FOILED CONJUGATION IN ALPHA-OXIMINO CARBOCATIONS

The 4-CHNOCH3 group is a cation-stabilizing group when placed in the p ara-position of a cumyl cation. The effect of this group on cumyl cati ons when flanked by adjacent methyl groups has now been determined. So lvolysis rates of 3,5-(CH3)(2)-4-(CHNOCH3)cumyl trifluoroacetates are somewhat slower than that of 3,5-dimethylcumyl trifluoroacetate. This is attributed to steric inhibition of the cation-stabilizing resonance effect of the p-oximino group. In a l-adamantyl system, where an alph a-oximino group has been placed directly adjacent to a developing cati onic center, solvolysis rates relative to l-adamantyl mesylate are slo wed by a factor of 10(8). This is attributed a cation-destabilizing in ductive effect where geometric constraints prevent stabilizing orbital overlap of the cationic center with the adjacent alpha-oximino group. This cation-destabilizing effect fades in the homoadamantyl and the b icyclo[3.3.1]nonyl systems, where rate-retarding effects are 1.6 x 10( 4) and 1.5 x 10(2), respectively. The behavior of geometrically constr ained alpha-oximino cations parallels that of analogously constrained allylic cations. Computational studies at the HF/6-31G level indicate that twisting the alpha-oximino group out of planarity with a tertiar y cationic center into a perpendicular arrangement decreases stabiliza tion by 21 kcal/mol. These studies suggest that conjugative interactio ns, and not ground state destabilization, are the most important facto rs in controlling rates of formation of alpha-oximino cations from mes ylates and trifluoroacetates.