1,4-addition of axylboronic acids and arylsiloxanes to α,β-unsaturated carbonyl compounds via transmetalation to dicationic palladium(II) complexes

A high reactivity of dicationic palladium(II) complexes for transmetalation of organoboron and -silicon compounds was demonstrated in the 1,4-addition of arylboronic acids or arylsiloxanes to acyclic and cyclic enones. [Pd(dppe)(PhCN)(2)] (SbF6)(2) and nitrile-free catalysts prepared in situ from Pd(acac)(2), dppben, and Cu(BF4)(2).6H(2)O or oxidation of Pd(dba)(2) With Cu(BF4)(2).6H(2)O in the presence of dppe or dppben effectively catalyzed the reactions of arylboronic acids at room temperature and arylsiloxanes at 75degreesC. A catalytic cycle involving transmetalation between Ar-[m] (m = B(OH)(2), Si(OMe3)) and [Pd](2+) to give an Ar-[Pd](+) intermediate, insertion of an enone into the C-Pd bond, giving a palladium enolate, and finally hydrolysis with water to regenerate [Pd](2+) species was proposed. The reaction between [Pd(dppe)(PhCN)(2)](BF4)(2) (4a) and PhB(OH)(2) in the presence of PPh3 allowed the first isolation of the transmetalation intermediate [Pd(Ph)(dppe)(PPh3)] (BF4) and characterization by X-ray analysis. The reaction of a series of para-substituted arylboronic acids with 4a showed a slightly negative rho value, (-0.54), demonstrating an electronic effect that was accelerated by the donating substituents.