SEMIEMPIRICAL AM1 CALCULATIONS OF THE SUBSTITUENT EFFECT ON ACYLKETENE DIMERIZATIONS

Dibenzoylketene has been found to dimerize in the usual way, i.e. [4 2] cycloaddition of the ketene C=C double bond as dienophile with a s econd acylketene molecule, to 3-acyl-alpha-pyrones. In contrast, dipiv aloylketene reacts with its carbonyl C=O double bond to a 1,3-dioxin-4 (2H)-one which contains an unusually insert alpha-oxo ketene functiona lity. This difference in site-selectivity is rationalized with the aid of semiempirical molecular orbital (AM1) calculations. Although both the dibenzoyl- and the dipivaloylketene reactions of the ketene C=C do uble bond are predicted to lead to thermodynamically more stable produ cts, cycloaddition of the carbonyl group should be favoured in the cas e of dipivaloylketene because of its lower activation energy. A differ ent cycloaddition pathway involving the ketene C=O group as dienophile should also be feasible.