Novel monometallic and dendritic SCS-pincer palladium complexes 2, 3 and 4 have been synthesized in good yields (60-89%) and high purity (palladium loading >97%). These complexes were successfully used as catalysts in the stannylation of cinnamyl chloride with hexamethylditin and in the catalytic auto-tandem reaction consisting of this stannylation followed by an electrophilic addition with 4-nitrobenzaldehyde, showing similar reaction rates and selectivities for all complexes. Dendritic complex 4 has furthermore been used in the compartmentalized catalysis of single and auto-tandem reactions, allowing catalyst reuse for four consecutive runs.

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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.

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