Palladium- or Nickel-Catalyzed Reactions of Alkenylmetals with Unsaturated Organic Halides as a Selective Route to Arylated Alkenes and Conjugated Dienes: Scope, Limitations, and Mechanism

Article abstract of DOI:10.1021/ja00242a024

Stereo- and regiodefined alkenylmetals containing Al, Zr, and Zn react with aryl and alkenyl iodides and bromides in the presence of catalytic amounts of Pd or Ni complexes containing phosphine ligands, such as PPh3, to give the corresponding cross-coupled products.Palladium catalysts permit nearly 100 percent stereospecificity in both alkenyl-aryl and alkenyl-alkenyl coupling reactions, whereas nickel catalysts lead to partial stereochemical scrambling in the alkenyl-alkenyl coupling.Although many other metals including Li, Mg, Cd, Hg, B, Si, Sn, Ti, and Ce were also used, the results were inferior to those obtained with Al, Zr, and Zn under the conditions used in the present study.The turnover numbers for the palladium-catalyzed reactions of PhI with (E)-1-octenylmetals containing Al, Zr, and Zn were 2,3, and > 2000 mmol of (E)-1-octenylbenzene (8) per mmol of Pd(PPh3)4 per hour at room temperature, respectively.The stoichiometric reaction of PhPd(PPh3)2I (6) with 1.2 equiv of (E)-1-octenylzinc chloride (7) in a 2:1 mixture of CD2Cl2 and THF was examined in detail.The reaction follows second-order konetics (k₂ = 2.9 L/(mol.min) at 0 deg C) to give 8 without the buildup of any intermediate.The results are consistent with a slow formation of 9 via transmetalation followed by its rapid reductive elmination to give 8 and "Pd(PPhe3)2".Addition of PhI to the reaction mixture rapidly gives 6 in 98 percent yield, supporting the plausibility of the proposed oxidative addition step.These results are consistent with the proposed mechanism consisting of oxidative addition of Pd(0) complexes, rate-determining transmetalation involving Pd(II) complexes, and rapid decomposition of diorganopalladium(II) species to produce the coupling products in one or more subsequent steps.The rate-determining transmetalation step provides an explanation for the effect of metals in organometallic reagents used stoichiometrically.