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

Abstract

Nitrilium ions Ar–CN⁺–Ar′ have been studied by laser flash photolysis in aqueous solutions containing 20% acetonitrile. The cations were generated by photoheterolysis of benzimidate esters Ar–CZN–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–CN⁺–Ph is only 50-fold shorter lived in water compared to Ph–CN⁺–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–CHN⁺(Me)–Ph. Thus, addition of water to analogous sp and sp² 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 σ, with ρ = −1.4. For the series where Ar was varied, the data correlated much better with σ⁺, although ρ⁺ 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 ∼10⁴, 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 k_az would be at or at least approaching the diffusion limit.