Normal acid/base behaviour in proton transfer reactions to alkoxy substituted carbenes: estimates for intrinsic barriers to reaction and pKa values

Abstract

Theoretical Eigen curves can be drawn through Brønsted data for dimethoxycarbene (1), phenyltrimethyl-siloxycarbene (3a), 4-methylphenyl(trimethylsiloxy)carbene (3b), 4-methoxyphenyl(trimethylsiloxy)carbene (3c), and β-naphthyl(trimethylsiloxy)carbene (3d). The Brønsted plots for these data are clearly curved with α values near 1 when proton transfer is thermodynamically unfavourable and α values near 0 when proton transfer is thermodynamically favourable, suggesting that these carbenes behave as "normal" Brønsted bases. Estimates of the intrinsic barriers (ΔG0‡) for proton transfer reactions and of the pKa values for the conjugate acids of the carbenes, extracted from these theoretical curves, have been made. The magnitudes of the intrinsic barriers (ΔG0‡) for these proton transfer reactions determined by Eigen and Marcus theories are all 1-5 kcal mol-1, suggesting that these reactions are intrinsically fast. Small intrinsic barriers also imply "normal" acid/base behaviour. Extrapolated pKa values are also the first estimates for the pKa values of the conjugate acids of carbenes 1 and 3a-3d. Key words: carbenes, proton transfer reactions, carbocations, Marcus theory, Brønsted plots.