Hammett correlations rearrangement

The role of substituents X on the mononuclear heterocyclic rearrangement (MHR) of 20 phenylhydrazones 54 of 3-benzoyl-5-phenyl-l,2,4-oxadiazole into the triazoles 55 (Equation 2) has been investigated, allowing the influence of X on the product distribution to be evaluated and first-order rate constants and Hammett correlations to be determined <1999T12885>. 

Products isolated from the thermal fragmentation of A-arylbenzamide oximes and A-arylbenzamide O-phenylsulfonyl oximes have been accounted for by invoking a free-radical mechanism which is initiated by the preferential homolysis of the N-O bond." Time-resolved IR spectroscopy has revealed that photolysis of A, A -diphenyl-l,5-dihydroxy-9,10-anthraquinone diimine affords acridine-condensed aromatic products via excited-state intramolecular proton transfer." The absolute and relative rates of thermal rearrangements of substituted benzyl isocyanides have been measured,and it has been found that the relative rates are independent of temperature and exhibit excellent Hammett correlations. Thionitrosoarene (25), thought to be generated by desulfurization of the stable A-thiosulfinylaniline (24), has been established" " as an intermediate in the formation of 3,3a-dihydro-2,l-benzisothiazole (26) from o-alkylthionitrosoarene (24). 

White et al.1A have obtained similar kinetic results for the acid-catalysed rearrangement of N-nitro-N-methylaniline, i.e. a first-order dependence on the nitroamine with a linear H0 plot of slope 1.19 for phosphoric acid, and a deuterium solvent isotope effect of about three, although the results have only been presented in preliminary form. Further, an excellent Hammett a+ correlation was claimed for thirteen para substituted nitroamines which gave a p value of —3.9. Since it is expected that the rate coefficients would correlate with a (rather than different basicities of the amines, the a+ correlation implies that the amino nitrogen is electron-deficient in the transition state,... 

The one-electron oxidation of iV-benzylphenothiazine by nitric acid occurs in the presence of /i-cyclodextrin, which stabilizes the radical cation by incorporation into its cavity. The reaction is inhibited by adamantane, which preferentially occupies the cavity. Novel Pummerer-type rearrangements of / -sulfinylphenyl derivatives, yielding /7-quinones and protected dihydroquinones, and highly enantioselective Pummerer-type rearrangements of chiral, non-racemic sulfoxides have been reviewed. A comprehensive study has demonstrated that the redox potential for 7- and 8-substituted flavins is linearly correlated with Hammett a values. DFT calculations in [3.3.n]pro-pellanes highlight low ionization potentials that favour SET oxidative cleavage of the strained central C-C bond rather than direct C-H or C-C bond attack. Oxidations and reductions in water have been reviewed. ... 

The values presented in the second column show the quantum yields of the aryl ester conversion and can be correlated with other values in Table V, only because in ethanol at 20°, 70-80% of the ester is converted to ortho-rearranged product.59 It was shown that the given data for para-substituted esters could be also correlated with Hammett s a constants (Eq. 2). 

The Hammett cr—p correlation has proved useful in studying substituent effects of aryl-substituted systems.55 In Section 6.1 we consider applications of this linear free-energy relationship to solvolysis with rearrangement. 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

Pearson et al. (1952) employed this approach to derive a series of substituent parameters for electron-deficient reactions of substituted benzenes. These constants, designated as sigmae, were based on a study of the Beckmann rearrangement of -substituted acetophenone oximes. These authors considered the rates of the rearrangement reaction of the oximes to deviate from the Hammett eq. (1). It is pertinent that, with the sole exception of the yi-OMe group, the deviations were not major. The entropy of activation, AS, for the -anisyl derivative was, however, 20 e.u. different from the essentially constant values for the other substituents. To remedy the deviations, Pearson and his associates suggested the sigmae constants. It was indicated that these constants were more suitable for the correlations of electron-deficient reactions than the conventional cr-values. 

The following table lists the most common substituents encountered in benzene rings and the neutral particles lost and observed on the mass spectrum.1 Complex rearrangements are often encountered and enhanced by the presence of one or more heteroatomic substituent(s) in the aromatic compound. All neutral particles that are not the product of rearrangement appear in parentheses and are produced alongside the species that are formed via rearrangement. Prediction of the more abundant moiety is not easy, as it is seriously affected by factors that dictate the nature of the compound. These include the nature and the position of any other substituents, as well as the stability of any intermediate(s) formed. Correlations of the data with the corresponding Hammett a constants have been neither consistent nor conclusive. 

Nevertheless, rearrangements of m- and /i-substituted benzoic acids were sensitive to polar influences as shown by their correlation in a Hammett relationship with p-T76 and since the decomposition of aryl azides was insensitive to such substitution (cf. section IV) it is tempting to consider both the sensitivity to polar groups and the higher... 

The l-aryl-2-methyl-2-propyl system (384) serves as a model for anchimerically unassisted behavior. The rate constants are well correlated with the regular Hammett a constants a value of p close to -1 indicates little if any charge delocalization into the aryl ring297, 316 No rearranged products are found317). 

These arguments do not invalidate the immense array of linear free energy relationships gathered over the past 70 years or so. Class II systems such as Hammett or Taft relationships can be excluded from these considerations because the two variables are independent. The following arguments can be employed to demonstrate experimentally that a Class I correlation such as a Bronsted or a Leffler Equation does not arise from a statistical artifact in a system under investigation. Rearranging the Leffler Equation (Equation 27) yields Equation (29). 

The rate of thermal rearrangement of 3-aryl-2,2-dimethylmethylene-cyclopropanes has been studied as a function of aryl substiments. Some of the data are given below. Examine the rate data for correlation with the Hammett a-substituent constants. What conclusion do you draw about the mechanism ... 

---