Radical reactions elimination, reductive

Claisen rearrangement, 1194-1195 dehydration, 622 elimination reactions, 393 oxidation, 625-626 radical reactions, 243-244 characteristics of, 162-164 comparison with laboratory reactions, 162-164 conventions for writing, 162. 190 energy diagram of, 161 reduction, 723-725 reductive animation, 932 substitution reactions, 381-383 Biological reduction, NADH and, 610-611... 

The reverse reaction is reductive elimination. No mechanism is implied in reaction (13.3). The addition may be stepwise, radical, electrophilic, or nucleophilic or concerted. Oxidative additions of H—H [reaction (13.1)] or H—R [reaction (13.2)] tend to be concerted. 

Moorthy PN, Hayon E (1975) Free-radical intermediates produced from the one-electron reduction of purine, adenine and guanine derivatives in water. J Am Chem Soc 97 3345-3350 Mori M, Teshima S-l, Yoshimoto H, Fujita S-l, Taniguchi R, Hatta H, Nishimoto S-l (2001) OH Radical reaction of 5-substituted uracils pulse radiolysis and product studies of a common redox-ambivalent radical produced by elimination of the 5-substituents. J Phys Chem B 105 2070-2078 Morin B, Cadet J (1995) Chemical aspects of the benzophenone-photosensitized formation of two lysine - 2 -deoxyguanosine cross-links. J Am Chem Soc 117 12408-12415 Morita H, Kwiatkowski JS,TempczykA(1981) Electronic structures of uracil and its anions. Bull Chem Soc Jpn 54 1797-1801... 

Analogous reactions are given by the minor radical cation [reactions (16) and (17)], and the secondary C(4 ) radicals can eliminate the other phosphate such as reaction (18) that is shown here as an example. Reduction and oxidation reactions depicted as examples in reactions (20) and (12) lead after release of unaltered bases [reactions (13) and (21)] to a series of dideoxypentoses and C(4 ) keto products, free and bound to DNA (Beesk et al. 1979 for the determination of these products see Chap. 13.2). 

Under hypoxic conditions, cellular enzymes reduce the benzotriazine di-N-oxide [(reaction (68) P450 reductase Cahill and White 1990 and NADPH may be involved Walton et al. 1992 Wang et al. 1993]. Upon microsomal reduction of tirapazamine the radical formed in reaction (68) has been identified by EPR (Lloyd et al. 1991). Using the pulse radiolysis technique, it has been shown that this radical has a pKd of 6 (Laderoute et al. 1988), and it is the protonated form that undergoes the DNA damaging reaction (Wardman et al. 2003). The rate constants of the bimolecular decay of the radical [reaction (70)] has been found to be 2.7 x 107 dm3 mol-1 s 1. The reaction with its anion is somewhat faster (8.0 x 108 dm3 mol-1 s 1), while the deprotonated radicals do not react with one another at an appreciable rate. From another set of pulse radiolysis data, a first-order process has been extracted (k = 112 s 1) that has been attributed to the water elimination reaction (72), and the tirapazamine action on DNA [reaction (74)] has been considered to be due to the resulting radical (Anderson et al. 2003). 

This is a radical reaction similar to the Birch reduction (see p. 148). In general benzyl groups are removed hydrogenolytically (see p. 260), but under these conditions elimination of the remaining double bond would occur. In the second step the carbonate is cleaved and the amine is deprotected to get the unprotected oligosaccharide 19. 

Nickel catalysis is a very active field in organometallic and organic chemistry (selected reviews [3-7]). Complexes of all oxidation states are active in two-electron transfer processes, such as oxidative addition or reductive elimination as well as in single electron transfer initiating radical reactions. Through these processes, oxidation states from Ni(0) to Ni(III) can be easily accessed under mild conditions. Occasionally, Ni(IV) intermediates were also proposed. Apart from the vast number of Ni(II) complexes, a number of organonickel(I) complexes were characterized by X-ray crystallography and their potency as active species in catalytic cycles tested [8-10]. Either radical or two-electron reactivity was observed. Recently, the structure of some alkylnickel(III) complexes was also structurally elucidated [11]. 

More recently Sternberg (8) has studied the dissolution of coal by reductive alkylation, which Involves a variety of reactions, Including free radical and elimination reactions. 

There are essentially two possibilities to accomplish two-electron or multielectron transfer at metal complexes without formation of one-electron transfer intermediates (e.g., radicals). Appropriate metal centers should have available stable oxidation states that differ by at least two emits. T5rpical examples that represent such photoredox reactions are reductive eliminations such as (X = halide, pseudohalide) (5) ... 

In the absence of the activating second carbonyl functionality, it is necessary to use more ingenious methods to produce the same net effect. These procedures more often than not involve radical reactions. Among them is the thermolysis of tert-butyl esters of peroxyacids 437, which are readily synthesized in a standard esterification of tert-butyl hydroperoxide with an acid chloride. Decarboxylation proceeds via an initial homolytic cleavage of the 0-0 bond, elimination of CO2, and reduction of the incipient alkyl radical by an added hydrogen atom donor such as 438 (Scheme 2.143). Examples showing the exceptional synthetic importance of this decarboxylation procedure will be presented later. 

Furthermore, this methodology was developed to afford the synthesis of e t-lycoricidine (248) using photolysis instead of a chemically mediated radical reaction (160). Photolysis of 4-(4/ )-hydroxy-6-[(2-methoxycar-bonyl-4,5-methylenedioxy)phenyl]-25,35-0-isopropylidenehex-5-ynal O-benzyloxime 245 (derived from D-glucose) in the presence of thiophenol under went intramolecular cyclization to produce l-(A-benzyloxy-amino)-5-phenylthiocyclohex-5-ene 246. Cleavage of the N-O bond and reductive elimination of the phenylthio group in 246 with Sml2 in THF... 

TAME, DIPE and ETBE were investigated in the study by Kim et al. [ 109]. They showed similar elimination rates. If pyrolysis were the main elimination mechanism, the volatility and thus the partitioning into the gas phase during cavitation would be of much influence. MTBE, ETBE, TAME and DIPE have different vapor pressures (345, 183, 105, 222mbar, respectively [109]), however, under the studies conditions no decisive difference in ehmination rates could be observed—another hint for the predominance of the radical-induced elimination path over direct pyrolysis. The addition of ozone to the reaction mixture resulted in an enhancement of the elimination rate for ETBE by the factor of 1.5 and for TAME of 2.1 [110]. The mixture of different ethers proved to yield a slower reduction rate than the ethers alone. 

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