Water elimination and rearrangement

In basic solution, Ura and Thy undergo a series of reactions as depicted in reactions (72)—(76) for Ura as an example (Fujita and Steenken 1981). Ura dissociates athighpH [equilibrium (72) for pfCa values see Table 10.11], Its OH-adductscan also be deprotonated at nitrogen leading to an oxidizing heteroatom-centered radical [reaction (76)]. 

The C(4)- OH-adduct of dGuo has oxidizing properties (note the mesomer-ic form with spin density at 0(6)). It eliminates water yielding the even more strongly oxidizing G [reaction (77) k = 6 x 103 s 1 in the absence of buffer Can-deias and Steenken 2000], 

A similar reaction has been suggested for the of dAdo that gives rise to A [reaction (78) k = 1.9 x 104s-1]. Again, the C(4)- OH-adduct is less oxidizing than A and reacts with TMPD with 2.3 x 107 dm3 mol-1 s 1, while A reacts with 2 x 109 dm3 mol-1 s-1 (Vieira and Steenken 1990). 

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Chemical Properties. Reactions of quaternaries can be categorized iato three types (169) Hoffman eliminations, displacements, and rearrangements. Thermal decomposition of a quaternary ammonium hydroxide to an alkene, tertiary amine, and water is known as the Hoffman elimination (eq. la) (170). This reaction has not been used extensively to prepare olefins. Some cycHc olefins, however, are best prepared this way (171). Exhaustive methylation, followed by elimination, is known as the Hoffman degradation and is important ia the stmctural determination of unknown amines, especially for alkaloids (qv) (172). 

The mechanism of the rearrangement is explained as shown in Scheme 19. Protonation of the 9-hydroxy group followed by its elimination and subsequent chloride attack at the 4a-carbon generates a chloroindolenine 126. Addition of water to the 9a-imine carbon atom of 126 gives 127. Concerted elimination of the chloride with rearrangement of the alkyl side chain attached to the 9a carbon atom results in 3,3-disubstituted oxindole structure 120a. 

Zwanenburg and Wagenaar148 have reported the rather unusual rearrangement of sulfone 81 to 82, after standing overnight at 0°, and suggested an elimination-addition mechanism, via initial isomerization of A3 to the A2-thiazoline-oxide with subsequent elimination and readdition of sulfmic acid, followed by spontaneous loss of water in a Pummerer-type aromatization reaction. 

Hematite forms by a combination of aggregation-dehydration-rearrangement process for which the presence of water appears essential. Structural details about this process at 92 °C were obtained from EXAFS (Combes et al. 1989 1990) face-sharing between Fe octahedra developed before XRD showed any evidence for hematite. It is followed by internal redistribution of vacancies in the anion framework and by further dehydration. The dehydration process involves removal of a proton from an OH group and this in turn leads to elimination of a water molecule and formation of an 0X0 linkage. The local charge inbalance caused by proton loss is compensated for by migration and redistribution of Fe " within the cation sublattice. 

Dihydro-1,2-dioxines (195) (available from dienes and 02) can be transformed into furans (197) by reduction and rearrangement (into 196) and subsequent elimination of water, usually by treatment with base. In some cases Co(II) and Fe(II) catalysts have been used (76TL4363, 77JOC1900, 77SCH9, 82ACS(B)31, 89JOC3475). 

Chemical Properties. Reactions of quaternaries can be categorized into three types Hoffman eliminations, displacements, and rearrangements. Thermal decomposition of a quaternary ammonium hydroxide to an alkene, tertiary amine, and water is known as the Hoffman elimination (eq. la). 

The reaction of arylsulfonyl-substituted sulfines (175) with diazomethane gives A3-l,3,4-thiadiazoline 1-oxides (176) which, however, are unstable and rearrange via an isomerization of the A3- to the A-thiadiazoline oxide. This is followed by an elimination and readdition of sulfinic acid and loss of water in a Pummerer-type aromatization to give the rearranged thiadiazole (177 Scheme 24) (73TL5009). 

As the OH addition reactions are controlled by kinetics rather than by thermodynamics, the primary OH adduct radicals are not necessarily the thermodynamically most stable radicals. Thus, a number of rearrangement and water elimination reactions take place. For example, Thy60H is more stable than ThySOH by 42 kj mol but Thy60H is formed preferentially. As has been shown in the case of 1,3-dimethyluracil in a detailed product study, acid catalysis converts the kinetically favored C6 OH adduct into the... 

Naumov S, von Sonntag C. (2008) The energetics of rearrangement and water elimination reactions in the radiolysis of the DNA bases in aqueous solution (e and OH attack). DFT calculations. Radiat Res 169 355-363. 

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