a 天然产物化学与小分子催化四川省高校重点实验室, 乐山师范学院化学学院, 四川乐山 614000;
b 四川文理学院化学化工学院, 四川达州 635000
B(C6F5)3-catalyzed chemoselective reduction of carbonyl compounds under water conditions
Sun Guofenga, He Yunqinga, Tian Chonga, Borzov Maxima, Hu Qishanb, Nie Wanlia
a Sichuan Province Key Laboratory of Natural products and Small Molecule Synthesis, Chemical Department of leshan Normal University, Leshan 614000, China;
b College of Chemistry and Chemical Engineering, Sichuan University of Arts and Science, Dazhou 635000, China
Recently, the research work concerning B(C6F5)3 catalyzed reduction of carbonyl compounds revealed that this Lewis acid B(C6F5)3 presents, actually, a rather water-tolerant system. This fact considerably broadens the scope of the water/base tolerant frustrated Lewis pairs (FLP) chemistry. In this research, an efficient chemoselective reduction of aldehydes and ketones to alcohols catalyzed by a Lewis acid catalyst B(C6F5)3 has been developed. It is the first report about the chemoselective reduction of carbonyl compounds under aqueous conditions catalyzed by FLPs with hydridosilanes as reducing agents. The selectivity and activity of different hydridosilanes and the influence of substituents in carbonyl compounds have been studied. The effect of water concentration on the chemoselectivity of the reaction has also been investigated. It has been found that a 2-3 fold excess of water relatively to hydridosilanes usually exhibits better selectivity and overall yields than in the equimolar case. The reduction reaction can even be successfully performed with pure water as a solvent without any loss of the reactivity. Such a procedure has been successfully applied to reduce 14 differently substituted aldehydes and ketones into alcohols with up to 100% yields under mild conditions, but failed in case of the diaryl substituted ketones. Both experimental and computational methods have been performed to confirm the possibility of the water mediated mechanism and the effects of different Lewis bases on the LB…H-OH…LA three-component aggregates. These mechanistic studies have revealed that such water mediation between a carbonyl compound and a catalyst advantageously (i) activates the C=O group by protonation and (ii) fixes the catalytic borane moiety by formation of a B-O bond, which to some extent prevents the direct hydrolysis of hydridosilane and makes the reaction possible under moist conditions. Detailed clarification of the actual role of water in the reduction reaction of question would promote the further propagation of FLP-catalyzed and related reactions into the field of "green" chemistry.