H-atom transfer

This kind of dynamieal speetroseopie analysis is not restrieted to fast primary IVR proeesses. It would apply just as well to the sPidy of eompletely unimoleeular reaetions, viz isomerizations sueh as H-atom transfer reaetions, for example CH2O f HCHO [97] HCN f HNC [98],and referenees eited therem), and HCCHf ... 

H atom transfer is another important unimolccular pathway for /-alkoxy radicals that have a suitably disposed hydrogen atom (Scheme 3.76)421,424,425... 

Margerum et al (Ref 9) photolyzed solns of aromatic nitrocompds in 95% ale using an unfiltered 400 watt mercury lamp. No compd was found to be phototropic which did not have a nitrogroup ortho to a benzyl hydrogen. They hypothesized that an intramolecular process involving an H-atom transfer was operative ... 

Wettermark (Refs 10, 11 12) studied o-nitrotoluene and dinitrotoluene, flash photolyzed in w and ale. He observed transients, and noted that the absorption spectra was a function of pH. He concluded that intramolecular H-atom transfer was involved... 

Pulsed source techniques have been used to study thermal energy ion-molecule reactions. For most of the proton and H atom transfer reactions studied k thermal) /k 10.5 volts /cm.) is approximately unity in apparent agreement with predictions from the simple ion-induced dipole model. However, the rate constants calculated on this basis are considerably higher than the experimental rate constants indicating reaction channels other than the atom transfer process. Thus, in some cases at least, the relationship of k thermal) to k 10.5 volts/cm.) may be determined by the variation of the relative importance of the atom transfer process with ion energy rather than by the interaction potential between the ion and the neutral. For most of the condensation ion-molecule reactions studied k thermal) is considerably greater than k 10.5 volts/cm.). 

Table II. Proton and H Atom Transfer Ion-Molecule Reactions...

Table III shows that in the gas phase at a pressure of 40 torr the relative rates of the H2 transfer reactions from the cyclopentane ion to the various additives differ drastically from those derived from liquid phase radiolysis experiments. This indicates that the changes in density may profoundly affect the relative rates of the two competitive reactions, Reactions 22 and 28. Experimental results, which will be described in a later publication, indicate that in the liquid phase an increased importance of the H2 transfer reaction to some of the additives occurs at the expense of the H atom transfer reaction, Reaction 23.

The carbon-centered radical R, resulting from the initial atom (or group) removal by a silyl radical or by addition of a silyl radical to an unsaturated bond, can be designed to undergo a number of consecutive reactions prior to H-atom transfer. The key step in these consecutive reactions generally involves the intra-or inter-molecular addition of R to a multiple-bonded carbon acceptor. As an example, the propagation steps for the reductive alkylation of alkenes by (TMSfsSiH are shown in Scheme 6. 

Alternatively, two propargyl radicals ( CH2-C=CH) can combine and undergo H-atom transfers, followed by cyclization, to yield benzene. 

The third fact that seemed to argue in favor of the occurrence of radicals 10 was the observation that reactions of a-tocopherol under typical radical conditions, that is, at the presence of radical initiators in inert solvents or under irradiation, provided also large amounts of two-electron oxidation products such as o-QM 3 and its spiro dimerization product 9 (Fig. 6.8).16,25,26 This was taken as support of a disproportionation reaction involving a-tocopheroxyl radical 2 and its hypothetical tautomeric chromanol methide radical 10, affording one molecule of o-QM 3 (oxidation) and regenerating one molecule of 1 (reduction). The term disproportionation was used here to describe a one-electron redox process with concomitant transfer of a proton, that is, basically a H-atom transfer from hypothetical 10 to radical 2. 

The Hj ion, recently detected in the interstellar medium via infrared transitions,25 can subsequently react with a variety of neutral atoms present in the gas. The reaction with oxygen leads to a chain of reactions that rapidly produce the hy-dronium ion H30+ via well-studied H atom-transfer reactions ... 

Rehm D, Weller A (1970) Kinetics of fluorescence quenching by electron and H-atom transfer. Isr J Chem 8 259... 

Hendry, D.G., Mill, T., Piszkiewicz, L., Howard, J.A., Eigenman, H.K. (1974) A critical review of H-atom transfer in the liquid phase chlorine atom, alkyl, trichloromethyl, alkoxy and alkylperoxy radicals. J. Phys. Chem. Ref. Data 3, 944-978. 

Establishment of a free radical mechanism via H-atom transfer for hydrogenation using HMn(CO)5 (see Section II,D), and possibly also HCo(CO)4 (see Section II,C), suggests that more serious consideration for such mechanisms should be given for other hydridocarbonyl catalyst systems, and indeed for other homogeneous catalysts systems in general. The pentacyanocobaltate(II) catalyst can certainly operate by such a mechanism (see Section II,D). 

Fig. 31.17 (a) Experimental observation of dihydrides in the PHOXIr+ system by NMR (S = THF). (b) The DFT-derived mechanism for Ir-catalyzed enantioselective hydrogenation involving the sequential addition of two molecules of dihydrogen, with a single H-atom transfer from each one (S = CH2CI2). 

FIGURE 2.38. Yields of electron transfer + protonation product vs. H-atom transfer product (Scheme 2.22) in constant-potential exhaustive electrolytes as a function of the competition parameter, a ECE electron transfer b DISP electron transfer. 

The strategy above provides a means to understand the competition between H-atom transfer and electron transfer + protonation in general. Additionally, it may be used to gather values for H-atom abstraction rate constants and kinetic isotope effects that are not readily accessible otherwise. It also provides guidelines for optimizing deuterium incorporation reactions. 

The competition between H-atom transfer and electron + proton transfer, exemplified by the reduction of aryl halides in Section 2.5.5, corresponds to the symbolic Scheme 6.4. 

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