Shilov system

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 partial oxidation. [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)] [Activation of C-H Bonds by Metal Complexes Shilov, A. E.; Shul'pin, G. B. "Chem. Rev."; 1997; 97(8); 2879-2932.] [Mechanistic Aspects of C-H Activation by Pt Complexes Lersch, M.; Tilset, M. "Chem. Rev."; 2005; 105(6); 2471-2526.] [Reactions of C-H Bonds in Water Herrerias, C. I.; Yao, X.; Li, Z.; Li, C.-J." Chem. Rev."; 2007; 107(6); 2546-2562.]

Overview

The Shilov system was discovered by Alexander E. Shilov in 1972 while investigating H/D exchange between isotopologues 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 Pt^IICl₂ in an aqueous solution with Pt^IVCl₆^2- 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 + Pt^IVCl₆^2- → RH₂OH + 2H⁺ + Pt^IICl₂ + 4Cl^-

Major Steps

The initial and rate limiting step involving the electrophilic activation of RH₂C-H by a Pt^II center to produce a Pt^II-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 thermodynamic sinks such as H₂O and CO₂.

In the next step the Pt^II-CH₂R complex is oxidized by Pt^IVCl₆^2- to a Pt^IV-CH₂R complex. There have been multiple studies to find a replacement oxidant that is less expensive than Pt^IVCl₆^2- or a method to regenerate Pt^IVCl₆^2-. 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 Pt^II-CH₂R species over the initial Pt^II species since Pt^IV complexes will not electrophilically activate a C-H bond. Such premature oxidation shuts down the catalysis.

Finally the Pt^IV-CH₂R species is nucleophilically attacked by ^-OH or ^-Cl resulting in the reductive elimination of HOCH₂R and ClCH₂R respectively and regeneration of the initial Pt^II species. This reductive elimination may be preceded 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.

References