Shilov system

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Shilov cycle The overall charge is omitted from the complexes since the exact coordination sphere of the active specie(s) is unknown.
Shilov cycle The overall charge is omitted from the complexes since the exact coordination sphere of the active specie(s) is unknown.

The Shilov system is a classic example of catalytic C-H bond activation and oxidation which preferentially activates stronger C-H bonds over weaker C-H bonds for an overall partials oxidation.[1][2][3] [4]

[edit] Overview

The Shilov system was discovered by Alexander E. Shilov in 1972 while investigating H/D exchange between isotopologes of CH4 and H2O catalyzed simple transition metal coordination complexes. The Shilov cycle is the partial oxidation of a hydrocarbon to an alcohol or alcohol precursor (RCl) catalyzed by PtIICl2 in an aqueous solution with PtIVCl62- acting as the ultimate oxidant. The cycle consists of three major steps, the electrophilic activation of the C-H bond, oxidation of the complex, and the nucleophilic oxidation of the alkane substrate. A equivalent transformation is performed industrially by steam reforming methane to syngas then reducing the carbon monoxide to methanol. The transformation can also performed biologically by methane monooxygenase.

Overall Transformation

RH3 + H2O + PtIVCl62- → RH2OH + 2H+ + PtIICl2 + 4Cl-

[edit] Major Steps

The initial and rate limiting step involving the electrophilic activation of RH2C-H by a PtII center to produce a PtII-CH2R species and a proton. The mechanism of this activation is debated. One possibility is the oxidative addition of a sigma coordinated C-H bond followed by the reductive removal of a the proton. Another is a sigma bond metathesis involving the formation of the M-C bond and a H-Cl or H-O bond. Regardless it is this step that kinetically imparts the chemoselectivity to the overall transformation. Stronger more electron rich bonds are activated preferentially over weaker more electron poor bonds of species that have already been partially oxidized. This avoids a problem that plagues many partial oxidation processes namely the over oxidation of substrate to the thermodynamic sinks such as H2O and CO2.

In the next step the PtII-CH2R complex is oxidized by PtIVCl62- to a PtIV-CH2R complex. There have been multiple studies to find a replacement oxidant that is less expensive than PtIVCl62- or a method to regenerate PtIVCl62-. It would be most advantageous to develop an electron train which would use oxygen as the ultimate oxidant. It is important that the oxidant preferentially oxidizes the PtII-CH2R species over the initial PtII species since PtIV complexes will not electrophilically activate a C-H bond. Such premature oxidation shuts down the catalysis.

Finally the PtIV-CH2R species is nucleophilically attacked by -OH or -Cl resulting in the reductive elimination of HOCH2R and ClCH2R respectively and regeneration of the initial PtII species. This reduction elimination may be proceeded by the loss of a spectator ligand to produce a five coordinate complex which is expected to reductively eliminate more readily. This step is the isoelectronic microscopic reverse of the oxidative addition of CH3I by Vaska's complex.

[edit] References

  1. ^ Reactions of alkanes in solutions of platinum chloride complexes. Gol'dshleger, N. F.; Es'kova, V. V.; Shilov, A. E.; Shteinman, A. A. Zhurnal Fizicheskoi Khimii 1972, 46(5)
  2. ^ Activation of C-H Bonds by Metal Complexes Shilov, A. E.; Shul'pin, G. B. Chem. Rev.; 1997; 97(8); 2879-2932.
  3. ^ Mechanistic Aspects of C-H Activation by Pt Complexes Lersch, M.; Tilset, M. Chem. Rev.; 2005; 105(6); 2471-2526.
  4. ^ Reactions of C-H Bonds in Water Herrerias, C. I.; Yao, X.; Li, Z.; Li, C.-J. Chem. Rev.; 2007; 107(6); 2546-2562.