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

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 PPh/sub 3/, to give the corresponding cross-coupled products. Palladium catalysts permit nearly 100% 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(PPh/sub 3/)/sub 4/ per hour at room temperature, respectively. The stoichiometric reaction of PhPd(PPh/sub 3/)/sub 2/I (6) with 1.2 equiv of (E)-1-octenylzinc chloride (7) in a 2:1 mixture of CD/sub 2/Cl/sub 2/ and THF was examined in detail. The reaction follows second-order kinetics (k/sub 2/ = 2.9 L/(mol.min) at 0/sup 0/C) to give 8 without the buildup of any intermediate.