The S_N1CB Mechanism in Non-Aqueous Solution

Abstract

The reaction between (NH₃)₅Co-NCMe³⁺ and dimethyl sulfoxide (Me₂SO) to produce (NH₃)₅Co-OSMe₂³⁺ and MeCN in Me₂SO solvent is general base-catalysed. The rates and rate laws have been determined for 10 substituted amines. First-order kinetics are observed, and the rates show a dependence which is less than first order on base (B) concentration. These facts are rationalized in terms of a common, reactive conjugate base (NH₃)₄(NH₂)CoNCMe²⁺, the solvolysis of which occurs at a rate competitive with its rate of reprotonation by BH⁺. Alternative mechanisms are considered. The measured rates, which span two orders of magnitude, do not correlate with base nucleophilicity, steric features of the amines, nor pKa values as measured for aqueous solution. Primary amines are very effective catalysts; secondary and especially tertiary amines are poorer catalysts. There appears to be no reaction at the bound nitrile function as there is in aqueous solution.