An exploratory study of ring closures of aryl radicals onto cyclopropyl- and oxiranyl-isocyanate acceptors

The idea that ring closures of C-centred radicals onto isocyanates could be made permanent by designing the cyclised radical to undergo a rapid onward beta-scission, was investigated for the 2-(2-isocyanato)cyclopropylphenyl and 2-(2-isocyanato)oxiranylphenyl radicals. The radical precursors, trans- and cis-1-bromo-(2-isocyanatocyclopropyl)benzene and (2-bromophenyl)-3-isocyanatooxirane, were prepared from the corresponding bromophenylcyclopropane and bromophenyloxirane carboxylic acids via Curtius rearrangements of the derived azides. The structure of the trans-2-(2-isocyanato)cyclopropylphenyl radical prevents cyclization, however, it was shown that isomerisation to the analogous cis-radical occurred, probably by scission of the disubstituted cyclopropane bond followed by internal rotation of the resulting resonance stabilised diradical. It was found, however, that the main product from homolytic reactions of both trans- and cis-isocyanatocyclopropyl compounds, with tributyltin hydride and tris(trimethylsilyl)silane, was the direct reduction product, trans-(2-isocyanatocyclopropyl)benzene. Only traces of cyclised products, that were probably 4,5-dihydrobenzo[c]azepin-1-one from the cyclopropane precursor and 5H-6-oxa-8-azabenzocyclohepten-9-one from the oxirane precursor, were detected. We conclude, therefore, that the rate of cyclization onto isocyanate acceptor groups must be slower in these systems than hex-5-enyl cyclization or that the reverse ring-opening process must be faster than for analogous radicals.