jo960073u_si_001.pdf (108.97 kB)
Foiled Conjugation in α-Oximino Carbocations
journal contribution
posted on 1996-05-17, 00:00 authored by Xavier Creary, Ziqi JiangThe 4-CHNOCH3 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-(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 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
108. 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 × 104 and 1.5 × 102, 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.