Silica-Immobilized Cyclic Urea Catalyst for the Reduction of Carboxylic Acids to Alcohols.

Catalytic reduction of carboxylic acids to the corresponding alcohols is a challenging but useful reaction. Since the conventional reduction methods such as hydrogenation and hydrosilylation largely rely on transition-metal catalysts, the development of heterogeneous organocatalysts for the reaction is an attractive research target. In this study, we achieved the reduction of benzoic acid derivatives to benzylic alcohols by hydrosilylation with a silica-immobilized cyclic urea catalyst followed by hydrolysis. The catalytic activity of immobilized cyclic urea was significantly higher than that of the free cyclic urea, silica supports, their physical mixture, or immobilized organocatalysts with different structures, such as cyclic carbonates and amines. Control experiments and Fourier transform infrared (FT-IR) spectroscopy revealed that the synergy between immobilized urea and silanol groups on the silica surface resulted in superior catalysis and that hydrosilylation of the silyl ester intermediate was the key step. It was proposed that urea and silanol concertedly activate the hydrosilane and silyl ester, respectively, to achieve efficient hydride transfer, which was supported by density functional theory (DFT) calculations on model reaction systems. This study demonstrates that the concerted activation of nucleophiles and electrophiles by silica-immobilized organocatalysts is effective for challenging molecular transformations.