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The remarkable versatility and selectivity of the 10-(trimethylsilyl)-9-borabicyclo[3.3.2]decanes (10-TMS-9-BBDs) in the allyl- and crotylboration of representative aldehydes are reported. The new reagents are prepared through air-stable crystalline pseudoephedrine borinic ester complexes of the 10-TMS-9-BBDs (4), which are available in 63% overall yield from B-MeO-9-BBN through a simple two-step procedure. These complexes 4 are directly converted to the corresponding B-allyl-10-TMS-9-BBDs (1) with allylmagnesium bromide, which either can be isolated (98%) or used in situ for the allylations. The remarkable enantioselectivity (96 to > or =99% ee) of these reagents in the rapid (<3 h), asymmetric allylboration process at -78 degrees C is only slightly diminished when it is conducted at 25 degrees C, a phenomenon attributable to its rigid bicyclic structure. In addition to providing the homoallylic alcohols 6 efficiently (68-80%), the procedure also permits the efficient recovery of 4 (68-84%) for the direct regeneration of 1. Alternatively, an oxidative workup procedure can be used for the preparation of 6. The reagent gives predictable stereochemistry and exhibits an extremely high level of reagent control in the allylboration of d-glyceraldehyde acetonide. A simple and efficient procedure has been developed for the preparation of all four geometric and enantiomeric isomers of the B-crotyl-10-TMS-9-BBDs (10) from optically pure enantiomers of B-MeO-10-TMS-9-BBD (3). These reagents 10 also add rapidly (<3 h) and efficiently to representative aldehydes at -78 degrees C, providing ready access to all four of the possible stereoisomers of the beta-methyl homoallylic alcohols 12-15 (69-92%) in high dr (> or =98:2) and ee (94-99%).

The syntheses of the optically pure asymmetric hydroborating agents 1 (a, R = Ph; b, R = TMS) in both enantiomeric forms are reported. These reagents are effective for the hydroboration of cis-, trans- and trisubstituted alkenes. More significantly, they exhibit unprecedented levels of selectivity in the asymmetric hydroboration of 1,1-disubstituted alkenes (28-92% ee), a previously unanswered challenge in the nearly 50 year history of this reagent-controlled process. For example, the hydroboration of alpha-methylstyrene with 1a produces the corresponding alcohol 6f in 78% ee (cf., Ipc2BH, 5% ee). Suzuki coupling of the intermediate adducts 5 produces the nonracemic products 7 very effectively (50-84%) without loss of optical purity.

The safe handling of borane complexes and alkylboranes (trialkylboranes, dialkylboranes, and trialkylborohydride reagents) and related workup issues are addressed. Oxidation of alkylboranes and quenching of reaction mixtures to safely decompose the reagent or products are discussed. Several case studies demonstrate large-scale applications of borane reagents for carboxylic acid reduction, reductive amination, and hydroboration followed by oxidation.

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When stabilized BH(3-)THF (BTHF) was added to a mixture of ketone and tetrahydro-1-methyl-3,3-diphenyl-1H,3H-pyrrolo[1,2-c][1,3,2]oxazaborole (MeCBS-ozaxaborolidine, MeCBS) catalyst 1, low enantioselectivities resulted. Several relative rate experiments showed that a borohydride species in BTHF catalyzed the nonselective borane reduction of ketones, effectively competing with enantioselective MeCBS reduction of ketones, lowering the overall selectivity of the reaction. Improved enantioselectivities in the reaction are obtained by reversing the mode of addition (ketone to BTHF and catalyst), lowering the concentration of NaBH(4) stabilizer in the BTHF solution (87-95% ee) and increasing the concentration or addition rate of BTHF. Decreased reaction temperature and increased catalyst loading only slightly improved the selectivity of the reaction. Upon reaction parameter optimization, simultaneous addition of substrate and BTHF to MeCBS catalyst stabilizer resulted in the highest overall enantioselectivities (96% ee) and diminished the effect of the borohydride. Alternatively, the addition of Lewis acids such as BF(3-)THF to the reaction mixture effectively destroyed the NaBH(4) stabilizer in BTHF solutions, restoring the enantioselectivity to acceptable levels.

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