Substitution reactions conjugate base mechanism

Once formed by this process, the carbene may undergo any of the normal carbene reactions (see p. 250). When the net result is substitution, this mechanism has been called the SnIcB (for conjugate base) mechanism. Though the slow step is an SnI step, the reaction is second order first order in substrate and first order in base. 

The kinetics of substitution of bipy or phen into Co (467, 574), Ni (144), and Pt (697) complexes have been reported. Several studies of the hydrolysis of complexes of form [Co(III)(bipy)2XY] may be found. For both cis and trans isomers where X = Y = CY, hydrolysis is instantaneous (581), whereas for the cis isomer with X = acetate and Y = acetate or OH , reaction is very slow as a conjugate base mechanism cannot operate and the first-order reaction is therefore independent of [OH ] (124). One NO2 group in acid dependent and under acidic conditions is thought to proceed via protonation of one nitro group (289, 472). The interconversion of the cis and trans isomers, where X = N02 and Y = H2O, has an overall rate constant equal to + A [H+], implying reactions for both OH and... 

Other cases in which second-order kinetics seemed to require an associative mechanism have subsequently been found to have a conjugate base mechanism (called S ICB, for substitution, nucleophilic, unimolecular, conjugate base in Ingold s notation ). These reactions depend on amine, ammine, or aqua ligands that can lose protons to form amido or hydroxo species that are then more likely to lose one of the other ligands. If the structure allows it, the ligand Irons to the amido or hydroxo group is frequently the one lost. 

Octahedral substitution is also affected by base catalysis according to the conjugate base mechanism (SjglCB). The rate constant for substitution of Cr in [Co(NH3)5CI] is over a million times faster for OH than it is for H2O. In fact, the rate law for the base catalysis reaction is complex second order first order in [Co(NH3)5CI] and first order in OH. In reality, however, the reaction takes place by proton abstraction, as shown by Equations (I7.32)-(I7.34) ... 

In this mechanism, the rate-determining step involves the dissociative reaction of the conjugate base. Because of this, the mechanism is known as the SN1CB mechanism, in which the substitution is... 

Bob s research interests and knowledge across chemistry were great. Throughout his career he retained an interest in biomimetic chemistry, specifically the study of metal ion-promoted reactions and reactions of molecules activated by metal ion coordination. His early interests in carbohydrate chemistry inspired him to study metal ion catalysis of both peptide formation and hydrolysis as well as studies in inorganic reaction mechanisms. He was particularly interested in the mechanisms of base-catalyzed hydrolysis within metal complexes and the development of the so-called dissociative conjugate-base (DCB) mechanism for base-catalyzed substitution reactions at inert d6 metal ions such as Co(III). 

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

The apparently first kinetic study of a metal-assisted electrophilic substitution in a Co(III) complex is recent. The bromination of Co(NH3)5imidH is complicated by the presence of different bromine species in solution (Brj, HOBr and Brj"). In addition, successive brominations of the coordinated imidazole occur. Rate data can be interpreted in terms of reaction of the conjugate base of the Co(III) complex with Brj, and a suggested mechanism for the first steps is (Rq = Co(NH3)5 ")... 

Alkanamines have acid strengths corresponding to Ka values of about 10 33, which means that their conjugate bases are powerfully basic reagents. Therefore they are very effective in causing elimination reactions by the E2 mechanism (Section 8-8) and aromatic substitution by the aryne mechanism (Section 14-6C). The following example illustrates this property in a useful synthesis of a benzenamine from bromobenzene ... 

A Michael addition consists of the addition of the enolate of an active-methylene compound, the anion of a nitroalkane, or a ketone enolate to an acceptor-substituted alkene. Such Michael additions can occur in the presence of catalytic amounts of hydroxide or alkoxide. The mechanism of the Michael addition is shown in Figure 13.67. The addition step of the reaction initially leads to the conjugate base of the reaction product. Protonation subsequently gives the product in its neutral and more stable form. The Michael addition is named after the American chemist Arthur Michael. 

Water leaves, generating a carbocation. This is the El mechanism. Substitution by the competing SNI mechanism is avoided because of the absence of good nucleophiles in the reaction mixture. In contrast, if an acid with a nucleophilic conjugate base, such as HCI, were used, substitution products would be formed. 

Tfce preferred synthetic route to these important intermediates is the Mannich reaction (Chapter 27), The compound is stored as the stable Mannich base and the unstable enone released by elimination of a tertiary amine with mild base, The same conditions are right for this elimination and for conjugate addition, Thus the aw-methylene compounds can be formed in the flask for immediate reaction with the enol(ate) nucleophile, The overall reaction from (3-amino carbonyl to 1,5-dicarbonyl appears to be a substitution but the actual mechanism involves elimination and conjugate addition,... 

The ability of azoles to electrophilic substitution reactions is determined by the activity of reagents, the basicity of substrates, and the acidity of media. This caused some uncertainty in the interpretation of results and complicated a comparison of the reactivity of various azoles. The situation has changed after Katritzky and Johnson [1] have reported the criteria allowing, with a sufficient degree of reliance, the establishment in what form (base or conjugative acid) the compound reacts. The information on the mechanism of nitration of azoles was basically borrowed from the extensive literature on the nitration of aromatic hydrocarbons [2-8] therefore, we have found expedient to discuss briefly some works in this field. 

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