Formation kinetic metal template reactions

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... 

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 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,... 

In terms of template reactions, this combination of kinetic and thermodynamic stability usually means that the metal ion remains co-ordinated to the macrocyclic ligand and the isolation of the metal complex of the macrocycle provides strong circumstantial evidence for the existence of a metal-directed process. This is particularly easy to establish if the incorporation of the metal ion into the macrocyclic ligand can be shown to be slower than the metal-directed formation reaction. 

An example of a kinetic template effect is provided by the reaction between this complex and l,2-bis(bromomethyl)benzene. Complexation orients the sulfur atoms of the quadridentate N2S2 ligand to favour ring formation in a two-step reaction with l,2-(BrCH2)2C H, as shown in Scheme 5.4. In the absence of the metal ion, reaction of the free ligand with l,2-(BrCH,)2C H4 gives polymeric materials. 

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 idea of an impurities-driven route had been speculated in prior arts. A mechanism for the initiation of the MTO process by organic impurities instead of any direct route from pure methanol and DME was also suggested [28,87]. Moreover, it was stated that the rate of formation of the initial reaction centers and, therefore, the duration of the kinetic induction period for the first C-C bond formation are governed by impurities [87]. According to Ref. [28], these first hydrocarbons could be the result of impurities present in the MeOH feed or could originate from the incomplete combustion of organic templates from the catalysts, or formed in situ due to, for example, a presence of some transition metal cations on the catalyst, or being formed with the help of the metallic surface of the reactor walls. 

The situation is even more clear cut in the formation of thioethers by alkylation of thiolate complexes. Such reactions have been described in Section 7.4.2.1.2, but in those examples new chelate rings were not formed. The reaction which led Busch to propose the kinetic template effect is a thiolate complex alkylation, which does indeed produce a new chelate ring (equation 40).110 162 163 This reaction is an example of the general type shown in equation (35) and it has been established clearly that the sulfur atoms remain coordinated to the metal ion throughout the... 

What should we do to observe a three-dimensional template effect First, we should choose a reaction type that we know to be effective for the formation of macrocyclic ligands and extend the methodology to a kinetically inert cP or d6 metal centre. Let us reconsider the reaction, that we first encountered in Fig. 6-11. In this reaction, a dioximato complex reacted with BF3 to give the nickel(n) complex of a dianionic macrocycle (Fig. 7-1). 

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