Photochemistry hydrogen atom abstraction

P. J. Wagner, B. -S. Park, Photoinduced Hydrogen Atom Abstraction by Carbonyl Compounds, in Organic Photochemistry, Vol. 11. A. Padwa (ed.), Marcel Dekker, New York, 1991, pp. 227-365 and references therein. 

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. 

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). 

Related photochemistry has also been examined with other functional groups such as phthalimides, which also abstract nearby hydrogens with the photoexcited carbonyl group. - Furthermore, since the hydrogen abstraction is performed by the half-vacant nonbonding orbital of a photoexcited ketone carbonyl, related chemistry is observed if the electron is removed electrochemically, not just photoexcited into a Tr -orbital. Electrochemical functionalization of nearby carbons has been reported in which, after hydrogen atom abstraction by an oxidized ketone, the resulting radical is electrochemically oxidized further to the carbon cation, which reacts with solvent (Scheme 7). ... 

While electron-transfer processes are common in inorganic photochemistry, excited-state atom transfer is limited to a small class of inorganic complexes. For U022 , the diradical excited state ( U-OO is active in alcohol oxidation (2). The primary photoprocess is hydrogen atom abstraction by the oxygen-centered radical. Photoaddition to a metal center via atom transfer has been observed for binuclear metal complexes such as Re2(CO)io (3-5). The primary photoprocess is metal-metal bond homolysis. The photogenerated metal radical undergoes thermal atom-abstraction reactions. Until recently, atom transfer to a metal-localized excited state had not been observed. 

Atherton, S. J., and Harriman, A. (1993). Photochemistry of intercalated methylene blue photoinduced hydrogen atom abstraction from guanine and adenine. /. Am. Chem. Soc. 115 1816-1822. 

Hydrogen atom abstraction dominates the excited-state reactivity profiles of iV-alkylphthalimides. This is exemplified by the photochemistry of h/-ethylphthalimide 32, where irradiation leads to formation of the benzazepinedione 33 (Scheme 18). Here, initial y-hydrogen abstraction is followed by biradical coupling and ensuing amidol ring opening. However, the yield of this process ca. 5%), as well as those of related N-alkylphthalimide photocyclizations, are not preparatively acceptable. 

Quinones undergo much interesting photochemistry. Triplet an-thraquinone lies 62.4 kcal above the ground state and readily abstracts hydrogen atoms from alcoholic solvents to yield semiquinone radicals.443... 

The UV photochemistry of phenol and related systems (such as indole, pyrrole, imidazole) is dominated by a hydrogen detachment reaction which is driven by repulsive 1ira states [33,35 10], For the isolated chromophores, the 1 mr -driven photodissociation has been explored in unprecedented detail by high-resolution photofragment translational spectroscopy [40], The OH (or NH) bond is broken homolytically, resulting in the formation of two radical species, the hydrogen atom and the phenoxy (or indolyl, etc.) radical. Ion pair formation (abstraction of protons) is energetically not feasible for isolated photoacids. 

Table 6, entry 48) whereas alkene isomerization is very slow [52]. Photochemistry is also used in the rhodium-catalyzed alkylborylation of alkanes in which boroalk-anes (Table4, entry 49) are obtained [53], An example of a gas-phase reaction is the photochemical mercury-catalyzed hydroxymethylation of cyclohexane to hydroxymethylenecyclohexane (Table 6, entry 50). This reaction is an example of the abstraction of a hydrogen atom from an alkane by an excited metal atom [54]. 

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