Mechanistic studies on the SCS-pincer palladium(II)-catalyzed tandem stannylation/electrophilic allylic substitution of allyl chlorides with hexamethylditin and benzaldehydes

摘要

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. Minor isomer, major importance: A mechanistic study of the SCS-pincer Pd-catalyzed auto-tandem reaction is described consisting of the stannylation of cinnamyl chloride, followed by an electrophilic allylic substitution of the primary tandem reaction product to yield homoallylic alcohols (see scheme; SCS=[2,6-(CH_2SPh)_2C_6H_3] ~-). This study pointed out that the anticipated stannylation product, cinnamyl trimethylstannane, is not a substrate for the second part of the tandem reaction.