Methyl bisulfate

Homogeneous catalysts have been reported, which can oxidize methane to other functionalized products via C-H activation, involving an electrophilic substitution process. The conversion of methane into methyl bisulfate, using a platinum catalyst, in sulfuric acid, has been described. The researchers found that a bipyrimidine-based ligand could both stabilize and solubilize the cationic platinum species under the strong acidic conditions and TONs of >500 were observed (Equation (5)).13... 

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. 

The oxidation takes place through the observable intermediate 1 to yield methyl bisulfate, which may be readily hydrolyzed to methanol. At a methane conversion of 50%, an 85% selectivity to methyl bisulfate was achieved. The second molecule of H2S04 reoxidizes Hg+ to Hg2+ completing the catalytic cycle. 

In an important development, Periana made Shilov-hke chemistry more practically useful with a series of methane conversion catalysts. The first such system involved Hg(n) salts in H2SO4 at 180°, the latter being both a solvent and a mild reoxidant (equation 3). Methane was converted to the methanol ester, methyl bisulfate, MeOSOsH, in which the -OSO3H provides a powerful deactivating group to... 

Periana et al. [46] reported selective catalytic oxidation of methane by sulfuric acid to produce methyl bisulfate at 180 °C. The reaction is catalyzed by mercuric ions. Sulfur dioxide is the product of sulfuric acid reduction. At methane conversion of 50%, 85% selectivity to methyl bisulfate is observed. The major side product is carbon dioxide. The mercury turnover efficiency is 10 s The ion reacts with methane as an electrophile substituting a proton and producing initially an intermediate methylated mercury complex, CHsHgOSOsH. The complex is formed in appreciable steady-state concentration and was observed directly by C and NMR spectroscopy. Under tbe reaction conditions, methyl mercuric bisulfate decomposes to produce methyl bisulfate, CHsOSOjH, and the reduced mercurous species, Hg2. The catalytic cycle is completed by reoxidation ofHg2 with H2SO4 to regenerate and to form other products, SO2 and HaO. 

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... 

Several catalytic systems catalyze methyl bisulfate synthesis without using heavy metals. For example, a catal3rtic amount of a radical initiator such as... 

K2S2O8 efficiently produces methyl bisulfate via methanesulfonic acid (eq. (27)) (47). 

Iodine catalyzes a similar methyl bisulfate synthesis from methane and fuming sulfuric acid. [I2][HS207] is proposed as the active catalytic species to electrophilically activate methane (eq. (28)) (48). 

As shown in equation (27), methanesulfonic acid can be transformed to methyl bisulfate, which is easily hydrolyzed to methanol. Hence, the synthesis of methanesulfonic acid is regarded as a methane conversion to methanol. Metal peroxides such as Ca02 catalyze the reaction of methane and fuming sulfuric acid to give methane sulfonic acid at a rather low temperature (eq. (29)) (49). 

Following up on their earlier report of surprisingly selective mercury-catalyzed oxidation of methane to methyl bisulfate by sulfuric acid (which was also the reaction medium) [52], Periana and coworkers discovered that a bipyrimidine complex of Pt(II) worked even better, generating the same product in over 70% yield - a remarkable achievement, given that selective oxygenation of methane to methanol or derivatives thereof rarely surpasses yields of a few percent. A mechanism closely akin to that of the Shilov system was proposed (Scheme 15), with SO3 replacing Pt(IV) as the oxidant to convert RPt(II) to RPt(IV) whether the initial C-H activation involved an RPt(IV)H intermediate or not was left an open question [52]. 

Although this process offers the advantage of driving a low temperature, carefully controlled oxidation of methane, thereby increasing the yield of methanol, it also utilizes sulfuric acid to produce the intermediate methyl bisulfate. The need for acid resistant containers to perform these reactions may raise costs of the process. And although the sulfuric acid is recovered and recycled into the process, the environmental benefits of this methane conversion are somewhat offset by the need to ship and store hazardous sulfuric acid. The trade-off between safer methane transport versus increased sulfuric acid transport and storage needs to be considered from the perspective of accidental releases. 

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