Di‐ and Triarylmethylium Ions as Probes for the Ambident Reactivities of Carbanions Derived from 5‐Benzylated Meldrum’s Acid

Abstract

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 (kC) of these reactions in DMSO followed the linear free-energy relationship lg k = sN (N+E), the nucleophile-specific reactivity parameters N and sN 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.

Benzhydrylium and tritylium ions: complementary probes for examining ambident nucleophiles

“…These observations led us to propose that highly reactive tritylium ions attack the oxygen atom of the carbanions 1 under conditions of kinetic control (Fig. 12) [61]. The O-attack of a trityl cation at the enolate is fast but reversible, and the backward reaction generates small concentrations of the tritylium ions that slowly oxidize 1 to the finally observed, thermodynamically preferred products of hydride transfer.…”

“…Product studies showed selective C-C bond formation between the carbanions 1 and benzhydrylium ions of widely different electrophilicity (from E = -7.02 to 0.00) [61]. The decay of the Ar 2 CH + absorbances in the presence of an excess of the carbanions 1 followed a mono-exponential function.…”

“…(1), the k C values for a certain carbanion 1 correlated linearly with the electrophilicities E of the employed benzhydrylium ions (Fig. 10), which allowed the determination of N and s N for the carbanions' C-nucleophilicities [61]. Interestingly, the benzylation at C-5 has little effect on the C-reactivity of these carbanions, as they have N values similar to that of the carbanion derived from the parent Meldrum's acid (in DMSO: N = 13.91, s N = 0.86 [18]).…”

“…As the subsequent hydride transfer occurs on the minute time scale at 20 °C, low temperature NMR techniques were required to characterize the O-adducts spectroscopically. At -20 °C, the life-time of the O-adduct 2 was sufficiently long to record 1 H and 13 C NMR spectra that provided evidence for the structure of the colorless transient species that forms under kinetic control [61].…”

“…The O-attack of a trityl cation at the enolate is fast but reversible, and the backward reaction generates small concentrations of the tritylium ions that slowly oxidize 1 to the finally observed, thermodynamically preferred products of hydride transfer. Equilibrium constants for the O-attack, K O , have been calculated to be in the range from 0.7 to 12 × 10 5 L mol -1 (acetonitrile, 20 °C) [61]. As the subsequent hydride transfer occurs on the minute time scale at 20 °C, low temperature NMR techniques were required to characterize the O-adducts spectroscopically.…”