The role of the dendritic support in the catalytic performance of peripheral pincer Pd-complexes

To investigate the effects of the dendrimer backbone on catalysis, a series of monomeric and dendritic SCS-pincer Pd-complexes was synthesized and tested in two different Pd(II)-catalyzed reactions. To this end, the three novel polar PAMAM dendrimer-immobilized SCS-pincer Pd-complexes 3, 4, and 5, and the two apolar carbosilane dendrimer-immobilized complexes 7 and 8 were compared to three monomeric analogues 1, 2 and 6. These complexes were investigated in the cross-coupling reaction between vinyl epoxide and styrylboronic acid and the auto-tandem reaction of cinnamyl chloride, hexamethylditin, and 4-nitrobenzaldehyde. The differences in catalytic rate and product selectivity for these complexes are described and discussed. For the cross coupling reaction, the PAMAM dendrimer-immobilized complexes were found to give a similar reaction rate, but a higher product selectivity than their monomeric counterparts. The carbosilane complexes showed a lower reaction rate and similar product selectivity. These observations are explained in view of dendrimer aggregation and peripheral group backfolding.