Reactivity of malonyl radicals. Synthesis of substituted dihydronaphthalenes by manganese(III) oxidation of diethyl .alpha.-benzylmalonate in the presence of alkynes

Oxidn. of di-Et α-benzylmalonate (I) by Mn(III) acetate in acetic acid at 70° in the presence of alkynes RC≡CR1 (R = cyclohexyl, CO2Et, Me3Si, Ph, CH2OH, R1 = H; R = R1 = Ph, Et, CO2Et, Pr; R = CO2Me, R1 = Me) leads to dihydronaphthalene derivs. II in moderate to good yields. A homolytic mechanism, involving oxidn. of I to the corresponding malonyl radical, its addn. to a triple bond, and intramol. homolytic arom. substitution of the vinyl radical adducts, is suggested. Propargylic hydrogen abstraction and dimerization at the malonic position for less reactive alkynes are the main side reactions obsd. Relative and abs. rates of addn. of α-benzylmalonyl radicals to representative alkynes and alkenes, deduced from competitive expts., indicate a lower reactivity toward alkynes than toward the corresponding substituted alkenes. Both SOMO-LUMO and SOMO-HOMO interactions in the transition state lower the activation energy of these homolytic addns. with alkenes, but only the SOMO-HOMO interaction dominates with the examd. alkynes. 2-Naphthoic acid derivs. can be efficiently obtained by oxidative decarboxylation of II with NaI and air