To Have and To Hold

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Science  06 Feb 2004:
Vol. 303, Issue 5659, pp. 731-733
DOI: 10.1126/science.303.5659.731e

Dendrimeric polymers possess a highly branched regular structure that can contain a variety of subnanometer-scale voids between their chains. This structure is reminiscent of binding pockets in enzymes; and, in principle, metal complexes incorporated into dendrimers could show improved catalytic activity because of favorable ligand interactions with the side groups of polymer chains and a high density of active sites. However, the number of systems in which dendrimer incorporation has improved catalytic activity are few.

Ooe et al. show that encapsulation of an allylic palladium catalyst, [PdCl(C3H5)]2, by a fifth-generation dendrimer (one formed by five rounds of adding branches) can improve yields in the Heck reaction, which couples an aryl halide to a compound with a double bond. The dendrimer arms contain amino acid groups, and 4-diphenylphosphinobenzoic acid was added to stabilize the active Pd(0) species through a phosphine ligand interaction. Stabilization was achieved at a ratio of P to Pd of unity, even though in solution, high P/Pd ratios typically stabilize Pd(0). The authors argue that the amino acid groups that bind the phosphine ligand donor produce a favorable polar environment around the Pd center. For the coupling of iodobenzene to n-butylacrylate, yields were >90%. — PDS

J. Am. Chem. Soc. 10.1021/ja038455m (2004).

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