Other Chelation-Controlled Systems

As is obvious from the numerous examples in previous sections, the presence of coordinating functionalities in substrate structures greatly influences the outcome of Mizoroki-Heck reactions. In this last section, we will briefly mention other representative chelation-controlled systems. 

(a) Oestreich, M. (2005) Neighbouring-group effects in Heck reactions. Eur. J. Org. Chem., 783-92 (b) Oestreich, M. (2008) Directed Mizoroki-Heck reactions, in Topics in Organometallic Chemistry, Vol. 24 (ed. N. Chatani), Springer-Verlag, Heidelberg, pp. 169-92. 

Andersson, C.-M., Larsson, J. and Hallberg, A. (1990) Chelation-controlled, paUadium-catalyzed vinylic substitution reactions of vinyl ethers. 2-Arylethanal equivalents from aryl halides. J. Org. Chem., 55, 5757-61. 

(a) Melpolder, J.B. and Heck, R.R (1976) A paUadium-catalyzed arylation of allylic alcohols with aryl halides. J. Org. Chem., 41, 265-72 (b) Chalk, A.J. and Magennis, S.A. (1976) PaUadium-catalyzed vinyl substitution reactions. I. A new synthesis of 2- and 3-phenyl substituted aUylic alcohols, aldehydes, and ketones from aUylic alcohols. J. Org. Chem., 41, 273-8. 

(a) Cacchi, S., Morera, E. and Ortar, G. (1984) PaUadium-catalyzed vinylation of enol triUates. 

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Syn 2-formyl-2-methyl-l,3-dithiane 1-oxide undergoes efficient cycloaddition reaction with Danishefsky s diene with excellent levels of diastereoselectivity in the presence of magnesium bromide at -78 °C (Scheme 9, Table 10). Chelation control models which are similar to those described previously by us, and others,54 were proposed to rationalize the observed stereoselectivity. The proposed model 29 for the syn system is shown below the structure of the major product diastereoi-... 

Fig. 4.30) with various substitution pattern at Cl and C2 were investigated. Experiments with alkyl substituents at C2, in place of the sulfur heteroatoms, eliminate the viability of a chelation-controlled selectivity and reveal the significance of geminal substitution [geminal substitution influenced the selectivity in other oxocarbenium ion systems [108]]. Perturbation of the Cl substituent of the oxocarbenium ion intermediate has little effect on reaction stereoselectivity, and analysis of this observation lends additional support for stereoelectronically preferred inside attack of the nucleophile. The results demonstrate that selective formation of the 1,4-c/s product 84 does not require a chelated transition stracture, reinforcing the utility of the inside attack model to analyze the reactivity of complex five-membered ring oxocarbenium ion intermediates. 

Citric acid is utilized in a large variety of food and industrial appHcations because of its unique combination of properties. It is used as an acid to adjust pH, a buffer to control or maintain pH, a chelator to form stable complexes with multivalent metal ions, and a dispersing agent to stabilize emulsions and other multiphase systems (see Dispersants). In addition, it has a pleasant, clean, tart taste making it useful in food and beverage products. 

The most important reaction of this type is the formation of imine bonds and Schiff bases. For example, salicylaldehyde and a variety of primary amines undergo reaction to yield the related imines, which can be used as ligands in the formation of metal complexes. However, it is often more desirable to prepare such metal complexes directly by reaction of the amine and the aldehyde in the presence of the metal ion, rather than preform the imine.113 As shown in Scheme 31, imine formation is a reversible process and isolation of the metal complex results from its stability, which in turn controls the equilibrium. It is possible, and quite likely, that prior coordination of the salicylaldehyde to the metal ion results in activation of the carbonyl carbon to amine nucleophilic attack. But it would be impossible for a precoordinated amine to act as a nucleophile and consequently no kinetic template effect could be involved. Numerous macrocyclic chelate systems have been prepared by means of imine bond formation (see Section 61.1.2.1). In mechanistic terms, the whole multistep process could occur without any geometrical influence on the part of the metal ion, which could merely act to stabilize the macrocycle in complex formation. On the other hand,... 

The a-amino acids have been reported to form stable complexes with transition metals. The derivatives of acetic acid are particularly noteworthy for their chelating ability. The most remarkable for the acetic acid derivatives, however, is ethylenediamine tetraacetic acid (often abbreviated EDTA or H4Y). The complexes of EDTA are remarkably stable, and have been investigated extensively from the industrial point of view. The substance has been used in water softening, electroplating, controlling of metal content of dye paths, in removing lead and other heavy metals from the human systems, and in the treatment of chlorosis in plants [7]. 

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