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

摘要

Theoretical Eigen curves can be drawn throughBronsted data for dimethoxycarbene (1), phenyltrimethyl-siloxycarbene (3a), 4-methylphenyl (trimethylsiloxy) carbene (3b),4-methoxyphenyl (trimethylsiloxy) carbene (3c), and #beta#-naphthyl (trimethylsiloxy) carbene (3d), The Bronsted plots forthese data are clearly curved with #alpha# values near 1 whenproton transter is thermodynamically unfavourable and #alpha#values near 0 when proton transter is thermodynamicallyfavourable, suggesting that these carbenes behave as "normal"Bronsted bases. Estimates of the intrinsic barriers (triangle openG_0~*) for proton transfer reactions and of the pKa values for theconjugate acids of the carbenes, extracted from these theoreticalcurves, have been made. The magnitudes of the intrinsic barriers(triangle open G_0~*) for these proton transfer reactionsdetermined by Eigen and Marcus theories are all 1-5 kcal mol~-1,suggesting that these reactions are intrinsically fast. Small intrinsicbarriers also imply "normal" acid/base behaviour. ExtrapolatedpKa values are also the first estimates for the pKa values of theconjugate acids of carbenes 1 and 3a-3d.