Platinum Shilov system

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... 

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. 

The activation of C-H bonds is one of the elementary steps in chemistry. Intensive research has lead to homogeneous as well as heterogeneous systems which can activate the strong C-H bonds (cf. Section 3.3.6). There are numerous experimental studies which have more recently often been accompanied by theoretical calculations. The two best known examples for the activation of methane are the so-called Shilov system K2PtCl4 [1], which was one of the first systems reported, and the [Pt(bpym)Cl2] system of Periana, which is currently the most active system reported for the direct, low-temperature, oxidative conversion of methane to methanol by platinum salts such as dichloro( /-2-[2,2 -bipyrimidyl])platinum(II) [Pt(bpym)Cl2] with yields of more than 70% and a selectivity of 80% [2]. 

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. 

CHs-Halide bond formation is a side reaction in the Shilov methane oxidation process (Scheme 24) [64]. Mechanistic analysis of several catalytic steps by Bercaw and coworkers showed that the formation of the carbon-chlorine bond takes place in parallel to the formation of methanol, often being the major reaction pathway [65]. The reaction most likely involves a nucleophilic attack of the chloride-anion at the coordinated methyl group of the Pt(IV) intermediate [66]. Thus, the overall mechanism is closely related to the organic SN2-type reaction. Further support for such a mechanism operating in Pt(lV) systems came from the Goldberg group which reported the competitive CH3-I and CH3-CH3 reductive elimination reactions in platinum phosphine complexes (Scheme 25) [67, 68]. 

In many other cases, oxidative additions of alkanes occur readily to transition-metal-alkyl complexes to generate hydride dialkyl intermediates that subsequently eliminate alkane and form a new metal-alkyl complex. For example, cations related to the alkyl hydrides of iridium formed by oxidative addition undergo reaction with alkanes at or below room temperature to generate new alkyl complexes (Equation 6.34). Cationic platinum complexes undergo similar reactions with substrates containing aromatic and aliphatic C-H bonds (Equation 6.35). " The C-H activation of the platinum complexes has been studied, in part, to understand and to develop systems related to the ones reported by Shilov that lead to H/D exchange, and oxidation and halogenation of alkanes. 

Several systems for selective catalytic reactions based on Shilov s system have been developed with oxidants more practical than platinum(IV). Periana reported two different systems for the oxidation of methane in sulfuric acid containing SO,. One of the catalysts is a simple mercuric halide, and reactions catalyzed by this mercury compound generated methyl sulfate with turnover frequencies of 10" s" . The second system is more reactive and is based on a platinum complex containing a bipyrimidine ligand (Equation 18.7). In this case, methane is converted to methyl bisulfate with 81% selectivity, greater than 500 turnovers, and a turnover frequency of 10 s" . These reactions are selective for the functionalization of methane to this methanol derivative because the electron-withdrawing... 

---