Dual Roles of Polyhydroxy Matrices in the Homocoupling of Arylboronic Acids Catalyzed by Gold Nanoclusters under Acidic Conditions

Abstract

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]

Framework‐Copper‐Catalyzed C−N Cross‐Coupling of Arylboronic Acids with Imidazole: Convenient and Ligand‐Free Synthesis of N‐Arylimidazoles

“…However, the catalyst was sensitive to water, and a notable decrease in the yield was observed if an aqueous mixture of solvents was used. The catalyst was highly specific towards N‐arylation, and no other homocoupling or conversion of the C−B bond into a C−O bond was found with any protic polar solvents, which was observed in previous studies under identical conditions . The reaction was highly time dependent, and shorter reaction times result in moderate conversions (Table , entry 10), whereas reaction times of 12 h afford an excellent yield of 99 % without any byproducts.…”

Gold‐Catalyzed Cross‐Coupling Reactions: An Overview of Design Strategies, Mechanistic Studies, and Applications

“…[345] Molecular O 2 dissolved in water was found to be criticalfor the reaction. Recent experimental [346] and computational [347] mechanistic studies have revealed that in addition to stabilizing and controlling the size of Au nanoparticles,P VP ligandsc ould also enhance the catalytic activity.T he electron transfer from PVP to Au impartsap artial negative chargeo n Au and facilitates the redoxc atalytic activity toward the CÀC coupling reaction assistedb yO 2 and H 2 O. Ts ukuda'sg roup later showedt hat under similar reaction conditions, the homocoupling of potassium aryltrifluoroborates could be catalyzed by Au:PVP nanoparticles. [348] The Au:PVP catalyst wasf ound to be ineffective for the aryl halide activation and forS uzuki-Miyaura-type cross-coupling reactions.…”

Pd‐catalyzed homocoupling of arylhydrazines via C―N cleavage under O2

“…It proceeded smoothly in solvent comprised of toluene and acetone (volume ratio 1:1) with a yield of 82% (Table 1, entry 15). Finally, we adjusted the constitution of toluene and acetone in the range of 9:1 to 1:9 ( Table 1, entries [16][17][18][19][20][21][22][23][24]. In general, increasing the toluene slightly favored the formation of biphenyl and a volume ratio of 3:1 was shown to be best ( Table 1, entry 19).…”