Alpine-Borane methyl ketone

Asymmetric Reductions. NB-Enantrane is an analog of B-3-Pinanyl-9-borabicyclo[3.3.1 Jnonane (Alpine-Borane ). Since the absolute configuration of nopol is the opposite of (+)-a-pinene, the reagent provides the opposite mode of asymmetric induction. The reagent provides higher asymmetric induction than Alpine-Borane for cases where the two groups flanking the ketone are relatively small, such as methyl or ethyl alkynyl ketones (eq 2). 

Methyl alkynic ketones are reduced with slightly lower efficiency and f-butyl alkynic ketones are reduced very slowly. In the latter case, dehydroboration of Alpine-Borane to give 9-BBN competes with the rate of reduction and the liberated 9-BBN reduces the ketone to give products of lower enantiomeric purity. This problem may be overcome by using high pressure or by using 6-10-cij -myrtanyl-9-BBN (eqs 5 and 6). ... 

Alpine-Borane is an effective, readily available reagent for the reduction of acetylenic ketones. This class of ketones is reduced somewhat slower than aldehydes but the reactions generally proceed at 25 °C. Straight chain alkyl acetylenic ketones are completely reduced after 8 hours at 25 °C using a 0.5 M solution of Alpine-Borane in tetrahydrofuran. More hindered internal acetylenic ketones may require 1 4 days under these conditions, but the process may be accelerated by performing the reaction in the absence of solvent. Acetylenic ketones are reduced faster than other ketones, and can also be reduced in the presence of a methyl ketone24. An example is the selective reduction of 7-nonyne-2,6-dione24. 

Some methyl acetylenic ketones are reduced with slightly less efficiency, but this can be improved by using NB-Enantrane11 or an analogous Alpine-Borane derivative with a substituent on the... 

Asymmetric reduction of ketones.2 Both Alpine-Borane and B-chlorodiisopino-campheylborane (Ipc2BCl) have one main defect for asymmetric reduction of prochiral ketones they show little cnantiosclectivity in reduction of dialkyl ketones in which the alkyl groups arc similar in size. This problem is now solved by reductions with Eap2BCl (1). Thus acctylcyclohexanc is reduced by 1 in 97% ce and isopropyl methyl ketone is reduced in 95% ee (65% yield). 

The use of elevated pressures (2,000-6,000 atm.) accelerates asym. reduction of prochiral ketones with B-3-pinanyl-9-borabicyclo[3.3.1]nonane while suppressing competing dehydroboration-reduction which erodes enantioselectivity at atm. pressure. E Neat Alpine-Borane at 0° treated with 3-acetylpyridine via a syringe under Nj, stirred for 20 min at room temp., the mixture [in a capped syringe] placed in a high-pressure cell, pressurized to 6,000 atm. for 1.5 days, and worked up by the standard oxidative procedure - (S)-methyl(3-pyridyl)carbinol. Y 67% (100% e.e. ... 

The chemoselectivity shown by Alpine-Borane allows one to achieve selective reduction of propargyl ketone in the presence of methyl ketone [14] (Eq. 26.10). 

As can be seen from the diagram, the reaction probably proceeds by P-hydride transfer via a boat-shaped transition state in which the less bulky substituent buttresses the methyl group of the pinene, accounting for the high selectivities seen with ketones containing the slender ethynyl group. Note that the pinene is regenerated and can be reconverted to Alpine borane as shown. In this way (+)-a-pinene can be seen merely as an asymmetric carrier for the hydride of 9-BBN. 

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