Kinetics and mechanism of the racemization of aryl allenes catalyzed by cationic gold(i) phosphine complexes

  • Harris R
  • Nakafuku K
  • Widenhoefer R
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Abstract

The kinetics of the racemization of aromatic 1,3-disubstituted allenes catalyzed by gold phosphine complexes has been investigated. The rate of gold-catalyzed allene racemization displayed first-order dependence on allene, and catalyst concentration and kinetic analysis of gold-catalyzed allene racemization as a function of allene and phosphine electron-donor ability established the accumulation of electron density on the phosphine atom and the depletion of electron density on the terminal allenyl carbon atoms in the rate-limiting transition state for racemization. These and other observations were in accord with a mechanism for allene racemization involving rapid and reversible inter- and intramolecular allene exchange followed by turnover-limiting, unimolecular conversion of a chiral gold η(2)-allene complex to an achiral η(1)-allylic cation intermediate through a bent and twisted η(1)-allene transition state. With respect to proper ligand selection, these studies reveal that both electron-poor phosphine ligands and polar solvents facilitate racemization.

Author-supplied keywords

  • allenes
  • allylic compounds
  • gold
  • pi complexes
  • racemization

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Authors

  • Robert J. Harris

  • Kohki Nakafuku

  • Ross A. Widenhoefer

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