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. [1][2] [3] [4]
The Shilov system was discovered by Alexander E. Shilov in 1969-1972 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 PtIICl2 in an aqueous solution with [PtIVCl6]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
RH + H2O + [PtCl6]2- → ROH + 2H+ + PtCl2 + 4Cl-
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 thermodynamic sinks such as H2O and CO2.
In the next step the PtII-CH2R complex is oxidized by [PtIVCl6]2- to a PtIV-CH2R complex. There have been multiple studies to find a replacement oxidant that is less expensive than [PtIVCl6]2- or a method to regenerate [PtIVCl6]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 PtII-CH2R species over the initial PtII species since PtIV complexes will not electrophilically activate a C-H bond of the alkane (although PtIV complexes electrophilically substitute hydrogens in aromatics - see refs. [1] and [2] ). Such premature oxidation shuts down the catalysis.
Finally the PtIV-CH2R undergoes nucleophilic attack by OH- or Cl- with the departure of PtII complex to regenerate the catalyst.