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 ((CRR3)-R-2=C=CH2) With acyl chlorides ((RCOCl)-C-1) and bispinacolatodiboron in the presence of PdCl2(CH3CN)(2) in toluene at 80 degreesC gave 2-acylallylboronates (CRR3)-R-2=C(COR1)CH2B-(OCMe2CMe2O) 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 (CR2R3=C(COR1)CH2SiMe3) in good to moderate yields. Several chloroformates ((ROCOCl)-O-4) also react with 1,1-dimethylallene (2a) and 5 to afford allylsilanes ((CRR3)-R-2=C(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 degreesC; 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.