M.C. White. Chem 153 Alkene/C-M insertions-296- Week of Novem ber 18, 2002 Dimerization, Oligomerization, Polymerization 3-100 R=CH3, H R MR dime R R terminaTion B-hydride elimination note that there is no oxidation state change to the metal throughout LnM R addition MmH+ L MH B-lydride L LMa. K, has been found to depend on the It has been observed that with early, high-valent metals (e.g. Zr(IV), d) the equilibrium lies to the left(K2> substituents on the olefin Increased I)whereas with late, low-valent metals(e.g. Pd(in), d substitution and steric bulk of the the equilibrium lies to the right(K2< 1). Electron density olefin leads to decreased rates of binding to the metal complex at the metal is thought to favor the hydrido-alkene Hoffmann JACS 1976(98)1729 species via stabilizing T-backbonding into the olefin T' Labinger Aciee 1976(15)333M.C. White, Chem 153 Alkene/C-M insertions -296- Week of November 18, 2002 LnMn H + R' K1 LnMn H R' K1 has been found to depend on the number and size of alkyl substituents on the olefin. Increased substitution and steric bulk of the olefin leads to decreased rates of binding to the metal complex. LnMn H R' ‡ LnMn R' H K2 β-hydride addition β-hydride elimination It has been observed that with early, high-valent metals (e.g. Zr(IV), d0) the equilibrium lies to the left (K2 > 1)whereas with late, low-valent metals (e.g. Pd(II), d8) the equilibrium lies to the right (K2 < 1). Electron density at the metal is thought to favor the hydrido-alkene species via stabilizing π-backbonding into the olefin π*. Hoffmann JACS 1976 (98) 1729. Labinger ACIEE 1976 (15) 333. Dimerization, Oligomerization, Polymerization LnMn R R = CH3, H LnMn R LnMn LnMn LnMn R R R H R LnMn R LnMn R n H R n oligomer, n= 3-100 polymer, n > 100 termination via β-hydride elimination propagation via insertion note that there is no oxidation state change to the metal throughout the cycle LnMn H R dimer termination via β-hydride elimination