Catalysis heterolysis

Two other theories as to the mechanism of the benzidine rearrangement have been advocated at various times. The first is the rc-complex mechanism first put forward and subsequently argued by Dewar (see ref. 1 pp 333-343). The theory is based on the heterolysis of the mono-protonated hydrazo compound to form a n-complex, i.e. the formation of a delocalised covalent it bond between the two rings which are held parallel to each other. The rings are free to rotate and product formation is thought of as occurring by formation of a localised a-bond between appropriate centres. Originally the mechanism was proposed for the one-proton catalysis but was later modified as in (18) to include two-protons, viz. 

In halogenated solvents, catalysis by a second bromine molecule, which assists the Br—Br bond heterolysis, is the main driving force. The role of the solvent is electrostatic, but the absence of an extensive Kirkwood relationship suggests that there is some other kind of contribution (Bellucci et al, 1985b). 

All schemes presented are similar and conventional to a great extent. It is characteristic that the epoxidation catalysis also results in the heterolytic decomposition of hydroperoxides (see Section 10.1.4) during which heterolysis of the O—O bond also occurs. Thus, there are no serious doubts that it occurs in the internal coordination sphere of the metal catalyst. However, its specific mechanism and the structure of the unstable catalyst complexes that formed are unclear. The activation energy of epoxidation is lower than that of the catalytic decomposition of hydroperoxides therefore, the yield of oxide per consumed hydroperoxide decreases with the increase in temperature. 

At pH < 7 the nitroxyl radicals do not undergo an observable heterolysis (khs 10 s ), but decay by bimolecular reactions. However, in basic solution an OH -catalyzed heterolysis takes place to yield the radical anion of the nitrobenzene and an oxidized pyrimidine. In the case of the nitroxyls substituted at N(l) by H (i.e. those derived from the free bases), the OH catalysis involves deprotonation at N(l) which is adjacent to the reaction site [= C(6)] (cf. Eq. 15) [26] ... 

Deprotonation provides the necessary electron push to kick out the electron pair joining C(6) with the nitrobenzene oxygen. If, however, N(l) is alkylated (as with the nucleosides and nucleotides), OH catalysis is much less efficient since it now proceeds by deprotonation from N(3) (with the uracils) or from the amino group at C(4) (with the cytosines). In these cases the area of deprotonation is separated from the reaction site by a (hydroxy)methylene group which means that the increase in electron density that results from deprotonation at N(3) is transferable to the reaction site only through the carbon skeleton (inductive effect), which is of course inefficient as compared to the electron-pair donation from N(l) (mesomeric effect) [26]. Reaction 15 is a 1 1 model for the catalytic effect of OH on the heterolysis of peroxyl radicals from pyrimidine-6-yl radicals (see Sect. 2.4). 

Solvolyses of the A(A -diphenylcarbamoylpyridinium ion (126) were found to be subject to specific and/or general base catalysis, which could be eliminated by addition of perchloric acid or increased, especially in fluoroalcohol-containing solvents, by addition of pyridine. The uncatalysed solvolyses in aqueous methanol and aqueous ethanol involve a weakly nucleophilically assisted (/ = 0.22) heterolysis and the solvolyses in the pure alcohols are anomalously slow. ... 

Mechanistically, the authors favored a thiourea 9-assisted heterolysis of the orthoester through hydrogen bonding as the entry into the catalysis cycle of the organocatalytic acetalization. The orthoester was suggested to serve as the source of the alcoholate, which rapidly attacks the carbonyl compound to form a... 

Hudson et a/.151,152 have concluded that the bimolecular solvolysis of ethyl chloroformate involves heterolysis of the carbon-chlorine bond and not heterolysis of the carbon-oxygen bond. Their data shows that the hydrolysis of ethyl chloroformate is a second-order reaction in water/acetone mixtures, methyl chloroformate reacting about 2.2 times as fast in 65% water/acetone at 50°C. Hydroxide ion accelerates the reaction (3.1 x 107 in 18% water/ acetone and 3.4 x 108 in 85% water/acetone) and catalysis by hydroxide ion was observed with pure water as solvent by Hall118. There is some disagreement about the value for the hydrolysis rate coefficient for ethyl chloroformate in water and in other solvents (Table 21). To date, the data of Queen153 (for pure water), Kivinen92 (for ethanol) and Liemiu101 (for methanol) must be considered the most accurate. 

If this reaction is carried out in the presence of alcohols, homogeneous catalysis of silane alcoholysis occurs, as will be discussed below. Heterolysis of Et3SiH in the highly electrophilic complex cis-Re(CO)4(PR3)(ri2 -HSiEt3)] [A] (R = Ph, Cy) occurs (Scheme 8) (85) much... 

Robinson, 1969a). It is probable that the hydrophobic nature of the phenyl groups of p-nitrophenyl diphenyl phosphate results in deep penetration of the neutral ester in the Stern layer, thus shielding the phosphoryl group from nucleophilic attack. Unlike other reactions between nucleophiles and neutral substrates catalyzed by cationic micelles (Bunton and Robinson, 1968, 1969a) and the hydrolysis of dinitrophenyl phosphate dianions in the presence of cationic micelles (Bunton et al., 1968), the catalysis of the hydrolysis of -nitrophenyl diphenyl phosphate by CTAB arises from an increase in the activation entropy rather than from a decrease in the enthalpy of activation. The Arrhenius parameters for the micelle-catalyzed and inhibited reactions are most probably manifestations of the extensive solubilization of this substrate. However, these parameters can be composites of those for the micellar and non-micellar reactions and the eifects of temperature on the micelles themselves are not known. Interpretation of the factors which affect these parameters must therefore be carried out with caution. In addition, the inhibition of the micelle-catalyzed reactions by added electrolytes has been observed (Bunton and Robinson, 1969a Bunton et al., 1969, 1970) and, as in the cases of other anion-molecule reactions and the heterolysis of dinitrophenyl phosphate dianions, can be reasonably attributed to the exclusion of the nucleophile by the anion of the added salt. 

In the case of good anionic leaving groups, such as phosphate cf. [38], sulfate [39], or halogen [40], these can be eliminated directly by bond heterolysis. These eliminations can be very fast even in the absence of base catalysis, the rate depending on the state of protonation of the leaving group (Table 2). 

A putative Cu -oxo species, formed from an acido-basic catalysis or from the heterolysis of the 0-0 bound of a Cu -hydroperoxo intermediate as in Eq. (23) of Fig. 15, was also proposed although its orbital populating seems unfavorable 1ST). Eqs. (24), (25) summarize the pathway that could be involved with this species during DNA oxidation events. A hydrogen atom abstraction on the DNA by the Cu -oxo species would produce a radical on DNA and a Cu -hydroxo species [Eq. (24)]. Then an eventual electron transfer between them may allow the oxidation of the radical to a cation and the regeneration of the initial Cu complex. 

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