Reduction 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). 

In 1951 Bothner-By first attempted asymmetric reductions based on the conversion of lithium aluminum hydride (LAH) into a chiral alkoxy derivative by reaction with (+)-camphor. Since this pioneering work, the use of chirally modified LAH reagents has been the focus of much attention. In 1979, the first virtually complete enantiofacial recognition of prochiral carbonyl compounds was accomplished by using LAH modified with optically pure 2,2 -dihydroxy-1,1 -binaphthyl and a simple alcohol (BINAL-H). Asymmetric reduction with chiral 2,5-dimethylborolane also gave alcohols in high optical yields." Recently, excellent results have been obtained using a chirally modified sodium borohydride... 

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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