Possible dimers from oxidative coupling

Scheme 2. Possible dimers from oxidative coupling of phenol.

The HPLC-MS/MS assay was also successfully applied to the measurement of UV-induced dimeric pyrimidine photoproducts [123, 124]. The latter lesions were released from DNA as modified dinucleoside monophosphates due to resistance of the intra-dimer phosphodiester group to the exonuclease activity during the hydrolysis step [125, 126]. The hydrolyzed photoproducts exhibit mass spectrometry and chromatographic features that allow simultaneous quantification of the three main classes of photolesions, namely cyclobutane dimers, (6-4) photoproducts, and Dewar valence isomers, for each of the four possible bipyrimidine sequences. It may be added that these analyses are coupled to UV detection of normal nucleosides in order to correct for the amount of DNA in the sample and obtain a precise ratio of oxidized bases or dimeric photoproducts to normal nucleosides. 

The scheme of reactions proposed to explain the products obtained is shown, after small modifications, in Scheme 8. Primary radicals 12 formed at the anodes produce with added 30 or 36 (equation lOe) the substituted benzyl or allyl radicals 38, which can dimerize to 39 or can couple with the added olefin to form radicals 40 or 41. For allyl radical (38) a 1,1 - or l,3 -coupling is possible yielding 41 and 40, respectively. Further couplings of 40 and 41 with the primary radical 12 produce 39 and head-to-tail dimer 42, respectively. It was evident from the products obtained that the coupling of 38 in the 1-position occurs 5 to 11 times faster than in the 3-position. However, for readily polymerizable olefins, rather polymerization occurs, in particular at graphite electrodes. At Pt electrodes both dimers 39 and 42 are formed, but for Cu electrodes exclusively dimers 39 were obtained with moderate yields. Thus, an indirect electrolysis including the oxidation of copper to Cu+ ions and their further reaction with 5 yielding intermediate RCu was considered, but not proved . ... 

This sequence explains Price s observations adequately and seems to be required in this particular case. The oxidative elimination of halide ion from salts of phenols does not always follow this course, however. In the peroxide-initiated condensation of the sodium salt of 2,6-dichloro-4-bromophenol (Reaction 23) molecular weight continues to increase with reaction time after the maximum polymer yield is obtained (Figure 5) (8). Furthermore, Hamilton and Blanchard (15) have shown that the dimer of 2,6-dimethyl-4-bromophenol (VIII, n = 2) is polymerized rapidly by the same initiators which are effective with the monomer. Obviously, polymer growth does not occur solely by addition of monomer units in either Reaction 22 or 23 some process leading to polymer—polymer coupling must also be possible. Hamilton and Blanchard explained the formation of polymer from dimer by redistribution between polymeric radicals to form monomer radicals, which then coupled with polymer, as in Reaction 11. Redistribution has indeed been shown to occur under... 

Scheme 7 displays a possibility of the synthesis of chiral 2-arylpropionic acids via the oxidative tranformation of (7 )-3-aryl-l-butenes. The requisite chiral olefins may be obtained by transition metal-catalyzed asymmetric coupling between a benzylic Grignard reagent and vinyl bromide (93 % optical yield) [28] or, more attractively, asymmetric hydrovinylation of an aromatic olefin with ethylene. The asymmetric combination of styrene and ethylene, giving the adduct 25 in 95 % ee, has been performed on a 10-kg scale with a dinuclear Ni catalyst formed from ( -allyl)NiCl2 and a unique chiral dimeric aminophosphine obtainable from (/ )-myrtenal and (5)-l-phenylethylamine [7a],... 

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