The regiochemistry and stereochemistry of the hydroboration of allylsilanes

Abstract

The hydroboration of a wide range of allylsilanes 3 and 5–21 is found to be generally regioselective for attachment of the boron to C-3 and hydrogen to C-2 of the allyl unit, and to be generally stereoselective in the sense 1, with attachment of the boron and the hydrogen anti to the silyl group in the most populated conformation. This stereochemistry is opposite in sense to that already known for the hydroboration-oxidation of the corresponding allylic alcohol derivatives. The degree of selectivity, both regio- and stereo-, is found to be greater with 9-BBN and other hindered boranes than with borane itself. Since the boron group and the dimethyl(phenyl)silyl group in the products can be successively converted into hydroxy groups with retention of configuration at both sites, the overall sequence is a stereocontrolled synthesis of 1,3-diols. The diols may be obtained with either relative stereochemistry by the appropriate choice of double bond geometry in the allylsilane, and, because the hydroxy groups are released separately, they may be easily differentiated. The structures and relative stereochemistries of all the products, major and minor, is proved by alternative syntheses and/or conversions into known compounds. The borane intermediates can also be used in carbon-carbon bond-forming reactions. A moderately high level (3 : 1) of 1, 3 induction is found both in nucleophilic attack on the 3-silyl ketone 46 and in the hydroboration of the homoallylsilane 61.