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Elementary Reactions Reaction Mechanisms

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Elementary Reactions Reaction Mechanisms Elementary Reactions Oxidative addition, reductive elimination RX + M RMX Transmetallation RM + M' M + RM' Migratory insertion, β-elimination M X M X Ligand exchange + L' ML ML' − L C.C. Tzschucke 4 Reaction mechanisms A reaction mechanism ... ... is a theorie deduced from available experimental data. ... can only be disproved ... cannot be proved to be true ... explains the available experimental data and... ... allows predictions that can be tested experimentally. C.C. Tzschucke 5 Ligand exchange: Trends associative dissociative L' + L' + L' ML ML ML' ML M ML' − L − L Type of complex 16-e− and 17-e− 18-e− complexes complexes Rate law First order in entering Zero order in ligand entering ligand Activation ∆S‡ < 0 ∆S‡ > 0 parameters Electronic effects basic entering ligand electrophilic metal Effect of departing small large ligand Steric effects more accessible metal more hindered metal C.C. Tzschucke 6 Ligand exchange: 16-e− Complexes d8 complexes, square-planar: Ni(II), Pd(II), Pt(II), Rh(I), Ir(I), Au(III) very common in catalysis usually associative: Y Y L X Y L X L Y L Y M M L M L M M L L L L L L −X L L L X X Cl Cl PEt PEt + PPh3 3 + py 3 N Pt N N Pt N − Cl− Pd Cl − Pd N Cl PPh − Cl 3 PEt3 PEt3 sometimes dissociative: + py Ph SMe2 Ph SMe2 Ph SMe2 Pt Pt Pt − SMe2 Ph SMe2 Ph Ph py PtBu2 PtBu2 PtBu2 CH3 + ethene Ir Ir Ir − propene PtBu2 PtBu2 PtBu2 C.C. Tzschucke 7 Ligand exchange: 18-e− Complexes d6 complexes, octahedral: Cr(0), Fe(II), Ru(II), Rh(III), Ir(III), Pt(IV), ... many more d10 complexes, tetrahedral: Ni(0), Pd(0), Pt(0), Cu(I) usually dissociative: slow fast Br Br Br OC CO OC + 13CO OC 13CO Mn Mn Mn OC CO − CO OC CO OC CO CO CO CO CO CO CO + L Ni Ni Ni OC CO − CO OC CO OC CO CO L k1 k2 MX M + X MY k−1 + Y d[MX] k1k2[MX][Y] rate law: − = ≈ k1[MX] if k−1[X] « k2[Y] dt k−1[X] + k2[Y] C.C. Tzschucke 8 Oxidative Addition Concerted mechanism: Addition of aryl halides, vinyl halides PPh3 PPh 3 − PPh3 Pd Pd Ph3P Pd PPh3 Ph P PPh3 − PPh 3 3 Ph3P PPh + PPh PPh3 3 3 Br slow Br fast Br Br Pd Pd PPh3 Rate depends on ... Ph3P Ph3P PPh3 PPh3 ... halide ... ligand − PPh3 ... aryl Br Ph3P ... metal Pd Ph3P Br PPh3 Pd Ph3P Similar mechanisms: Addition of H2, C-H-Bonds, disilanes, ... C.C. Tzschucke 9 Oxidative Addition SN2 mechanism: Addition of alkyl halides, allyl carbonates I I H H CH H 3 CH3 PPh3 Cl PPh3 PPh Cl Cl 3 Cl PPh3 Ir Ir Ir Ir Ph P Ph P CO Ph P 3 CO 3 3 CO Ph3P CO I O O MeO O MeO O R R PPh3 R − − MeOCO2 Pd Pd Pd Ph3P PPh Ph P 3 3 PPh3 Ph3P PPh3 C.C. Tzschucke 10 Reductive Elimination MePh2P CH3 L Pd H3CCH3 + (MePh2P)2PdLn MePh2P CH3 r.t. R PhPh PhPh P P 95 °C PPh3 + Fe Pt PPh3 R R + Fe Pt PPh P P 3 Ph Ph Ph Ph R Hartwig JACS 2004 126 13016 N N N N Et − e− N Et − e− N Et Fe Fe Fe N Et N Et N Et N N N ∆ slow fast H2CCH2 + H2CCH2 Et H CCH CH3 3 3 H C + + 3 H3CCH3 CH3 H3C C.C. Tzschucke 11 Migratory Insertion, β-Elimination H3C O H3C O Carbonyl Insertion CH3 OC CO OC + PPh3 OC PPh3 Mn Mn Mn OC CO OC CO OC CO CO CO CO e.g. Monsanto Process, carbonylation, decarbonylation Olefin Insertion CC CC C C MX MX M X X = H, alkyl, aryl e.g. Heck reaction, hydrogenation, olefin polymerization C.C. Tzschucke 12 Dimerization vs. Polymerization H3C CH2 H3C CH3 CH2 H2C H2 HC HC CH C CH CH CH N 2 N CH2 N 2 N CH3 N CH Pd Pd CH2 Pd Pd Pd 3 N CH2 N H3C N H N N H CH3 CH3 H3C HC CH2 CH HC H3C H2 H2C CH2 C CH N CH3 N 2 Pd Pd N N H H3C H3C CH2 CH2 H CCH H2CCH2 2 2 x H C x H C H H2C 2 H2C 2 2 C CH C N C N N CH2 N 2 N CH2 H H Pd Pd Pd Pd CH Pd H CCH 2 H C CH 2 2 x N H N H N H N 2 2 x N CH2 H2C CH3 H2C CH3 + CH2=CH2 H2C CH3 H CCH 2 2 x H2C CH Brookhart JACS 1995 117 1137 C.C. Tzschucke 14.
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