Lewis acid-promoted oxidative addition of thioimidates to Pd(0)

The isomeric S-methyldihydropyrrins 9-Z and 9-E exhibit markedly different behavior in Pd(O)catalyzed cross-coupling reactions. Thioimidates 9-Z are readily converted to imines 10-Z employing Pd(0)/AlkZnI. Under identical conditions 9-E are inert. Oxidative addition to Pd(O) requires activation by Zn or other Lewis acids, which is sterically unfavorable with 9-E. Analogous results were obtained with the related thioimidates 11-E,Z as well as with methylthiopyridines 19alpha-gamma. In the case of both 11 and 19 oxidative addition to Pd(O) was greatly facilitated in the presence of BF3.Et2O. The importance of Lewis acid activation to Pd(O) oxidative addition in such substrates appears to be a general phenomenon not previously recognized.