[122-56-5] · C12H27B · Tri-n-butylborane · (MW 182.20)
Physical Data: bp 108-110 °C/20 mmHg, 90-91 °C/9 mmHg; d 0.747 g cm-3.
Solubility: sol most organic solvents.
Form Supplied in: colorless liquid; commercially available.
Purification: distillation from CaH2.
Handling, Storage, and Precautions: moisture sensitive. Use in a fume hood.
Highly stereoselective reduction of b-hydroxy ketones is realized via boron chelates.1 Dibutylboronates of b-hydroxy ketones are formed in situ from b-hydroxy ketones and tributylborane with the catalytic action of air in THF. Treatment of the mixture with Sodium Borohydride at -78 °C then affords syn-1,3-diols stereoselectively (eq 1). The selectivity is better when the reduction is carried out at a lower (-100 °C) temperature. In some instances, triisobutylborane gives a better selectivity than tributylborane, but the reduction proceeds more slowly.
For example, this stereoselective reduction is utilized in the synthesis of a b-hydroxy-d-lactone HMG-CoA reductase inhibitor (eq 2).6 This procedure is also applicable to the stereoselective reduction of syn-a-substituted b-hydroxy ketones.1 The reduction gives 1,3-diols with syn,syn stereochemistry exclusively (eq 3). However, the stereoselectivity of reduction of anti-a-substituted b-hydroxy ketones depends on the alkyl substituents, R1 and R2 (eq 4).
In the presence of an equimolar amount of tributylborane (Triethylborane is also effective), but-2-enyllithium reacts with aldehydes to give the anti-homoallyl alcohols predominantly. For example, in the reaction with benzaldehyde the anti-isomer is obtained in 74% yield, along with the syn-isomer in 15% yield (eq 5).
The chlorination of alkanes with Chlorine, t-Butyl Hypochlorite, or N-Chlorosuccinimide proceeds at 20 °C in carbon tetrachloride, in the presence of a catalytic amount of tributylborane, even in the dark, giving monochloroalkanes. For the chlorination of 2,3-dimethylbutane, 3-methylpentane, and n-heptane, the regioselectivity is dependent on the chlorinating reagents, as shown in eq 6. Trihexylborane and tricyclohexylborane are also effective initiators.
Di-n-butylboryl Trifluoromethanesulfonate, which is useful for cross-aldol reactions of ketones with aldehydes (eq 8), is prepared from equimolar amounts of tributylborane and Trifluoromethanesulfonic Acid (eq 7).4
Alkylthiodibutylboranes are easily prepared from tributylborane and thiols (eq 9).5 The reaction of these boranes with ketene generates vinyloxyborane intermediates, which react with ketones, aldehydes, or nitriles to afford b-hydroxy or b-imino thioesters (eq 10).7
Alkylthiodibutylboranes also react with a,b-unsaturated ketones to generate vinyloxyborane intermediates, which react with aldehydes to afford b-hydroxy ketone derivatives (eq 11).7
Koichi Narasaka & Yujiro Hayashi
The University of Tokyo, Japan