Ortho substituent effects on transfer of aryl groups from oxygen to carbon in adducts from diaryloxycarbenes and DMAD

Abstract

Diaryloxycarbenes, generated by thermolysis of 2,2-diaryloxy-5,5-dimethyl-Δ3-1,3,4-oxadiazolines in benzene, reacted with dimethyl acetylenedicarboxylate (DMAD) to afford triesters. The triesters arose by aryl-group transfer from oxygen to carbon (ipso substitution). A cyano group in the para position had been shown earlier to favour the ipso substitution whereas a para-methoxy group disfavoured it. Surprisingly, the ortho-methoxy substituent also favoured ipso substitution. As a tentative rationale, we had proposed that a vinylogous diaryloxycarbene intermediate, with dipolar properties, is formed when a diaryloxycarbene attacks at the sp-carbon of DMAD and that an ortho substituent, with one or more unshared electron pairs, can stabilize the intermediate by donating electron density to the positive site. In the present work, support for that interpretation and for the importance of a steric effect, was provided by examining the competition between (i) p-tolyl and phenyl; (ii) o-tolyl and phenyl; (iii) o-iodophenyl and phenyl; (iv) o-iodophenyl and p-chlorophenyl; and (v) o-bromophenyl and p-chlorophenyl as potential migrating groups.Key words: aromatic substitution, carbene, ipso, diaryloxycarbene, oxadiazoline.