Regioselective reduction of the homoisoflavone system with dialkylboranes in the presence of palladium catalysts

(E)-3-Benzylidene-4-chromanones were regioselectively reduced to 3-benzyl-4-chromanones through the 1,4-addition of dialkylboranes, especially 9-borabicyclo[3.3.1]nonane, in excellent yields. The reaction was efficiently promoted by some palladium catalysts. Among them, dichloro[1,1'-bis(diphenylphosphino)ferrocene]palladium(II) was most effective, followed by palladium(II) chloride. Furthermore, the reaction was characteristic of the structurally rigid exo-cyclic alpha,beta- unsaturated ketones, such as (E)-3-benzylidene-4-chromanone, while no reaction occurred in the case of 3-benzyl-4H-chromen-4-one, which is an endo-cyclic one. 2-Benzylidene-3,4-dihydro-1(2H)-naphthalenone and 2-benzylideneindanone were similarly reduced at an olefin moiety only as the result of a 1,4-reduction.