Di- and triarylmethylium ions as probes for the ambident reactivities of carbanions derived from 5-benzylated Meldrum's acid

The kinetics of the reactions of carbocations with carbanions 1 derived from 5-benzyl-substituted Meldrum's acids 1-H (Meldrum's acid=2,2-dimethyl-1,3- dioxane-4,6-dione) were investigated by UV/Vis spectroscopic methods. Benzhydryl cations Ar₂CH⁺ added exclusively to C-5 of the Meldrum's acid moiety. As the second-order rate constants (k_C) of these reactions in DMSO followed the linear free-energy relationship lg k=s _N(N+E), the nucleophile-specific reactivity parameters N and s _N for the carbanions 1 could be determined. In contrast, trityl cations Ar₃C⁺ reacted differently. While tritylium ions of low electrophilicity (E<-2) reacted with 1 through rate-determining β-hydride abstraction, more Lewis acidic tritylium ions initially reacted at the carbonyl oxygen of 1 to form trityl enolates, which subsequently reionized and eventually yielded triarylmethanes and 5-benzylidene Meldrum's acids by hydride transfer.