And radical abstraction

Alternatively, a retro-ene reaction cleaves the O10-C11 bond and gives a highly unsaturated ketene. The ketene can undergo cycloaromatization to give a diradical intermediate. H- abstraction and radical-radical recombination then give the product. 

Arnold and coworkers reported a series of reactions between benzophe-nones and monoalkenes. In most cases the oxetanes were the major products and could be isolated in good yields [49]. Benzaldehyde 55, which has a triplet 7771 as the reactive state [50], is less chemoselective and gave with cyclohexene besides oxetane 56 also several hydrogen abstraction and radical coupling products 57-58 (Sch. 15) [51]. 

Irradiation of solid 1 1 complexes of benzaldehyde 89 and P-cyclodextrin 2 resulted in enantioselective intermolecular hydrogen abstraction and radical coupling to afford (/ )-( — )-benzoin 90 as a major product in up to 15% ee and 56% chemical yield. 4-Benzoylbenzaldehyde was obtained as a minor product (Scheme 20) [22]. This reactivity can be understood from the presence of guest-to-host (2 2) complexes generated by the head-to-head association of two 1 1 inclusion complexes. 

The photochemical reactions of arenecarboxylic acid esters with alkenes has received recent attention by Cantrell. - For example, irradiation of 2,3-dimethyl-2-butene and methyl benzoate gave a mixture of alkoxyoxetane (56), carbonyl-alkene metathesis product (57) and ketone (58), resulting from alkoxy radical allylic hydrogen abstraction and radical recombination. Such alkoxyoxetane photoproducts are... 

Kamachi T, Toraya T, Yoshizawa K. Computational mutation analysis of hydrogen abstraction and radical rearrangement steps in the catalysis of coenzyme B 12-dependent diol dehydratase. Chem. Eur. J. 2007 13 7864-7873. 

The transient ketocarbene is also able to effect both intramolecular and intermolecular H-abstraction. The fact that a triplet state of the carbene is often implicated follows from observations that sensitized photolyses frequently enhance the abstraction pathway. Abstraction and radical combination results in the formal insertion of the carbene into the C—H (or S—H, N—H, etc.) bond. Insertions may also occur into C—C, C—O, C—S, C—Hal, etc. bonds the mechanisms are often not known, but a singlet state of the ketocarbene is probably involved in many cases. Reactions at soft basic centers (e.g. —S or —Br ) generally proceed by the preliminary formation of an ylide. The multiplicity of the possible pathways is illustrated in Scheme 7, equation (14) (photolysis in dioxane affords only 27, 57% and 28, 43%), and Scheme 8. ... 

The triplet state of the alkyne (265) is involved in the photochemical reactions with di-isopropyl ether. The proof that a triplet is involved was obtained from quenching and sensitization studies. The products (266, 267) obtained from the reaction arise from hydrogen-abstraction and radical-combination reactions. 

Acetyl chloride undergoes the expected fission into radicals (MeCO and Cl) when irradiated (254 nm) in ethereal solution.82 The major product (8) (72% at 18 °C) is formed by hydrogen abstraction and radical combination within the solvent cage. The other product (9) (28% at 18 °C) is formed by combination of two MeCHOEt radicals, presumably as a result of escape of this radical from the solvent cage in which it is formed. The proportion of (9) diminishes with decreasing temperature (21% at —45 °C). Propionyl chloride behaved similarly. Irradiation of the thioacetates (10) results in both S-alkyl and S-acyl bond cleavage.23... 

However, the main product (20) is thought to arise by hydrogen abstraction and radical combination. 

Epstein JL, ZhangX, Doss GA, et al. Interplay of hydrogen abstraction and radical repair in the generation of single- and double-strand DNA damage by the esperamicins. J Am Chem Soc. 1997 119 6731-6738. 

Other common reactions are hydrogen abstraction and radical additions to double bonds. These reactions are denoted with single-headed arrows as combinations of the two kinds of electron pushing given just above. Combining the kinds of steps shown here, with different radical reactants, will allow you to write the electron pushing for most radical reactions. 

SCHEME 3 Potential mechanism of hyaluronan strand cleavage as a result of hydrogen abstraction and radical formation on C(4) of the glucuronic acid unit (adapted from [19]). 

The photolysis of cyclopentene and norbomene 3 in inert solvent pentane results in several rearranged products from hydrogen abstraction and radical coupling [9]. 

Figure 4.5. Reaction scheme for the formation ofstarting radicals from TPO a) a-cleavage of the C-P bondforming C-centered acyl and P-centered phosphinoyl radicals and b) possible H-atom abstraction and radical shift from the C-centered acyl radical [NEU 06]...

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