Shilov alkane oxidation

Ab initio studies of the Shilov alkane oxidation using soluble platinum salts have considered oxidative addition and a-bond metathesis pathways for the reaction... 

Using the analogy of model reactions of alkane oxidation in mixtures ofFe(II) and dioxygen in solvents, a mechanism invoking the formation of intermediate with an iron-carbon bond followed by interaction with soxygen was proposed (Waller and Fimscomb, 1996 Shilov, 1997). 

Karasevich, E.I.,Kulikova, V.S., Shilov, A.E., and Shteinman, A.A. (1998) Biomimetic alkane oxidation based on metal complexes, lisp. Khim. 67, 376-390. 

Shilov and coworkers discovered the oxidation of methane to methanol by mixtures of Pt and Pt, and aroused the Holy Graft-pursuing for electrophilic C H activation and subsequent alkane oxidation. The diimine complexes of Pt(II) methyl are indeed found to facilitate smooth benzene activation, resulting in formation of methane via Pt (Me)(Ph)(H) intermediates (Scheme 10). Such Pt(II)/Pt(IV) involved C-H activation reactions have been widely extended to a variety of nitrogen-donor ligands, whose electronic and steric effects shed light on the reaction mechanisms (see Section 7.1). 

Shilov Chemistry and Related Electrophilic Systems for Homogeneous Alkane Oxidation... 

In 1983 Shilov et al. proposed a mechanism for Pt-catalyzed alkane oxidation consisting of three basic transformations [Eq. (12.29)] (a) activation of the alkane by Pt" to generate an alkyl Pt" intermediate, (b) two-electron oxidation of the alkyl Pt11 intermediate to generate an alkyl PtIV species, and (c) reductive elimination of... 

Shilov s initial proposal for the meehanism of this Pt-catalyzed alkane oxidation involved ... 

Shilov proposed a mechanism for the alkane oxidation consisting of three basic transformations (Equation (32)). 

Synthetic organic chemistry applications employing alkane C-H functionalizations are now well established. For example, alkanes can be oxidized to alkyl halides and alcohols by the Shilov system employing electrophilic platinum salts. Much of the Pt(ll)/Pt(rv) alkane activation chemistry discussed earlier has been based on Shilov chemistry. The mechanism has been investigated and is thought to involve the formation of a platinum(ll) alkyl complex, possibly via a (T-complex. The Pt(ll) complex is oxidized to Pt(iv) by electron transfer, and nucleophilic attack on the Pt(iv) intermediate yields the alkyl chloride or alcohol as well as regenerates the Pt(n) catalyst. This process is catalytic in Pt(ll), although a stoichiometric Pt(rv) oxidant is often required (Scheme 6).27,27l 2711... 

The authors point out that the dependence of the site of electrophilic attack on the ligand trans to the hydride in the model systems may be important with respect to alkane activation. If the information is transferable to Pt-alkyls, protonation at the metal rather than the alkyl should be favored with weak (and hard ) a-donor ligands like Cl- and H20. These are the ligands involved in Shilov chemistry and so by the principle of microscopic reversibility, C-H oxidative addition may be favored over electrophilic activation for these related complexes. 

During the 1970s Shilov published extensively on the reactions of alkanes in aqueous solutions of platinum(II) complexes [3]. The reactions are typically carried out in aqueous hydrochloric acid as solvent at <100°C with chloride salts of Pt(II) as catalyst and the chloride salts of Pt(IV) as the stoichiometric oxidant. Typical reaction yields, based on added methane, are less than 3% with >75% selectivity to methanol and methyl chloride. It was proposed the reaction proceeded via C-H activation to generate alkyl platinum intermediates in reactions with alkanes and later results are consistent with this proposal [4]. This system is one of the first systems proposed to operate via the C-H activation reaction and to generate potentially useful functionalized products. The key disadvantages of the Shilov system were the low rates (catalyst tum-over-frequency, TOF, <10 s ), short catalyst life (turnover-number, TON, <20), and the use of Pt (IV) as a stoichiometric oxidant. 

Shilov A.E. (1997) Metal Complexes in Biomimetical Reactions. N2 Fixation, Activation and Oxidation of Alkanes, Chemical Models of Photosyntheses, CRC Press, Boca Raton, New-York, pp. 302... 

Shilov chemistry, developed from 1970, employs [Pt(II)CLt] salts to oxidize alkanes RH to ROH or RCl with modest efficiency. Pt(IV) is an efficient (but economically impractical) primary oxidant that makes the process catalytic. This discovery strongly contributed to the continuing activity in CH activation. Periana developed a related and much more efficient system for methane oxidation to methanol using 2,2 -bipyrimidine ligands and sulfuric acid as solvent. In this case, the sulfuric acid is the primary oxidant and the methanol formed is protected by being converted in situ to MeOSOsH, an ester that strongly resists further oxidation. This area is more fully described under the entry Alkane Carbon-Hydrogen Bond Activation. 

Shilov s catalytic process using a mixture of Pt(II) and Pt(IV) salts for the conversion of methane into methanol and methyl chloride in aqueous solution makes a chemical paradigm for alkane functionalization. One of the new approaches finds oxidative functionalization of methane is catalyzed by a (bipyrimidine)platinum(II) complex in concentrated sulfuric acid at 100 In... 

For the Shilov system a mechanism was proposed which consists of three basic steps (1) activation of the alkane by a Pt" species, followed by (2) a two-electron oxidation forming a Pt intermediate and (3) reductive elimination of the oxidized alkane as shown in Scheme 4. 

Shilov and his coworkers were the first to demonstrate metal-mediated alkane functionalization in water [ 15f]. They showed that simple Pt(II) complexes, such as PtCl42, will activate and oxidize the C-H bonds of alkanes, including methane and ethane. Sen [34],BercawandLabinger [35], and Horvath... 

Recent reviews (a) references 2a-d, 12a. (b) Arndtsen BA, Bergman RG,Mobley TA, Peterson TH (1995) Acc Chem Res 28 154. (c) Davies JA, Watson PL, Greenberg A, Liebman JF (eds) (1990) Selective hydrocarbon oxidation and functionalization. VCH, New York, Chaps 1-5. (d) Hill CL (ed) (1989) Activation and functionalization of alkanes. Wiley, New York, (e) Shilov AE, Shul pin GB (1997) Chem Rev 97 2879. (f) Shilov AE (1984) Activation of saturated hydrocarbons by transition metal complexes. D. Reidel, Dordrecht... 

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