Asymmetric Metal Catalyzed Formation of Sulfur Ylides

Abstract

Sulfur ylides are important reactive intermediates in organic chemistry and are readily generated by using copper catalysts. These catalysts typically give a 50:50 mixture of two sulfur ylides that are related as non-superimposable mirror images. However, copper catalysts bearing shapeselective ligands should produce mixtures of ylides having a percent excess of one over the other. To test this hypothesis, the selectivity for one of the two sulfur ylide isomers was tested. Because sulfur ylides ( 4) are too reactive to be directly isolated and measured, they were designed to undergo an intra-molecular rearrangement known as the [2,3]-sigmatropic rearrangement (Figure 1). This transfers the center of handedness, depicted by the asterisk, from the sulfur to the configurationally stable carbon. The resulting product mixture was tested for isomeric excess. By varying the ligands and diazo compounds (2), a selectivity up to 24% was achieved.