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–Cu0001 u0001N –Aru0003 have been studied by laser flash photolysis in aqueous solutions containing 20%nacetonitrile. The cations were generated by photoheterolysis of benzimidate esters Ar–CZu0001 u0001N–Aru0003 (Z = –OC6H4-4-nCN) with 4-cyanophenoxide as the photochemical leaving group. Rate constants for the reaction with water (kw),nazide ion (kaz) and hydroxide (kOH) were measured. The cation Ph–Cu0001 u0001 u0001Nu0002–Ph is only 50-fold shorter lived in waterncompared to Ph–Cu0001 u0001 u0001Nu0002–iPr (2); thus the effect of replacing an N-alkyl group with N-phenyl is modest. These twoncations are also shown to have similar lifetimes to iminium analogs, e.g. Ph–CHu0001 u0001Nu0002(Me)–Ph. Thus, addition of waternto analogous sp and sp2n hybridized systems occurs at a similar rate, and the increased steric access to the nitriliumnplays at most a modest role. For the series where substituents in Aru0003 were varied with Ar equal to phenyl, thenHammett plot for log kw correlated with σ, with ρ = u00041.4. For the series where Ar was varied, the data correlatednmuch better with σu0002, although ρu0002 was only u00040.6. This remarkably small effect of substituents in Ar contrasts with theneffects seen in benzylic carbocations, but is consistent with the nitrilium structure, with most of the positive chargenlocated on the nitrogen. Rate constant ratios kaz : kw, including the N-alkylnitrilium ion 2, are constant at ∼104n, withnvalues of kaz well below the diffusion limit, even for the most reactive nitrilium ions. This is very different behaviournfrom that of carbocations and arylnitrenium ions of similar lifetimes in water. For these cations the rate constants kaznwould be at or at least approaching the diffusion limit.