Hammett constants reactions, benzyl

Gas-phase nucleophilic substitution reactions of Y-benzyl chlorides and X-phenoxide or X-thiophenoxide nucleophiles have been investigated by using the PM3 semiempirical MO method. The structure of the transition state was examined. The values of the gas-phase Hammett constants px and py are much greater than for the solution reactions, but a theoretical cross-interaction constant pxy (ca —0.60 for both phenoxides and thiophenoxides) agrees well with an experimental value of —0.62 for the thiophenoxide reactions in MeOH at 20 °C. Other work by the same group has involved theoretical studies of competitive gas-phase 5 n2 and E2 reactions of NCCH2CH2CI with HO and An ab initio method at the 6-31-l-G level was... 

Oxidation with ozone, under physiological conditions, follows the rate order uric acid ascorbic acid > glutathione. The amounts of ozone absorbed and antioxidant consumed have been simulated with a mathematical model and reaction rate constants of the oxidations have been evaluated.194 Various facets of transition metal-catalysed oxidation of benzylic compounds with ozone have been reported. The correlation of the effect of substituents with Hammett constants and steric factors has been discussed. The reaction seemed to proceed via a radical mechanism.195... 

The addition of two ortho-methyl groups to cumyl derivatives also results in a decrease in the Hammett reaction constant from p+ = —4.3 for cleavage of X-[8]-(4-nitrobenzoate) to p+ = —3.0 for cleavage of X-[10]-(4-methoxy-benzoate).27 This provides additional support for the conclusion that rotation about the CAr—Ca bond, which will minimize destabilizing steric interactions between methyl groups in the transition state, also reduces stabilizing resonance electron donation from the ring substituent to the benzylic carbon (Scheme 9). 

The ion pairing between the enolate of 5 and the catalyst should make the asymmetric induction sensitive to the electronic effects of substituents on the N-benzyl group. A Hammett plot of log ee/eeQ vs the substituent constant o of the para N-benzyl substituted catalysts (R = CH3O, CH3 H, F, Cl, CF3) gave a reaction constant of p - 0.21 + 0.02 with ee s in the range of 60% to 92% demonstrating that substituents with increasing... 

In order to confirm the radical character of 51 and to extend its utility, oxidations of ary-lacetic acids to the corresponding ketones, aldehydes or alcohols have been conducted. Competitive decarboxylation reactions of phenylacetic acid and p-substituted phenylacetic acids were carried out. The ratio of the rate constants for the decarboxylation of various substituted phenylacetic acids relative to that of phenylacetic acid was found to decrease on decreasing the electron density at the benzylic carbon. Consequently, compound 51 shows an electrophilic oxidation ability towards arylacetic acids, giving a Hammett p value of —0.408. 

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 . 

HA compounds is not necessary for the formation of a polyester. Nevertheless, an acceleration effect of HA compounds on the rate of copolymerization was detected later 36 57 74), even for systems in which proton donors are directly bound to monomers 67). This effect is not the sum of the contributions from the tertiary amine and the proton donor but even stronger. Hence, proton donors display a cocatalytic effect. Concerning the effect of HA compounds Tanaka and Kakiuchi 36) established a linear correlation between Hammett s ct constants and the logarithm of the gelation time for various substituted derivatives of benzoic acid, benzyl alcohol and phenol, and positive reaction parameters q were found in all cases. This means that electron-withdrawing substituents increase the effect of HA compounds, or their effect becomes more pronounced with increasing hydrogen atom acidity. 

There are five papers on carbamate chemistry of interest.6,84-87 The mechanism of the reaction in MeCN of /V-mcthyI-/V-phcnyIcarbamoyI chlorides (94) with benzyl-amines is believed to be Sf2 based on Hammett p values, a cross-interaction constant pxy of —0.14, ka/ko values for the /V-dcutcriatcd benzylamines all <1, and low activation enthalpies.84 The aminolysis of /7-nitrophcnyl /V-phenylcarbamatcs in acetonitrile involving the T1 (7) was discussed earlier.6 Solvolysis-decomposition of N- -adamantyl-/V-/7-tolylcarbamoyl chloride (95) in hydroxylic solvents involves a facile slow ionization (SWl mechanism) giving a cation which eliminates ArNCO to... 

The Hammett reaction constants p = 2.7138 and 3.0157 were determined for addition of water to meta-substituted l-(4-methoxyphenyl)ethyl carbocations (80-X, Scheme 44A) and 1-4-methoxybenzyl carbocations (p-Me-1 +-X), respectively. The value of p = 2.7 for addition of water to 80-X is 36% that of the value of = 7.6, the slope of Hammett-type plots equilibrium constants for addition of water to 80-X to form the alcohol.138 This shows that the addition of water proceeds through a transition state where partial bond formation to the nucleophile results in a 36% change in the interaction between the m-substituent and the cationic benzylic carbon, compared with the complete loss of this interaction at the water adduct. Scheme 44A therefore proceed through a transition state in which there is a 36% change in the interaction between the m-substituent and positive charge at the benzylic carbon due to partial bond formation to the water nucleophile. 

The quaternization of 1-substituted imidazoles is a facile reaction which leads to a stable quaternary salt via an 5n2 reaction (Scheme 16) which may be affected by steric factors. It has been shown (78AHC(22)7l) that the effects of fV-aryl substituents on rate constants for the quaternization of imidazoles can be correlated using a Hammett equation. The value of p (-0.45) for the ethylation of imidazoles shows little sensitivity to substituent effects, but this is to be expected since the basic pK values of the fV-aryl compounds do not vary widely, and the rate of alkylation must depend on the basicity of the nitrogen being quaternized. Certainly the expected sequence of reactivities, 1-methyl > 1-benzyl > 1-phenyl, is observed in the ethylation of 1-substituted imidazoles with iodoethane in ethanol or acetone, and although only qualitative observations are available, l-methyl-4-and -5-chloroimidazoles react less readily than 1-methylimidazole. Considerable experimental difficulty is experienced in quaternizing nitro-substituted imidazoles. 

The kinetics for the Christmas rearrangement of 9-benzyl- and of variously 4-substituted 9-phenyl-8-azapurine-6-thiones were found to be first order, and the rate constants and equilibrium constants were recorded. A plot of log forward component versus Hammett s sigma values for the 4-phenyl substituents was rectilinear. The 8-azapurine isomer was disfavored by electron-withdrawing substituents and by increased temperatures. In dimethyl sulfoxide, the equilibrium favored the 8-azapurine isomer more than in alcohols. Ultraviolet monitoring of the reaction, in both directions, detected no buildup of any acyclic intermediate." ... 

Polymer Tacticity. Our initial results on the polymerization of several different p-substituted-a-methylstyrene monomers indicated that there was some relationship between polymer stereoregularity and both the type of initiator and substituent in these monomers ( ). However, our recent investigations with a much wider variety of monomers, catalysts and cocatalysts revealed that the classical approach to analyzing substituent effects in organic reactions, the use of the Hammett pa relationship, gave no simple and self-consistent relationship between tacticity and the a (or a ) constant for the para-substituent. These results are summarized in the data in Table I for the cationic polymerization of a-methylstyrene and a series of five p-substituted-a-methylstyrene monomers initiated with two different Friedel-Crafts catalysts, TiCl and SnCl, either alone or with a cocatalyst benzyl chloride (BC) or t-butyl chloride (TBC), in methylene chloride at -78°C. Where a cocatalyst was used, the initiator was presumably a carbonium ion formed by the following reaction ... 

Rate constants for three distinct competing processes were separately determined solvolysis k, unimolecular k, and bimolecular k2. The Hammett equation with cross-terms was applied to the effects of substituents X in the nucleophile and Z in the leaving group on the analysed rate constants, but in most cases the pxz term was negligible. The Hammett equation with cross-terms has also been applied to the reactions of Z-substituted benzyl X-benzenesulfonates with Y-substituted thiobenzamides in acetone at 45 The findings pz < 0 and pyz > Pxz indicate that this reaction proceeds by a dissociative 5ivr2 mechanism. 

The value of the Hammett equation in estimating reaction rates is illustrated as follows. Suppose it is required to know the rate of alkaline hydrolysis of p-nitrobenzyl acetate in 56% acetone at 25°C. p for the general reaction is +0.760 and the rate constant for the hydrolysis of benzyl acetate is 6.995 X 10 1 mole" sec". The substituent constant for p-nitro is +0.778. The required rate constant is obtained from the equation ... 

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