Carbosilylation of allenes catalyzed by palladium complexes: A new efficient route to substituted allylic silanes

An aromatic or olefinic halide RX (C6H5I, p-CH3COC6H4I, p-NO2C6H4I, p-CH3OC6H4I, o-CH3OC6H4I, m-C2H5OCOC6H4I, (Z)-C2H5OCOCH=CHI, 3-iodocyclopent-2-en-1-one, 1-iodothiophene, 1-iodonaphthalene, 3-bromo-5,5-dimethylcyclohex-2-en-1-one, or alpha-bromostyrene), Bu3SnSiMe3 (2), and 1,1-dimethylallene (3a) undergo three-component coupling reaction in toluene in the presence of Pd(dba)(2) (dba = dibenzylideneacetone) to give an allylic silane (Me)(2)C=C(R)CH2SiMe3 in good to excellent yields. When X = I, the yields are substantially higher than when X = Br or Cl (no reaction). This carbosilylation reaction is highly regioselective, with the R group adding to the middle carbon and the silyl group to the unsubstituted terminal carbon of 3a. Monosubstituted allenes R " CH=C=CH2 (R " = cyclo-C6H11, n-Bu, and Ph) also undergo carbosilylation with PhI and 2, producing R " CH=C(Ph)CH2SiMe3 stereoselectively with Z/E between 98/2 and 80/20. Bulkier organic halides and allenes give products with higher Z/E ratios. Based on known palladium-allene and -allyl chemistry, we propose a mechanism to account for this palladium-catalyzed three-component coupling reaction.