Carbosilylation of Allenes Catalyzed by Palladium Complexes:  A New Efficient Route to Substituted Allylic Silanes

An aromatic or olefinic halide RX (C 6 H 5 I, p -CH 3 COC 6 H 4 I, p -NO 2 C 6 H 4 I, p -CH 3 OC 6 H 4 I, o -CH 3 OC 6 H 4 I, m -C 2 H 5 OCOC 6 H 4 I, ( Z )-C 2 H 5 OCOCH CHI, 3-iodocyclopent-2-en-1-one, 1-iodothiophene, 1-iodonaphthalene, 3-bromo-5,5-dimethylcyclohex-2-en-1-one, or α-bromostyrene), Bu 3 SnSiMe 3 ( 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)CH 2 SiMe 3 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 CH 2 (R‘ ‘ = cyclo-C 6 H 11, n -Bu, and Ph) also undergo carbosilylation with PhI and 2, producing R‘ ‘CH C(Ph)CH 2 SiMe 3 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.