Exploring the vinylogous reactivity of cyclohexenylidene malononitriles: switchable regioselectivity in the organocatalytic asymmetric addition to enals giving highly enantioenriched carbabicyclic structures

J Am Chem Soc. 2014 Aug 6;136(31):11107-14. doi: 10.1021/ja5054576. Epub 2014 Jul 24.

Abstract

Modulation of the complex reactivity of cyclohexenylidene malononitriles using diverse β-aryl-substituted enals and proper organocatalytic modalities resulted in divergent asymmetric reaction patterns to furnish angularly fused or bridged carbabicyclic frameworks. In particular, use of remotely enolizable dicyanodienes 1, under one-pot sequential amine/NHC catalysis, led to [3 + 2] cycloaddition to afford ε,δ-bonded spiro[4.5]decanone structures 5. Alternatively, modifying the standard amine catalysis by adding a suitable chemical stimulus (p-nitrophenol cocatalyst) switched the reactivity decidedly toward a domino [4 + 2] cycloaddition to afford γ',δ-bonded bicyclo[2.2.2]octane carbaldehydes 8. Products invariably formed in good yields, with rigorous chemo-, regio-, diastereo-, and enantiocontrol. Experimental evidence, including carbon isotope effects measured by (13)C NMR, were indicative of the rate (and stereochemistry) determining step of these transformations and suggested a stepwise mechanism for the [4 + 2] cycloadditive pathway.

MeSH terms

  • Aldehydes / chemistry*
  • Bridged Bicyclo Compounds / chemistry*
  • Catalysis
  • Cycloaddition Reaction
  • Cyclohexenes / chemistry*
  • Methane / analogs & derivatives
  • Methane / chemistry
  • Nitriles / chemistry*
  • Spiro Compounds / chemistry
  • Stereoisomerism
  • Substrate Specificity

Substances

  • Aldehydes
  • Bridged Bicyclo Compounds
  • Cyclohexenes
  • Nitriles
  • Spiro Compounds
  • cyclohexene
  • carbene
  • Methane