An evaluation of the combined application of hammett reaction constant and primary kinetic isotope effects to hydride equivalent transfer reactions

✍ Scribed by R. Srinivasan

• Book ID: 102927960
• Publisher: John Wiley and Sons
• Year: 1992
• Tongue: English
• Weight: 587 KB
• Volume: 24
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550240806

✦ Synopsis

Abstract

Bunting et al. have suggested that a combined application of Hammett ρ values (or Brønsted exponents) and primary kinetic isotope effects to hydride equivalent transfer reactions involving 1,4‐dihydronicotinamides and organic oxidants is capable of revealing whether this transfer occurs in a single step or follows a two step e^−^ + H° transfer mechanism (J.W. Bunting, V.S.F. Chew, G. Chu, N.P. Fitzgerald, A. Gunasekara, and H.T.P. Oh, Bioorg. Chem., 12, 141 (1984)). We have used this approach with respect to the reduction of flavins by 1‐substituted 1,4‐dihydronicotinamides. Compelling lines of evidence are presented to demonstrate that this reaction follows the single step hydride transfer mechanism. We also present an analysis of the Brønsted exponent and the primary kinetic isotope effect data for the flavin reaction. It is shown that although a simple application of these parameters does indeed favor the two step e^−^ + H° transfer mechanism for flavin reduction, various other empirical data provide conclusive evidence for the single step hydride transfer mechanism. We attribute this discrepancy to the occurrence of nonequilibrium solvation of the transition state. We have considered several mechanistic complexities which could produce seemingly inconsistent Hammett and primary isotope effect values, and have concluded that these complexities must be considered in interpreting the mechanism of a hydride‐equivalent transfer reaction from kinetic parameters. © John Wiley & Sons, Inc.