Substituent groups boron hydrides

The ligands R can be exclusively halide, amide, hydride, or smart leaving groups like trimethylsilyl (TMS) or trimethylstannyl, or combinations of them. Some, but never all, of the sites R may be occupied by saturated or unsaturated organic substituents. In the case of boron hydrides, the Lewis base (D) adducts are preferred because they are much easier to handle than the pure boranes. 

Redistribution of substituents tends to be especially facile for halides, hydrides, and alkyls of Groups I—III nontransition elements because these compounds are electron-deficient. Bridging groups are present in many of these compounds. Even in the boron trihalides that are not bridged, a bridged transition state making use of the empty valence shell orbitals is possible, so that redistribution can occur with a relatively low activation energy (113) ... 

The cis isomer was favored ( 9 1). This is a rare example of deactivated pyridine undergoing borohydride reduction. The answer lies in the carbonyl group, which undoubtedly undergoes reduction to give the borate ester 39. This boron complex will have a significant steric requirement. It is known that increasing the size of the 1 substituent increases the proportion of hydride attack at the 4 position, and hence formation of piperidine, in pyridinium salts. Likewise, initial hydride attack will now be directed toward the 4 position, and subsequently the piperidine 38 will be formed. 

Steric bulkiness of substituents in ketones (25), for example, makes it possible to differentiate two pairs of lone-pair electrons (a or b in 25) on carbonyl oxygen for coordinating with the Lewis acidic center of boron in oxazaborolidine (28). The complex (29) formed from 28 with catecholborane binds ketone (32) stereospecifically by using the lone-pair electrons at the less hindered side so as to direct the smaller substituent Rs toward the bulky N-tert-butyl group. The intramolecular hydride transfers to the favorably coordinated carbonyl group as shown in 30, Scheme 1.82, results in asymmetric reduction of ketones. Reduction of trifluoromethyl and methyl ketones (25) (R = CF3 or CH3) by this system afforded the corresponding alcohols (26 or 27) with an opposite stereochemistry (Scheme 1.82). 

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