How Does Organic Structure Determine Organic Reactivity? Nucleophilic Substitution and Alkene-Forming Elimination Reactions of a-Carbonyl and a-Thiocarbonyl Substituted Benzyl Derivatives

摘要

The effect of a-(N,N-dimethylcarbamoyl) and alpha-(Af,Af-dimethylthiocarbamoyl) substituents on the rate constants for partitioning of a-substituted H4-methoxyphenyl)ethyl carbocations between nucleophilic addition of 50:50 (v:v) MeOH/H_2O (k_s, s~(-1)) and deprotonation by this solvent (kt, s~(-1)) have been investigated. The data show that these a-amide and a-thioamide substituents result in 80-fold and >=30 000-fold decreases, respectively, in k_s for capture of the 4-methoxybenzyl carbocation bysolvent, but that they lead to much smaller changes in kt for deprotonation of the corresponding a-substituted l-(4-methoxyphenyl)ethyl carbocations by solvent. The large effect of the a-thioamide substituent on the partitioning of a-substituted 1-phenylethyl carbocations between formation of the products of solvolysis and elimination is therefore due primarily to the effect of this a-substituent on k_s for capture of the carbocation by solvent. The results of experimental and computational studies areconsistent with the conclusion that the relative magnitude of the rate constants k_s and fee for partitioning of a-substituted 1-phenylethyl carbocations is strongly controlled by the relative thermodynamic stabilities of the neutral products of these reactions.