C鈭扚 and C鈭扝 Bond Activation of Fluorobenzenes and Fluoropyridines at Transition Metal Centers: How Fluorine Tips the Scales
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- 作者：Eric Clot ; Odile Eisenstein ; Naseralla Jasim ; Stuart A. Macgregor ; John E. McGrady ; Robin N. Perutz
- 刊名：Accounts of Chemical Research
- 出版年：2011
- 出版时间：May 17, 2011
- 年：2011
- 卷：44
- 期：5
- 页码：333-348
- 全文大小：1494K
- 年卷期：v.44,no.5(May 17, 2011)
- ISSN：1520-4898
文摘
In this Account, we describe the transition metal-mediated cleavage of C鈭扚 and C鈭扝 bonds in fluoroaromatic and fluoroheteroaromatic molecules.
The simplest reactions of perfluoroarenes result in C鈭扚 oxida tive addition, but C鈭扝 activation competes with C鈭扚 activation for partially fluorinated molecules. We first consider the reactivity of the fluoroaromatics toward nickel and platinum complexes, but extend to rhenium and rhodium where they give special insight. Sections on spectroscopy and molecular structure are followed by discussions of energetics and mechanism that incorporate experimental and computational results. We highlight special characteristics of the metal鈭抐luorine bond and the influence of the fluorine substituents on energetics and mechanism.
Fluoroaromatics reacting at an ML2 center initially yield 畏2-arene complexes, followed usually by oxidative addition to generate MF(ArF)(L)2 or MH(ArF)(L)2 (M is Ni, Pd, or Pt; L is trialkylphosphine). The outcome of competition between C鈭扚 and C鈭扝 bond activation is strongly metal dependent and regioselective. When C鈭扝 bonds of fluoroaromatics are activated, there is a preference for the remaining C鈭扚 bonds to lie ortho to the metal.
An unusual feature of metal鈭抐luorine bonds is their response to replacement of nickel by platinum. The Pt鈭扚 bonds are weaker than their nickel counterparts; the opposite is true for M鈭扝 bonds. Metal鈭抐luorine bonds are sufficiently polar to form M鈭扚路路路H鈭扻 hydrogen bonds and M鈭扚路路路I鈭扖6F5 halogen bonds.
In the competition between C鈭扚 and C鈭扝 activation, the thermodynamic product is always the metal fluoride, but marked differences emerge between metals in the energetics of C鈭扝 activation. In metal鈭抐luoroaryl bonds, ortho-fluorine substituents generally control regioselectivity and make C鈭扝 activation more energetically favorable. The role of fluorine substituents in directing C鈭扝 activation is traced to their effect on bond energies. Correlations between M鈭扖 and H鈭扖 bond energies demonstrate that M鈭扖 bond energies increase far more on ortho-fluorine substitution than do H鈭扖 bonds.
Conventional oxidative addition reactions involve a three-center triangular transition state between the carbon, metal, and X, where X is hydrogen or fluorine, but M(d)鈭扚(2p) repulsion raises the activation energies when X is fluorine. Platinum complexes exhibit an alternative set of reactions involving rearrangement of the phosphine and the fluoroaromatics to a metal(alkyl)(fluorophosphine), M(R)(ArF)(PR3)(PR2F). In these phosphine-assisted C鈭扚 activation reactions, the phosphine is no spectator but rather is intimately involved as a fluorine acceptor. Addition of the C鈭扚 bond across the M鈭扨R3 bond leads to a metallophosphorane four-center transition state; subsequent transfer of the R group to the metal generates the fluorophosphine product. We find evidence that a phosphine-assisted pathway may even be significant in some apparently simple oxidative addition reactions.
While transition metal catalysis has revolutionized hydrocarbon chemistry, its impact on fluorocarbon chemistry has been more limited. Recent developments have changed the outlook as catalytic reactions involving C鈭扚 or C鈭扝 bond activation of fluorocarbons have emerged. The principles established here have several implications for catalysis, including the regioselectivity of C鈭扝 activation and the unfavorable energetics of C鈭扚 reductive elimination. Palladium-catalyzed C鈭扝 arylation is analyzed to illustrate how ortho-fluorine substituents influence thermodynamics, kinetics, and regioselectivity.
The simplest reactions of perfluoroarenes result in C鈭扚 oxida tive addition, but C鈭扝 activation competes with C鈭扚 activation for partially fluorinated molecules. We first consider the reactivity of the fluoroaromatics toward nickel and platinum complexes, but extend to rhenium and rhodium where they give special insight. Sections on spectroscopy and molecular structure are followed by discussions of energetics and mechanism that incorporate experimental and computational results. We highlight special characteristics of the metal鈭抐luorine bond and the influence of the fluorine substituents on energetics and mechanism.
Fluoroaromatics reacting at an ML2 center initially yield 畏2-arene complexes, followed usually by oxidative addition to generate MF(ArF)(L)2 or MH(ArF)(L)2 (M is Ni, Pd, or Pt; L is trialkylphosphine). The outcome of competition between C鈭扚 and C鈭扝 bond activation is strongly metal dependent and regioselective. When C鈭扝 bonds of fluoroaromatics are activated, there is a preference for the remaining C鈭扚 bonds to lie ortho to the metal.
An unusual feature of metal鈭抐luorine bonds is their response to replacement of nickel by platinum. The Pt鈭扚 bonds are weaker than their nickel counterparts; the opposite is true for M鈭扝 bonds. Metal鈭抐luorine bonds are sufficiently polar to form M鈭扚路路路H鈭扻 hydrogen bonds and M鈭扚路路路I鈭扖6F5 halogen bonds.
In the competition between C鈭扚 and C鈭扝 activation, the thermodynamic product is always the metal fluoride, but marked differences emerge between metals in the energetics of C鈭扝 activation. In metal鈭抐luoroaryl bonds, ortho-fluorine substituents generally control regioselectivity and make C鈭扝 activation more energetically favorable. The role of fluorine substituents in directing C鈭扝 activation is traced to their effect on bond energies. Correlations between M鈭扖 and H鈭扖 bond energies demonstrate that M鈭扖 bond energies increase far more on ortho-fluorine substitution than do H鈭扖 bonds.
Conventional oxidative addition reactions involve a three-center triangular transition state between the carbon, metal, and X, where X is hydrogen or fluorine, but M(d)鈭扚(2p) repulsion raises the activation energies when X is fluorine. Platinum complexes exhibit an alternative set of reactions involving rearrangement of the phosphine and the fluoroaromatics to a metal(alkyl)(fluorophosphine), M(R)(ArF)(PR3)(PR2F). In these phosphine-assisted C鈭扚 activation reactions, the phosphine is no spectator but rather is intimately involved as a fluorine acceptor. Addition of the C鈭扚 bond across the M鈭扨R3 bond leads to a metallophosphorane four-center transition state; subsequent transfer of the R group to the metal generates the fluorophosphine product. We find evidence that a phosphine-assisted pathway may even be significant in some apparently simple oxidative addition reactions.
While transition metal catalysis has revolutionized hydrocarbon chemistry, its impact on fluorocarbon chemistry has been more limited. Recent developments have changed the outlook as catalytic reactions involving C鈭扚 or C鈭扝 bond activation of fluorocarbons have emerged. The principles established here have several implications for catalysis, including the regioselectivity of C鈭扝 activation and the unfavorable energetics of C鈭扚 reductive elimination. Palladium-catalyzed C鈭扝 arylation is analyzed to illustrate how ortho-fluorine substituents influence thermodynamics, kinetics, and regioselectivity.