Studies of Acyl and Thioacyl Isocyanates. VIII. Cycloaddition Reactions of Benzoyl and Thiobenzoyl Isocyanates with Carbodiimides

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Abstract
The investigations of cycloaddition reactions of benzoyl and thiobenzoyl isocyanates with carbodiimides have shown that the adducts formed from benzoyl isocyanates and N,N′-dicyclohexylcarbodiimide are the corresponding (4+2) cycloadducts, and not the (2+2) cycloadducts proposed by Neidlein. The reaction of isocyanates with N,N′-diphenylcarbodiimide at 0°C gave (2+2) cycloadducts, which were thermally isomerized into the corresponding (4+2) cycloadducts. Benzoyl isocyanates reacted with N-cyclohexyl-N′-phenylcarbodiimide to give (4+2) cycloadducts of isocyanates to the cyclohexyl-N=C bond in the carbodiimide. On treatment with neutral alumina, (4+2) cycloadducts were easily isomerized into 1,3,5-triazines. Thiobenzoyl isocyanate reacted with both the N=C bonds of the carbodiimide to afford two isomeric (4+2) cycloadducts. It has been found that benzoyl isocyanates reacted with N-phenyl-N′-o-tolylcarbodiimide to yield the corresponding (4+2) cycloadducts of isocyanates to the o-tolyl-N=C bond and/or the isomeric 1,3,5-oxadiazin-6-ones, depending on the nature of the substituent in the isocyanate. Reaction paths for the formation of the product are also discussed.

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