MECHANISTIC INSIGHT INTO ALLYLMETAL-THIOACETAL REACTIONS EMPLOYING 2-ACETOXY-2-PHENYLACETALDEHYDE THIOACETALS

The mechanism of allymetal-thioacetal reactions has been investigated by using 2-acetoxy-2-phenylacetaldehyde thioacetals in the presence of trimethylsilyl triflate. The results unamibguously revealed the occurrence of both SN1- and SN2-type reactions. Upon treatment with the monothioacetal 1, allylsilane 3a and -germane 3b provided allylation product 5 in a ca. 80:20 syn-anti ratio irrespective of the stereochemistry of 1. With trimethyl- and tributylallyltin (3c and 3d), the syn product 5a was obtained from the u-monothioacetal and the anti product 5b from l-monothioacetal. These results indicate that the reactions of less nucleophilic silicon and germanium compounds proceed through an SN1 mechanism while organotin compounds react through an SN2-type mechanism. Reaction of the dithioacetal 2 furnished predominantly the syn product 6 with both allylsilane and -stannane, indicative of an SN2-type mechanism. These results reflect the inferior ability of a sulfenyl group to stabilize an a-carbocation, relative to a methoxyl group. © 1990, American Chemical Society. All rights reserved.