M.C. White Chem 153 Mechanism-52 Week of september 24th, 2002 Ligand dis sociation△orhv ordinatively and electronically unsaturate unactivated C-H bonds Rh-co hvor△ rhi Roll 18e 16 18 Light-promoted ligand dissociation M-CO M-CO prop bond order =1 ond order=0 Bergman JACS 1994(116)9585
M.C. White, Chem 153 Mechanism-52- Week of September 24th, 2002 Ligand dissociation: ∆ or hv Bergman JACS 1994 (116) 9585. coordinatively and electronically unsaturated complexes capable of oxidatively adding into unactivated C-H bonds. RhI OC 18 eCO hv or ∆ CO RhI OC 16 e- RhIII OC 18 e- H RhI OC H proposed intermediate Light-promoted ligand dissociation σ σ∗ M-CO bond order = 1 σ σ∗ M-CO bond order = 0 hv
M.C. White Chem 153 Mechanism-53 Week of september 24th, 2002 ligand dissociation: weakly coordinating solvents First generation Crabtree hydrogenation catalyst P(Me)Pl PE P(Me)Phz cat H,, solvent(S) R a glimpse into the catalytic cycle +(PF6) H +(PF6) dissociative Ph Me)B, H displacement Phd Me)Pu Phx Me)p, H P(MpH P(Me)Ph P(Me)Ph catalytically active species Solvent (s) Turnover frequency (toF) 10 0 3800 1900 TOF =mol reduced substrate/mol catalyst/h Crabtree Acc Chem Res 1979(12)331
M.C. White, Chem 153 Mechanism-53- Week of September 24th, 2002 Ligand dissociation:weakly coordinating solvents Crabtree Acc Chem Res 1979 (12) 331. First generation Crabtree hydrogenation catalyst Solvent (S) O CH2Cl2 Turnover Frequency (TOF) ~10 0 0 5100 3800 1900 TOF = mol reduced substrate/mol catalyst/h Ir P(Me)Ph2 P(Me)Ph2 (PF6-) S IrIII Ph2(Me)P H P(Me)Ph2 H R R + S IrIII Ph2(Me)P H P(Me)Ph2 H S IrIII Ph2(Me)P H P(Me)Ph2 H S R cat. H2, solvent (S) R catalytically active species S A glimpse into the catalytic cycle dissociative displacement (PF6- + ) (PF6- + ) (PF6- + )
M. C. White. Chem 153 Mechanism-54 Week of September 24th, 2002 Non-coordinating solvents no suc ch thing C11 C10 (BAre c13 H N3 C12 C1 The first isolated chloromethane-metal complex. There are also similar complexes formed with CH,Ch, and Cl, ch that have been characterized by nmr. Bergman JACS 2001(123)11508
M.C. White, Chem 153 Mechanism-54- Week of September 24th, 2002 Non-coordinating solvents: “no such thing” N Ir N PMe3 CH3 N Cl N N N H B H3C + (BArf-) = Bergman JACS 2001 (123) 11508. The first isolated chloromethane-metal complex. There are also similar complexes formed with CH2Cl2, and Cl3CH that have been characterized by NMR
M.C. White Chem 153 Mechanism-55 Week of september 24th, 2002 Oxidative Addition reductive elimination Oxidative Addition(OA): metal mediated breaking of a substrate o-bond and formation of lor 2 new M-L bonds. OA requires removal of 2 electrons from the metal's d electron count. This is reflected in a two unit increase in the metals oxidation state. The formation of i or 2 new M-L o bonds is accompanied by an increase in the metals coordination number by l or 2 units respectively. The latter results in a 2 unit increase in the electron count of the metal complex(e.g. 16 e to 18 e"). Currently, OA of low valent, electron rich metals to polar substrates is the best way to form M-C o bonds within the context of a catalytic cycle. The term oxidative addition confers no information about the mechanism of the reaction R oxidative addition or LMn+2R X- "Nu:" E+u reductive elimination Reductive elimination(RE): microscopic reverse of oxidative addition where two M-L o bonds are broken to form one substrate o bond. RE esults in the addition of two electrons into the metal d electron count this is reflected in a two unit decrease in the metal s oxidation state The breaking of 2 M-L o bonds is accompanied by a decrease in the metal's coordination number by 2 units. The result is a 2 unit decrease in the electron count of the metal complex(e. g. 18e-to 16 e-). The two M-L o bonds undergoing reductive elimination must be oriented cis to each other. Currently, RE is the most common way to form C-C bonds via transition metal complexes General oa mechanisms Concerted (generally for non-polar substrates Nucleophilic displacement(generally for polar substrates) LM L M LM L、M B Radical(both non-polar and polar) L-M LM L, MQ+)
M.C. White, Chem 153 Mechanism-55- Week of September 24th, 2002 Oxidative Addition/Reductive Elimination ·Oxidative Addition (OA): metal mediated breaking of a substrate σ-bond and formation of 1or 2 new M-L σ bonds. OA requires removal of 2 electrons from the metal's d electron count. This is reflected in a two unit increase in the metal's oxidation state. The formation of 1 or 2 new M-L σ bonds is accompanied by an increase in the metal's coordination number by 1 or 2 units respectively. The latter results in a 2 unit increase in the electron count of the metal complex (e.g.16 e- to 18 e-). Currently, OA of low valent, electron rich metals to polar substrates is the best way to form M-C σ bonds within the context of a catalytic cycle. The term oxidative addition confers no information about the mechanism of the reaction. R X M (n+2) R X LMn + oxidative addition reductive elimination L or M(n+2) L R + X- "Nu:" "E+" Reductive elimination (RE): microscopic reverse of oxidative addition where two M-L σ bonds are broken to form one substrate σ bond. RE results in the addition of two electrons into the metal d electron count. This is reflected in a two unit decrease in the metal's oxidation state. The breaking of 2 M-L σ bonds is accompanied by a decrease in the metal's coordination number by 2 units. The result is a 2 unit decrease in the electron count of the metal complex (e.g. 18e- to 16 e-). The two M-L σ bonds undergoing reductive elimination must be oriented cis to each other. Currently, RE is the most common way to form C-C bonds via transition metal complexes. A B Concerted (generally for non-polar substrates) LxMn LxMn A B ‡ LxM(n+2) A B 3-centered TS Nucleophilic displacement (generally for polar substrates) A X LxM(n+2) A + XLxMn A C Radical (both non-polar and polar) LxM(n+1) A C · LxM(n+2) A C + + + LxMn A X ‡ δ+ δ- + LxMn General OA Mechanisms: cis addition