Hydrogen radical abstraction

Photo-induced Diels Alder reaction occurs either by direct photo activation of a diene or dienophile or by irradiation of a photosensitizer (Rose Bengal, Methylene Blue, hematoporphyrin, tetraphenylporphyrin) that interacts with diene or dienophile. These processes produce an electronically excited reagent (energy transfer) or a radical cation (electron transfer) or a radical (hydrogen abstraction) that is subsequently trapped by the other reagent. 

Radical-mediated silyldesulfonylation of various vinyl and (a-fluoro)vinyl sulfones 21 with (TMSlsSiH (Reaction 25) provide access to vinyl and (a-fluoro)vinyl silanes 22. These reactions presumably occur via a radical addition of (TMSlsSi radical followed by /)-scission with the ejection of PhS02 radical. Hydrogen abstraction from (TMSlsSiH by PhS02 radical completes the cycle of these chain reactions. Such silyldesulfonylation provides a flexible alternative to the hydrosilylation of alkynes with (TMSlsSiH (see below). On oxidative treatment with hydrogen peroxide in basic aqueous solution, compound 22 undergoes Pd-catalyzed cross-couplings with aryl halides. 

Scheme 3.62. Domino radical hydrogen abstraction-cyclization procedure in the synthesis towards Aspidosperma alkaloids.

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

Reaction 7.74) [84], That is, (TMS)3Si radical added to the double bond of allyl sulfides, giving rise to a radical intermediate that undergoes (3-scission with the ejection of the thiyl radical. Hydrogen abstraction from the silane completes the cycle of these chain reactions. 2-Functionalized allyl tris(trimethylsilyl)si-lanes (71) have been employed in the radical-based allylation reactions. 

Rate of Initiation. Rate and mode of formation of the first few radicals. Rate of initial attack of the hydrocarbon molecule by free radicals (hydrogen abstraction). 

Rate of Initial Attack of Hydrocarbon Molecule by Free Radicals (Hydrogen Abstraction). 

Hydrogen Abstraction by Carbon-centred Radicals. . Hydrogen Abstraction by Heteroatom-centred Radicals... 

Photolysis of benzoin and benzoin methyl ether in the presence ctf the phenoxynitrone gave rise to e.sj. signals from both phenoxy and nitroxide radicals. Hydrogen abstraction by benzoin and benzoin ethers would yield radicals of the type... 

A study of the photodeconjugation reaction of the ester (136) in the presence of a variety of optically active amines such as (IR,2S)-l-phenyl-2-isopropylamino propanol has shown that the product can be obtained with an enantiomeric excess of about 70 X. Irradiation of the alkynylketones (137) in alcohols (methanol, ethanol or propanol) affords the furan derivatives (138) by a free radical hydrogen abstraction path. Excitation of the cyclohexenone derivatives (139) yields the cyclized products (140) the structures of which were verified by X-ray crystallography. The formation of the amide products is reminiscent of a Norrish Type II process. 

The thymidine derivatives (146) and (147) undergo cleavage of a C-C bond on irradiation. These reactions are typical Norrish Type I processes and provide a route to study C-3 -DNA radicals. Hydrogen abstraction by the radicals yields a 1 1 mixture of the threo and erythro derivatives (148). The reactions from the P-isomers (146) are generally more efficient than from the a-isomer (147). A study of the photochemical reactivity of the deoxyuridine derivative (149) has been reported. This novel compound is an electron-accepting nucleo base. It has been used as a means of cleaving DNA. The photochemical fission occurs specifically at the 5 -G of 5 GG3 sequences. 

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