Studies on the Reactivity of Migrating Group in [2,3]-Sigmatropic Rearrangement of Sulfur Ylides

Abstract

The [2,3]-sigmatropic rearrangement of sulfur ylides is unique and useful reaction in organic synthesis. In this study, the reactivity of sulfides containing three different migrating groups (propargyl, allyl, and allenyl) in [2,3]-sigmatropic rearrangement of sulfur ylides has been compared. The competition reactions of phenylethyldiazoacetate with sulfides through sulfonium ylide [2,3]-sigmatropic rearrangement are designed under Rh(II)- or Cu(I)-catalyzed reaction conditions. Both intra- and intermolecular competitions of sulfides bearing two different migrating groups have been carried out. The ratio of products has been determined by H-1 NMR in order to compare the reactivity of different sulfides bearing allyl, propargyl or allenyl groups. Obvious disparity of the reactivity of these sulfides in [2,3]-sigmatropic rearrangement has been observed. Experimental data indicate that the tendency of preferential [2,3]-sigmatropic rearrangement has the following order: propargyl sulfide>allyl sulfide>allenyl sulfide. Catalysts such as Rh-2(OAc)(4), Rh-7(O2CCF3)(4), and Cu(CH3CN)(4)PF6 ligated with a series of diimine ligands have been investigated for these reactions. Rh(II) complexes are found more efficient than Cu(I) complexes, and Rh-2(O2CCF3)(4) is more efficient than Rh-2(OAc)(4). The efficiency of different catalytic system has been explained based on the proposed reaction mechanism. The reaction catalyzed by Rh(II) complexes is suggested to be different from that catalyzed by Cu(I) complexes. In the case of Cu(I)-catalyzed reaction, Cu(I)-bonded sulfur ylide is considered as the predominant intermediate, while the [2,3]-sigmatropic rearrangement is suggested to proceed through free ylide. Both steric hindrance and electronic properties of ligands influence the ratio and selectivity in Cu(I)-catalyzed reactions. This study provides useful information for further investigation of [2,3]-sigmatropic rearrangement of sulfur ylides.