Methane insertion reaction

An unprecedented carbene insertion reaction was observed on reaction of the cationic re-arene ruthenium amidinates with trimethylsilyldiazo-methane (Scheme 145, TFPB = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate). 

The mechanism proposed for carbene-abstraction and carbene-insertion reactions is based on the calculations of Dewar (MINDO/2) and Hoffmann (extended Hiickel) Hoffmann dealt only with the concerted reactions of singlet carbenes, whereas Dewar discussed both singlet and triplet carbene reactions. The calculations of Dewar s ) for the reaction of triplet methylene with methane gave the following results ... 

Another chemical approach to the chemical conversion of methane involves organometallic reactions.85-89 Interesting work with iridium complexes and other transition metal insertion reactions (rhodium, osmium, rhenium, etc.) were carried out. Even iron organometallics were studied. These reactions take place in the coordination spheres of the metal complexes, but so far the reactions are stoichiometric and noncatalytic.77 In terms of synthetic hydrocarbon chemistry, these conversions are thus not yet practical, but eventually it is expected that catalytic reactions will be achieved. 

What effect do shocks have on the gas phase synthesis of complex interstellar molecules This question has been investigated at least for hydrocarbons through six carbon atoms in complexity by Mitchell (1983, 1984). He has found that if a shock passes through a dense cloud where much of the carbon is already in the form of carbon monoxide, complex hydrocarbons are not formed in high abundance. However, if a shock passes through a diffuse cloud, of density approximately 103 cm-3, where much of the cosmic abundance of carbon is in the form of C+ and to a lesser extent C, a different scenario is present. As the shock cools, the C+ and C, which remain in appreciable abundance for up to 10s yrs after the shock passage, react via many of the reactions discussed above as well as others to produce a rich hydrocarbon chemistry. The net effect is that large abundances of hydrocarbons build up as the cloud cools and eventually reaches a gas density of 3 x 104 cm-3. Do these results bear any relation to the results obtained from ambient gas phase models In both types of calculations, hydrocarbon chemistry appears to require the presence of C+ and/or C both to synthesize one-carbon hydrocarbons such as methane and then, via insertion reactions, to produce more complex hydrocarbon species. Condensation reactions do not appear to be sufficient. 

The insertion reaction with ethane and propane is a few kilocalories per mole more exothermic than with methane. The observed lack of decomposition of the mercaptans produced from these higher homologs demonstrates the marked effect on product lifetime resulting from the availability of additional internal degrees of freedom in the mercaptan molecule and the capacity of the mercaptan for stabilization by equiparti-tion of its excess energy. 

The effect of electrical fields on the radiolysis of ethane has been examined by Ausloos et and this study has shown that excited molecules contribute a great deal to the products. The experiments were conducted in the presence of nitric oxide, and free-radical reactions were therefore suppressed. The importance of reactions (12)-(14) was clearly demonstrated by the use of various isotopic mixtures. Propane is formed exclusively by the insertion of CH2 into C2H6 and the yield is nearly equal to the yield of molecular methane from reaction (14). Acetylene is formed from a neutral excited ethane, probably via a hot ethylidene radical. Butene and a fraction of the propene arise from ion precursors while n-butane appears to be formed both by ionic reactions and by the combination of ethyl radicals. The decomposition of excited ethane to give methyl radicals, reaction (15), has been shown by Yang and Gant °° to be relatively unimportant. The importance of molecular hydrogen elimination has been shown in several studies ° °. ... 

Intermolecular reactions between coordinatively unsaturated metal atoms and the C-H groups of relatively unreactive hydrocarbons such as cyclohexane and even methane have been reported (Scheme 1.4). ° Insertion reactions of carbenes R R C into C-H or other bonds are clearly of the same type, the carbene carbon atom playing the role of the coordinatively unsaturated metal... 

Ab initio molecular orbital theory has been applied by Olah and coworkers to investigate the reactions of NO and the protonitrosonium ion HNO with methane. The reaction path was found to involve attack of NO on carbon instead of C-H bond insertion in accord with the studies of Schreiner et al. It was, however, pointed out that this is the consequence of the ambident electrophilic nature of NO and does not represent a general electrophilic reaction pathway for the reactions of methane. In fact, Schreiner and coworkers suggested that the electrophilic substitution of methane occurs by substitution of the nonbonded electron pair of methane instead of insertion of the electrophile into a C-H bond via 3c-2e bonding. Nonbonded electron pair formation in methane, however, can be considered only when methane would tend to flatten out (58) from its tetrahedral form, but this would be prohibitively energetic (>100 kcal mol ) and thus unlikely. 

Figure 2. Spin pairing schemes for Y atom insertion reaction into methane.

A similar approach can be applied for the Y atom insertion into the C-H bond of alkenes and other alkanes. Our calculation by the Cl method in a 6-311-H-G(2d,2p) basis set with a complete active space for 8 electrons in 8 orbitals (Is orbital of carbon atom is frozen) predicts that the vertical S-T excitation energy in methane is around 11 eV (11.37 eV or 262 kcal/mol). Following the above approximation it is equal -2 Jch- From Eq. (9) the activation energy for the yttrium atom insertion reaction, Eq. (1) M=Y, should be 17.6 kcal/mol. This simple estimation is in a good agreement with very accurate ab initio calculations, Ea = 20.7 kcal/mol [15]. 

Figure 4. Qualitative potential energy surfaces cross sections along the reaction path of the Y atom insertion reaction into methane. Schematic correlation diagram for doublet states.

Protolysis also is a useful way of preparing Z —CO— adducts with the early transition metal centers. Typically, the early transition metal alkyls have carbanionic character and readily react with the acidic metal carbonyl hydrides. In the reaction of Cp2ZrMe2 with CpMo(CO)3H, methane is evolved and Cp2ZrMe2(OC)Mo(CO)2Cp forms (60,61). A low-frequency Z —CO— stretching frequency is observed at 1545 cm - and the formulation is confirmed by an X-ray structure determination. When this complex is placed under a CO atmosphere, a migratory insertion reaction occurs (Scheme 1) to produce a species having both f/2-acetyl and Z —CO— ligands. 

The gas-phase reactions of PH/ (x = 0,1,2) ions with methane have been studied by a number of groups " These ions are reactive and undergo a series of insertion reactions to produce ions containing phosphorus carbon bonds (equation 67-69). The reaction between the methyl cation and phosphine also produces ions containing phosphorus-carbon bonds (equation 70) ". ... 

Mechanistically, both reactions are electrophilic insertion reactions into the methane C—H bonds. In the platinum insertion reaction subsequent chlorolysis of the surface... 

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