Mechanism and origins of ligand-controlled selectivities in [Ni(NHC)]-catalyzed intramolecular (5 + 2) cycloadditions and homo-ene reactions: a theoretical study

J Am Chem Soc. 2013 Jan 30;135(4):1456-62. doi: 10.1021/ja309873z. Epub 2013 Jan 17.

Abstract

The mechanism and origins of selectivities in [Ni(NHC)]-catalyzed intramolecular (5 + 2) cycloadditions and homo-ene reactions of vinylcyclopropanes (VCPs) and alkynes have been studied using density functional theory. The preferred mechanism involves oxidative alkyne-alkene cyclization to form a metallacyclopentene intermediate, in contrast to a cyclopropane cleavage pathway in the reaction with Rh(I) catalysts. The selectivity between the (5 + 2) and homo-ene products is determined in the subsequent competing reductive elimination and β-hydride elimination steps. Two similar-sized N-heterocyclic carbene (NHC) ligands, SIPr and ItBu, yielded reversed product selectivity, favoring the (5 + 2) and homo-ene products respectively. This is attributed to the anisotropic steric environment of these NHC ligands, which positions the bulky substituents on the ligand toward different directions and leads to distinct steric control in the reductive elimination and β-hydride elimination transition states.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Alkynes / chemistry
  • Catalysis
  • Cyclization
  • Cyclopropanes / chemistry
  • Ethers / chemical synthesis*
  • Ethers / chemistry
  • Heterocyclic Compounds / chemistry
  • Ligands
  • Methane / analogs & derivatives
  • Methane / chemistry
  • Molecular Structure
  • Nickel / chemistry
  • Organometallic Compounds / chemistry*
  • Quantum Theory*

Substances

  • Alkynes
  • Cyclopropanes
  • Ethers
  • Heterocyclic Compounds
  • Ligands
  • Organometallic Compounds
  • carbene
  • Nickel
  • Methane