Reactivities of diarylmethyl and triarylmethyl cations with primary amines in aqueous acetonitrile solutions. The importance of amine hydration

Article abstract of DOI:10.1021/ja00031a041

By use of the technique of laser flash photolysis, rate constants k(RNH₂ have been directly measured for the reactions of primary amines RCH₂NH₂ (R = CH₃CH₂, CH₃OCH₂, NCCH₂, CF₃) with diarylmethyl cations (D⁺) in acetonitrile/water solutions. In 100% acetonitrile the reactions approach the diffusion limit, 5 × 10⁹ M^-1 s^-1, although they are slower, k(RNH₂) for a given cation increasing with increasing amine basicity and for a given amine increasing with decreased electron donation from substituents in D⁺. In the mixed solvents the rate constants decrease in a regular fashion with increasing water content. The changes can be large, being on the order of 10-100 proceeding from 100% acetonitrile to 100% water. Moreover, the rate-retarding effect of water is more pronounced with more basic amines, with the consequence that in water-rich solutions the reactivity order no longer parallels amine basicity. Plots of log k(RNH₂) versus pk_a(RNH₃⁺) not only are curved but also show a change in the sign of their slope on progressing from weakly basic amines (positive β_nuc) to strongly basic ones (negative β_nuc). This behavior is explained by a mechanism in which a hydrated amine RNH₂-HOH is unreactive and an equilibrium desolvation to form the unhydrated amine precedes reaction with the cation. Quantitative treatment is carried out, using the rate constants in 100% acetonitrile to model the reaction of the free amine. This approach reproduces the experimental data within an average of ±0.04 log unit and results in equilibrium constants for the desolvation with the expected β= -0.2 dependency on amine basicity. Rate constants have also been measured in 33% acetonitrile/water for a series of triarylmethyl cations ranging from 4,4'-(Me₂N)2T⁺ to 4,4'-(CF₃)₂T⁺. The β_nuc values for these are all positive, with a clear trend for β_nuc to decrease with increasing cation reactivity, this being true even for the relatively stable cations 4,4'-(Me₂N)₂T⁺, 4-Me₂NT⁺ and 4,4′,4″-(MeO)₃T⁺. Thus, amine nucleophiles do not adhere to the N₊ constant selectivity relation, even for stable cations.