Methane elimination

CI3 SiRhCl(H)(PPh3)2/HSiCl3 135). The methane elimination is established for Me3SiOs(CO)4H/MeAuPPh3 168). 

Returning to reaction 6, Schemes I and II and Figure 7 suggest that the loss of hydrogen and ethane result from a common intermediate and that a distinct intermediate is responsible for the elimination of methane. A comparison of the product distributions measured using the ion beam instrument with the relative metastable yields recorded with the reverse sector instrument supports this conjecture (Table HI) in that the ratio of hydrogen to ethane loss is approximately the same and methane elimination is diminished in importance in the metastable data (38). From... 

A transition state for the direct methane elimination from the Pt(IV) complex having two PH3 ligands was not observed. Phosphine loss occurred concomitantly with the reductive elimination. However, the authors were able to estimate an activation barrier of ca. 16 kcal/mol for direct elimination from this Pt(IV) complex (PH3)2Cl2PtCH3(H) using artificial restraints for the geometry optimization. This value is very close to the 16.5 kcal barrier obtained for reductive elimination... 

Despite the increasing munber of theoretical studies of individual steps, a full picture of the gas-phase chemistry has not yet been deduced. Both the physical transport effects as well as the gas-phase chemistry have to be taken into consideration. Even the most elaborate mechanisms [83-86] rely on the possibly erroneous experimental activation energies for GaMes pyrolysis in the gas phase. Neither GaN etching [96] at high temperatures nor pressure-dependent rates [95] have been included in the mechanisms. In addition, the cyclic oligomers [Mc2GaNH2]x with x = 2,3 are taken either as dead ends or as direct precursors to GaN via further methane elimination, whereas a rad-... 

In contrast to the results obtained with -alkanes and cycloalkanes, H2 elimination is found to be an unimportant process in the photolysis of neopentane [107]. At 7.6 eV the methane elimination and direct C-C bond cleavage to radicals are the predominant processes ... 

With an increase in photon energy, the importance of the radical-forming reaction increases at the expense of the methane elimination process. 

Conceivable mechanisms for methane elimination from Os(CO)3(CCH3)(H)L involve either a seven-coordinate transition state or direct migration of CH3 onto H. 

Methane elimination from the thermally unstable alkoxy(thiolato) complex [Ti(Me)(OC6H3Pr2,-6>,o )(SEt)(T75-C5H5)] yielded a similar thioacetal-dehyde complex.85 In the reaction of hexafluorobut-2-yne with the thiolato complex [Co2(CO)6(SC6F5)4], a dinuclear complex with a bridging thioke-tone ligand was also formed.212... 

Cleavage of the gold-methyl bond occurred in the reaction of [AuMe(PPh3)] with [H2Os3(CO)i0] (2/5), where methane elimination was accompanied by formation of the mixed-metal cluster [ H AuOs(CO)10(PPh3)]. 

The vertical electron affinity (EA) of acetone is given as —1.51 eV by Jordan and Burrow386. Lifshitz, Wu and Tiernan387 determine—among other compounds—the excitation function and rate constants of the slow proton transfer reactions between acclone-Ih, acetone-Dg and other ketones. The acetone enolate anion has been produced in a CO2 laser induced alkane elimination from alkoxide anions by Brauman and collaborators388-390. These show, e.g. that the methane elimination from t-butoxide anion is a stepwise process ... 

C—H activation of the solvent however, upon addition of a strong donor (PMes) dihydrogen elimination is induced, affording TpPt(Me)(PMe3) (377). Definitive explanations for this observation, the facility of H/D scrambling and reluctance toward methane elimination have yet to be established, but are the subject of on-going experimental124 and theoretical studies.125... 

A propensity for H/D scrambling with solvent, in preference to methane elimination, has also been noted for the thermolysis (55—70 °C) of the related TpPtMe(H)2 (347) in 1 1 Cf,Hf,/CHyOD, which affords the fully deuterated 347-d5 over 6 h, with no evidence for loss of methane.124... 

Likewise, irradiation of AfejReO also leads to methane elimination and the formation of H2C = Re(0)Me2, as shown in Eq. 2, via methyl radical involvement [7,8]. 

However, the complex is thermally unstable, and rapidly decomposes as its solution is warmed to RT. As measured by GC-MS, the volatile products of the reaction contain Et3SiCl and (Et3Si)20 as two major components, characteristic of heterolytic cleavage of the r 2-Si H bond. The (Et3Si)20 was presumably formed by the reaction of Si Et, with adventitious H20, and Et3SiCl was formed by attack of SiEt, on CD2C12 solvent. Pd black also forms, possibly due to methane elimination from an unobserved Pd methyl hydride complex and instability of the resulting Pd(0) species. 

Studies [61,62] have shown that at 90°C, the adduct reacts to form a six member ring, cyclo(trimmido-hexamethyitrigallium), with the release of one methane molecule per Ga atom. The overall reaction proceeds in two steps, as indicated by EQNS (2) and (3). The first step is methane elimination to form (CH3)2Ga NH2, followed by the oligomerisation of this compound. 

Ion abundances for methane loss from metastable (CH3CD2CHD2)t ions have been found to be in the ratios 87 1 12 (CH4 CH3D CH2D2) after correction for the numbers of equivalent pathways [250]. The reaction is a 1, 2 elimination, so these results indicate both a large primary and a large secondary deuterium isotope effect (see Sect. 7.5.4). On the basis of these intramolecular isotope effects, a mechanism involving a non-classical transition state with a CCH three-centre bond has been proposed for this methane elimination [250]. 

Table XXY shows that most of the methane is formed by molecular elimination. Just as with other hydrocarbons it is difficult to know the origin of the methane eliminated as a molecule. It may arise in a primary process...

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