Preparation and intramolecular cyclization of bis(carbodiimides).: Synthesis and X-ray structure of 1,3-diazetidine-2,4-diimine derivatives

Aza-Wittig reactions of bis(iminophosphorane) 1 derived from 2,2'-diazidobiphenyl with aromatic isocyanates provided dibenzo[d,f]-1,3-diazetidino[1,2-a]diazepine derivatives 2 in moderate yields. Similar results can be achieved from the reaction of 2,2'-bis(isothiocyanato)biphenyl 4 with aryliminophosphoranes. Treatment of bis(isothiocyanate) 4 with arylamines and further dehydrosulfurization of the resulting bis(thioureas) leads to the seven-membered ring guanidines 8. N-2-(2'-Azido)biphenyI-N'-arylcarbodiimides 13 react with 1 equiv of triphenylphosphane to give zwitterionic compounds 15, which undergo either hydrolytic cleavage to afford the guanidines 8 or react with 1 equiv of aromatic isocyanates to provide 2. Cross-experiments suggest that the: conversion 15 --> 2 takes place through a nonisolable bis(carbodiimide) that undergoes an intramolecular [2 + 2] cycloaddition at the final step. The reaction of bis(iminophosphorane) 1 with an excess of carbon dioxide leads to a mixture of the tricyclic 1,3-diazetidine derivative 6 and the 14-membered cyclic bis(carbodiimide) 23, which decomposes by thermal treatment. Compound 6 can also be obtained along with the seven-membered cyclic urea derivatives 24 from the reaction of bis(iminophosphorane) 1 with the carbon dioxide source Boc(2)O/DMAP system. A comprehensive mechanistic scheme for the aza-Wittig reactions studied is conveniently presented. The molecular and crystal structures of 1-(4-methoxyphenyl)-2-(4-methoxyphenyl)iminodibenzo[d,f]-1,3-diazetidino-[1,2-a]diazepine (2c) and [a,c]bis[dibenzo[d,f][1,3]diazepino]-1,3]diazepino]-1,3-diazetidine (6) have been determined by X-ray analysis.