Hammett correlation amides

Electronic effects. Nucleophilic attack is favoured by electron-withdrawing groups on the amide and the acyloxyl side chains. Interpolated bimolecular rate constants at 308 K for the series of para-substituted /V-acetoxy-/V-butoxybenzamides 25c, 26b-g and 26i (Table 5) gave a weak but positive Hammett correlation with a constants ip = 0.13, r = 0.86).42,43 These Sn2 reactions are analogous to those of aniline and substituted pyridines with phenacyl bromides, which have similar Arrhenius activation energies and entropies of activation in methanol (EA= 14-16 kcal mol-1, AS = — 27 to —31 calK-1 mol-1) and 4-substituted phenacyl halides afforded a similar Hammett correlation with pyridine in methanol (cr, p — 0.25).175... 

Nucleophilic attack by iV-methylaniline is favoured by electron-withdrawing groups on the amide and acyloxyl side chains. A series of / ara-substituted Af-acetoxy-Af-butoxy-benzamides (138) (Table 6) gave a weak but positive Hammett correlation with a constants (p = 0.13, r = 0.86) °. The analogous reactions of pyridine with para-substituted phenacyl halides in methanol afforded a similar Hammett correlation a, p = 0.25) . The bimolecular rate constants for the limited series of Ai-benzoyloxy-A-benzyloxybenzamides (139) in Table 6 correlated strongly with Hammett a constants (p = 1.7, r = 0.97) °. Stabilization of developing carboxylate character supported the computed charge redistribution in the transition state ... 

These observations, along with kinetic isotope effect studies and Hammett correlation studies, support a concerted elimination by a-bond metathesis involving a four-membered transition state (Eq. 2) [23]. A large kinetic isotope effect is observed for the loss of methane from methyl amide complexes lb and lh (Eqs. 3 and 4), comparable to those observed by Buchwald and coworkers for formation of zirconium rj2-thioaldehyde complexes [25] and by Bercaw and coworkers for formation of tantalum rf- imine complexes [5a] through similar transition states. 

Mutagenic A-acyloxy-A-alkoxybenzamides undergo 5n2 reactions at the amide nitrogen with A-methylaniline and ring-substituted anilines.Hammett correlations for various reaction series are presented. These reactions model the possible mode of interaction between the mutagens and nucleic acids, which may be responsible for their biological activity. 

Reactivities and activation parameters for pyrolytic unimolecular first-order elimination reactions of A -acetylurea, A -acetylthiourca, /V,7V -diacetylthiourea and N-acetylthiobenzamide have been interpreted with reference to those for other amide derivatives.55 The first-order rate constants for pyrolysis of RCONHCSNHCe R (R = Me, R = H R = Ph, R = H, 4-N02, 3-C1, 4-C1, 4-Me) have also been measured at 423-500 K and correlated with Hammett [Pg.378]

Coordination compounds with organonitrile ligands are of considerable interest due to the increased susceptibility of the coordinated nitrile to nucleophilic attack by reagents such as hydroxide ion. Rate accelerations of 10 in base hydrolysis (nitrile-> amide) have been observed. Two new aromatic organonitrile complexes of [Co(NH3)5] with 2- and 4-nitroben-zonitrile have been prepared, and their base hydrolysis to coordinated carboxamides studied. The second-order rate constants for base hydrolysis are 180 4 and 510 90 M s at 25°C and / = 1.0 M for the ortho and para isomers, respectively. Rate constants for the hydrolysis of 11 coordinated aromatic nitriles follow a Hammett-type correlation with log koH = l-93o- + 1.30 at 25 C and / = 1.0 M. Carbon-13 NMR studies of the free and coordinated nitriles indicate similar chemical shifts, so that the variation in the slope of the Hammett plots for free- and coordinated-nitrile hydrolysis is a transition-state effect rather than a ground-state phenomenon. 

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