First determination of absolute rate constants for the reaction of aroyl-substituted benzyl carbanions in water and DMSO.

The prompt generation of carbanions II and III within the duration of the nanosecond laser pulse provides a way of evaluating absolute rate constants for their two decay pathways, protonation and cyclization, the latter resulting from an intramolecular nucleophilic carbanion displacement of iodide tethered at the end of the lateral alkyl chain. Absolute rate constants are given for both carbanions (II and III) and show that the intra-S(N)2 reaction is favored in aprotic media, such as dimethyl sulfoxide (DMSO), while protonation is the dominant reaction in basic aqueous media.