Coupling reactions concerted reaction mechanism

Turning to pathway c), the concerted-reaction mechanism, we have formulated two approaches to predicting the rate constant, a double-adiabatic and a two-dimensional approach (4,5). In the double-adiabatic theory, the electron is considered to be coupled to two nuclear modes, a solvent (orientational polarization) mode that is treated classically in view of its low... 

Recently, Fu and coworkers have shown that secondary alkyl halides do not react under palladium catalysis since the oxidative addition is too slow. They have demonstrated that this lack of reactivity is mainly due to steric effects. Under iron catalysis, the coupling reaction is clearly less sensitive to such steric influences since cyclic and acyclic secondary alkyl bromides were used successfully. Such a difference could be explained by the mechanism proposed by Cahiez and coworkers (Figure 2). Contrary to Pd°, which reacts with alkyl halides according to a concerted oxidative addition mechanism, the iron-catalyzed reaction could involve a two-step monoelectronic transfer. 

Intramolecular coupling reactions of group 4 imino-acyl complexes yielding the corresponding free imines are rather rare and mainly involve titanium complexes [35 — 38]. The mechanism of these reactions still remains unclear although a concerted reductive elimination step, that initially leads to an -irnine intermediate complex of the type II, has already been postulated [35] (Fig. 4). 

The second step of the arylation process involves a coupling reaction between two of the ligands linked to the bismuth atom. Various mechanistic pathways were considered to be possible. Study of the relative migratory aptitudes of aryl groups indicated that the C-phenylation reaction with either a phenol or a p-dicarbonyl does not follow an ionic pathway. Although these migratory aptitudes were of the same order as a free-radical type, the relative ratios are more consistent with a non-synchronous concerted mechanism.26... 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

The reaction sequence includes (1) an oxidative addition of C-X bond of the aromatic substrate Ar-X to a Pd° center, (2) a substitution of OR for X in the LnPd (Ar)X intermediate, and (3) a reductive elimination of the C-O bond from the Pd center. The ability of palladium(ll) alkoxides bearing p-hydrogen atoms to undergo p-hydride elimination imposes some limitations on the type of alkoxide groups that can be involved in these C-O coupling reactions [2]. The C(sp )-0 reductive elimination reactions from Pd and Pt centers have also been studied computationally [4]. The reactions were suggested to proceed via a concerted three-center mechanism. 

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