Carbodiimide from isocyanates

Carbodiimide, diphenyl-, 43,31 Carbodiimides, from isocyanates, 43, 31 Carboxylation of saturated hydrocarbons containing tertiary hydrogens, 44, 3... 

Gade and coworkers reported that titanium hydrazinediido complexes act as effective catalysts for hydrohydrazination reactions involving substituted hydrazines and carbodiimides giving rise to aminoguanidines in good yields. On the other hand, Sundermeyer developed titanium phthalocya-nine complexes as catalysts for the synthesis of highly pure carbodiimides from isocyanates under solvent-free reaction conditions. ... 

Isocyanates undergo dimerization reactions by a [2+2] cycloaddition across their C=N bonds to give diazetidinediones 3. The isomeric unsymmetrical dimers have never been isolated but they are postulated to be intermediates in the formation of carbodiimides from isocyanates. The isocyanate dimers usually dissociate back to the monomers on heating. Therefore, they are considered to be masked isocyanates. The dimerization of isocyanates requires the use of a base or a Lewis acid as a catalyst, and often isocyanate trimers are formed as coproducts. 

Calcium hydride, for drying of lert-butyl alcohol, 41, 7 CARBODIIMIDE, DrPHENYL-, 43, 31 Carbodiumdes, from isocyanates, 43, 31... 

A (pentamethylcyclopentadienyl)iridium chelating guanidinate complex has been conveniently prepared by treatment of [Cp IrCl2]2 with N,N, N"-th-p-tolylguanidine and base in THF at room temperature followed by recrystallization of the green product from toluene and pentane (Scheme 154). Insertion reactions of the product with heterocumulenes (diaryl carbodiimides, aryl isocyanates) have been investigated. It was found that the complex serves as highly active catalyst for the metathesis of diaryl carbodiimides with each other and for the more difficult metathesis of diaryl carbodiimides with aryl isocyanates (cf. Section V.C). ... 

Prolonged heating of free isocyanates in the absence of catalysts has been reported recently to give poor yields of carbodiimides unless a slow stream of nitrogen was passed through the boiling isocyanate [18]. In the presence of phosphorus catalysts [18, 19, 27-39], aromatic carbodiimides are obtained in high yield under mild conditions from isocyanates but not isothiocyanates [18]. Aliphatic isocyanates react more slowly, but improved yields are obtained in... 

The presence of the carboxylic group will cause further breakdown. The removal of this group by the reaction with carbodiimide helps stop continued breakdown of the polyester. Carbodiimides are made from isocyanates. Their reaction with the carboxylic acid generated in the hydrolysis is illustrated in Figure 2.42. 

The use of dicarbodiimides as monomers in polyaddition reactions have not as yet found wide utility. However, polymers containing carbodiimide groups are known, and further nucleophilic reactions of these polymers with numerous substrates are reported. Carbodiimides, generated in situ from isocyanates are used as catalyst in the formation of polyamides from diisocyanates and dicarboxylic acids. Also, homoleptic lanthanide amidinates, made from carbodiimides, exhibit high catalytic activity for the ring opening polymerization of e-caprolactone at room temperature. ... 

The alkyl-, alky laryl- and diarylcarbodiimides are the diimides derived from carbon dioxide, however, no direct formation of carbodiimides from amines and carbon dioxide is known. Interestingly, carbodiimides can be obtained from amines and carbon dioxide via a switteri-onic titanium complex (see Section 2.2.8). The major starting materials for the synthesis of carbodiimides are isocyanates, 1,3-disubstituted ureas or 1,3-disubstituted thioureas. The synthesis of isocyanates requires the use of the toxic carbonyl chloride or its oligomers. A book on the synthesis and reactions of isocyanates appeared in 1996. ... 

Symmetrical carbodiimides, i.e., molecules with the same substituent on both nitrogen atoms, are best prepared from alkyl or aryl isocyanates in the presence of a phospholene oxide catalyst, the byproduct being carbon dioxide gas. No solvent is required for this reaction. Unsymmetrical carbodiimides or alkylarylcarbodiimides are also obtained from isocyanates, either by reaction with amines and subsequent dehydration of the intermediate... 

Many other carbodiimides with two different substituents are conveniently synthesized from isocyanates and iminophosphoranes. For example, carbodiimides 67 with a phosphorous substituent in the a-position are obtained from the isocyanate 65 and the iminophos-phorane 66 (R = i-Pr or Ph). °... 

From Phosphoramidates. As well as iminophosphoranes phosphoramidates 86 also react with isocyanates to give carbodiimides 87 as shown by Wadsworth and Emmons in 1964. This is also a snitable method to synthesize carbodiimides with different substituents, from isocyanates. 

In Table 2.1 the [2+2] cycloadducts derived from carbodiimides and isocyanates are listed. 

Table 2.1 l2+2j Cycloadducts derived from Carbodiimides and Isocyanates... 

In this reaction phenyl isocyanate is generated in an exchange reaction of the diphenylcar-bodiimide with iron pentacarbonyl, and the unusual reaction product is formed from the carbodiimide, the isocyanate and two eqnivalents of the acetylene derivative. 

Attempts to synthesize N-heterocyclic substituted carbodiimides from 1,2,4-triazine imino-phosphoranes 19 and isocyanates result in the isolation of the corresponding... 

I became involved in carbodiimide chemistry in my research work on isocyanates at the former Donald S. Gilmore Research Laboratories of the Upjohn Company in North Haven, CT. Carbodiimides are readily synthesized from isocyanates using a phospholene oxide catalyst. This reaction can be conducted without a solvent, and the byproduct is carbon dioxide. We used this reaction in the manufacture of a liquid version of MDI (4,4 -diisocyanatodiphenylmethane), which today is sold in huge quantities worldwide. By reacting MDI with dicarboxylic acids in a vented extruder we manufactured a family of thermoplastic polyamide elastomers, which are sold today by the Dow Chemical Company. Also, N-sulfonylcarbodiimides were synthesized for the first time in our laboratories. They are the precursors of the antidiabetic sulfonamides, such as Upjohn s Tolbutamide (Orinase). Because of the close relationship of isocyanates with carbodiimides we studied many linear and cyclic carbodiimide reactions, especially their cycloaddition reactions. 

Similar reactions occur on treatment of benzalaniline, carbodiimides, phenyl isocyanate and N-sulfinylaniline with iminium chlorides For example, from chlorodimethylformiminium chloride and phenyl isocyanate the 1 1 adduct CXXXVII is obtained, which on thermolysis (with elimination of carbonyl chloride) or hydrolysis yields the formamidine CXXXVIH... 

Reviews on the synthesis and chemistry of carbodiimides are given in [1248-1250]. Carbodiimides are mainly synthesized in one of three ways from ureas or thioureas, from isocyanates, or from isocyanides. Several reagents have been employed in carbodiimide synthesis phosgene [1252, 1253], dimethylphosgenimi-nium chloride [1254], tiiphosgene [561, 562], phosphorus pentoxide [1255], phos-phoryl chloride [1256], triphenylphosphine dibromide [758, 1257-1261], triphenylphosphine/tetrahalomethanes [1262, 1263], iminophosphoranes [1264-1277], Mitsunobu reagent [1278, 1279], p-tosyl chloride [1280, 1281], and CDC [1137] oxidative additions have also been used [1282-1284]. 

Table 3.16 [2- -2] Cycloadducts derived from carbodiimides and isocyanates...

The reaction is usually carried out at high temperatures (of about 200°C) in a polar solvent, such as tetramethylene sulfone, and the polyamide formation can be accelerated by the addition of l-phenyl-3-methyl-2-phospholene 1-oxide as catalyst. However, in the case of a two-step process, the reaction time of the first step must be carefully controlled, since the catalyst can also play a role in the formation of carbodiimides from two terminal isocyanate groups [36], These carbodiimides can then further react and lead to crosslinking [36], In most cases [34-39], the polymers are prepared with 4,4 -methylene bis(phenyl-isocyanate) (MDI), using adipic acid, isophtahc acid, azelaic acid, or a mixture of two of them (in order to accelerate the solubilization of the polyamide phase in the solvent) and a polyether based on tetramethylene oxide, ethylene oxide, or a mixture of propylene oxide and ethylene oxide. 

Other six-membered rings with two heteroatoms are also obtained from reaction of diketene with imidates, cyanamides, carbodiimides, isocyanates, oxa2olines, or other multiftmctional compounds. 

Other Reactions. 2-Dioxolaniniines, 2-oxathiolaniniines, and 2-oxa2o1idinimines result from the reaction of propylene oxide with isocyanates, isothiocyanates, and carbodiimides, respectively (103,104). 

Ionic polymers are also formulated from TDI and MDI (43). Poly(urethane urea) and polyurea ionomers are obtained from divalent metal salts of /)-aminohen2oic acid, MPA, dialkylene glycol, and 2,4-TDI (44). In the case of polyureas, the glycol extender is omitted. If TDI is used in coatings apphcations, it is usually converted to a derivative to lower the vapor pressure. A typical TDI prepolymer is the adduct of TDI with trimethyl olpropane (Desmodur L). Carbodiimide-modified MDI offers advantages in polyester-based systems because of improved hydrolytic stabihty (45). Moisture cure systems based on aromatic isocyanates are also available. 

Cycloadditions of isocyanates and their derivatives with vinyiaziridines were first reported by Alper and coworkers. From their previous studies of cydoadditions to vinylepoxides or alkylaziridines, they investigated cydoadditions to vinyiaziridines and found that such reactions with isocyanates, carbodiimides, or isothiocyanates in the presence of catalytic amounts of Pd(OAc)2 (2 mol%) and PPh3 (10 mol%) at room temperature afforded five-membered ring products 249 in 34—97% yields (Scheme 2.61) [91]. When an aziridine 247 possessing an alkyl substituent at the... 

An effective synthesis of the pyrido[T,2 l,2]imidazoI5,4-c]isoquinoline system 41 uses the 3-aceta (or benza)mido-2-phenylimidazo[l,2-a]pyridine 40 and phosphorus oxychloride. An alternative approach using the carbodiimide 42, obtained from the corresponding iminophosph(Mane and an isocyanate, is also successful <95H(41)2019>. 

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