Shilov System - Major Steps

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 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 2- to a PtIV-CH₂R complex. There have been multiple studies to find a replacement oxidant that is less expensive than 2- or a method to regenerate 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. and ). 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.