Methyl transfers equation

A rapid scission of the Me—B bond (on the picosecond timescale) generates the methyl radical (CH ) and BMe3. The coupling of pyridine and methyl radicals within the solvent cage completes the methyl transfer (equation 47). 

The homologation of methanol [equation (4)] is an area of increasing interest. A range of metal carbonyls (e.g., Rh, Fe, Mn, or Ru) will, in the presence of amine, for example, MesN, catalyze this reaction the amine B promotes the formation of carbonyl anions and activates the methanol [equations (5) and (6)]. The catalytic pathway is shown below with methyl transfer [equation (9)] being the slow step. It is suggested that ethanol does not undergo further homologation because attack of McsN upon ethyl formate is sterically less favorable than attack upon methyl formate [equation (7)]. ... 

Equation (3.34) without the H X ) and the H 2la terms is identical to the Marcus equation for methyl transfer reactions (Ref. 13). This equation predicts, at the range ( AG0 < a), a linear relationship between AAG0 and AAg by... 

When R is CH3 the process is called methyl transfer. For such reactions, the work terms and are assumed to be very small compared to AG° and can be neglected, so that the Marcus equation simplifies to... 

The D/A complexation in equation (41) is further substantiated by infrared and NMR studies. These observations suggest that an initial thermal electron transfer within the D/A charge-transfer complex generates an ion-radical pair, and a rapid methyl transfer subsequently completes the 1,4-addition (equation 42). 

Thermal methyl transfer. An orange solution of [iQ +, BMeT] in tetrahydrofuran loses its color in 1 h at room temperature to afford the adduct 1,2-dihydro-1,2-dimethylisoquinoline together with BMe3 in quantitative yield (equation 44). 

Note that the photolysis of various pyridinium borate salts at — 78°C (to prevent thermal reaction) affords the same methyl-transfer products as obtained in thermal reactions (i.e., equation 44). 

Comparison of thermal and photochemical activation. The identical color changes that accompany the thermal and photochemical methyl transfer in various [Py+, BMe ] salts suggests that pre-equilibrium charge-transfer complexation is common to both processes. Moreover, the methyl transfer either by charge-transfer photolysis or by thermal activation of [Py+, BMeT] leads to the same products, which strongly suggests common reactive intermediates (i.e., the radical pair in equation (46)) for both thermal and photochemical processes. 

Recently, this view of secondary a-deuterium KIEs has had to be modified in the light of results obtained from several different theoretical calculations which showed that the Ca—H(D) stretching vibration contribution to the isotope effect was much more important than previously thought. The first indication that the original description of secondary a-deuterium KIEs was incorrect was published by Williams (1984), who used the degenerate displacement of methylammonium ion by ammonia (equation (4)) to model the compression effects in enzymatic methyl transfer (SN2) reactions. 

In the [2 + 2] cycloadditions of 10 with iV-phenylmaleimide and dimethyl fumarate, the major cycloadducts were formed with a very high degree of ee transfer from 1,3-dimethylallene8. Similar results were obtained in the reaction of 10 with 1,1-dichloro-2,2-difluoroethene. The reaction with less reactive 1,1-diphenylethene did not lead to cycloadduct formation, but resulted in racemization of the chiral 1,3-dimethylallene instead9, which implies reversible formation of the diradical intermediate in this case. Finally, the cycloaddition of 1,3-dimethylallene to methyl propiolate (14) afforded two cycloadducts, 15 and 16, to which >40% of the initial ee had been transferred (equation 5)11. 

With the notable exception of rhodium, Group VIII metal-peroxo complexes are generally reluctant to react with simple alkenes by nonradical pathways. However, such an oxygen transfer has been shown to occur in the reaction of 180-labeled [(AsPh3)4Rh02]+C104" with terminal alkenes under 02-free, anhydrous conditions, producing lsO-labeled methyl ketone (equation 52).131... 

Fig. 2 A simplified version of Fig. I. The point A locates the transition states for systems obeying the Ritchie equation (11) for nucleophilic addition to R. Transition states for methyl transfer reactions occur in the shaded area...

We now explore whether the pattern of reactivity predicted by the Marcus theory is found for methyl transfer reactions in water. We use equation (29) to calculate values of G from the experimental data where, from (27), G = j(JGlx + AG Y). The values of G should then be made up of a contribution from the symmetrical reaction for the nucleophile X and for the leaving group Y. We then examine whether the values of G 29) calculated for the cross reactions from (29) agree with the values of G(27) calculated from (27) using a set of values for the symmetrical reactions. The problem is similar to the proof of Kohlrausch s law of limiting ionic conductances. 

Fig. 12 Test of the Marcus theory for methyl transfers in H,0. The graph compares the values of G(29) calculated by equation (29) from experimental free energies with C,7) calculated by equation (27) from the Cx x for the symmetrical reactions...

Isotopic substitution is a small enough change to be equivalent to differentiation and hence we may use equation (36). In order to simplify the algebra we assume either that a which we have shown above to be true for many reactions, or that the thermodynamic term, AGXY, is smaller than the kinetic terms. This will be true when the methyl transfer is between oxygen bases of the same type. We can then write... 

Alkylation of N-unsubstituted tetrazoles with diazomethane. N-Unsubstituted tetrazoles 24 react with diazomethane providing isomeric 1- and 2-methyltetrazoles 215 and 216 in a ratio close to that observed in alkylation of the respective tetrazolates with dimethyl sulfate or methyl iodide (Equation 23) <2000H(53)1421>. A possible reason for this similarity is that a (fast) proton transfer from the heterocyclic NH-acid (cf. Section 6.07.5.3.2) to diazomethane occurs in the first stage. Then, in the rate-limiting stage, the resulting tetrazolate anion reacts with the protonated diazomethane. Unfortunately, a detailed study of this reaction presents experimental difficulties since the determination of diazomethane concentration in solutions is always troublesome. 

A similar result obtains on transfer hydrogenation of 2 with Pd-HCOOH in the presence of isobutyl methyl ketone (equation I) to provide 6. 

A significant observation for cobaloximes as B12 models was the reduction of MeCo L4(B) by Cr(aq) +, which gave Co L4(B) and Cr(Me)(aq) + (equation 37). These products are indicative of an inner-sphere (see Inner-sphere Reaction) electron transfer with Me as the bridging ligand. This result has promoted further studies, notably methyl transfer between cobaloximes and nickel tetraazacycle complexes, which provides a possible model for the methylcobalamin alkylation of CO hydrogenase. ... 

By treatment with NaH, isoxazolidine 196 underwent N-O bond cleavage to afford 198. The process is believed to occur through deprotonation of the carbamate nitrogen followed by proton transfer with simultaneous N-O bond cleavage to imine 197, and subsequent loss of methyl glyoxylate (Equation 23) <2001JOC6046>. 

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