Mechanism of the Stille Reaction. 2. Couplings of Aryl Triflates with Vinyltributyltin. Observation of Intermediates. A More Comprehensive Scheme

• Published in: Journal of the American Chemical Society 2000-12, Vol.122 (48), p.11771-11782
• Main Authors: Casado, Arturo L, Espinet, Pablo, Gallego, Ana M
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja001511o

The mechanism of the [PdL4]-catalyzed couplings between R−OTf (R = pentahalophenyl; L = PPh3, AsPh3) and Sn(CHCH2)Bu3 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 = AsPh3, 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 SE2(cyclic) mechanism proposed in Part 1 (Casado, A. L.; Espinet, P. J. Am. Chem. Soc. 1998, 120, 8978−8985), giving the coupling product RCHCH2 directly. For X = TfO or L, rather stable intermediates trans-[PdR(CHCH2)L2] are detected, supporting an SE2(open) mechanism. The key intermediates undergoing transmetalation in the conditions and solvents most commonly used in the literature have been identified. The operation of SE2(cyclic) and SE2(open) pathways emphasizes common aspects of the Stille reaction with the Hiyama reaction where, using R2SiF3 that is chiral at the α-carbon of R2, retention or inversion at the transmetalated chiral carbon can be induced. This helps us to understand the contradictory stereochemical outcomes in the literature for Stille couplings using R2SnR3 derivatives that are chiral at the α-carbon of R2 and suggests that stereocontrol of the Stille reaction might be achieved.