Foiled Conjugation in α-Oximino Carbocations

1996-05-17T00:00:00Z (GMT) by Xavier Creary Ziqi Jiang
The 4-CHNOCH<sub>3</sub> group is a cation-stabilizing group when placed in the para-position of a cumyl cation. The effect of this group on cumyl cations when flanked by adjacent methyl groups has now been determined. Solvolysis rates of 3,5-(CH<sub>3</sub>)<sub>2</sub>-4-(CHNOCH<sub>3</sub>)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 <i>p</i>-oximino group. In a 1-adamantyl system, where an α-oximino group has been placed directly adjacent to a developing cationic center, solvolysis rates relative to 1-adamantyl mesylate are slowed by a factor of 10<sup>8</sup>. This is attributed a cation-destabilizing inductive effect where geometric constraints prevent stabilizing orbital overlap of the cationic center with the adjacent α-oximino group. This cation-destabilizing effect fades in the homoadamantyl and the bicyclo[3.3.1]nonyl systems, where rate-retarding effects are 1.6 × 10<sup>4</sup> and 1.5 × 10<sup>2</sup>, respectively. The behavior of geometrically constrained α-oximino cations parallels that of analogously constrained allylic cations. Computational studies at the HF/6-31G* level indicate that twisting the α-oximino group out of planarity with a tertiary cationic center into a perpendicular arrangement decreases stabilization by 21 kcal/mol. These studies suggest that conjugative interactions, and not ground state destabilization, are the most important factors in controlling rates of formation of α-oximino cations from mesylates and trifluoroacetates.