The Nature of Azo-Substituted Carbocations: N–N π‑Electron Stabilization versus Nitrogen Nonbonding Electron Stabilization

Computational
and experimental studies reveal two different modes
of cation stabilization by the phenylazo group. The first mode involves
a relatively weak conjugative interaction with the azo π-bond,
while the second mode involves an interaction with the nitrogen nonbonding
electrons. The 4-phenylazo group is slightly rate-retarding in the
solvolysis of cumyl chloride and benzyl mesylate derivatives but rate-enhancing
in the solvolysis of α-CF3 benzylic analogs. The
phenylazo group can become a potent electron-donating group in cations
such as [Me2CNNPh]+.
Nonbonding electron stabilization can be strong enough to offset the
very powerful γ-silyl stabilization. In aromatic cyclopropenium
and tropylium cations, the demand for stabilization is quite low,
and the mode of phenylazo stabilization reverts back to the less-effective
π-stabilization. The solvolysis of cis-4-phenylazo
benzyl mesylate is faster than that of trans-4-phenylazo
benzyl mesylate. Products formed suggest a stepwise ionization, cation
isomerization, and nucleophile capture mechanism. Computational studies
indicate a vanishingly small barrier for the isomerization of the cis-cation intermediate to the trans-cation.