Shilov cycle

This cycle, often referred to as the Shilov-cycle converts methane into methanol and chloromethane in homogeneous aqueous solution at mild temperatures of 100-120 °C (11). However, while Pt(II) (added to the reaction as PtCl ) serves as the catalyst, the system also requires Pt(IV) (in the form of PtCle-) as a stoichiometric oxidant. Clearly, this system impressively demonstrates functionalization of methane under mild homogeneous conditions, but is impractical due to the high cost of the stoichiometric oxidant used. A recent development by Catalytica Advanced Technology Inc., often referred to as the Catalytica system used platinum(II) complexes as catalysts to convert methane into methyl-bisulfate (12). The stoichiometric oxidant in this case is S03, dissolved in concentrated H2S04 solvent. This cycle is depicted in Scheme 3. 

Such a reaction could be quite useful on an industrial scale, but unfortunately, it is stoichiometric in highly expensive Pt complex. Since its discovery in 1972, Shilov chemistry has been the focus of numerous investigations, many of which have sought to replace the Pt(IV) stoichiometric oxidant with a cheaper alternative. In the course of this research, the mechanism of the Shilov C-H bond activation has been elucidated. The following scheme shows the catalytic cycle involved. 

The Shilov Pt(II) system for methane functionalization has been studied using computations. For example, using MCl2(H20)2 (M = Pt or Pd) systems as models for Shilov-type reactions, the overall catalytic cycle was studied using a combination of... 

Scheme 24 Proposed catalytic cycle for the Shilov methane oxidation process. Under typical reaction conditions, chloromethane is formed parallel to methanol...

The mechanism for the catalytic oxidation of methane by Pt(II) was first put forth by Shilov. The first step of the catalytic cycle involves the formation of a methylplatinum(II) intermediate 16 via the C-H activation of hydrocarbon methane with Pt(II) 15. A methylplatinum(IV) species 17 is obtained by the oxidation of methylplatinum(II) intermediate 16 (second step). The formation of the product takes place through reductive elimination from the methylplatinum(IV) 17 either via coordination to water or by the nucleophilic attack at the carbon by an external nucleophile such as water or chloride ion. Considerable amount of experimental data supports has been offered for supporting the given general mechanism (Fig. 6). 

However, the mechanistic scheme for this catalytic cycle (Shilov system) is a topic of debate and there have been alternative proposals for of oxidative addition pathways that have been put forth, and experimental evidence provided to support the theory. Excellent review articles have focused on catalytic oxidation of alkanes, however herein we will discuss only those examples for the synthesis of bi(hetero)aryls involving an electrophilic aromatic substitution mechanism. 

The first example of a metal-catalyzed oxygen atom insertion into the C-H bond was the reaction found by Shilov and Shteinman and their coworkers in 1972 (for reviews, see References Ih and 5). These authors demonstrated that Pt Cl4 ion could catalyze H/D exchange in methane in a D2O/CD3COOD solution and, if Pt Cls " is added, the latter oxidizes methane to methanol (Shilov chemistry). The catalytic cycle in which ct-methyl complexes of platinum(ll) and platinum(lV) are involved is shown in Fig. 1.1. 

In a more complicated photoassisted reaction described by Shilov [33], the simultaneous formation of hydrogen (P) from water (S) and aldehydes or ketones (P) from alcohols (S) is due to the consecutive light absorption by V2+ and V3+ complexes in one cycle. 

Alkane functionalization is a highly desirable chemical transformation. The Shilov system [112], based on Pp and complexes, constimtes an early proposal for hydrocarbon oxidation in aqueous solution (Fig. 7). The rate-limiting step of the catalytic cycle, namely, C-H bond activation, was investigated by Vidossich et al. by means of Car-Parrinello molecular dynamics simulations in explicit water [113]. The authors considered the PtCl2(H20)(CH4) species, in both cis and trans configurations, solvated with 32 water molecules in a cubic box of 10 A edge treated under periodic boundary conditions. 

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