How Does a Reaction Change Its Mechanism? General Base Catalysis of the Addition of Alcohols to 1-Phenylethyl Carbocations1

Rachel Ta-Shma, William P. Jencks

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59 Scopus citations

Abstract

Structure-reactivity correlations are reported for general base catalysis of the addition of alcohols to l-(4-(di-methylamino)phenyl)ethyl and l-(4-methoxyphenyl)ethyl carbocations in 50:40:10 H2O:CF3CH2OH:ROH. The addition of trifluoroethanol to the relatively stable ((dimethylamino)phenyl)ethyl cation is catalyzed by substituted acetate ions with ß = 0.33, which is larger than ß= 0.23 for addition to the (methoxyphenyl)ethyl cation. Catalysis is more important for the more stable carbocation, but it decreases faster with increasing alcohol basicity. For the “water catalysis” of alcohol addition to 1-phenylethyl carbocations there is an increased sensitivity to the basicity of ROH with increasing carbocation stability (a “Hammond effect”). This indicates a small involvement of proton transfer in the transition state and is consistent with simple hydrogen bonding of ROH to a base; it is described by a positive interaction coefficient pyy= (Richard, J. P.; Jencks, W. P. J. Am. Chem. Soc. 1984, 106, 1396). However, for the acetate-catalyzed reaction there is no significant increase in the sensitivity to ROH basicity with increasing carbocation stability. This represents a decrease in the pyy coefficient and a shift toward the negative pyy coefficient that is expected for a fully concerted, coupled mechanism. This change in and other changes in interaction coefficients describe interrelated changes in the transition state structure that accompany changes in reaction mechanism. They may be described by third derivatives of log k, such as for these 1-phenylethyl carbocations, pxyy = 0.011.

Original languageEnglish
Pages (from-to)8040-8050
Number of pages11
JournalJournal of the American Chemical Society
Volume108
Issue number25
DOIs
StatePublished - 1986
Externally publishedYes

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