Electron-donating substituents Electronic parameters

Since HF calculations have a tendency to underestimate the N—N and the C—N bond lengths in triazoles [98JPC(A)620, 98JPC(A) 10348], the structural parameters should be computed at least at the DFT or MP2 levels. This is particularly true if electron-donating substituents are attached to the ring. Nitrogen NMR shielding tensors were computed for a set of methylated triazoles and tetrazoles but will be discussed in the context of tetrazoles (cf. Section IV,B). 

In the oxidation of alkanethiols to disulfides with chloramine-T (CAT), in alkaline solution, the proposed reactive species are hypochlorous acid and TsNCl- anion. A correlation of reaction rate with Taft s dual substituent parameter equation yielded p = -5.28 and 5 = -2.0, indicating the rate-enhancing effect of electron-donating substituents.133 Michaelis-Menten-type kinetics have been observed in the oxidation of atenolol with CAT in alkaline solutions. TsNHCl is assumed to be reactive species. A mechanism has been suggested and the activation parameters for the rate-determining step were calculated.134 The Ru(III)-catalysed oxidation of diphenyl... 

Rates of oxidation of para-substituted arylphosphines with singlet oxygen show good correlation with the Hammett a parameter (p = —1.53) and with the Tolman electronic parameter. The only products are the corresponding phosphine oxides. However, for ortho-substituted phosphines with electron-donating substituents, there are two products, namely a phosphinate formed by intramolecular insertion and phosphine oxide. Kinetic analyses demonstrated that both products are formed from the same intermediate, a phosphadioxirane. VT NMR experiments showed that perox-idic intermediates can only be detected for highly hindered and very electron-rich arylphosphines 243... 

Kinetics of reaction of the di- and trisubstituted acids 7a, 7c and 7d with anthocyanins were moderately enhanced compared to 7e (Figure 5). With the amino moiety in 7b, however, rate of formation was accelerated by a factor larger than 100 (data not shown). These observed reactivities coincide with the expected order of reaction rates for para-substituent effects when electron-deficient transition states are involved, as given by Brown s af constants 27,28). It is known that electron-donating substituents can accelerate reactions involving the olefinic double bond of cinnamates. Negative Hammett reaction parameters p have been obtained,... 

The flash photolysis of benzothiazolino-spirochromenes (207 R = Me, CH Ph, Ph, OMe, SMe, etc.-, R = Me, Et, or Pr R = H, SMe, NO2, or OMe R = OMe, H, CHiOMe, CHjOCHaCHaOCHaCHjMe, or CHiSEt) was studied. The electron-donating substituents decrease the extent of photolysis. Hammett correlations involving inductive, resonance, and steric parameters were estabhshed for three compounds the steric effect is the most important polar solvents decrease the extent of photolysis. The kinetics of A -methylation of substituted 2-(2-furanyl)benzothiazoles have been determined spectrophotometrically at 80°C. The electron-donating methyl group accelerates the reaction, whereas the halogens have the opposite effect. 

On the basis of the linear correlations between Vmin and ABDE for electron-accepting substituents and p max and ABDE for electron-donating substituents, we decided to investigate if a dual parameter relationship of the following type could correlate the ABDE of all phenols ... 

This probably because the cytotoxicities of the more unstable compounds (with the most negative c values) are underestimated, due to rapid inactivation of drug by hydrolysis during the Ih exposure, and these effects are fortuitously compensated for by the fact that the c parameter scale terminates at ca. -0.16 for all highly electron-donating substituents. 

It is known that carbonyl oxides are predominantly formed at carbon atoms with electron-donating substituents [19]. Excellent correlations of the regioselectivities of MO fiiagmentation with electron donation by substituents (as measured by Hammett and Taft parameters) have been obtained, consistent with the effects expected for stabilization of a zwite-rionic carbonyl oxide [20], According to Ref. [23], for polyisoprenes the ratio between the two intermediates, DCI and MCI, is 64 36. 

Likewise, Kishan and Sundaram (1985, 1980) report that substituted phenacyl bromides (CgHjCOBr) are oxidized (in 40-70% acetic acid/H2S04) without complex formation. The reaction is catalyzed by acid, but the pseudo-first-order fits depend on the initial [Ce(lV)]. This observation is attributed to the presence of an unreactive Ce(IV) trimer, which has been reported to be present in acetic acid solutions. The rate of reaction of phenacyl bromides substituted with either electron-withdrawing or electron-donating substituents is faster than that of unsubstituted phenacyl bromide. The activation parameters for the p-methyl and p-methoxyl substituents suggest that a different mechanism operates for these systems. Where comparable substituents exist, the rates for oxidation of phenacyl bromides compare favorably with those for benzaldehyde dted above. 

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