Stereoselective Olefination of N‐Sulfonyl Imines with Stabilized Phosphonium Ylides for the Synthesis of Electron‐Deficient Alkenes

Abstract

AbstractAn unprecedented protocol has been developed for thestereoselective synthesis of structurally diverse electron‐deficient alkenes in moderate to excellent yields from readily accessible N‐sulfonyl imines and stabilized phosphonium ylides. Significantly, the olefination reaction of N‐sulfonylimines with nitrile‐stabilized phosphonium ylides affords an array of α,β‐unsaturated nitriles with high Z selectivity, and the reactions with ester‐, amide‐, and ketone‐stabilized phosphonium ylides afford α,β‐unsaturated esters, amides, and ketones with high E selectivity, respectively. Spectroscopic analysis of the reaction mixtures and trapping of the intermediates allow plausible mechanisms to be proposed. Initialimine/ylide addition leads to the formation of betaines that cyclize to form 1,2‐azaphosphetanes that subsequently eliminate iminophosphoranes to yield alkenes. For the synthesis of electron‐deficient 1,2‐disubstituted alkenes, the presence of an electron‐withdrawing group in the betaine allows rapid interconversion between its two diastereomers through proton transfer. The Z/E selectivity for alkene synthesis is determined by the different rates at which the two betaine diastereomers form the corresponding 1,2‐azaphosphetane diastereomers. In contrast, the Z/E selectivity for the synthesis of electron‐deficient trisubstituted alkenes originates from the diastereoselective addition of stabilized phosphonium ylides to N‐sulfonyl imines.