Hammett study

Hammett studies of the dissociation constants of benzoylacetanilides XCgH4COCHZCONHCgH4Y permitted the determination of reasonable quantitative relationship to the above value for S02Ph. 

Ashby and Yu have studied the kinetics of reduction of benzophenone with TIBA in ether and showed that the overall kinetic rate expression is second order, first order in TIBA and first order in ketone (151). The observed activation parameters were AG - 18.8 kcal/mol AH = 15.8 kcal/mol and AS = - 10.1 e.u. The negative entropy of activation is consistent with a cyclic transition state for the rate-determining hydride-transfer step. A Hammett study gave a value of p = 0.362, supporting nucleophilic attack by the aluminum alkyl on the carbonyl group in the rate-determining step. 

Complex (1) is a catalyst for selective oxidation of benzylic, allylic alcohols to aldehydes, and secondary alcohols to ketones using r-butyl hydroperoxide. Primary aliphatic alcohol oxidation failed. The use of cumyl hydroperoxide as radical probe discounted the involvement of i-BuO /t-BuOO. Hammett studies p = -0.47) and kinetic isotope effects kn/ku = 4.8) have been interpreted as suggesting an Ru—OO—Bu-i intermediate oxidant. 

The acidifying influence of the sulfonyl group, combined with its ability to transmit electronic effects is apparent from results of Hammett studies of the dehydrochlorination of P-RC6H4SO2CH2CH2CI and RC6H4S02CH2CHClPh, on reaction with EtsN the nearly identical positive p values indicate that for each series reaction proceeds via carbanion formation." ... 

In systems where steric interference is not a factor, C-H insertion at a methylene site is strongly preferred over that at methyl sites. A striking example of this effect is the reaction with 4-ethyltoluene (Eq. 24) [137]. The only C-H activation product formed is 202, derived from C-H insertion at the methylene site. A Hammett study on the benzylic C-H activation indicated that the transition-state build-up of positive charge at the benzylic position is stabilized by resonance. 

The C-H activation of benzylic positions further underscores the interesting interplay that exists between electronic and steric influences in this chemistry. The C-H activation of 4-ethyltoluene results in clean insertions at the benzylic methylene site to form a 4 1 diastereomeric mixture of 10a and 10b [Eq. (5)] [18]. A Hammett study using competition reactions between various substituted ethylbenzenes revealed that the C-H activation is strongly favored in systems containing electron-donating para substituents (q=-1.2) [18]. 

The following transformations occur by different mechanisms. Show them and predict what a Hammett study would show for each of them. 

Non-stereospecific cyclopropanation reactions of the diazafluorenylidene (10), generated by photolysis of the diazo compound, indicated a triplet carbene.18 Competition experiments suggested a singlet-triplet equilibrium at room temperature and a Hammett study of additions to substituted styrenes indicated that the carbene reacts as an electrophile (p = —0.65). 

O Shea, K. and Cardona, C., Hammett study on the Ti02-catalyzed photooxidation of para-substituted phenols a kinetic and mechanistic analysis, /. Org. Chem., 59, 5005, 1994. 

Addition of dimethylsulfonium methylide (122) to various Michael acceptors (121), followed by alkylation, has been reported to produce functionalized 1-substituted alkenes (124), arising via the unprecedented elimination (123), rather than the usual cyclopropanation products. In silyl substituted substrates, where a facile Peterson-type olefination is possible from the adduct, elimination took place instead. Aryl-substituted Michael acceptors (121 R1 = Ar) underwent a similar olefination to give 1-substituted styrene derivatives with moderate yields along with a side product, which arose by nucleophilic demethylation from the adduct of dimethylsulfonium methylide and arylidene malonates. Hammett studies revealed that selectivity for olefination versus demethylation increases as the aryl substituent becomes more electron deficient.164... 

Reactivity-selectivity relationships are obtainable in carbene chemistry provided an independent selectivity parameter is found. Under such circumstances the relative rate data then serve as a measure of relative reactivity. This has been done with a number of Hammett studies. Thus the addition of dichlorocarbene to compounds [ 1 ] — [3] (listed in Table 15) indicates that a reactivity-selectivity relationship is obtained using p as a measure of selectivity. A more reactant-like transition state is obtained for the more reactive substrate in accordance with the Leffler- Hammond postulate. 

The effects of substituents on the basic nature of imidazole are summarized in Section 4.02.1.3.4. Tables 1 and 2 list the basic pKa values of some representative imidazoles and benzimidazoles. From these it can be seen that methyl and other alkyl substituents exert a weak, base-strengthening inductive effect which is additive. Aromatic substituents decrease basic strength, while groups attached to these aryl rings exert their normal behavior. In fact, Hammett studies have permitted prediction with reasonable accuracy and correlation of p/Ca values for a variety of imidazoles. In a study of meta- and para-substituted 1-phenylimidazoles Pozharskii <70CHE194) has shown not only that 1-phenylimidazole is a... 

JCS(P1)2703, 95JCS(P1)2543], Reaction of (2-methyl-l-propenyl)triphe-nylbismuthonium tetrafluoroborate with KO Bu in the presence of an excess of styrenes in dichloromethane at — 78°C yields the corresponding alkylidene cyclopropanes and triphenylbismuthine [96JCS(P1)1971], A Hammett study of this reaction has shown that free alkylidene carbene is generated under simultaneous elimination of the bismuthine. These base-promoted reactions are illustrated in Scheme 3.6. 

Arylazides will decompose in the presence of [Co(TPP)] and unsaturated hydrocarbons to generate both aziridine and allylic amine product mixtures [88-90], Such a process is capable of oxidizing benzylic C-H bonds as well, though poor catalytic efficiency and problems with product over-oxidation limit the utility of these reactions [91, 92], Detailed kinetics analysis and Hammett studies have led Cenini and coworkers to propose a mechanism that does not involve a Co imido complex, as had been previously suggested. The observation of a Co(III) species by UV/vis spectroscopy strongly implicates a one-electron pathway for this particular amination method. 

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