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 Pd-II-catalyzed auto-tandem reaction consisting of the stannylation of cinnamyl chloride 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 Pd-II complexes in the auto-tandem reaction and on the formation and possible involvement of Pd-0 species during prolonged reaction times. This has led to optimized reaction conditions in which the overall tandem reaction proceeds through SCS-pincer Pd-II-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.