Bimolecular methyl transfer

Figure 39. Bimolecular methyl transfer following oxidation of [Pt Me2(a-diimine)].

Chemical and electrochemical oxidation of these [Pt Me2(a-diimine)] species in MeCN was later studied in more detail in the Tdset group (38). Irreversible electrochemical waves were again observed, and bulk electrolysis revealed consumption of 1.1-1.6 FmoP indicative of a le oxidation process. (Electro)chemical bulk oxidation leads to only marginal formation of methane or ethane, and almost quantitative formation of the species [Pt Me3(a-diimine)(NCMe)] and [Pt Me(a-diimine)(NCMe)] in a 1 1 ratio. It was proposed that the short-lived [Pt Me2(a-diimine)] " " decomposes via a bimolecular methyl transfer from one platinum to another. This explains the product distribution and the lack of products derived from free alkyl radicals. This could either involve a reaction between two [Pt °Me2(a-diimine)] + intermediates or a reaction of [Pt° Me2(a-diimine)] + with the starting material [Pt Me2 (a-diimine)]. 

Bimolecular Decomposition of the Transient Complexes Methyl Transfer Reactions Rearrangement of the Carbon-Skeleton of R (H20)sCrCH02+ as a Hydride Transfer Reducing Agent Alkene Complexes... 

The modeling of the bimolecular /3-methyl transfer to a coordinated propene molecule was attempted by Cavallo and others. DFT calculations on the CEZr-(isobutyl) system and combined QM/MM calculations on the Me2C(3-f-Bu-l-Ind)2Zr(isobutyl)+ sys-tem229 gave extremely high activation barriers, which ruled it out as a viable chain release mechanism. However, experimental evidence is building up that it can also occur via associative displacement with the monomer. 

Alkylation at the corrin-boimd Co(I) center normally proceeds via the classical bimolecular nucleophihc substitution (SN2) mechanism, where the Co(I)-corrin acts as a supernucleophile [91,132 [. However, in certain cases alkylation occurs via a two-step one-electron transfer path, where Co(I)-corrins act as strong one-electron reducing agents and the process goes via Co(II)-corrin intermediates [84], In complete corrins, such as Bi2s (40 ), either pathway results in methylation at the -face (see Fig. 11), which allows the nucleotide to coordinate at the a-face of methylcobalamin (3) [86,134]. When the nucleotide base has been changed from a DMB-base to an imidazole little effect on the thermodynamics of the methyl transfer reaction occurs [68]. 

Tsukahara and Wilkins studied the product of the reaction of CoCNHjjjCmbpy), where mbpy is l-methyl-4,4 -bipyridinium, with COj " at pH 7.2 and 25°C. The COj was formed by pulse radiolysis. The initial product was assigned as the radical complex CoCNHjjjCmbpy ), which then undergoes intramolecular electron transfer (1 = 8.7x10 s" ) and bimolecular electron transfer to Co(NH3)5(mbpy) (1 2 = 5.4x10 s" ). 

Xylene isomerization is a test reaction which is claimed to require moderately strong Bronsted acid sites to proceed. One reason for this is the very good stabilization of the formed carbenium ions over the benzene ring. The reaction proceeds via a benzenium ion and the rate-Umiting step in this reaction is the intramolecular methyl transfer. Besides the monomolecular mechanism, xylene isomerization can also proceed via a bimolecular reaction pathway as outlined by Morin et al. [ 172]. They determined the contributions of both pathways and determined the contribution of the monomolecular reaction, which they propose to compare activity and acidity in zeolites. These findings emphasize that the reaction pathway should be known in order to properly estimate acidity. Especially for large pore zeolites, this may be a problem. 

Jencks (1969) has pointed out that the role of the enzyme itself is particularly difficult to understand in bimolecular displacement reactions in which general acid-base catalysis would give little advantage, or is not possible. As examples of such reactions Jencks included methyl transfer from S-adenosylmethionine to nicotinic acid to form the N-meth-yl pyridinium, trigonelline (Joshi and Handler, 1960), or to methionine to form the new sulfonium, S-methylmethionine (Greene and Davis,... 

The mechanism of the oxidation of 2-mercapto-5-methyl-l,3,4-thiadiazole (McMT) to its disulfide dimer and its subsequent reduction was examined with a combined approach employing experimental data and digital simulation. To elucidate the influence of proton transfers on these redox processes, special attention was paid to the influence of various bases and proton donors on both the oxidation and reduction reactions. In particular, McMT oxidation is facilitated by a rapid bimolecular proton transfer from McMT to weak bases such as pyridine that produces McMT , the thiol-ate form, which is then oxidized. There is no such facilitation in the presence of the sterically hindered base 2,6-di-r-butylpyridine, suggesting that the facilitation occurs through the formation of a discrete hydrogen-bonded complex/ ... 

Nosaka and Fox determined the quantum yield for the reduction of methyl viologen adsorbed on colloidal CdS particles as a function of incident light intensity. Electron transfer from CdS to MV " competes with electron-hole recombination. They derived a bimolecular rate constant of 9 10 cm s for the latter process. 

In recent years, there have been many significant advances in our models for the dynamics for proton transfer. However, only a limited number of experimental studies have served to probe the validity of these models for bimolecular systems. The proton-transfer process within the benzophenone-AL A -di methyl aniline contact radical IP appears to be the first molecular system that clearly illustrates non-adiabatic proton transfer at ambient temperatures in the condensed phase. The studies of Pines and Fleming on napthol photoacids-carboxylic base pairs appear to provide evidence for adiabatic proton transfer. Clearly, from an experimental perspective, the examination of the predictions of the various theoretical models is still in the very early stages of development. 

---