The Shilov system is a classic example of
catalytic
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
C-H bond activation and
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
which preferentially activates stronger C-H bonds over weaker C-H bonds for an overall partial oxidation.
[C. I. Herrerias, X. Yao, Z. Li, C.-J. Li, Reactions of C-H Bonds in Water, ''Chem. Rev.''; 2007; 107(6); 2546-2562.]
Overview
The Shilov system was discovered by
Alexander E. Shilov in 1969-1972 while investigating H/D exchange between isotopologues of CH
4 and H
2O catalyzed simple
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
coordination complexes
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
. The Shilov cycle is the partial oxidation of a hydrocarbon to an
alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
or alcohol precursor (RCl) catalyzed by
PtIICl2 in an aqueous solution with
IVCl6sup>2−">t
IVCl
6sup>2− acting as the ultimate oxidant. The cycle consists of three major steps, the
electrophilic
In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carri ...
activation of the C-H bond, oxidation of the complex, and the
nucleophilic
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
oxidation of the
alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...
substrate. An equivalent transformation is performed industrially by
steam reforming
Steam reforming or steam methane reforming (SMR) is a method for producing syngas (hydrogen and carbon monoxide) by reaction of hydrocarbons with water. Commonly natural gas is the feedstock. The main purpose of this technology is hydrogen product ...
methane to
syngas
Syngas, or synthesis gas, is a mixture of hydrogen and carbon monoxide, in various ratios. The gas often contains some carbon dioxide and methane. It is principly used for producing ammonia or methanol. Syngas is combustible and can be used as ...
then reducing the
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
to
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
. The transformation can also performed biologically by
methane monooxygenase
Methane monooxygenase (MMO) is an enzyme capable of oxidizing the C-H bond in methane as well as other alkanes. Methane monooxygenase belongs to the class of oxidoreductase enzymes ().
There are two forms of MMO: the well-studied soluble form (s ...
.
Overall Transformation
RH + H
2O +
6">tCl6sup>2− → ROH + 2H
+ + PtCl
2 + 4Cl
−
Major steps
The initial and rate limiting step involving the electrophilic activation of RH
2C-H by a Pt
II center to produce a Pt
II-CH
2R species and a proton. The mechanism of this activation is debated. One possibility is the
oxidative addition
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidat ...
of a sigma coordinated C-H bond followed by the reductive removal of the proton. Another is a
sigma-bond metathesis In organometallic chemistry, sigma-bond metathesis is a chemical reaction wherein a metal-ligand sigma bond undergoes metathesis (exchange of parts) with the sigma bond in some reagent. The reaction is illustrated by the exchange of lutetium(III) ...
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-CH
2R complex is oxidized by
IVCl6">tIVCl6sup>2− to a Pt
IV-CH
2R complex. There have been multiple studies to find a replacement oxidant that is less expensive than
IVCl6">tIVCl6sup>2− or a method to regenerate
IVCl6">tIVCl6sup>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-CH
2R species over the initial Pt
II species since Pt
IV complexes will not electrophilically activate a C-H bond of the alkane (although Pt
IV complexes electrophilically substitute hydrogens in aromatics - see refs.
/sup> and /sup> ). 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.
References
{{Reflist
Organometallic chemistry
Catalysis