Enantioselective Reduction of Prochiral Ketones by Catecholborane Catalysed by Chiral Group 13 Complexes

Abstract

LiGaH4, in combination with the S,O-chelate 2-hydroxy-2'-mercapto-1,1'-binaphthyl (MTBH2), forms an active catalyst for the asymmetric reduction of prochiral ketones, with catecholborane as the hydride source. Enantioface differentiation is on the basis of the steric requirements of the ketone substituents. Aryl/ n-alkyl ketones are reduced in 90-93% ee and RC(O)Me (e.g. R = iPr, cycloC6H11, tBu) in 60-72% ee. Other borane sources and alternative catalyst structures based on indium do not form enantioselective catalysts.

Catalytic Hydroboration of Organic Nitriles Promoted by Aluminum Complex

“…[9][10][11][12][13][14] As a result, innumerable procedures have been evolved to accomplish the addition of boranes to nitriles. All these procedures require more or less effective catalysts based on transition-metal complexes such as Co, Fe, Ru, [15][16][17][18][19][20][21][22][23] alkaline earth metal (Mg), [24][25][26][27][28][29][30][31][32][33][34][35] but the processes culminate in frustrated Lewis pairs. [36][37][38] On the other hand, while the reduction of aryl and alkyl nitriles can be achieved using stoichiometric quantities of maingroup reducing agents such as LiAlH 4 and NaBH 4 , [39] the combustible nature of these reagents and large quantities of inorganic waste by-products they generate render the process unattractive, and hence reductive hydroboration is preferable in order to provide further functionality to the resultant amine.…”

Asymmetric Reduction of Ketones with Catecholborane Using 2,6-BODOL Complexes of Titanium(IV) as Catalysts

“…[6] In addition to the diol ligands mentioned above, a Ga III complex that contained two monothiobinaphthol ligands (4) was reported by Ford and Woodward to give moderate-to-high ees ( 93 %) in the reduction of ketones with catecholborane. [7,8] In connection with another project, we became interested to see whether 1,3-diol systems, based on the bicyclo[2.2.2]octane framework, could be used as ligands for the Ti IVcatalyzed reduction of ketones with catecholborane as the reductant. The rigidity of the bicycles and the many possible orientations of metal-coordinating atoms or groups constitute interesting qualities for ligand construction.…”

Diverse Activation Modes in the Hydroboration of Aldehydes and Ketones with Germanium, Tin, and Lead Lewis Pairs

“…An enantioselective hydroboration based on a metal‐free oxazaborolidine was published in 1987 by Corey and co‐workers, and Woodward et al. described enantioselective reductions of ketones with a chiral Ga complex in 2000 . The groups of Hill, Jones, Kinjo, and Stasch recently developed main‐group hydroboration catalysts based on magnesium, tin, and phosphorus hydrides –.…”