Pd Catalysis on Dendronized Solid Support:  Generation Effects and the Influence of the Backbone Structure

Abstract

Recent studies revealed that catalysts, prepared on dendronized support, frequently exhibit enhanced activity and selectivity as compared to their non-dendronized analogues. Regretfully, in early studies of the supported dendritic catalysis, no particular attention was paid to the coordinative nature of the dendritic backbone. In this study, we functionalized Wang polystyrene support with three types of dendritic templates: poly(aril benzyl ether), poly(aryl benzyl thioether), and poly(aryl benzyl amine). These dendronized resins were further decorated with phosphine ligands on the periphery and complexed with a Pd(0) catalytic precursor. The catalysis of the Heck and Suzuki reactions of bromobenzene with the first to third generation supported dendritic catalysts was examined and compared to that of the non-dendritic analogues. All of the examined reactions revealed a positive dendritic effect, reflected in up to 5-fold increase in yield, in the most prominent case. The reasons for the observed effect are the proximity of the ligating sites translated into reduced cross-linking and, probably, the increased distance of the catalyst from the polymer matrix. We proved, however, that the latter could not be achieved with a linear spacer. Although the Suzuki reaction was rather insensitive to the backbone structure, the Heck reaction catalysis at 80 degrees C exhibited substantial sensitivity to the nature of the dendritic backbone, with the polyether structure demonstrating the best outcome. This is the first demonstration of the influence of the coordinative ability of the backbone on the activity of a supported dendritic catalyst.

Converting homogeneous to heterogeneous in electrophilic catalysis using monodisperse metal nanoparticles

“…In a more rigorous test for the formation of a homogeneous species, a 3-phase test 47,48 was employed with Wang resin-bound substrate 14 (Figure 6a). In the presence of homogeneous PtCl 2 , 26% conversion of resin bound substrate to product was observed.…”

“…The striking improvement in long-term activity of the smaller sized PAMAM dendrimer capped NPs over the larger PVP capped NPs is complemented by the differing recycling ability based on capping agent. Both results illustrate the effect of size and capping agent, which would not be possible if homogeneous catalysts were generated as the active species even if a "release and capture" dynamic were present.In a more rigorous test for the formation of a homogeneous species, a 3-phase test 47,48 was employed with Wang resin-bound substrate 14 (Figure 6a). In the presence of homogeneous PtCl 2 , 26% conversion of resin bound substrate to product was observed.…”

“…In a more rigorous test for the formation of a homogeneous species, a 3-phase test 47,48 was employed with Wang resin-bound substrate 14…”

Highly efficient silica gel-supported 1,2-diaminocyclohexane-Pd catalyst for Suzuki-Miyaura and Sonogashira coupling reactions

“…Palladium catalyst are among the most popular transition metal catalysts widely used in numerous organosyntheses and synthetic transformations [1,2]. Homogeneous Pd catalysts suffer various difficulties both commercially and environmentally.…”

Elucidation of Architectural Requirements from a Spacer in Supported Proline‐Based Catalysts of Enantioselective Aldol Reaction

“…Positive dendritic effects were observed in the past in supported organometallic catalysis, but their possible explanations were based (in those cases discussed) on the proximity and cooperativity of the ligating sites in the metal coordination events, but not on cooperative interaction of two ligands or complexes with the substrates. [8] In the case of organocatalysts such as proline, however, it is tempting to suggest cooperative action of two proximal proline units on the substrates, in order to explain the aforementioned effect. …”