The last two decades have witnessed remarkable progress in the development of nanosized gold catalysts for a wide range of oxidation processes.[1] Polymer-protected colloidal gold, in which the gold clusters are dispersed quasi-homogeneously in a medium, can provide a unique catalytic system for many types of organic reactions. [1,2] The purpose of the polymeric matrix is not limited to stabilizing and regulating the size of the colloidal nanogold; it can also control the reactivity and selectivity of the catalytic reaction. For example, by donating electrons to the gold surface, poly(1-vinylpyrrolidin-2-one) (PVP) shows a greater catalytic activity in the aerobic oxidation of alcohols than is the case with other protective polymers such as poly(vinyl alcohol) or poly(allylamine).[3] Herein, we report dual roles of the polyhydroxylated biopolymer matrices chitosan and starch in the stabilization of gold clusters and in the activation of substrates, which result in an increase in the overall catalytic reactivity and selectivity of oxidative homocoupling reactions of arylboronic acids, [4, 5] even under aqueous acidic conditions. Basic conditions are a prerequisite for the promotion of transition-metal-mediated homocoupling reactions of arylboronic acids because of the requirement for formation of tetra-coordinated boronate intermediates for the transmetalation process, [6] which leads to product formation after reductive elimination on metal clusters. We have previously reported that PVP-stabilized colloidal gold (Au/PVP) catalyzes the homocoupling reaction of phenylboronic acid under basic aqueous conditions to give 72 % of the homocoupling product, biphenyl, together with 23 % of phenol as a byproduct (Scheme 1, conditions a). [4]

… plays dual roles as a stabilizer of nanogold and as an activator of arylboronic acids in homocoupling reactions in acidic aqueous media. In their Communication on page 55 ff., H. Sakurai et al. demonstrate, by using 11 B NMR spectroscopy, that the tetra-coordinated boronate species are generated inside the chitosan (or starch) matrix even under acidic conditions, resulting in a high reactivity and selectivity for aerobic homocoupling of arylboronic acid. The result could expedite the development of designer catalysts consisting of combinations of functionalized polymers and metal nanoclusters.