Aromatic ligands, electrophilic attack

General Order of Rate Constants. The rate constants of electrophilic reactions of aromatic ligands and their metal complexes fall in the order fo, > kML > kffL. The difference between these rate constants becomes greater as the activity of the attacking reagent decreases. When L is a phenolate, HL is the phenol when L is an amine, HL is the corresponding ammonium derivative. The possible synthetic applications of this sequence can be appreciated from the fact that 8-hydroxyquinoline is usually sulfonated with 15 to 30% oleum, while its copper (II) complex can be readily sulfonated in 70% sulfuric add (5). 

These equations show the general theoretical basis for the empirical order of rate constants given earlier for electrophilic attack on an aromatic ligand L, its metal complex ML, and its protonated form HL, one finds kt > n > hl. Conflicting reports in the literature state that coordination can both accelerate electrophilic aromatic substitution (30) and slow it down enormously (2). In the first case the rates of nitration of the diprotonated form of 0-phenanthroline and its Co(III) and Fe(III) complexes were compared. Here coordination prevents protonation in the mixed acid medium used for nitration and kML > h2l. In the second case the phenolate form of 8-hydroxyquinoline-5-sulfonic acid and its metal chelates were compared. The complexes underwent iodination much more slowly, if at all, and kL > kML ... 

Whereas co-ordination to a metal is seen to have relatively little effect upon the reactivity of aromatic ligands with electrophiles, this is not the case in reactions with nucleophiles. In the same way that we might expect co-ordination to a metal ion to reduce the tendency for attack of an aromatic ring by electrophiles, we might expect the attack of nucleophiles to be enhanced. The majority of studies in this area have been concerned with the dramatic effects that copper salts have on the reactivity of aromatic ligands. Copper salts are... 

It is convenient to consider heteroaromatic ligands in two classes - 7t-excessive, five membered rings typified by pyrrole, furan and thiophen, and TC-deficient six-membered rings typified by pyridine. The 7i-excessive heterocycles are usually extremely reactive towards electrophilic attack and, with the exception of thiophen, do not exhibit the chemical inertness often associated with aromatic benzene derivatives. Conversely, the TT-deficient heterocycles are extremely inert with respect to electrophilic attack. Paradoxically, it is the high reactivity of the five-membered rings and the inertness of the six-membered rings that give rise to common synthetic problems. The usual methods for the... 

Those donor atoms and/or groups are examined as donor centers in the problem of competitive coordination on which, due to the molecular structure of the ligand, the most favorable conditions are created for electrophilic attack by the metal, after taking into account the acceptor properties of the metal and the conditions of complex-formation reactions [19]. Such donor centers are mostly elements of a few main subgroups belonging to Groups V and VI of the Periodic Table, and also the unsaturated, aromatic, and heteroaromatic compounds which form the fundamentals of modern ligands (Chap. 2). 

Perdenteration of the methylene hnker affords a relatively kinetically stable complex, which allows for the monitoring of exogenons snbstrate oxidations. When (7) is exposed to cold (-95 °C) acetone solntions of the lithium salts of para-substituted phenolates, clean conversion to the corresponding o-catechols is observed. Deuterium kinetic isotope effects (KIEs) for these hydroxylation reactions of 1.0 are observed, which is consistent with an electrophilic attack of the peroxo ligand on the arene ring. An electrophilic aromatic substitution is also consistent with the observation that lithium jo-methoxy-phenolate reacts substantially faster with (7) than lithium / -chloro-phenolate. Furthermore, a plot of observed reaction rates vs. / -chloro-phenolate concentration demonstrated that substrate coordination to the metal center is occurring prior to hydroxylation, and thus may be an important feature in these phenolate o-hydroxylation reactions. 

Reactions are achieved with benzene and its derivatives (including those deactivated for electrophilic attack), polycyclic aromatics, heteroaromatics and transition-metal complexes of aromatic ligands. 

In the aluminum (III) chelate of salicylic acid, as well as in its copper (II) chelate, the positions susceptible to electrophilic attack are ortho and para to the hydroxy group. Since the course the reaction takes in the presence of the metal ions is quite different from that in their absence, it can be concluded that substitution occurs in the metal chelates rather than in the free ligands. This reaction is noteworthy in that it is one of the few reported instances in which a metal ion in a chelate ring has a marked influence on the course of a substitution reaction in an adjacent aromatic ring. 

Electrophilic attack on coordinated polyenyl ligands is uncommon, but electrophilic attack on the Ti -cyclopentadienyl ligands in certain complexes has become synthetically valuable. For example, the acetylation of ferrocene (Equation 12.75) is a reliable Friedel-Crafts process " and generates derivatives used to make many ferrocenyl ligands. Similar electrophilic aromatic substitution reactions of haloboranes on ferrocene (shown in Equation 12.76) and on the anion generated from deprotonation of tungstenocene dihydride (Equation 12.77) leads to borylcyclopentadienyl complexes. " ... 

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