Shilov processes

Thus, many groups have sought alternative oxidants. A polyoxometaUate (POM) has been shown to act as a mediator of oxidation by 0 (Equation 18.9). In this case, the reaction of methane with O in the presence of Periana s catalyst supported on HjPVjMOjjO j as acid and mediator of oxidation has been reported to form a mixture of methanol and acetaldehyde. The mechanism of the formation of the acetaldehyde product from methane is not firm, but is proposed to occur by oxidative coupling of methane with formaldehyde, which would be generated from methanol. These reactions occur with modest turnover numbers of about 30, but the use of and a POM is a clear advance over the original Shilov process with platinum(IV) as the stoichiometric oxidant. 

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 Shilov method, discussed in Section 10.4.1, has been accomplished as a catalytic process.327 Aqueous platinum chlorides can be used as catalysts for the homogeneous catalytic chlorination of methane. The reaction is carried out at 100-125°C with chlorine to yield methyl chloride that is partially hydrolyzed to... 

Of great interest are Shilov s works in the branch of self-induction, which is a specific case of conjugated reactions. Fie characterized it as follows Examples are known, when one of final products is the inducer and, consequently, the inducer is formed during the process itself therefore, its concentration is increased during interaction, increasing, in turn, the secondary reaction rate, i.e. speeding it up [1, p. 13],... 

The objectives set before investigators in the branch of conjugated processes, formulated by Shilov in the early 1900s, are generally still valid ... 

Hitherto, we have only discussed Shilov s classic work in which using the example of conjugated oxidation reaction the general theory of conjugated processes was properly achieved. However, a series of subsequent works should be mentioned which developed investigations in the field of conjugated reactions. Without attempting a full treatment of the subject, let us concentrate on the most important results for scientific and applied purposes. 

Meanwhile, based on the ideas of primary oxidation by molecular oxygen in which oxidants more active than the initial 02 were formed (e.g. H202, 03, etc.), Shilov suggested that this fact was sufficient for consideration of these processes from the position of conjugated reactions. He believed that for chemical induction, the possibility of primary and secondary reactions proceeding separately is not of fundamental importance. Therefore, the possibility of selecting a secondary reaction for the primary one is important in this case. 

Shilov [5] has limited considerations in regard to the final equation for conjugating reactions and determination of an intermediate substance which stipulates the initiating action of the primary reaction. However, this is not nearly complete enough to describe the secondary reaction mechanism, which should be changed during the conjugation process. 

For benzene hydroxylation an analytical system [37] was successfully used at the interface. This system contains Fe3+ hydrophobic complexes, which promote the process intensification. It is shown [38, 39] that compared with hydrophobic complexes, Fe3+ complexes with the phase transfer—tertiary ammonium salts and crown ethers—display more effective action. At 20-50 °C, owing to the use of trimethylacetylammonium bromide as the phase transferring agent, benzene is successfully hydroxylated in the two-phase water-benzene system in the presence of Fe3+ ions [40], Hence, it is Shilov s opinion [41] that in the case of cytochrome P-450 a radical reaction is probable. It produces radicals, which then transform in the cell, as follows ... 

Shilov devoted his review [12] to the development of selective and non-waste processes. He discusses the questions of organized molecular system (or ensembles) construction. Note that in the case of usual catalysis, catalytic sites freely interact with substrate molecules. This process is characterized by the absence of complementary activity. This is the reason for the relatively low selectivity of catalytic reactions. However, from positions of applied catalysis, such a catalytic system possesses clear advantages ... 

The activation and transformation of C-H bonds in alkanes by homogeneous transition metal catalysts should to be a topic of further research. No processes are close to commercialization on a technical scale. So far the organometallic approach by Shilov is one of the most promising ways of producing a practical system [55]. 

The Four-Electron Hypothesis. This hypothesis, apparently favored by Shilov (101,102), postulates that nitrogenase works by a series of two four-electron processes. In the reduction of dinitrogen, the first step would be the production of N2H4 while the second would be the production of 2NH3 and H2 which nicely explains the 1 1 stoichiometry for dinitrogen and dihydrogen discussed above. The residual reduction of... 

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... 

At 600-700 °C carbon tetrachloride thermally decomposes mainly to chlorine and hexachloroethane > by a first-order process. Shilov and Sabirova have reported an activation energy of 55.1 kcal.mole for the decomposition, and have proposed a short-chain mechanism involving the following steps... 

NN Semenov, AE Shilov and Gl Likhtenstein (1975) Multielectron oxidation-reduction processes in the chemistry and biology. DokI Akad NaukSSSR 221 1374-1377... 

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