Hydrogen atom abstraction intermolecular

The intermediates which are generated are free radicals. The hydrogen-atom abstraction can be either intramolecular or intermolecular. Many aromatic ketones react by hydrogen-atom abstraction, and the stable products are diols formed by coupling of the resulting a-hydroxyben2yl radicals ... 

Hydrogen Abstraction Photoexcited ketone intermolecular hydrogen atom abstraction reactions are an interesting area of research becanse of their importance in organic chemistry and dne to the complex reaction mechanisms that may be possible for these kinds of reactions. Time resolved absorption spectroscopy has typically been nsed to follow the kinetics of these reactions but these experiments do not reveal mnch abont the strnctnre of the reactive intermediates. " Time resolved resonance Raman spectroscopy can be used to examine the structure and properties of the reactive intermediates associated with these reactions. Here, we will briefly describe TR experiments reported by Balakrishnan and Umapathy to study hydrogen atom abstraction reactions in the fluoranil/isopropanol system as an example. 

The rate of this intramolecular isomerization depends on the chain length, with the maximum in the case of a six-atomic transition state, i.e., when the tertiary C—H bond is in the (3-position with respect to the peroxyl group [13]. For the values of rate constants of intramolecular attack on the tertiary and secondary C—H bond, see Table 2.9. The parameters of peroxyl radical reactivity in reactions of intra- and intermolecular hydrogen atom abstraction are compared and discussed in Chapter 6. 

A molecule of linear alkyl ether possesses a very convenient geometry for intramolecular hydrogen atom abstraction by the peroxyl radical. Therefore, chain propagation is performed by two ways in oxidized ethers intermolecular and intramolecular. As a result, two peroxides as primary intermediates are formed from ether due to oxidation, namely, hydroperoxide and dihydroperoxide [62],... 

Many of the limitations of C—C bond formation by C —H insertion outlined for intermolecular reactions (Section 1.2.1.) can be overcome by making the reaction intramolecular. Thus, hydrogen atom abstraction followed by intramolecular radical-radical coupling or radical addition to an alkene are increasingly popular processes. Two-electron carbene insertions, either thermal or transition metal catalyzed, have also been used extensively. In either case, ring construction involves net C—C bond formation at a previously unactivated C-H site. 

The major intermolecular reaction of triplet aryl nitrenes in solution is hydrogen atom abstaction to form primary amines. For a photoaffinity reagent bound to a receptor, this would result in a failure to couple. However, it is possible that the intramolecular photochemistry of aryl azides is more relevant, and here numerous examples of insertion by triplets have been noted. Presumably, these are two step processes hydrogen atom abstraction, followed by radical coupling (cf. Figs. 2.1 and 2.3). 

Alternatively, intermolecular hydrogen atom abstraction can take place by reaction of the radical end of one chain with the middle of another chain. Long-chain branching results from this kind of reaction. 

The synthetic potential of fhe intermolecular radical additions would be dramatically enhanced by developing conditions compatible wifh primary radicals. Less stable T radicals (versus 2° or 3°) often suffer premature reduction by hydrogen atom abstraction processes and are impractical under Ets B/O2 initiation conditions due to fhe competition with ethyl radicals. These considerations led us to seek alternatives to triethylborane. 

This follows the ionisation potential of the amino groups. Both conventional and microsecond flash photolysis confirmed the involvement of intramolecular hydrogen atom abstraction and this is illustrated by the mechanistic processes in scheme 3 for an N-methyl derivative and scheme 4 for a diethylamino derivative. Here it is seen that the N-methylpiperazine derivative, does not undergo intermolecular hydrogen atom abstraction. Intramolecular hydrogen... 

Nitroarenes display strong absorption in the near-UV region and are efficiently photoreduced.1187 An intermolecular version of this reaction is depicted in Scheme 6.187. The photoreduction is initiated by hydrogen atom abstraction. Substituted nitrobenzenes 393, where X is an electron-donating group (X = p-Me, /)-OMe) or nitrobenzene itself (X = H) are photoreduced in the presence of propan-2-ol to the corresponding... 

The two ketyl units that are part of the ion quadruplet undergo (1) interionic dimerization to give pinacolates, (2) intraionic disproportionation via hydrogen atom transfer from one unit to the other to give alcoholate and enolate, and (3) intermolecular hydrogen atom abstraction from the solvent molecule (THF) in the solvent shell to provide the alcoholate. Thus ketyl radicals appear to undergo the three basic reactions that are characteristic for free radicalsld. 

Intramolecular and intermolecular hydrogen atom abstractions by excited acetoxy groups are also proposed (38,39). 

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