Mechanism of ipso aromatic substitution by reaction of aryloxy(methoxy)carbenes and diaryloxycarbenes with DMAD

Some aryloxy(methoxy)carbenes and diaryloxycarbenes attack dimethyl acetyle nedicarboxylate (DMAD) with aryl group transfer to an alkyne carbon of DMAD . In this study diaryloxycarbenes with different aryl groups that could be transferred competitively, were generated in the presence of DMAD to probe for the mechanism of that ipso aromatic substitution. It was found that a p ara electron-withdrawing substituent, relative to an electron-donating subs tituent, facilitated migration of an aryl group. Mechanisms in accord with these findings involve initial nucleophilic attack by the carbene at an alk yne carbon of DMAD. That step is followed by either nucleophilic, ipso atta ck on the aromatic ring or by electron transfer, from the side chain of the aromatic ring into the ring itself.