Zinc Salt Catalytic Potential Chiral Ketone Asymmetric Silicon Hydrogen Addition Reaction Research

The preparation of chiral secondary alcohols from prochiral ketones via asymmetric catalysis is an important transformation in organic synthesis for the development of biologically active molecules as well as in the fields of pharmaceuticals, agrochemicals, fragrance and flavour. The methods of asymmetric reduction of the prochiral ketones include hydrogenation, hydrogen transfer and asymmetric hydrosilylation (AHS), etc. Among these methods mentioned, AHS appears to be the most attractive way because of the advantages of simplicity of the procedure combined with the use of inexpensive and stable silanes as reducing agents. AHS of prochiral ketones had been known since the early1970s, and thereafter, several transition metals complexes such as rhodium with chiral phosphine, oxazoline or N-heterocyclic carbene complexes.Recently, the use of zinc species in hydrosilylation reactions became an interesting research field. Chandrasekhar et al reported an inexpensive and safe reagent system comprising of polymethylhydrosiloxane (PMHS) and ZnCl2, which has been developed for the selective reduction of carbonyl compounds to corresponding alcohols at room temperature. Furthermore, zinc complexes of chiral secondary amines were introduced by Mimoun as chiral catalysts for the enantioselective hydrosilylation of ketones. The other chiral zinc complexes such as zinc-macrocyclic oligoamine complexes, diethyl zinc/chiral diamines,(S,S)-bis(oxazolinylphenyl)amine/FeCl2/zinc salt or zinc powder were also efficient catalysts for AHS. Zinc complexes with chiral Schiff base derived from readily available and inexpensive a-amino acids exhibited excellent catalytic properties for asymmetric hydrosilylation of ketones.Several chiral ligands containing (R,R)-diaminocyclohexane moieties and pyrrole, furan or benzene have been synthesized. These ligands were tested in enantioselective zinc-catalyzed hydrosilylation reactions. Furthermore, excellent enantioselectivities were obtained when the ligands containing (R,R)-diaminocyclohexane moieties and furan rings were used. For comparison, zinc chloride combined with different potassium carboxylate salts and ligands were also tested for the catalytic hydrosilylation reactions, and zinc complexes with these chiral ligands prepared were tested for the catalytic asymmetric hydrosilylation of prochiral ketones, and the results showed that excellent enantiomeric excess values (85%ee) were obtained, which are the prominent examples of catalytic asymmetric hydrosilylation of prochiral ketones catalyzed with zinc complexes in the presence of readily available and inexpensive (R,R)-diaminocyclohexane moieties and pyrrole, furan or benzene.