Competitive coordination

Competitive coordination of ambident heterocyclic ligands in metal complexes 97UK434. 

One of the problems related to the Lewis acid activation of a,/ -unsaturated carbonyl compounds for the reaction with a nitrone is the competitive coordination of the nitrone and the a,/ -unsaturated carbonyl compound to the Lewis acid [30]. Calculations have shown that coordination of the nitrone to the Lewis acid can be more feasible than a monodentate coordination of a carbonyl compound. However, this problem could be circumvented by the application of alkenes which allow a bidentate coordination to the Lewis acid which is favored over the monodentate coordination. 

The principle of hard and soft acids and bases and the problem of competitive coordination in complex compounds. A, D. Garnovskii, D. A. Osipov and S. B. Bulgarevich, Russ. Chem. Rev. (Engl. Transl), 1972, 41, 341-359 (441). 

Copper(II) triflate is quite inefficient in promoting cyclopropanation of allyl alcohol, and the use of f-butyl diazoacetate [164/(165+166) = 97/3%] brought no improvement over ethyl diazoacetate (67/6 %)162). If, however, copper(I) triflate was the catalyst, cyclopropanation with ethyl diazoacetate increased to 30% at the expense of O/H insertion (55%). As has already been discussed in Sect. 2.2.1, competitive coordination-type and carbenoid mechanisms may be involved in cyclopropanation with copper catalysts, and the ability of Cu(I) to coordinate efficiently with olefins may enhance this reaction in the intramolecular competition with O/H insertion. 

Different behavior was found for the p-toluenesulfonate and chloride derivatives containing ligands derived from pyrimidine. This could be due to a competitive coordination of these anions against the heterocyclic ligands. For the chloride complexes, a clear resonance broadening is observed at room temperature. Considering the chemical shifts it must be accepted that the free ligand is the dominant species. 

Saturated heterocycles are important ligands. They are flexible models for the problem of competitive coordination of the hard and soft acids with the nonconjugated donor sites. Analysis of these problems is of interest for the chemistry of complex and heteroaromatic compounds. The number of publications in the 1990 s shows that the interest to the problems analyzed in the present review is still enormous. 

The theory of regioselective synthesis of metal complexes is based on the ideas of competitive coordination and the principle of hard and soft acids and bases (HSAB, Sec. 1.2.2). 

The idea of competitive coordination as the capacity of di- and polydentate ligands to react with metals on different donor centers was introduced in 1972 [1], further developed in a monograph [2], and a series of reviews and original publications [3-18], Its general approach is based on the following statements [1,19] ... 

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). 

The majority of metal ions discussed before (Sec. 3.1.2) are included as acceptors into the problem of competitive coordination [1,2,19]. To be applied to the problem of competitive coordination, the HSAB principle (Sec. 1.2.2) [1,2,11,19] emphasizes preferential binding of hard metals by hard donor centers and soft electrophilic reagents (metal centers) by soft nucleophilic atoms or groups (mainly unsaturated, aromatic, or heteroaromatic systems). This statement applies only if... 

Infrared and NMR-spectral analysis, and x-ray diffraction data, testify [42-54] that in case of complexes of the already discussed pseudohalide ions, the competitive coordination can be explained by the HSAB principle hard Pearson acids are bound with hard N-center, and soft acids with soft X- donor (S, Se) centers. This situation allows us to obtain directly the coordination compounds of pseudohalides with a definite localization mode of the coordination bond, i.e., to carry out the regioselective synthesis on the basis of the higher stability of complexes which are obtained as a result of hard-hard or soft-soft interactions [2]. 

On the basis of a large amount of experimental material, it was shown that there are much more complex compounds of N-coordinated azines [synthetic method A, transformation (4.13)] in comparison with complexes with analogous n-coordinated ligands (method B). The mentioned reactions (4.13) represent a modern state of competitive coordination of pyridine and its C-substituted derivatives [11,15,55,56] ... 

The possibility of controlled preparation of ICC having different sizes for the metal-cycles (inner-chelate isomery), by using chelating ligands with some inner-cyclic donor atoms, is related to the regioselective synthesis [11,17]. This type of competitive coordination (Sec. 4.1.1) is especially characteristic for azo compounds 4, 916 [482, 483 Scheme (4.69) for details see Sec. 2.2.2.5.4]. 

Regioselective synthesis is examined mainly on the basis of the conception of competitive coordination and the principle of hard and soft acids and bases [14]. A description of polyhedron-programmed synthesis is given, taking into consideration thin structure compounds to be used as ligands (number, nature, and mutual situation of donor centers and the presence and character of organic fragments, annealed to a metal-cycle). 

Indeed, whilst C usually reacts rapidly with oxygen, it can occasionally undergo competitive coordination to an alcohol, producing the copper derivative F that might undergo hydrogen transfer and loss of the hydrazine substituent, resulting in the inactive complex G (36,37). 

In the case of secondary alcohols, competitive coordination of the OH function and oxygen to C largely favors the latter and the... 

According to Cabassi et al. the reduction of polymerization rates by aromatic compounds is caused by competitive coordination of monomer and arenes to vacant Nd-sites. The following coordination equilibrium was put forward in order to account for the observed effects (Scheme 12) [165,166]. 

Scheme 12 Competitive coordination to Nd between dienes and arenes [161], reproduced with permission from Wiley-VCH Verlag GmbH Co. KGaA...

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