Cyclopropanation of benzylidenemalononitrile with dialkoxycarbenes and free radical rearrangement of the cyclopropanes

Thermolysis of 2-cinnamyloxy-2-methoxy-5,5-dimethyl-Delta (3)-1,3,4-oxadiaz oline (1a) and the analogous 2-benzyloxy-2-methoxy compound (1b) at 110 deg reesC, in benzene containing benzylidenemalononitrile, afforded products of apparent regiospecific addition of methoxycarbonyl and cinnamyl (or benzyl ) radicals to the double bond. When the thermolysis of 1a was run with adde d TEMPO, methoxycarbonyl and cinnamyl radicals were captured. Thermolysis o f the 2,2-dibenzyloxy analogue (1c) in the presence of benzylidenemalononit rile gave an adduct that is formally the product of addition of benzyloxyca rbonyl and benzyl radicals to the double bond. In this case, a radical addi tion mechanism could be ruled out, because the rate constant for decarboxyl ation of benzyloxycarbonyl radicals is very large. A mechanism that fits al l of the results is predominant cyclopropanation of benzylidenemalononitril e by the dialkoxycarbenes derived from the oxadiazolines, in competition wi th fragmentation of the carbenes to radical pairs. The cyclopropanes so for med then undergo homolytic ring-opening to the appropriate diradicals. Subs equent beta -scission of the diradicals to afford radical pairs, and coupli ng of those pairs, gives the final products. Thus, both carbene and radical chemistry are involved in the overall processes.