Photolysis hydrogen atom abstractions

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

The restricted motion of molecules and of fragments such as free radicals formed by photodissociation results in interesting differences in the photochemistry of some molecules in solution or as guests in inclusion compounds. To take one example, the aliphatic ketone 5-nonanone can yield fragmentation or cyclization products via the biradical formed through intramolecular hydrogen atom abstraction (Figure 8.18). In the photolysis of the inclusion compound the cyclization is the preferred reaction, and there is a marked selectivity in favour of the ay-isomer of the cyclobutanol. 

Banerjee, A. and Falvey, D.E. (1998) Direct photolysis of phenacyl protecting groups studied by laser flash photolysis an excited state hydrogen atom abstraction pathway leads to formation of carboxylic acids and acetophenone. Journal of the... 

The major source for atmospheric FCO is expected to be photolysis of CF20 and CFCIO, but the advent of hydrofluorocarbons, such as CF3CFH2 (HFC-134a), as CFC-alternatives now provides an additional source by means of hydrogen-atom abstraction from CHFO by OH, F or Cl. Yet another source is addition of atomic fluorine to carbon monoxide. 

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. 

Laser flash photolysis of [CpM(CO>3]2 (M = W, Mb, and Cr) provides a convenient source of CpM(CO)3, an organometallic free radical with 17 valence electrons. It is a transient and highly reactive species. Depending on the circumstances and the other reagents present, the radical will dimerize, undergo halogen and hydrogen atom abstraction reactions, and electron transfer reactions. With tetramethyl-phenylenediamine, there is a cyclic process of electron transfer steps, the net result of which is the catalyzed disproportionation of the metal radical. 

Photolysis at lower temperatures and in the solid state decreases the extent of the hydrogen atom abstraction due to its higher activation energy compared to radical dimerization. Additionally, the lower mobility of the radicals in the solid state increases the cage reaction of the pairwise generated radicals. This way unsymmetrical diacylperoxides form unsymmetrical dimers with less than 1% of the symmetrical product [29]. Peroxides from carboxylic acids, being optically active at the a-carbon, couple photochemically in the solid with only a minor loss of optical activity [30]. 

The sensitized photolysis of tetrasubstituted cyclopropenes that do not have a C-C double bond in the side chain at C3 leads to hydrogen atom abstraction from a 3-alkyl group to form a diradical, which may either cyclize or disproportionate (see Section 1.1.6.2.1.) to give exolendo-l and ( /Z)-2. ... 

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... 

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