Unusual Palladium-Catalyzed Silaboration of Allenes Using Organic Iodides as Initiators: Mechanism and Application

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Abstract
A highly regio- and stereoselective method for the synthesis of various 2-silylallylboronates 7 from allenes 1 and 2-(dimethylphenylsilanyl)-4,4,5,5-tetramethyl[1,3,2]dioxaborolane (5) catalyzed by palladium complexes and initiated by organic iodides is described. Treatment of monosubstituted aryl and alkylallenes RCHCCH2 (1a−m) and 1,1-dimethylallene (1n) with borylsilane 5 in the presence of Pd(dba)2 (5 mol %) and organic iodide 3a (10 mol %) afforded the corresponding silaboration products 7a−n in moderate to excellent yields. This catalytic silaboration is totally regioselective with the silyl group of 5 adding to the central carbon and the boryl group to the unsubstituted terminal carbon of allene. Furthermore, the reactions show very high E stereoselectivity with the Z/E ratios lying in the range from 1/99 to 7/93. In the absence of an organic iodide, silaboration of 1 with 5 still proceeds, but gives products having completely different regiochemistry as that of 7. The silaboration chemistry can be applied to the synthesis of homoallylic alcohols. Treatment of allenes (1) with borylsilane 5 and aldehydes 14 in the presence of Pd(dba)2 (5 mol %) and 3a (10 mol %) at 80 °C in ethyl acetate for 5 h afforded homoallylic alcohols 15a−p in one pot in good to excellent yields, with exceedingly high syn selectivity (>93%). Mechanistic pathways involving an unusual palladium-catalyzed three-component assembling reaction of dimethylphenylsilyl iodide, allene 1, and borylsilane 5 were proposed to account for these catalytic reactions.

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