N-arylbenzonitrilium ions. Photochemical generation and effect of substituents in the phenyl rings on lifetimes in water and reactivity with azide ion

摘要

Nitrilium ions Ar-C=N+-Ar' have been studied by laser ash photolysis in aqueous solutions containing 20% acetonitrile. The cations were generated by photoheterolysis of benzimidate esters Ar-CZ=N-Ar' (Z = -OC6H4-4-CN) with 4-cyanophenoxide as the photochemical leaving group. Rate constants for the reaction with water (k(w)), azide ion (k(az)) and hydroxide (k(OH)) were measured. The cation Ph-C=N+-Ph is only 50-fold shorter lived in water compared to Ph-C=N+-iPr (2); thus the effect of replacing an N-alkyl group with N-phenyl is modest. These two cations are also shown to have similar lifetimes to iminium analogs, e.g. Ph-CH=N+(Me)-Ph. Thus, addition of water to analogous sp and sp(2) hybridized systems occurs at a similar rate, and the increased steric access to the nitrilium plays at most a modest role. For the series where substituents in Ar' were varied with Ar equal to phenyl, the Hammett plot for log k(w) correlated with sigma, with rho = -1.4. For the series where Ar was varied, the data correlated much better with sigma(+), although rho(+) was only -0.6. This remarkably small effect of substituents in Ar contrasts with the effects seen in benzylic carbocations, but is consistent with the nitrilium structure, with most of the positive charge located on the nitrogen. Rate constant ratios k(az) : k(w), including the N-alkylnitrilium ion 2, are constant at similar to10(4), with values of k(az) well below the diffusion limit, even for the most reactive nitrilium ions. This is very different behaviour from that of carbocations and arylnitrenium ions of similar lifetimes in water. For these cations the rate constants kaz would be at or at least approaching the diffusion limit. [References: 29]