Shilov system

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 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-

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 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 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


Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать реферат

Look at other dictionaries:

  • Carbon–hydrogen bond activation — or C−H activation may be defined as a reaction that cleaves a carbon–hydrogen bond. Often the term is restricted to reactions involving organometallic complexes and proceeding by coordination of a hydrocarbon to the inner sphere of metal, either… …   Wikipedia

  • C-H bond activation — Carbon hydrogen bond activation or CH activation may be defined as a reaction that cleaves a carbon hydrogen bond. Often the term is restricted to reactions involving organometallic complexes and proceeding by coordination of a hydrocarbon to the …   Wikipedia

  • CH-Aktivierung — Die CH Aktivierung kann definiert werden als Reaktion, die eine Kohlenstoff Wasserstoff Bindung spaltet. Der Begriff wird meist für Reaktionen verwendet, die über organometallische Komplexe unter Koordination des Kohlenwasserstoffs an ein Metall… …   Deutsch Wikipedia

  • Organoplatinum — chemistry is the chemistry of organometallic compounds containing a carbon to platinum chemical bond, and the study of platinum as a catalyst in organic reactions.[1][2][3] Organoplatinum compounds exist in oxidation state 0 to IV, with oxidation …   Wikipedia

  • Dirac delta function — Schematic representation of the Dirac delta function by a line surmounted by an arrow. The height of the arrow is usually used to specify the value of any multiplicative constant, which will give the area under the function. The other convention… …   Wikipedia

  • Eigenvalue, eigenvector and eigenspace — In mathematics, given a linear transformation, an Audio|De eigenvector.ogg|eigenvector of that linear transformation is a nonzero vector which, when that transformation is applied to it, changes in length, but not direction. For each eigenvector… …   Wikipedia

  • Eigenvalues and eigenvectors — For more specific information regarding the eigenvalues and eigenvectors of matrices, see Eigendecomposition of a matrix. In this shear mapping the red arrow changes direction but the blue arrow does not. Therefore the blue arrow is an… …   Wikipedia

  • Intel — For other uses, see Intel (disambiguation). Coordinates: 37°23′16.54″N 121°57′48.74″W / 37.3879278°N 121.9635389°W / 37.3879278; 121.9 …   Wikipedia

  • Athlon 64 — The Athlon 64 is an eighth generation, AMD64 architecture microprocessor produced by AMD, released on September 23, 2003.cite web|url=http://www.amd.com/us en/Corporate/VirtualPressRoom/0,,51 104 543 10218 74465,00.html|title=AMD Ushers in Era of …   Wikipedia

  • Advanced Micro Devices — Infobox Company company name=Advanced Micro Devices, Inc. company logo= company type=Public (nyse|AMD) foundation=1969 founder=W. Jerry Sanders III Edwin J. Turney Additional co founders location city=Sunnyvale, California, US key people=Derrick… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”