Mechanism of the stille reaction. 2. Couplings of aryl triflates with vinyltributyltin. Observation of intermediates. A more comprehensive scheme

The mechanism of the [PdL4]-catalyzed couplings between R-OTf (R = pentahalophenyl; L = PPh3, AsPh3) and Sn(CH=CH2)Bu-3 has been studied. The addition of LiCl favors the coupling for L = AsPh3 in THF but retards it for L = PPh3. Separate experiments show that for L = AsPhs, LiCl accelerates the otherwise very slow and rate-determining oxidative addition of the aryl triflate to [PdL4], leading to trans[PdRClL2]. Therefore, the overall process is accelerated. For L = PPh3, the rate-determining step is the transmetalation. Complex trans-[PdRXL2],with X = Cl, is formed in the presence of LiCl, whereas an equilibrium mixture mainly involving species with X = TfO, L, or S (S = solvent) is established in the absence of LiCl. Since the transmetalation is slower for X = Cl than for the other complexes, the overall process is retarded by addition of LiCl. The transmetalation in complexes trans-[PdRXL2], with X = Cl, follows the S(E)2(cyclic) mechanism proposed in Part 1 (Casado, A. L.; Espinet, P. J. Am. Chem. Sec. 1998, 120, 8978-8985), giving the coupling product R-CH=CH2 directly. For X = TfO or, L rather stable intermediates trans[PdR(CH=CH2)L-2] are detected, Supporting an SE2(open) mechanism. The I;ey:intermediates undergoing transmetalation in the conditions arid solvents most commonly used in the literature have been identified.;The operation of S(E)2(cyclic) and S(E)2(open) pathways emphasizes common aspects of the Stille reaction with the Hiyama reaction where, using (RSiF3)-Si-2 that is chiral at the alpha -carbon of R retention or inversion gt the transmetalated chiral carbon can be-induced. This helps us to understand the contradictory stereochemical outcomes in the literature for Stille couplings using (RSnR3)-Sn-2 derivatives that are chiral Bt the a-carbon of R-2 and suggests that stereocontrol of the Stille reaction might be achieved.