A molecular orbital study of alkoxy allyl and vinyl anions

Abstract

The substitution of an alkoxy group for hydrogen is to stabilize by inductive effects both the vinyl and ally1 anions. Since the vinyl carbanion center is more localized, substitution of an OR group adjacent to this center produces a more dominant stabilizing effect than for the ally1 system. Superimposed upon the inductive stabilization is a trend of destabilization of the n system of the ally1 moiety and to a lesser extent the carbanion center of the vinyl anion by the oxygen lone pair orbitals. An interaction diagram for the T orbitals of an ally1 fragment interacting with oxygen lone pair orbitals of the -OR group is shown in Figure 1. The lower molecular orbitals of S and A synzaetry are localized predominantly on the oxygen lone pair orbitals while the high energy orbitals are essentially on the ally1 fragment with some oxygen lone pair character mixed in an antibonding manner. The overall effect is a four-electron destabilizing interaction between the w-donor orbitals on the oxygen with the filled II orbitals of the ally1 moiety. The OR group also has a destabilizing effect in the vinyl anion. In 1 is shown an alkoxy vinyl anion along with optimized values for some of the important structural parameters. The optimized value of 1.45 /! for the C-O bond length in the alkoxy vinyl anion represents an increase