Phenyl isocyanate dimerization reactions

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 ... 

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. 

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. 

This reaction is the reverse of the dimerization described in Section IV,A. Early workers found that phenyl isocyanate could be produced on thermolysis of diphenylfuroxan,329 and benzonitrile oxide was found to rearrange... 

When phenyl isocyanate was heated above 135°C with aluminum chloride, it dimerized to 3-phenylquinazoline-2,4-dione (moderate yield).43 The reaction proceeded similarly when a high-boiling solvent replaced the salt.44... 

EP.15 The dimerization of phenyl isocyanate in the liquid phase is performed in a tubular reactor, according to the following reaction ... 

Some isocyanates also react with themselves to form thermally reversible dimer structures, the so-called uretidinediones. Such self reaction is apparently confined to aromatic isocyanates, and is illustrated with phenyl isocyanate in Fig. 8.4. The dimerization reaction is catalyzed vigorously by trialkyl phosphines and less by tertiary amines such as pyridine. 

Anionic copolymerization of isocyanates with chloral (67), cyanopropionalde-hyde (68), and acrolein dimer (69) is also known. For example, reaction of phenyl isocyanate with chloral in DMF at -68°C, using potassium cyanide as initiator, affords chloroform-soluble copolymers 10, which melt at about 230°C (67). 

Another type of cycloaddition reaction is observed when dimethoxycarbene is generated in the presence of phenyl isocyanate. In this case the dimethoxycarbene dimerizes to form tetramethoxyethylene which undergoes a [2+2] cycloaddition to form -lactams in 49-68 % yields 28. ... 

Cycloadditions The [4+2] cycloaddition reactions of carbodiimides with phenyl-carbonyl isocyanate, phenylcarbonyl isothiocyanate and thiocarbamoyl isothiocyanate have been discussed above. In the dimerization reactions the functional carbodiimides react as both the diene and the dienophile. Unsaturated carbodiimides, generated in situ, can be trapped with N=N bond- or C=N bond-containing substrates. 

In the reaction of Ph3P=C=C=NPh with phenyl isocyanate a Wittig-type reaction is observed, generating a new heterocumulene, which dimerizes under the reaction conditions to form the four-membered ring dimer 5 in 53 % yield... 

Some of the physical constants of pyrrole and of a selection of its derivatives are collected in Table 4,1, The boiling point of pyrrole is higher than might have been expected, and the closer similarity in this property of 1-methylpyrrole to, say, toluene suggests that in pyrrole the imino group is responsible for some sort of association (see below). The characterization of pyrrole and simple alkylpyrroles through the formation of crystalline derivatives is not always easy. Picrates are usually unstable. In cases where picric acid causes dimerization, the dimer picrate is often a satisfactory derivatively, 233 xhe reaction with phenyl isocyanate (p. 66) is useful. 

More recently, Belzner et al. reported a new type of oxygen transfer reaction from isocyanates to bis[2-(dimethylaminomethyl)phenyl]silylene (8)18 which was thermally generated from the corresponding cyclotrisilane 7, and they obtained some convincing results of the involvement of silanone 9 (Scheme 3). However, they found that silanone 9 is not stable enough to be isolated. Only cyclic di- and trisiloxanes 10 and 11 (i.e., the cyclic dimer and trimer of the silanone 9) were obtained together with the corresponding isonitrile as other main products when... 

The first examples of urea-containing dimeric capsules were discovered independently by Rebek and Bohmer [165,166]. These are based on calix[4]arene ethers, in which the lower rim ether units help to fix the calixarenes in a cone conformation via intramolecular interactions. Figure 60 shows the generic structure of such systems. Synthetically, the urea calixarenes are readily prepared. The calixarenes are first nitrated, followed by a reduction of these groups into amines. The urea derivatives are fixed to the upper rim by reaction of the p-amino calixarenes with isocyanates. Many systems were studied using differently substituted ureas which contain either short alkyl chains or simple phenyl derivatives. Studies also involved the changing of the lower rim ether substituents. 

Oxazepam (116) can be converted into a range of 3-(disubstituted-amino) derivatives, in moderate yield, by its reaction with the appropriate 2-amino-4,5-dihydro-l,3,2-dioxaphosphole. A direct route to the 3-amino-derivatives has also been reported.l,3-Dihydro-2//-l,4-benzodiazepin-2-ones react with most isocyanates to give the 1-carbamoyl derivative however, (114), on reaction with trichloroacetyl isocyanate and subsequent hydrolysis, gave the 3-amido-derivative. The latter has been converted into a range of 3-esters.Continuing work on the electrolytic reduction of 2,3-dihydro-l,4-diazepinium salts has shown that the AW-dibenzyl-6-phenyl-compound gives a diazepine dimer and an unexpected rearrangement product, which is 1,4-di-imidazolidinyl-butadiene. ... 

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