Commun. 1985, 171;
Makromolecular Chemie, 1981, 182, 3411; teaches the use of desulfurylating agent
utilized for the synthesis of carbodiimide starting from thiourea; such as p-
methoxybenzenetellurinic acid anhydride, di-2-pyridyl sulfite and l,l'-thiocarbonyldi-

2,2'-pyridine and 2,4-dichloro-5-nitropyrimidine. All these methods use expensive
reagents for carbodiimide synthesis.
Reference is drawn to Tetrahedron Letters 1968, 29, 2473 and Tetrahedron 1970,
26, 5731 wherein carbodiimides were prepared from 1, 3-disubstituted thioureas,
diethyl azodicarboxylate and triphenylphosphine refluxed in benzene or toluene. Uses
of carcinogenic solvent and expensive reagents are the major detraction of the
processes.
Chem. Commun. 1997, 347 is directed to a method wherein the synthesis of N, N'-
dialkyl carbodiimide was achieved by catalytic condensation of amines and isonitrile
at 100 °C using a palladium complex catalyst, oxygen and molecular iodine. The use
of expensive palladium catalyst and requirement of elevated temperature is the
drawback of the method.
Inspite of the huge number of teachings flowing from the background art, such
above said processes for the synthesis of alkyl and aryl carbodiimides suffer from
either one or many of the drawbacks which include: use of expensive/ toxic/
lachrymatic reagents, expensive dehydrating agents and catalysts; need of strongly
basic and harsh reaction conditions; use of hazardous halo solvent and carcinogenic
solvents such as benzene; low yield alongwith long reaction times and thus less
efficient; moisture sensitive reagent needing inert atmosphere techniques thus
difficult in handling.
In light of the above said there is a continuous need in the art to develop a process
for the manufacture of alkyl and aryl carbodiimides in an industrial scale which would
circumvent all the aforementioned difficulties associated with the process.
More particularly, it would be clearly apparent from the discussion above that there
is a strong need in the art to provide for a process for the manufacture of alkyl and
aryl carbodiimides in an industrial scale that would be: energy efficient; yield high
purity product in good yields; economic with regard to the kind of reagents used and
eco-friendly. More importantly, the method employed for product isolation in pure
from is also an important tool to assess the feasibility of the said process.

OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide for a process for the
manufacture of alkyl and aryl carbodiimides in an industrial scale that would be
efficient, economic, environment friendly and importantly would yield highly pure
products in good yield.
Another object of the invention is to provide for a clean simple process for a large
scale manufacture of alkyl and aryl carbodiimides of general formula RN=C=NR'.
Yet another object of the invention is to provide for a process for manufacturing alkyl
and aryl carbodiimides of general formula RN=C=NR' that would be economically
viable, eco-friendly and energy efficient for large scale and faster production of the
said carbodiimides under mild conditions.
Yet further object of the present invention is directed to the manufacture of alkyl and
aryl carbodiimides involving inexpensive, non-toxic and easily available starting
materials and non-corrosive reagents.
A further object of the invention is to provide for a process for the manufacture of
alkyl and aryl carbodiimides that would be simple and easily operable under mild
reaction conditions.
Yet another object of the invention is to provide for a process for the manufacture of
alkyl and aryl carbodiimides that would be energy efficient with no requirement for
any external heating.
It is a further object of the invention to provide for a process for the manufacture of
alkyl and aryl carbodiimides that would be economical and can be accomplished
within a short time.
Still another object of the invention is to provide for a process for the manufacture of
alkyl and aryl carbodiimides that would results in highly pure product specifications.

Yet another object of the invention is to provide for a process for the manufacture of
alkyl and aryl carbodiimides wherein the desired products of alkyl and aryl
carbodiimides would be obtained in quantitative yields.
SUMMARY OF THE INVENTION
This according to the basic aspect of the present invention there is provided a
process for the preparation of carbodiimides of general formula RN=C=NR' wherein
R, R' is an arkyl or an aryl group and -NCN- is the carbodiimide group comprising of
(a) reacting or decomposing 1, 3-disubstituted thioureas with thiophilic halogen in
the presence of a base in organic solvent and (b) obtaining therefrom the said
carbodiimides.
In accordance with a preferred aspect of the process of the present invention the
said step of obtaining the carbodiimides by decomposing the 1,3-disubstituted
thioureas in organic solvent comprises of (i) separating the precipitated sulphur and
evaporating the solvent (ii) redissolving the residue in a water immiscible solvent,
washing with water, (iii) drying the water immiscible layer with anhydrous Na2SO4
followed by (iv) evaporation of the solvent layer and (v) purifying over a short silica
gel column (eluted with Hexane) to thereby obtain the said alkyl and aryl
carbodiimides of general formula RN=C=NR'.
In accordance with another preferred aspect of the process of the present invention
the said step of obtaining carbodiimides of the said general formula RN=C=NR'
wherein R, R' is an alkyl and aryl group selected from the group of aliphatic and
aromatic amines comprising of cyclohexyl, phenyl, 4-methyl phenyl, 4-bromophenyl,
4-chloro phenyl, 2-methoxy phenyl, 4-methoxy phenyl, 2,4-dimethyl phenyl, 2-
chloro phenyl, 3-nitrophenyl, 2,6-dimethylphenyl, 2-(3,4-dimethyl-phenyl)-
ethylamine group and wherein -N=C=N- is the carbodiimide group.
Advantageously, the process in accordance with the invention comprises providing
carbodimides wherein the said carbodiimide of general formula RN=C=NR' comprises
R=R'or R*R'.
The said thiophiiic halogen is selected from I2, Br2 or its equivalents such as
tribromides selected from the tribromides of tetramethylammonium,

tetraethylammonium, tetrabutylamonium, cetyltriethylammonium,
pyridinehydrobromideperbromide, phenyltrimethylammonium,
benzyltriethylamonium, DABCO, DBU, bmim, MPHT and l,l'-(ethane-l,2-diyl)
dipyridinium bis-tribromide preferably I2.
The said base is preferably selected from triethylamine, pyridine, DBU, NH3, NaH,
NaOH, KOH, NaHCO3, Na2CO3, K2CO3 and most preferably Et3N.
The said organic solvent comprise aprotic solvent preferably CH2CI2, CHCI3, toluene,
THF, hexane, CH3CN and the like and most preferably ethylacetate.
In accordance with another aspect of the invention in the above process, the molar
ratio of the said 1, 3-disubstituted thiourea to thiophilic halogen is in the range of 1:
1 to 1: 1.1 preferably 1: 1 more preferably 1: 1.05 and the molar ratio of the said 1,
3-disubstituted thioureas to base is in the range of 1:1.5 to 1: 2.5 preferably 1:2.
Importantly also, in the above process the said thiophilic halogen is added in a
controlled manner over a period of 15-20 minutes to an ice-cooled solution of the
said 1, 3-disubstituted thiourea and said base in said solvent.
In accordance with an aspect of the above process the 1, 3-disubstituted thiourea is
suspended in organic solvent preferably ethylacetate and cooled in an ice bath.
Advantageously in the above process the product obtained comprise a yield% of 58-
93% and purity of > 98%.
The details of the invention, its objects and advantages are explained here under in
greater detail in relation to non-limiting exemplary illustrations as per the following
examples:
DETAILED DESCRIPTION OF THE INVENTION
As already disclosed herein before the present invention comprises of a simple and
cost-effective and environment friendly process for manufacture of alkyl and aryl
carbodiimides of the general formula RN=C=NR', wherein R is an alkyl and aryl

group selected from the group of aliphatic and aromatic amines comprising of
cyclohexyl, phenyl, 4-methyl phenyl, 4-bromophenyl, 4-chloro phenyl, 2-methoxy
phenyl, 4-methoxy phenyl, 2,4-dimethyl phenyl, 2-chloro phenyl, 3-nitrophenyl, 2,6-
dimethylphenyl, 2-(3,4-dimethyl-phenyl)-ethylamine group and wherein -N=C=N- is
the carbodiimide group.
The above mentioned alkyl and aryl carbodiimides are obtained in almost
quantitative yields from the reaction involving a thiophilic halogen preferably iodine
mediated decomposition of 1, 3-disubstituted thiourea in aprotic or polar protic
solvent preferably ethylacetate (aprotic) at room temperature.
The 1, 3-disubstituted thiourea are easily prepared by an established procedure by
reacting aryl or alkyl isothiocyanate with aryl or alkyl amine.
Once the synthesis of 1, 3-disubstituted thiourea is accomplished, a thiophilic
reagent preferably iodine proved to be an effective reagent for the decomposition it
to the desired carbodiimide in good yield within 15-20 minutes in the presence of
triethylamine or NaHCO3.
Some of the previously reported methods although giving better yields are quite
expensive and environmentally unsafe and hence not economically viable and
environmentally acceptable for large scale production. The reagent iodine for the
decomposition of thiourea is non-corrosive, inexpensive and environmentally safe.
Thus the method of the present invention provides for an economically viable
process for the preparation of carbodiimide. The reaction is rapid and facile and
accomplished under mild conditions whereby the product obtained is in high yields
and purity.
The reagent iodine for the decomposition of 1, 3-disubstituted thioureas is non
corrosive, inexpensive and environmentally safe. Thus the method provides an
economically viable process for the preparation of carbodiimides. The reaction is
rapid and facile and accomplished under mild conditions. The product obtained is in
high yields and high purity after chromatographic purification.

Example I: Preparation of 1,3-disubstituted thiourea
Phenyl isothiocyanate (675 mg, 5 mmol) in ethanol 5 mL was added aniline (505 mg,
5 mmol) and the reaction mixture was stirred for 30 minutes. After completion of the
reaction solvent ethanol was evaporated and the crude product as obtained was used
as such for the subsequent reaction.
Synthesis for the preparation of the 1, 3-disubstituted thiourea was carried out as
depicted below in Scheme 1

Scheme 1
Example II: Manufacture of alkyl and aryl carbodiimide from 1. 3-
disubstituted thiourea in organic solvent
The 1, 3-diphenyl thiourea (456 mg, 2 mmol) obtained as above following Example I,
a base preferably triethylamine (558 µL, 4 mmol) was suspended in a solvent,
ethylacetate (10 mL), which is cooled prior to reaction in an ice bath for 5 minutes.
The reaction or decomposition of 1, 3-disubstituted thiourea was effected by the
addition of a thiophilic halogen, iodine (558 mg, 2.2 mmol) by adding it in a
controlled manner pinch wise. The reaction being exothermic addition of iodine in
pinches was necessary while cooling for the reaction to proceed and to minimize the
side products. Iodine was added in pinches over a period of 15-20 minutes. When
the iodine addition was complete and after 10 minutes a TLC experiment on the
reaction mixture revealed that the reaction was complete and overall time taken for
the completion of the reaction was ~30 minutes. The precipitated sulphur was
filtered and ethylacetate was evaporated in rotary evaporator under reduced
pressure. The residue was redissolved in a water immiscible solvent, hexane (b. p.
60-70 °C). The hexane layer was washed with water and dried over anhydrous
Na2SO4 and subsequently evaporated on a rotary evaporator. The crude product so
obtained was further purified over a short silica gel column (eluted with 100 %
hexane) to give 1, 3-diphenylcarbodiimide in 86 % isolated yield. The above

synthetic procedure followed is further briefly outlined below in Scheme 2
hereunder.

Where R = alkyl or aryl; R=R' or ≠ R'
Scheme 2
The above examples are given by way of illustration of the present invention and
therefore should not be construed to limit the scope of the present invention. All
percentages are by weight.
The above exemplary illustration of the process of the invention clearly reveals the
efficient, economic and environment friendly method for the preparation of alkyl and
carbodiimides directed to overcome the difficulties associated with what is already
known in the art in its preparative process. Importantly, it is found by way of the
invention that Iodine being highly thiophilic (electrophilic) in nature can interact with
sulphur (nucleophile) thus assisting the desulphurization process of 1, 3-
disubstituted thiourea. This is facilitated by the abstraction of NH protons by
triethylamine / NaHCO3 present as a base in the reaction medium. Formation of
elemental sulfur as the sole byproduct of the reaction supports the mechanism as
proposed in Scheme 3. Performing the reaction under ice-cold conditions minimizes
the side products. Thus the preferred use of iodine as a thiophilic reagent favours
decomposition of pre-synthesized 1, 3-disubstituted thioureas to carbodiimides.
Controlled addition of iodine in pinches over a time span of 15-20 minutes in the
reaction flask is a prerequisite to prevent side reactions. The mechanism for the
formation of alkyl and aryl carbodiimides is further illustrated in the following
Scheme 3.

Scheme 3
Thus the examples clearly illustrate an overall simple, fast and efficient process for
the manufacture of alkyl and aryl carbodiimides.
The above process is quick in yielding the said carbodiimides in a short time span of
~30 minutes, more preferably within 15 minutes and in > 98% pure form in good
yields of 58-93 %. It is also noteworthy that the process is very economical for the
inexpensive nature and easy commercial availability of the reagents employed.
Advantageously the process provided in this invention is environmentally safe for the
non corrosive, non toxic nature of the reagents and mild basic conditions needed for
the process. Accordingly the process provided in this invention is favourable with
respect to easy handling of the reagents which are moisture insensitive and non
lachrymators.
The advantage associated with the above said process also resides in the easily
isolable and harmless byproduct such as sulphur which can be separated by simple
filtration. The above process is advantageously also energy efficient since no external
heating is required.
It is thus possible by way of the present invention to provide for a process for the
manufacture of alkyl and aryl carbodiimides which is efficient in terms of energy
needed for activation for the formation of products from reactants, time required and
high purity associated with the isolated yields of the product. The process is
environment friendly especially in terms of non toxic nature of the reagents needed,
mild reaction conditions employed and the harmless by product obtained. It is easy
to operate in terms of the moisture insensitive and non lachrymator reagents needed
for the purpose.

WE CLAIM:
1. A process for the preparation of carbodiimides of general formula RN=C=NR'
wherein R, R' is an arkyl or an aryl group and -NCN- is the carbodiimide group
comprising of (a) reacting or decomposing 1, 3-disubstituted thioureas with
thiophilic halogen in the presence of a base in organic solvent and (b)
obtaining therefrom the said carbodiimide.
2. A process as claimed in claim 1 wherein said step of obtaining the
carbodiimides by decomposing the 1, 3-disubstituted thioureas in organic
solvent comprises of (i) separating the precipitated sulphur and evaporating
the solvent (ii) redissolving the residue in a water immiscible solvent, washing
with water, (iii) drying the water immiscible layer with anhydrous Na2SO4
followed by (iv) evaporation of the solvent layer and (v) purifying over a short
silica gel column (eluted with hexane) to thereby obtain the said alkyl and aryl
carbodiimides of general formula RN=C=NR'.
3. A process as claimed in anyone of the preceding claims comprising providing
carbodiimides of the said general formula RN=C=NR' wherein R, R' is an alkyl
and aryl group selected from the group of aliphatic and aromatic amines
comprising of cyclohexyl, phenyl, 4-methyl phenyl, 4-bromophenyl, 4-chloro
phenyl, 2-methoxy phenyl, 4-methoxy phenyl, 2, 4-dimethyl phenyl, 2-chloro
phenyl, 3-nitrophenyl, 2,6-dimethylphenyl, 2-(3,4-dimethyl-phenyl)-
ethylamine group and wherein -N=C=N- is the carbodiimide group.
4. A process as claimed in any one of the preceding claims wherein the said
carbodiimide of general formula RN=C=NR' comprises R=R' or R≠R'.
5. A process as claimed in any one of the preceding claims wherein the said
thiophilic halogen is selected from I2, Br2 or its equivalents such as
tribromides selected from the tribromides of tetramethylammonium,
tetraethylammonium, tetrabutylamonium, cetyltriethylammonium,
pyridinehydrobromideperbromide, phenyltrimethylammonium,
benzyltriethylamonium, DABCO, DBU, bmim, MPHT and l,l'-(ethane-l,2-diyl)
dipyridinium bis-tribromide preferably I2.

6. A process as claimed in any one of the preceding claims wherein the said base
is preferably selected from triethylamine, pyridine, DBU, MH3, NaH, NaOH,
KOH, NaHCO3, Na2CO3, K2CO3 and most preferably Et3N.
7. A process as claimed in any one of the preceding claims wherein the said
organic solvent comprise aprotic solvent preferably CH2CI2, CHCI3, toluene,
THF, hexane, CH3CN and the like and most preferably ethylacetate.
8. A process as claimed in any one of the preceding claims wherein the molar
ratio of the said 1, 3-disubstituted thiourea to thiophilic halogen is in the
range of 1: 1 to 1: 1.1 preferably 1: 1 more preferably 1: 1.05 and the molar
ratio of the said 1, 3-disubstituted thioureas to base is in the range of 1:1.5 to
1: 2.5 preferably 1: 2.
9. A process as claimed in any one of the preceding claims wherein the said
thiophilic halogen is added in a controlled manner over a period of 15-20
minutes to an ice-cooled solution of the said 1, 3-disubstituted thiourea and
said base in said solvent.
10. A process as claimed in anyone of the preceding claims wherein the 1, 3-
disubstituted thiourea is suspended in organic solvent preferably ethylacetate
and cooled in an ice bath.
11. A process as claimed in anyone of the preceding claims wherein the product
obtained comprise a yield% of 58-93% and purity of >98 %.
12. A process for the preparation of carbodiimides of general formula RN=C=NR'
wherein R, R' is an alkyl or an aryl group and -N=C=N- is the carbodiimide
group substantially as herein described and illustrated with reference to the
accompanying examples.

A cost-effective, eco-friendly and energy efficient process for the preparation of high
purity alkyl and aryl carbodiimides by the direct treatment of 1, 3-disubstituted
thioureas with thiophilic halogens in the presence of base in organic solvent
demonstrating good yields and high purity of the products. The process involves safe and inexpensive reagents and is energy efficient and is therefore favoured for wide scale industrial application and use.

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Patent Number

270484

Indian Patent Application Number

998/KOL/2009

PG Journal Number

01/2016

Publication Date

01-Jan-2016

Grant Date

28-Dec-2015

Date of Filing

27-Jul-2009

Name of Patentee

INDIAN INSTITUTE OF TECHNOLOGY

Applicant Address

INDIAN INSTITUTE OF TECHNOLOGY, GUWAHATI, GUWAHATI-781039, ASSAM, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	PATEL, BHISMA KUMAR	INDIAN INSTITUTE OF TECHNOLOGY, GUWAHATI, DEPARTMENT OF CHEMISTRY, GUWAHATI-781039
2	GHOSH, HARISADHAN	INDIAN INSTITUTE OF TECHNOLOGY, GUWAHATI, DEPARTMENT OF CHEMISTRY, GUWAHATI-781039
3	ALI, ABDUR REZZAK	INDIAN INSTITUTE OF TECHNOLOGY, GUWAHATI, DEPARTMENT OF CHEMISTRY, GUWAHATI-781039

PCT International Classification Number

C07D209/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA