Hydrogen transfer side chains

A chain of hydrogen-bonded side chains apparently provides a pathway for transfer of an impaired electron from the active site to the Tyr 122 radical and from there to the radical generating center.363 The tyrosyl radical can be destroyed by removal of the iron by exposure to 02 or by treatment of ribonucleotide reductases with hydroxyurea, which reduces the radical and also destroys catalytic activity ... 

Analogous side-chain oxidations occur in various biosynthetic pathways. The neurotransmitter norepinephrine, for instance, is biosynthesized from dopamine by a benzylic hydroxylation reaction. The process is catalyzed by the copper-containing enzyme dopamine /3-monooxygenase and occurs by a radical mechanism. A copper-oxygen species in the enzyme first abstracts the pro-R benzylic hydrogen to give a radical, and a hydroxyl is then transferred from copper to carbon. 

Tram-anular interactions, which would create an active radical site via hydrogen transfer through 98, cannot be invoked to explain the specific loss of a CH3 radical from the ether side chain. This conclusions is based upon the following experimental observations. The radical cation of the tetrafluoro substituted compound 101 eliminates CH3, but loss of CH3 from the para-isomer 102 is not observed. If a transanuiar process according to 97- 98 were operative, then such a reaction is not expected to be suppressed upon substitution of H by F as is known for many examples from the field of photochemistry of fluoro substituted compounds41 (23). 

In aliphatic amines (diethylamine or triethylamine) the intramolecular hydrogen abstraction is quenched almost completely. Instead, smooth photoreduction of the nitro group without participation of the side chain is observed with 1,3,5-tri-fezf-butyl-2-nitrobenzene (5) and 14, R = C(CH3)3 ). Products derived from the respective phenylhydroxylamines were isolated in both cases. Again, an electron transfer, which does not seem to suffer from steric restrictions, is operative (see also Section A. 1.3). 

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