NOVEL SYN OXIDATIVE ADDITION OF ALLYLIC HALIDES TO OLEFIN COMPLEXES OF PALLADIUM(O) AND PLATINUM(O)

Stereochemistry of the oxidative addition of 5-(methoxycarbonyl)-2-cyclohexenyl chloride (1a) and bromide (1b) and 5-methyl-2-cyclohexenyl chloride (1c) to palladium(0)-olefin and platinum(0)-olefin complexes to give the corresponding (eta3-allyl)palladium and -platinum complexes was examined as a function of solvent and olefin ligands. Novel syn addition (>90% selectivity) occurred in the reactions of the trans isomers of 1a-c with palladium complexes containing electron-withdrawing olefinic ligands (maleic anhydride, dimethyl fumarate, fumaronitrile, dibenzylideneacetone) carried out in benzene, CH2Cl2, or THF. Somewhat lower syn selectivity (80-43%) was found in the reactions between these palladium complexes and the cis isomer of 1a or between the analogous platinum complexes and 1a-trans under the same conditions. Anti oxidative addition dominated in the reactions of 1a-trans with electron-withdrawing olefin-palladium complexes in acetonitrile or DMSO, or with electron-donating olefin-palladium complexes in benzene or CH2Cl2. The crystal structure of Pd(trans-(5-methoxy-carbonyl)(1-3-eta)-cyclohexenyl)(Cl)(PPh3) was determined. Crystal data: C26H26O2ClPPd, fw = 543.32, monoclinic, space group P2(1)/n, a = 12.717 (2) angstrom, b = 9.881 (2) angstrom, c = 19.772 (3) angstrom beta = 105.38 (2)-degrees, V = 2395.7 (6) angstrom3, Z = 4, D(c) = 1.507 g cm-3, R = 0.043 for 4150 reflections (\F(o)\ > 3sigma(\F(o)\)). The six-membered ring adopts a pseudochair conformation, with COOMe occupying the equatorial position. A reaction path for the syn oxidative addition is proposed with the aid of the relative rates of the oxidative addition observed for a series of methyl-substituted allylic chlorides, ClCHR1Cr2=CR3R4. Cross coupling of 1a with RBu3Sn (R = phenyl, vinyl) with net retention of configuration was achieved by the use of a catalyst precursor, [Pd(eta3-C3H5)Cl]2/maleic anhydride. Evidence is given which shows that the catalytic cycle involves two stereoretentive key reactions, namely, oxidative addition of 1a with Pd(0) and reductive elimination of the (eta3-allyl)palladiums with organotins, both requiring maleic anhydride as an indispensable ligand to palladium.