Hydrogen atom donor, and

R2P(0)H) and phosphonates (R02P(0)H) as hydrogen atom donors and the corresponding phosphonyl radicals as chain carriers (Scheme 3). 

Hydrogen bonding means sharing a hydrogen atom between one atom that has a hydrogen atom (donor) and another atom that has a lone pair of electrons (acceptor) ... 

Aminium radical 67, produced in reactions of TTOC carbamates, can be self trapped in useful yield (Scheme 24). Trapping with hydrogen atom donors and with Ph2Se2 are competitive with this self trapping reaction (91JOC1309). 

Photolysis of [Rh(tfacac)3] (tfacac is the unsymmetrically substituted 1,1,1-trifluoromethyl-acac) reveals the existence of two photoinduced reaction paths the relative efficiency of the two paths is dramatically solvent dependent.1140 In cyclohexane, mer- cis isomerization is the only observed photoreaction, but if ethanol or 2-propanol is added to the solvent, the photoisomerization efficiency decreases, and photodecomposition occurs. The nature of the photodecomposition products is not specified, but the enhanced photoreactivity in the presence of tri(n-butyl)stannane, a hydrogen atom donor, and flash and continuous photolysis studies in mixed-solvent systems strongly implicate hydrogen atom abstraction from the solvent as a key step in the photodecomposition of wer-[Rh(tfacac)3] and suggests that the photo reactive states have considerable radical character .1140 Analysis of quantum efficiencies implies that at least two distinct photoproduced excited states must be involved. 

The acyl radicals decayed either by decarbonylation or by radical-radical recombination. Decarbonylation reaction rates were found to decrease in more polar solvents, indicative of the more polar nature of acyl radicals relative to their decarbonylation products. Second-order rate constants for the reaction of a variety of acyl radicals with halogen atom donors, hydrogen atom donors, and oxygen were determined by monitoring the decay of the acyl radical, the growth of the product, or, when possible, both. The observed reactivity was roughly comparable with that of simple alkyl radicals. 

HydrOXylaminGS. A relatively new stabilizer chemistry, commercially introduced in 1996, (18) based on the hydroxylamine functionality, can serve as a very powerful hydrogen-atom donor and free-radical scavenger (19), as illustrated in Figure 3. 

The resonance stabilized benzofuranyl (lactone) radicals can either reversibly dimerize or react with other free radicals. Model experiments have demonstrated that this class of chemistry behaves as a powerful hydrogen atom donor and are also effective scavengers of carbon-centered and oxygen-centered free radicals (21) (see Fig. 5). 

The byproduct HX can frequently influence the course of reaction, by both re-adding to elimination products and serving as an efficient hydrogen atom donor, and is best removed from the irradiation mixture as it is formed. Triethylamine can be used as a scavenger in nonpolar solvents. In polar solvents, however, it leads to increased formation of the reduction product RH because of competing reaction via the amine-halide exciplex 1 (Scheme 1). The poorer electron donor HO" has been found to be a suitable HX scavenger in alcohoHc solvents. ... 

Givens et al. examined the photorelease of phosphate esters using f-butyl alcohol and methanol as solvents, the former being a poor hydrogen atom donor and both exhibiting increased polarity compared with 1,4-dioxane. The results correlated well with those of Anderson and Reese in that the major photoproduct was the rearranged ester, not the photoreduction product (Eq. (69.7)). The amount of 13 was decreased to 21% in methanol and 14% in t-butyl alcohol. Further investigation of the solvent dependency for the release of phosphates revealed a solvent isotope effect with deuterated vs. protiated methanol as the solvent (Table 69.4). The formation of p-methoxyacetophenone was suppressed by a factor of five when... 

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