Selectivity Enhancement in Dynamic Kinetic Resolution of Secondary Alcohols through Adjusting the Micro-Environment of Metal Complex Confined in Nanochannels: A Promising Strategy for Tandem Reactions

Abstract

Dichloro­(η⁶-<i>p</i>-cymene) (1-butyl-3-cyclohexyl-imidazolin-2-ylidene) ruthenium­(II) (<b>RuL</b>) was synthesized and confirmed. Five heterogeneous catalysts with similar ruthenium cores were prepared by chemical immobilization method using various silica-based supports, including mesoporous silica SBA-15 of different pore sizes (<b>Ru/Si-9</b>, <b>Ru/Si-8</b>, and <b>Ru/Si-7</b>), nonporous silica particles (<b>Ru/SiO</b>_<b>2</b>), and surface trimethylsilylated SBA-15 (<b>Ru/SiMe</b>). The dynamic kinetic resolution (DKR) of 1-phenylethanol, which includes metal–enzyme bicatalytic racemization in tandem with stereoselective acylation, gave product in 99% yield and 0% ee with homogeneous catalyst <b>RuL</b>, whereas the heterogeneous <b>Ru/Si-8</b> exhibited high catalytic activity and enantioselectivity (up to 96% yield and 99% ee). The racemization and acylation abilities of different catalysts were analyzed. The influences of pore size and surface properties for heterogeneous catalysts were investigated, and the nanocage effect was found to be the key factor in stereoselectivity. The catalyst <b>Ru/Si-8</b> performed well in reactions with various substrates and can be reused for at least seven times