Ketones 2,5-dimethylborolane

Dimethylborolane has been shown to be effective in asymmetric reduction of ketones (Imai, T. Tamura, T. Yamamuro, A. Sato, T. Wollmann, T. A. Kennedy, R. M. Masamune, S., J. Am. Chem. Soc., 1986, 108, 7402-7404). Using the frontier orbitals of a borolane and a ketone, show the probable course of the initial interaction between the two. 

Oxaborolidine, 9-borabicyclo[3.3.1]nonane, and 2,5-dimethylborolane derivatives as catalysts for the asymmetric reduction of ketones 92S605. 

Asymmetric Reduction of Ketones. A reagent system consisting of (/ ,/ )-2,5-dimethylborolane (1.0 equiv) and the corresponding borolanyl mesylate (0.2 equiv) reduces a variety of prochiral ketones with asymmetric induction in the range of 80-100% ee. The reagent system is prepared in situ by addition of 1.4 equiv of Methanesulfonic Acid to a solution of the lithium dihydridoborate, prepared as in eq 5 above (eq 7). 

Masamune and cowotkers48 designed a set of chiral rrans-2,5-dimethylborolane-based reagents that show remarkable enantioselectivities in hydroboration,49 ketone reduction50 and crotyl addition to aldehydes.51 The chirality transfer also turns out to be highly efficient in the aldol reaction. Initial experiments were aimed at defining the structural parameters which affect the E(0) Z(0) ratios of the enolates prepared from various alkanethioates with 2,5-dimethylborolanyl triflate (90) under standard conditions... 

The unusual observation that neither the borane nor the borohydride alone are effective, prompted an investigation of the mechanism of reduction21. It is postulated that the ketone forms a complex with 2.5-dimethylborolanvl methanesulfonate. This complex then undergoes reduction by monomeric 2,5-dimethylborolane. 

Masamune s Chiral 2,5-Dimethylborolanes Enantioselective reduction of aliphatic ketones like 2-butanone, 4-methyl-2-pentanone, and several others where both the appendages of the carbonyl group are stericaUy similar is very chaUenging. Masamune s dimeric ligand (RJi)- (Scheme 2.138) or (5,5)-2,5-dimelhylborolane (not shown) is very efficient in reducing those types of ketones [66]. 

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