Methane electrophilic activation

This general mechanistic scheme is widely accepted, but there are several different pathways possible for each of the three steps. For example, the reductive elimination could proceed via an Sn2 mechanism or via a concerted mechanism involving a three-center transition state. Many research activities have been devoted to the investigation of these detailed questions, which have been reviewed extensively by Stahl [3], while here only a summary is given (1) for the reductive elimination it could be concluded that an Sn2 mechanism is operative (2) the oxidation of RPt" to RPt does not occur by alkyl transfer, but by a two-electron transfer from RPt to Pt and (3) the electrophilic activation of the alkane is the most difficult part to investigate. At present none of proposed pathways shown in Scheme 5 could be discounted also, the experimental observation of H/D exchange in methane can be explained by both mechanisms. 

One final report of alkane activation has been reported by Moiseev. The mechanism of the reaction was not investigated, but this system might be classified as an electrophilic activation of methane, either of the Shilov type or of the concerted four-center type (Fig. lc) where X=triflate. Reaction of methane with cobalt(III)triflate in triflic acid solution leads to the formation of methyltriflate in nearly stoichiometric quantities (90% based on Co) (Eq. 18). Carbon dioxide was also observed, but not quantified. Addition of 02 led to catalysis (four turnovers) [79]. 

There has been a number of recent reports on metal-catalyzed electrophilic activation of methane and ethane. For two reasons much of the work in the area has been carried out in strong acids. First, the conjugate bases of strong acids are... 

We have previously shown that the mononuclear zirconium hydride complexes 1 activate, under very mild conditions, the C-H bond of alkanes, including methane [7], The mechanism involves a four center intermediate, as proposed earlier for electrophilic activation of C-H bonds by group 3, 4 and lanthanides d° complexes [8], Given the similarities of the energies of dissociation of C-H and Si-H bonds, it is not surprising at ail that activation of Si-H bonds occurs with 1. Reactions of H/D exchange, followed by in situ IR spectroscopy, reveal that all types of silanes are activated, i.e. primary, secondary and even tertiary silanes [9],... 

In 1993 Bergman discovered that an iridium (iii) methyl cation was capable of undergoing an exchange of the methyl group with other alkanes in a process that looked similar to the electrophilic activation of alkanes by Shilov s Pt(n) complex (Equation (17)). Theoretical treatment of this system provided evidence that the actual pathway involved oxidative addition of the alkane to give an Ir(v) dialkylhydride that then underwent reductive elimination of methane. ... 

Electrophilic Activation by Electron-deficient Complexes. Alkane activation by Pt(ll) complexes was reported nearly 30 years ago. Methane was successfully converted to methyl chloride and methanol using equimolar amounts of Pt(lV) complexes as oxidants (13,14). A postulated methylplatinum(IV) intermediate is observed by NMR (eq. (6)) (13,14). 

Recently, the electrophilic activation of methane by electron-deficient metal complexes has been clearly demonstrated using various metals such as Pt(II), Ir(IIl), and rare-earth metals (eqs. 7-9) (15-20). It is noteworthy that the oxidation states of the metals are unchanged in these reactions. 

Synthesis of Methyl Sulfate and Methanesulfonic Acid. The electrophilic activation of methane in sulfuric acid catalyzed by metal complexes was initially developed using a mercury salt as the catalyst (eq. (24)) (33). 

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). 

Conversion of 8.40 to 8.41 is an example of electrophilic activation of an alkane (see Section 2.3.5). In this case the electrophile is Pt and the alkane is of course methane. Under strong acidic conditions, the noncoordinated nitrogen atoms of BIPYM may also be protonated. This increases the electrophiUcity of the metal ion. 

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... 

Other soft, electrophilic metals, including mercury(ll) analogs, are also known to activate methane.16... 

The reactivity of the closely related system TpMe2PtMeH2 toward electrophiles in arene solvents has also been reported recently (68). The boron-based Lewis acid B(C6F5)3 induced elimination of methane and formation of an aryl(dihydrido) platinum(IV) complex via arene C-H activation (Scheme 17, A -> C). The active acid may be either B(C6F5)3 or alternatively a proton generated from B(C6F5)3 and trace water. It was proposed that the acid coordinates to a pyrazole nitrogen (shown in Scheme 17, B) forming an intermediate five-coordinate platinum(IV) complex, which readily eliminates methane. 

Fig. 4. Relevant structures for the discussion of methane activation by (bipyrimi-dine)PtCl2 Methane complex of Pt(II) (A) methyl(hydrido)platinum(IV) complex, the product of the oxidative addition (B) transition state for intramolecular deprotonation of the methane complex ( cr-bond metathesis , sometimes also called electrophilic , C) intermolecular deprotonation of the methane complex ( electrophilic pathway , D).

Phosphine complexes are generally regarded more electron-rich than the corresponding ammine complexes, and which pathway is preferred under these electronic conditions has also been investigated. For trans-PtCl2(PH3)2, oxidative addition has been calculated to be much more favorable than the electrophilic pathway for the activation of methane... 

Other metals capable of electrophilic substitution of C-H bonds are salts of palladium and, environmentally unattractive, mercury. Methane conversion to methanol esters have been reported for both of them [29], Electrophilic attack at arenes followed by C-H activation is more facile, for all three metals. The method for making mercury-aryl involves reaction of mercury diacetate and arenes at high temperatures and long reaction times to give aryl-mercury(II) acetate as the product it was described as an electrophilic aromatic substitution rather than a C-H activation [30],... 

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... 

In the absence of a substituent at the 1-position, 3-hydroxymethylindoles are also unstable under basic conditions, due to the activation of the heterocyclic ring to electrophilic attack by the initial removal of the proton from the 1-position. Under these conditions the bis(3-indolyl)methanes are formed. However, it has been noted that under neutral conditions 3-hydroxymethyl-2-phenylindole is converted into 2-phenylindole with the extrusion of formaldehyde <79HC(25-3)l). It has been suggested that the bulky 2-phenyl group inhibits the alternative formation of the indolylmethane. 

These results can be interpreted in terms of protosolvation of the nitronium ion. While the monocationic nitronium ion is a sufficiently polarizible electrophile to react with strong nucleophiles such as olefins and activated arenes, it is generally not reactive enough to react with weak nucleophiles including methane. Partial or complete protonation of the nitronium oxygen then leads to the superelectrophilic species 8. The... 

Superelectrophilic activation has also been proposed to be involved, based upon the reactivity of carbocations with molecular hydrogen (a a-donor).16 This chemistry is probably even involved in an enzymatic system that converts CO2 to methane. It was found that A. A -menthyl tetrahy-dromethanopterin (11) undergoes an enzyme-catalyzed reaction with H2 by hydride transfer to the pro-R position and releases a proton to give the reduced product 12 (eq 15). Despite the low nucleophilicity of H2, cations like the tert-butyl cation (13) are sufficiently electrophilic to react with H2 via 2 electron-3 center bond interaction (eq 16). However, due to stabilization (and thus delocalization) by adjacent nitrogen atoms, cations like the guanidinium ion system (14) do not react with H2 (eq 17). 

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