Mechanistic insights in gold-stabilized nonclassical carbocations: gold-catalyzed rearrangement of 3-cyclopropyl propargylic acetates

J Am Chem Soc. 2010 Apr 7;132(13):4720-30. doi: 10.1021/ja909013j.

Abstract

The reaction of 3-cyclopropyl propargylic carboxylates with Au(I) and Au(III) catalysts affords selectively 5-(E)-alkylidenecyclopentenyl acetates via [3,3]-sigmatropic rearrangement of the carboxylic moiety followed by cyclopropyl ring opening and cyclization. DFT calculations have been performed, supporting a two-step no-intermediate mechanism along the cyclization coordinate. The stereoselective formation of the exocyclic alkenes is kinetically controlled in the first of these events. Although stereospecific in nature through a gold-stabilized nonclassical carbocation, the chirality transfer in these cyclopentannulations is not complete. Computational and experimental evidence is provided for a Au-promoted cyclopropyl ring opening/epimerization/ring closure in both cis- and trans-cyclopropyl settings, which competes with the cyclization event, thus eroding the stereochemical information transfer. When tertiary acetates were used, products of both 1,2- and 1,3-acyloxy migration processes could be isolated, supporting the competitive coexistence of these two pathways along the reaction profile, as suggested also by DFT calculations.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Catalysis
  • Cations / chemistry
  • Computer Simulation
  • Cyclization
  • Cyclopropanes / chemical synthesis*
  • Cyclopropanes / chemistry
  • Gold / chemistry*
  • Models, Chemical
  • Molecular Structure
  • Phenylacetates / chemical synthesis*
  • Phenylacetates / chemistry
  • Stereoisomerism

Substances

  • 3-((2-methylpropenyl)cyclopropylpropargyl)-1-phenylprop-2-ynyl acetate
  • Cations
  • Cyclopropanes
  • Phenylacetates
  • Gold