Allenoates in Enantioselective [2+2] Cycloadditions: From a Mechanistic Curiosity to a Stereospecific Transformation

J Am Chem Soc. 2018 Nov 21;140(46):15943-15949. doi: 10.1021/jacs.8b10008. Epub 2018 Nov 5.

Abstract

Identification of a novel catalyst-allenoate pair allows enantioselective [2+2] cycloaddition of α-methylstyrene. To understand the origin of selectivity, a detailed mechanistic investigation was conducted. Herein, two competing reaction pathways are proposed, which operate simultaneously and funnel the alkenes to the same axially chiral cyclobutanes. In agreement with the Woodward-Hoffmann rules, this mechanistic curiosity can be rationalized through a unique symmetry operation that was elucidated by deuteration experiments. In the case of 1,1-diarylalkenes, distal communication between the catalyst and alkene is achieved through subtle alteration of electronic properties and conformation. In this context, a Hammett study lends further credibility to a concerted mechanism. Thus, extended scope exploration, including β-substitution on the alkene to generate two adjacent stereocenters within the cyclobutane ring, is achieved in a highly stereospecific and enantioselective fashion (33 examples, up to >99:1 er).

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cycloaddition Reaction
  • Molecular Structure
  • Naphthalenes / chemistry*
  • Stereoisomerism
  • Styrenes / chemical synthesis*
  • Styrenes / chemistry

Substances

  • Naphthalenes
  • Styrenes
  • vinyltoluene
  • allenolic acid