Regio- and stereoselective synthesis of boryl-substituted allylsilanes via transition metal-catalyzed silaboration

Regio- and stereo-controlled synthesis of boryl-substituted allylsilanes via transition metal-catalyzed additions of silylboranes to unsaturated organic compounds is described. Nickel-catalyzed reactions of (dimethylphenylsilyl)pinacolborane with 1,3-dienes, vinylcyclopropanes, and vinylcyclobutanes yielded 4-, 5-, and 6-boryl-substituted allylsilanes, respectively. Palladium-catalyzed addition of the silylborane to allenes took place at the more substituted C=C bond to yield 2-borylallylsilane selectively. The 2-borylallylsilanes served as useful allylation reagents in Lewis acid-mediated reactions with acetals and aldehydes. In addition to the simple allylation reactions, a cascade reaction to form the trans-9-boryl-1,2-benzooxadecalin skeleton and a cyclization reaction to form cyclic alkenylboranes were achieved by the use of 2-borylallylsilanes as key reagents. Reactions of methylenecyclopropanes were catalyzed by palladium and platinum catalysts. The reaction course, however, depended upon the substrate structure and the catalyst employed. For instance, cycloalkylidenecyclopropanes yielded 2-cycloalkylidene-3-boryl-1-silylpropanes selectively in the presence of a palladium catalyst, while 3-cycloalkylidene-3-boryl-1-silylpropanes were obtained selectively in the corresponding platinum-catalyzed reactions.