Hammett correlations esters

In reactivity, the same series of mutagens exhibited positive Hammett correlations for AaiI acid catalysed solvolysis (p — + 0.32) and SN2 reactivity with A-methyl-aniline (p — + 1.7), hydroxide (p — +0.55) and L-cysteine ethyl ester (p — + 1.1), all reactions in which the benzoyloxyl group leaves with electrons and which are therefore accelerated by electron-withdrawing groups. The negative Hammet p-value for... 

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 . 

The rate of acetate fragmentation depends upon the nature of the alkyl group, with the relative rates of elimination of ethyl, isopropyl and /-butyl acetates being 1 26 1660, respectively, at 400 However, Hammett correlations for fragmentation of a- and 3-afyl-substituted acetates show little sensitivity to aryl substituents.The rates of fragmentation of esters derived from other carboxylic acids are dependent upon the pK of the parent acid, but this does not appear to influence significantly the regioselectivity... 

The pA a and log-on for the dissociation of the conjugate acid and hydroxide ion catalysed elimination from substituted ethyl benzimidate esters obey the Hammett correlations ... 

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. 

A good example for a detailed Hammett correlation study is the rhodium(II)-catalyzed decomposition of a-diazo esters. The most likely mechanism for the a-diazo ester decomposition process involves the initial complexation of the nega-... 

The Hammett correlation study on the Rh(II)-catalyzed decomposition of diazo esters 1 indicates that the reaction is accelerated by the presence of electron-donating groups in flie aromatic ring. The excellent correlation with ct suggest that the second step of flie reaction (formation of the Rh(II) carbene intermediate 4) is more sensitive to electronic effects than the initial pre-equilibrium. 

The ionization of benzoic acids in water at 25° was used by Hammett as the standard reaction for the original qp treatment (2a). This reaction and several analogous reactions, e.g., ionization and ester saponification rates of benzoic acids, cinnamic acids, and phenylpropiolic acids, gives ap correlations of relatively high precision. Taft and Lewis classified such reactions in an A category (2f). Reexamination of these A reactions, as well as additional analogous data which have become available subsequently, provided eight reaction series of data of apparently comparable reliability. In the para position, each of these sets of data meets the necessary condition of a minimal basis set... 

For Z-a-benzoylaminocinnamic acid and methyl ester substrates, with various donor and acceptor substituents in the phenyl ring, there was no correlation between the Hammett o--values of para substituents and optical purity of the product (239). The DIOP systems hydrogenate Z isomers more rapidly than E isomers, and induce a greater optical bias. N-Acetyl substrates always gave higher optical purities than the JV-benzoyl substrates, and similarly acid substrates were better than the corresponding methyl esters (239). 

The efficient enantioselective alkylation of the P/y-unsaturated ester 39 was achieved1351 by use of the N-anthracenylmethyl catalyst 12 (R=benzyl, X=Br) together with CsOH-H20 under phase transfer conditions analogous to those in the alkylation of the O Donnell imine 23, as shown in Scheme 13. The enantioselectivity of the alkylation correlates with Hammett o constants, and the N,N-dimethylamino substituents in 39 showed the most effective enantioselectivity. The tight ion pair in which the enolate... 

Rates of reaction were moderately fast. Glutathione reacted rapidly in DMS0-rf6/D20 ( 2 ° ca 2 X 10 lmol s ) and a series of A-benzoyloxy-A-benzyloxybenzamides (139) and (L)-cysteine ethyl ester in methanol-(f4 reacted with low a values (8-16 kcal mol ) and negative AS (—19 to —43 calK mol ), similar to their reaction with anilines. Bimolecular rate constants at 298 K (Table 8) correlated positively with Hammett a constants with slightly lower sensitivity p =. as opposed to p =. l for A-methylaniline). ... 

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). 

Substituent effects on the A,u I reaction have been studied by Bender and Chen55. These authors measured the rates of hydrolysis of a series of 4-substituted 2,6-dimethylbenzoates in 9.70 M sulphuric acid at 25°C, and found that the values for the first-order coefficients with 4-methoxy, 4-methyl, 4-unsubstituted and 4-bromo-compounds (5.0, 0.37, 0.033 and 0.01 x I0 4 sec-1, respectively) are satisfactorily correlated by the Hammett equation, following cr+ with a slope p = —3.22. Since the esters are not fully protonated in 9.70 M H2SOj, part of this factor is due to the effects of the 4-substituent on the protonation equilibrium, p for the protonation of substituted benzoic acids is about — l35, but is likely to be considerably smaller for di-ortho-substituted compounds, since the conjugative interaction of the p-substituents with the protonated carboxyl group requires coplanarity with the ring. 

Exercise 26-37 Would you expect a Hammett type of relationship to correlate data for the dissociation of acids of the following type with rate data for hydrolysis of the corresponding esters Explain. 

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