Hydrogen abstraction by carbonyl

The prototype of photoreduction reactions is hydrogen abstraction by carbonyl compounds in presence of suitable H-donors. Such H-atom transfer may be visualized to occur first by transfer of an electron followed by proton transfer. An electron deficient centre is the seat of reaction and the efficiency of the reaction depends on the nucleophilic nature of the donor. 

Scheme 12 Asymmetric synthesis via 8-hydrogen abstraction by carbonyl group.

The photochemical hydrogen abstraction by carbonyl compounds, which has already been discussed in the last section, will be used to illustrate the procedure. The left-hand side of Figure 6.27 shows the dominant VB structures of the ground state and of the excited (n,jr ) and states of the... 

Prolonged irradiation of the endo-Diels-Alder adduct formed between duroquinone and trnns,trans-hexa-2,4-diene yields the cage dione (930). Similar reaction of the exo-Diels-Alder adduct of quinone and diene, in which intramolecular allylic hydrogen abstraction by carbonyl oxygen or enone carbon is not impeded, results in the forma-... 

With compounds containing C—H bonds y to the carbonyl moiety, one often observes intramolecular hydrogen abstraction, in contrast to the in-termolecular hydrogen abstraction by aryl ketones discussed earlier in this... 

HYDROGEN ABSTRACTION BY GROUPS OTHER THAN THE CARBONYL... 

Norrish Type II photocyclizations have been employed in many instances in the synthesis of nitrogen heterocycles. Type II cyclizations are the result of an intramolecular hydrogen abstraction by an excited carbonyl group followed by cyclization of the resulting biradical. Hydrogen abstraction from the y-carbon atom is normally preferred. The introduction of a nitrogen atom... 

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. 

A dienol is also formed via hydrogen abstraction by the excited carbonyl group from a second enone molecule in (4.14). This dienol tautomerizes in C6F6 to give the (3,y-unsaturated ketone selectively, the overall reaction thus being deconjugation of the a,(3-unsaturated ketone415K... 

Oxidation of aliphatic ketones in trifluoroacetic acid leads to hydrogen abstraction by the carbonyl oxygen radical-cation fonning a carbon radical, then further oxidation of the radical to the carbocation and migration of this centre along the carbon chain by a series of hydride transfer steps. Long chain ketones yield a mixture of alcohol trifluoToacetates by reaction of the carbocation centres with the solvent [3]. 

There is some contribution due to / -scission of the alkyl radical formed by the type I process, particularly in the MIPK and tBVK polymers. Loss of carbonyl occurs from photoreduction or the formation of cyclobutanol rings, and also from vaporization of the aldehyde formed by hydrogen abstraction by acyl radicals formed in the Norrish type I process. As demonstrated previously (2) the quantum yields for chain scission are lower in the solid phase than in solution. Rates of carbonyl loss are substantially different for the copolymers, being fastest for tBVK, slower for MIPK, and least efficient for MVK copolymers (Table I and Figure 1). 

Figure 41. Distance and orientational parameters associated with the efficiency of y-hydrogen abstraction by a carbonyl oxygen atom. (Copied with permission [282a].)...

In photochemistry, intermolecular and intramolecular hydrogen abstractions by excited carbonyl oxygen are often encountered [64]. If an intramolecular (3-hydrogen... 

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