Highly Enantioselective Transfer Hydrogenation of Prochiral Ketones Using Ru(II)‐Chitosan Catalyst in Aqueous Media

Abstract

AbstractUnprecedentedly high enantioselectivities are obtained in the transfer hydrogenation of prochiral ketones catalyzed by a Ru complex formed in situ with chitosan chiral ligand. This biocompatible, biodegradable chiral polymer obtained from the natural chitin afforded good, up to 86 % enantioselectivities, in the aqueous‐phase transfer hydrogenation of acetophenone derivatives using HCOONa as hydrogen donor. Cyclic ketones were transformed in even higher, over 90 %, enantioselectivities, whereas further increase, up to 97 %, was obtained in the transfer hydrogenations of heterocyclic ketones. The chiral catalyst precursor prepared ex situ was examined by scanning electron microscopy, FT‐mid‐ and ‐far‐IR spectroscopy. The structure of the in situ formed catalyst was investigated by 1H NMR spectroscopy and using various chitosan derivatives. It was shown that a Ru pre‐catalyst is formed by coordination of the biopolymer to the metal by amino groups. This precursor is transformed in water insoluble Ru‐hydride complex following hydrogen donor addition. The practical value of the developed method was verified by preparing over twenty chiral alcohols in good yields and optical purities. The catalyst was applied for obtaining optically pure chiral alcohols at gram scale following a single crystallization.