Mechanistic Studies on the SCS-Pincer Palladium(II)-Catalyzed Tandem Stannylation/Electrophilic Allylic Substitution of Allyl Chlorides with Hexamethylditin and Benzaldehydes

This paper describes a mechanistic study of the SCS-pincer PdII-catalyzed auto-tandem reaction consisting of the stannylation of cinnamyl chlo- ACHTUNGTRENUNGride with hexamethylditin, followed by an electrophilic allylic substitution of the primary tandem-reaction product with 4-nitrobenzaldehyde to yield homoallylic alcohols as the secondary tandem products. As it turned out, the anticipated stannylation product, cinnamyl trimethylstannane, is not a substrate for the second part of the tandem reaction. These studies have provided insight in the catalytic behavior of SCS-pincer PdII complexes in the auto-tandem reaction and on the formation and possible involvement of Pd0 species during prolonged reaction times. This has led to optimized reaction conditions in which the overall tandem reaction proceeds through SCS-pincer PdII-mediated catalysis, that is, true auto-tandem catalysis. Accordingly, this study has provided the appropriate reaction conditions that allow the pincer catalysts to be recycled and reused.