Hammett correlation oxidation

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

Few other reactions of series of substituted pyridines have been investigated extensively. Dondoni, Modena, and Todesco have measured the rate of N-oxidation of a limited series of pyridines and found a good correlation with normal u-values with a p-value of — 2.23. The A-alkylation of pyridines with alkyl iodides in nitrobenzene has been studied by Brown and Cahn and by Clarke and Rothwell. Unfortunately, the only data available are for the parent compound and for alkyl derivatives, and, since the a-values for the various alkyl groups in a given position are substantially constant, this leaves a correlation of only three independent points. However, the rates of A-alkylation of the j8- and y-alkyl derivatives are so nearly equal that it appears as if no correlation existed. Clarke and Rothwell have also studied the alkylation with allyl bromide in nitromethane at various temperatures, and in this case a more extensive series is available. The authors state that no overall Hammett correlation is obtained however, the j8-substituted derivatives fall on one straight line and the y-derivatives on another one with a different slope. The data are shown in Fig. 2. The line for the j8-compounds, p = — 2.53 0.31, r = 0.95, is seen not to be very good the line for the y-derivatives, p = — 1.42 0.06, r = 0.99, is much more satisfactory. It does not seem likely that the discrepancy is due to the intervention of resonance effects, since in this case one would expect the correlation for the y-derivatives to be poorer than that for the j8-analogs. More extensive studies with a wider variety of substituents would seem very desirable. 

The single-electron reduction and oxidation of Co(salen) is solvent dependent as a result of the available coordination sites perpendicular to the CoN202 plane.1220 Furthermore, substituents on the phenyl rings modulate the observed redox potentials and subsequently the 02 binding constants. Hammett correlations are obtained.1221 Potentiometric titrations were performed to determine the 02 binding constants and species distribution as a function of pH for a variety of Schiff base Co complexes.1222... 

Another method capable of inducing a TEMPO-catalysed oxidation is the one reported by Minisci and coworkers, which exploits Mn(II)-Co(II) nitrates in AcOH solution at 20-40 °C in the presence of 02 °. Primary and secondary alcohols, including aliphatic ones, are oxidized very efficiently. This procedure gave a good Hammett correlation vs. a in the oxidation of X-substituted benzyl alcohols with a large p value of —1.44... 

A number of substrates having a benzylic ether moiety were reacted with 51 to afford the corresponding benzylic esters in good yields (equation 84). For evaluating the effects of p-substiments on the oxidation of a series of benzylic ethers, a competitive oxidation of p-substimted benzylic propyl ethers with 51 was carried out. The Hammett correlation plot for the oxidation reaction gave a better correlation of the relative ratio factors with the a rather than with the a+ substituent constants and afforded a reaction constant p+ = —0.57 (r = 0.99). This p+ value shows that 51 is an electrophilic species and appears to be comparable to the p+ value of —0.65 for benzylic hydrogen abstraction from dibenzyl ethers by the benzoyloxy radicaP . 

Oxidation of Cu with O2 is markedly slower than the analogous oxidation by TEMPO. The existence of a copper-centered dehydrogenation step was supported by investigating kinetic isotope effects and Hammett correlation studies using different substituted benzyhc alcohols. The /1-hydrogen abstraction was postulated to occur in a concerted mechanism with an -coordinated TEMPO radical bgand (Scheme 4). As such, this TEMPO-mediated copper-catalyzed oxidation of alcohols bears resemblance... 

The pK values for pyridine 1-oxides are lower than those for the corresponding pyridines (Table 1). Electronic effects of substituents on pKa values of yV-oxides are in the same direction as in the pyridines but smaller in magnitude. This has been quantified by a Hammett correlation for the pKa values of 3- and 4-substituted pyridine 1-oxides that yields a p value of 2.09, which compares with one of 5.71 for a series of similarly substituted pyridines. 

In some cases, the effect of reactant structure may outweigh the influence of catalyst nature. This is seen by comparison with the dehydration of aliphatic secondary alcohols and substituted 2-phenylethanols on four different oxide catalysts (Table 4). With aliphatic alcohols, the slope of the Taft correlation depended on the nature of the catalyst (A1203 + NaOH 1.2, Zr02 0.3, Ti02—0.8, Si02—2.8 [55]) whereas for 2-phenyl-ethanols, the slope of the corresponding Hammett correlation had practically the same value (from —2.1 to —2.4) for all catalysts of this series [95]. The resonance stabilisation of an intermediate with a positive charge on Ca clearly predominates over other influences. 

The half-wave potentials for polarographic reduction of 2- and 3-substituted phenazines and phenazine-A -oxides follow a Hammett correlation only for the latter substituents,206 although independent investigations on phenazines afford a general correspondence.207-210... 

The Hammett correlation for substituted carboxylic acids is demonstrated in Figure 5.26. The Hammett constant om fails for aliphatic compounds, and the derived constant o must be used to predict accurate Hammett correlations. The least-substituted carboxylic acid, formic acid, was used as the reference compound. The Hammett correlation for substituted carboxylic acids (CAs) demonstrates that the CAs substituted by electron-withdrawing substituents, such as Cl, oxidize the fastest. CAs substituted by electron-donating groups, such as CH3 and NH2, oxidize more slowly than those substituted by electron-withdrawing substituents. The reaction pathway for substituted carboxylic acids is shown in Figure 5.27. These trends are different for phenols and alkanes, because the reaction site is at the election pair located at the oxygen atom. 

The o scale was developed to incorporate the effect of through-resonance. Both types of correlations included ortho- as well as meta- and para-substituted phenoxides, and the only outliers are compounds that exhibit strong intramolecular hydrogen bonding because this effect is not incorporated in most molecular descriptors. The rate constants for oxidation of phenoxide anions give good Hammett correlations to cr constants, as shown in the following equation ... 

The Hammett correlation for nitrobenzenes is almost identical to the correlation at pH 9 therefore, the degradation of substituted benzenes at pH 3 can also be described by the same hole oxidation mechanism. Figure 9.21 demonstrates the oxidation of nitrobenzene by positive hole. At pH 3, substituted benzenes are oxidized by the formation of a positive hole. An electron transfer from nitrobenzene to Ti02 creates this positive hole. 

The Hammett correlations for substituted benzenes were similar for elementary hydroxyl radical reaction and UV/Ti02 because both correlations have negative slopes. The similarity of the correlations may be due to the same oxidation species, such as hydroxyl radical. For both reactions, the benzenes substituted with electron withdrawing groups were difficult to degrade. 

Quantitative structure-activity relationship models can be used to predict kinetic rate constants for compounds similar in chemical structure under UV /TiOz photocatalytic oxidation. In formulating the Hammett correlations, the descriptors, such as on s and o, proved to be successful in deriving equations for aromatic and aliphatic compounds respectively. For the case of aromatic compounds, the ores descriptor formulated better Hammett correlations than the om descriptor. 

The classic reaction catalyzed by peroxidases is the one-electron oxidation of phenols. The guaiacol assay for peroxidase activity, an assay that derives from the earliest observation of peroxidase activity [70], involves the one-electron oxidation of guaiacol to a free radical that undergoes subsequent radical-radical combination to give a colored dimeric product (Fig. 5.13) [71]. The ability of substituted phenols to act as substrates and reduce HRP Compound I is controlled by the electronic nature of the substituent [72], At pH 7.0, the linear Hammett correlation observed between the substituent a value and the rate of Compound I reduction adheres to the equation log kx/ku = 6.92a, where kx is the rate of the... 

The stoichiometry of the oxidations was examined with the most reactive of the dyes used in the present study, l-(4-methoxyphenylazo)-2-naphthol-6-sulfonate, to minimise problems from the slow self-reaction of 1. This revealed that the reaction requires four oxoiron(IV) species to oxidise each molecule of dye. The consequence of this result is that the true k2 value for the reduction of 1 by an azonaphthol dye in this study is a quarter of the value shown in Table 1. This correction does not, however, affect the Hammett correlations and p values. 

Examination of the measured second order rate constants for the dye oxidations shows that the reaction is significantly favoured by electron-donating substituents the 4-methoxylated compound (9) reacts 274 times faster than the 4-nitro analogue (3). Furthermore, the excellent linear Hammett correlation of log(second order rate constant) with and the p value of-1.66 confirm this conclusion and show that all the substrates are oxidised by the same mechanism. However, the size of the p value is larger than that obtained from the earlier oxidation of phenols (p -1.10) and is larger than expected for a mechanism involving H atom abstraction in the rate determining step. A possible explanation is that the substituents have two effects on the reaction which reinforce each other they influence H atom abstraction by the electrophilic oxidant, as noted previously this is favoured by electron-donation, and they shift the position of the azo-hydrazone equilibrium. 

This catalyst is exceptionally active and allows the Heck reaction to be performed on iodobenzene at 30 °C. The catalyst is inhibited by addition of a 300-fold excess of Hg. Hammett correlation of the reaction on pora-substituted bromobenzenes gave a p = 2.7, which is in accord with an oxidative addition on Pd(0).TEM showed the presence of nanoparticles in a solution of the catalyst which was pre-treated with n-butyl acrylate, NaOAc and TBAB. After addition of iodobenzene nanoparticles could no longer be detected showing that the oxidative addition of iodobenzene to the colloids is indeed a very fast reaction. Reaction with haloarenes and acrylate esters attached to a solid support gave the Heck products in excellent yields, proving that the catalyst is not heterogeneous in nature. 

Hammett correlations for styrene epoxidations have been reported for a variety of metal-catalyzed oxidations. In iron porphyrin systems, p values between -0.83 and -0.94 have been reported for PhIO reactions.Groves and Watanabe l reported a p value of -1.9 for styrene epoxidations where the intermediate is presumed to be the iron(IV) 0x0 porphyrin cation radical, generated by reaction of Fe(TMP)Cl with MCPBA at -50 °C. It is noteworthy that p values for reactions of iodosylbenzene and hypochlorite catalyzed by Fe(TDCPP)Cl were found by Collman et al. not to be identical. The p values for PFIB (pentafluoroiodosylbenzene) and PhIO were -0.86 and -0.91, respectively, whereas the p value for LiOCl plus a phase transfer catalyst was -0.57. Those authors suggested that either different oxidants were formed in the iodosylbenzene and hypochlorite reactions or differences in the reaction conditions were responsible for the different p values. In our reactions, the smaller p value for the H2O2 reaction relative to those for the PhIO and MCPBA reactions suggests that different intermediates are responsible for the oxygenations. 

The oxidative addition of p-substituted aryl iodidesf f ° and triflates to Pd°(PPh3>4 follows a Hammett correlation with a positive slope and is thus faster for aryl derivatives substituted by electron-withdrawing groups. The following order of reactivity has been established in DMF at 20... 

Pd°(dba)2 + nP(p-Z-CgH4)3 mixtures behave as for PPh3 (Scheme The values of the equilibrium constants (Eq. 4 of Scheme 3) have been determined as a function of the Z substituent. For a given value of n(n> 2), the equilibrium between Pd°(dba)L2 and Pd°L3 lies more in favor of Pd°(dba)L2 when the phosphine is more basic (Hammett correlation for with p = + 4). Similarly, when n = 2, Pd°L2 is less easily released from Pd°(dba)L2 (Eq. 2 of Scheme 3) when the phosphine is more basic. The oxidative addition involves Pd°L2 as the reactive species (Schemae 4), which is more reactive when the phosphine is more basic. Thus, two opposite effects are at hand when the basicity of the phosphine is varied, so that the resulting overall rate constants do not follow linear Hanunett correlations. Instead, bell-shaped curves are obtained whose maximum value and position depend on n, that is, on the phosphine concentration (Figure... 

The oxidative addition of the palladium(O) complexes generated from Pd(OAc)2 and P(p-Z-C6H4)3 follows a linear Hammett correlation with a negative slope (p = -2.8) establishing, as expected, that the more basic the phosphine, the more reactive the palla-dium(O) complex. ... 

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