Phenyl isocyanate dimer

A special technique of trimerization has been described by Kogon 24, 25). Phenyl isocyanate reacts with ethyl alcohol to form a urethane (ethyl carbanilate). At 125° a substantial yield of ethyl a,7-diphenyl allophanate is observed as well as a small amount of phenyl isocyanate dimer. However, when A-methyhnorpholine (NMM) is added as a catalyst, the reaction is altered and the product is triphenylisocyanurate (isocyanate trimer) in high yield. The reaction sequence is believed to be ... 

The same compound was obtained when phenyl isocyanate dimer was treated with COF under similar conditions. 

Phenyl-3,3-diethyl urea Diphenyl urea 1,2-Bis(2 -phenylureido) ethane Phenyl isocyanate dimer 3312 3442, 3425, 3325 (shoulder) 3449, 3329, 3305 (max.) 3447, 3329 3440 (max.) 3300 (shoulder)... 

C13H2 4N40, 2-Ethyl-2-(5 -dimethylamino-4 ,4 -dimethyl-4 H-imidazol-2 -yDbutyramide, 45B, 210 C14H10N2O2, Phenyl isocyanate dimer, 40B, 206 Cl4H13O3NCI2r 0,0 ,N-Trimethylpyoluteorin, 35B, 163 Cl4H14BrN30, 1,3-Dimethyl-2-(p-bromobenzoyl-cyanomethylene)-imidazo-lidine, 40B, 53... 

Since some catalysts initiate dimerization and trimerization under specific conditions, equilibria between monomeric isocyanates/catalyst, dimer/catalyst and trimer/catalyst exist. For example, interrupting the trimerization of phenyl isocyanate with a penta-substituted guanidine catalyst after short periods of time leads to the formation of the phenyl isocyanate dimer. ... 

Buckles and McGrew [J. Am. Chem. Soc. 88 (15), 1966] have studied the dimerization of phenyl isocyanate in liquid solution in the presence of a catalyst. 

When a toluene solution of a mixture of cyclotrisilane 34 and cyclohexyl isocyanate (or f-butyl isocyante) was heated at 70 °C, cyclic di- and trisiloxanes 37 and 38, i.e. the cyclic dimer and trimer of the silanone 36, were obtained together with the corresponding isonitrile RN=C. The formation of 37 as well as 38 was completely suppressed in the presence of hexamethylcyclotrisiloxane (19 D3) instead, quantitative conversion of 35 into 39, the formal insertion product of the silanone 36 into the Si—O bond of D3, occurred (Scheme 14). Since neither cyclodisiloxane 37 nor cyclotrisiloxane 38 reacted with D3 under the reaction conditions, the possibility that 37 or 38 is the precursor of 39 was ruled out. Whereas the oxidation of 35 with cyclohexyl and t-butyl isocyanates proceeded with exclusive formation of 37 and 38 (as the silicon-containing compounds) the reaction of 35 with phenyl isocyanate resulted in the formation of 37 in low yield. Furthermore, in this case the presence of D3 did not totally suppress the formation of 37. According to the authors, these results indicate that the oxidation of 35 with cyclohexyl and f-butyl isocyanates appears to use other reaction channels than that with phenyl isocyanate. 

Historically, Hofmann reported in 1858 the first example of diazetidines by dimerization of phenyl isocyanate in the presence of triphenylphosphine <1858MI349>. 

The dimerization of aryl isocyanates to l,3-diarylazetidin-2,4-diones is one of the classical methods for the synthesis of 1,3-diazetidinones. In 1993 trimerization of phenyl isocyanate catalyzed by a fluoride ion was also reported, and a small amount of l,3-diazetidin-2,4-dione was obtained at room temperature <1993JOC1932>. 

Dimerization of phenyl isocyanate, catalyzed by lanthanide complexes, has been reported by Deng et al. <2003CHJ574>. A number of lanthanide complexes were tried and Sm(SPh)3(hmpa)3 was found to be the most effective catalyst. Conversion was as high as 96% with 2500 1 of substrate to catalyst ratio (Scheme 47). 

Dehydration of primary nitroalkanes with phenyl isocyanate or acetic anhydride in the presence of catalytic triethylamine affords nitrile oxides, which may be trapped as their 1,3-dipolar cycloadducts or allowed to dimerize to the corresponding furoxans. Other dehydrating agents that have been used include diketene, sulfuric acid and, when the a-methylene group is activated by electron-withdrawing groups, boron trifluoride in acetic anhydride, trifluoroacetic anhydride with triethylamine, and nitric acid in acetic acid. 

The P-alkyl iminophosphoranes are considerably more reactive than the P-aryl compounds, but iminophosphoranes with alkoxy or chloro groups are also known to participate in the reaction. N-alkyl- or N-aryltrichloroiminophosphoranes are usually obtained as four membered ring dimers, which on thermolysis generate the monomers. For example, thermolysis of the dimeric N-methyltrichloroiminophosphorane 63 in o-dichlorobenzene in the presence of phenyl isocyanate affords N-methyl-N -phenylcarbodiimide 64 in 57 % yield. 

Catalysts which have been found to promote dimerization of phenyl isocyanate include pyridine (11), methylpyridine (12), triethylamine (13), X-methyl- (or ethyl-)morpholine, triethylphosphine, and other alkyl or alkyl-arylphosphines (14, 15). Alkylphosphines bring about a very violent polymerization since they act as active catalysts and the polymerization is quite exothermic. Triphenylphosphine is inactive. Alkyl-arylphosphines are not as active as alkylphosphines and permit better control of the reaction. Another convenient method (14, 16) for control of phosphine-catalyzed dimerization involves the addition of an alkylating agent such as benzyl chloride in an amount stoichioraetrically equivalent to the substituted phosphine present. Complete deactivation of the catalyst results. By this means the reaction may be mitigated or even quenched and then activated by the addition of more catalyst. 

Isocyanate dimers are solids having a fairly high melting point. For example, the dimer of phenyl isocyanate melts at 175°. As expected, diisocyanates can polymerize to form resins. Much attention has been given to the physical properties of dimers including mixed dimers (2, 17), but such properties are not of primary concern here. 

For catalyzed dimerization of phenyl isocyanate to diphenylcarbodiimide, see 3-Methyl-1 -ethyl-3-phospholene-1 -oxide. 

A similar siloxide elimination also occurs for both phenyl isocyanate and diphenyl-ketene giving the heterocumulenes 12 and 13, which are both stable to heat, water and air, and do not dimerize if R = Ar (equation 22)29. 

Phenyl isocyanate inserts the Si-P bond of bis-silylphosphines to give the adduct which eliminates siloxane to yield the dimer with a four-membered C2P2 ring (equation 23). 

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