Highly Regio- and Stereoselective Acylboration, Acylsilation, and Acylstannation of Allenes Catalyzed by Phosphine-Free Palladium Complexes:  An Efficient Route to a New Class of 2-Acylallylmetal Reagents

A new method for the synthesis of substituted 2-acylallylmetal reagents in a highly regio- and stereoselective fashion involving a three-component assembly of allenes, acyl chlorides, and bimetallic reagents (B−B, Si−Si, and Sn−Sn) catalyzed by phosphine-free palladium complexes is described. Treatment of various allenes (CR²R³CCH₂) with acyl chlorides (R¹COCl) and bispinacolatodiboron in the presence of PdCl₂(CH₃CN)₂ in toluene at 80 °C gave 2-acylallylboronates in moderate to good yields. The acylsilation of allenes with acid chlorides and hexamethyldisilane (5) proceeded successfully in the presence of Pd(dba)₂ in CH₃CN affording the corresponding allylsilanes (CR²R³C(COR¹)CH₂SiMe₃) in good to moderate yields. Several chloroformates (R⁴OCOCl) also react with 1,1-dimethylallene (2a) and 5 to afford allylsilanes (CR²R³C(COOR⁴)CH₂SiMe₃) in 66−70% yields. Acylstannation of allenes could also be achieved by slow addition of hexabutylditin (10) to the reaction mixture of acyl chloride (or chloroformate) and allene 2a in CH₃CN in the presence of Pd(dba)₂ at 60 °C; the corresponding 2-substituted allylstannanes were isolated in moderate to good yields. The above catalytic reactions are completely regioselective and highly stereoselective. A mechanism is proposed to account for the catalytic reactions and the stereochemistry.