9-Borabicyclo[3.3.2]decanes and the asymmetric hydroboration of 1,1-disubstituted alkenes.

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.

Alkylboranes in the Suzuki-Miyaura Coupling: Stereochemical and Mechanistic Studies.

Both erythro and threo isomers of B-(3,3-dimethyl-1,2-dideuterio-1-butyl)-9-BBN (6) were prepared from 3,3-dimethyl-1-butyne (4) through a hydroboration-deuteronolysis-hydroboration sequence employing first 9-BBN-H and then 9-BBN-D, or in reverse order, respectively. Employing the Whitesides protocol, the stereochemistry of B --> Pd alkyl group transfer in the Suzuki-Miyaura coupling of 6 to PhBr has been found to occur with complete retention of configuration with respect to carbon. For the coupling process, the Lewis acidity of the boron plays an important role with B-alkyl-9-BBN (10) forming [HO(R)-9-BBN](-)(1) (12) with the added base, in marked contrast to their B-alkyl-9-oxa-10-borabicyclo[3.3.2]decane counterparts (R-OBBD, 11) which do not. This behavior parallels their coupling rates with the exclusive reaction of 10 over 11 in competitive experiments. Five five possible roles were demonstrated for the added base in the coupling: (1) the formation of 12, (2) the hydrolysis of Ph(Ph(3)P)(2)PdBr (14) to provide monomeric Ph(Ph(3)P)(2)PdOH (15), (3) the complexation of HOBR(2) byproducts which can compete with 10 for base, (4) accelerated coupling rates for 11, and (5) catalyst regeneration. Kinetic studies reveal that the couplings are zero-order in the borane but for 10 exhibit a first-order dependence on [PhBr] (i.e., oxidative addition), while for 11 exhibit a first-order dependence on [OH(-)(1)] (i.e., Pd(II)X hydrolysis). These data are interpreted in terms of attack of 14 by 12 to form a hydroxo &mgr;(2)-bridged intermediate 8(a) [PhL(2) Pd <-- (OH)BR(9-BBN)]. This provides the precursor to transmetalation through a four-centered transition state 9. Because the analogous hydroxyborate complex is absent for 11, 14 is hydrolyzed by OH(-)(1) forming 15 in a slower process, with this ultimately reacting with 11 to form a related intermediate 8(b) [PhL(2) Pd(OH) --> BR(OBBD)] which also collapses to products through 9.

Asymmetric allyl- and crotylboration with the robust, versatile, and recyclable 10-TMS-9-borabicyclo[3.3.2]decanes.

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%).

Role of NaBH(4) stabilizer in the oxazaborolidine-catalyzed asymmetric reduction of ketones with BH(3-)THF.

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.