Enantioselective Potassium-Catalyzed Wittig Olefinations

We report asymmetric potassium–isothiourea–boronate-catalyzed
Wittig olefinations of 4-substituted cyclohexanones with non-stabilized
phosphorus ylides to afford highly enantioenriched axially chiral
alkenes. The optimal catalyst features an unusual macrocyclic amide–potassium–boronate
chelate. Kinetic and spectroscopic analyses are consistent with a
Lewis acid mechanism for the catalytic olefination that results in
the formation of the oxaphosphetane adduct under cryogenic conditions.
Thermal fragmentation of the oxaphosphetane to the alkene product
occurs after the reaction is complete. Computational studies indicate
that cycloaddition proceeds via a stepwise mechanism involving enantiodetermining
polar 1,2-addition to afford an intermediate potassium betaine complex.