Advances in the stereoselective synthesis of antifungal agents and ...

Chapter 6 146[P(Ph) 3 PdCl 2 ]CuDIPEA.HClRHCuI; DIPEAR[Pd(Ph 3 )P]R2RRR[P(Ph) 3 ]Pd°XR'R'Ph 3 PPh 3 P2+PdXPh 3 PPh 3 P2+PdRCuXCuRScheme 6.16: Crosscoupling mediated by Pd 0 .The coupling reaction appears to involve Pd° catalysis, the speciesbeing generated by reduction of palladium II in the organopalladiumhalide by attack of the copper acetylide anion. The thus formed bis(triphenylphophine)dialkynyl palladium undergoes reductive eliminationof disubstituted acetylene to form bis(triphenylphosphine) Palladium(0).The thus resulting Palladium(0) species enters the catalytic cycle (scheme6.16) by oxidative addition of para substituted aryliodide, followed byalkylation to generate again a palladium II species. Finally, reductiveelimination leads to a coupling product and the palladium(0) catalyst isregenerated. This mechanism is the most accepted one for the couplingbetween the aryliodide and acetylenic compounds. It seems reliable alsofor the coupling between arylpropynols 58 and 2-iodophenol. The secondstep of the synthesis of benzofurans or indols might involve more thanone mechanism. In fact, in the literature 4 it is reported that Pd° cancatalyze the annulation step (Scheme 6.17).