Metal template reactions equilibrium

Metal template reactions, 1, 416, 433 equilibrium kinetic, 1, 434 thermodynamic, 1, 434 Metal tolerance amino acid complexes, 2, 964 plants, 2, 963 Metal toxicity... 

Simple a-diimines are hydrolytically unstable, but can be stabilized as metal complexes by virtue of the formation of stable five-membered chelate rings.68 69 a-Diketones and glyoxal undergo metal template reactions with amines to yield complexes of multidentate ligands such as (34),70 (35)71 and (36).72>73 In the last case, the metal exerts its stabilizing influence on the a-diimine partner in an equilibrium process (Scheme 5). The same phenomenon occurs with amino alcohols74 75 in addition to amino thiols. The thiolate complexes (37) can be converted to macrocyclic complexes by alkylation in a kinetic template reaction (Scheme 5).76 77... 

Mercury, tris(l,10-phenanthroline)-structure, 64 Mercury(II) complexes masking agent, 536 Mercury electrodes potential range aqueous solution, 480 Metal carbonyls structure, 16 Metallocenes nomenclature, 126,127 Metallochromic indicators, 554 Metallofluorescent indicators, 558 Metallothionein proteins, 142 Mettd-metal bonding, 137,169 gravimetry, 525 history, 21,23 nomenclature, 122, 123 Metal nitrosyls structure, 16 Metal-phthalein metallochromic indicator, 557 Metal template reactions, 416,433 equilibrium kinetic, 434 thermodynamic, 434 Methane, dichloro-... 

Two possible roles for the metal ion in a template reaction have been delineated (Thompson Busch, 1964). First, the metal ion may sequester the cyclic product from an equilibrium mixture such as, for example, between products and reactants. In this manner the formation of the macrocycle is promoted as its metal complex. The metal ion is thus instrumental in shifting the position of an equilibrium - such a process has been termed a thermodynamic template effect. Secondly, the metal ion may direct the steric course of a condensation such that formation of the required cyclic product is facilitated. This process has been called the kinetic template effect. 

The template effect has been recognized to show two or three aspects. The thermodynamic template effect involves the removal of a product from an equilibrium by stabilization of its metal complex (Scheme 29). This phenomenon has also been termed the equilibrium template effect,112 but some authors5 draw a distinction between these two types of effects. They reserve the equilibrium template effect for reactions which result in the formation of different products in the metal-assisted and metal-free reactions. 

Define the terms "equilibrium template reaction and "kinetic template reaction as applied to reactions involving transition metal ions, giving an example of each. 

Some believe that there are two main template effects kinetic and thermodynamic [8]. The latter is responsible for an increase in the yield of the complex with ligands formed in situ in the presence of metal ions, which bind products that result from ordinary reactions and to withdraw them from the reaction medium. These procedures are not true template reactions since they do not satisfy the above-mentioned conditions, and the metal ion causes equilibrium shift only. It is impossible to distinguish between kinetic and thermodynamic contributions to the template effect, since the coordination to the metal ion simultaneously causes both steric... 

The macrocyles formed depend on the overall proportions of 1 and 2, the diamine chain length and the type of cation used as the template. Of importance is the fact that the imine formation is a reversible reaction. Expressed in the DCC sense, one can say that all products, the macrocycies and the nonrepresented oligomers and polymers, are in constant equilibrium, and that all the species represented in Fig. 22 constitute a virtual combinatorial library (VCL). From this library, the high proportion of a specific macrocycle can be obtained by the use of the appropriate conditions (type of metal template, relative proportion of reactants). 

Different roles for the metal ion in a template reaction have been dehneated and termed thermodynamic template effect and kinetic template effect In the first case, the metal ion picks out the macrocyclic ligand from an equilibrating mixture of products, thus driving the reaction equilibrium... 

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,... 

Myriad polydentate aza-macrocycles have been reported 41. The extent of the subject forces limitation of this discussion to only macrocycles containing a pyridine or dipyridine subunit. Most of these coronands have been synthesized by a SchifF base condensation of an aldehyde or ketone with a hfc-primary amine in the presence of a metal ion. The metal ion acts as a template, resulting in dramatic increases in yield of the desired cyclic product over linear polymerization products42 46. Lindoy and Busch45 have described this effect in two ways, kinetic and thermodynamic. If the metal ion controls the steric course of a series of stepwise reactions, the template effect is considered to be kinetic. If the metal ion influences an equilibrium in an organic reaction sequence by coordination with one of the reactants, the template effect is termed thermodynamic. It is the kinetic effect that is believed to be operative in most metal ion-assisted (in situ) syntheses of... 

The template centre plays the key role in matrix reactions. If the steric course of macrocyclisation, or other multistep reaction, is directed and facilitated by the ligsons spatial structures, and their activation by coordination to the metal ion or another centre, then the process is controlled by the kinetic coordination template effect. If the metal ion (or another centre) sequesters one of the components from an equilibrium mixture (starting ligsons and oilier competing molecular species) and, as a consequence, shifts the position of the equihbriiun towards the formation of the desired product in the form of its metal complex, then the thennodynamic (or equilibrium) coordination template effect is considered to be operative for the reaction. 

The template synthesis of macrocyclic complexes rra 5-[M(L9)]" (M"" = Ni, Co +, Fe +) from 1-phenyl-1,2-propanedione and 1,3-diaminopropane occurs [44] according to Scheme 1-2. Reaction (1) represents the organic reaction prior to addition of a metal salt. When a methanolic solution of the latter is added to the reaction mixture, the metal-directed condensation is realised, producing the carbi-nolamine and final products. It is obvious that reaction (2) is a consequence of shifting the position of tautomeric equilibrium under the influence of a metal ion exclusively towards [M(L9)] , the latter subsequently being transformed into [M(L10)]"+. That the condensation reaction actually takes place within the metal coordination sphere has been demonstrated by the isolation of the corresponding cobalt(III) macrocyclic complexes with L9. 

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