| Abstract
| - The 4-CHNOCH3 group is a cation-stabilizing group whenplaced in the para-position of a cumylcation. The effect of this group on cumyl cations when flanked byadjacent methyl groups has nowbeen determined. Solvolysis rates of3,5-(CH3)2-4-(CHNOCH3)cumyltrifluoroacetates are somewhat slower than that of 3,5-dimethylcumyl trifluoroacetate. Thisis attributed to steric inhibition of the cation-stabilizing resonance effect of thep-oximino group. In a 1-adamantyl system,where an α-oximino group has been placed directly adjacent to adeveloping cationic center, solvolysis rates relative to 1-adamantyl mesylate are slowed by a factor of108. This is attributed a cation-destabilizing inductive effect where geometric constraints preventstabilizing orbital overlapof the cationic center with the adjacent α-oximino group. Thiscation-destabilizing effectfades in the homoadamantyl and the bicyclo[3.3.1]nonylsystems, where rate-retarding effectsare 1.6 × 104 and 1.5 × 102, respectively.The behavior of geometrically constrained α-oximinocations parallels that of analogously constrained allylic cations.Computational studies at theHF/6-31G* level indicate that twisting the α-oximino group out ofplanarity with a tertiary cationic center into a perpendicular arrangement decreases stabilizationby 21 kcal/mol. Thesestudies suggest that conjugative interactions, and not ground statedestabilization, are the mostimportant factors in controlling rates of formation of α-oximinocations from mesylates andtrifluoroacetates.
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