Reaction template

A major feature of template reactions is geometric formation of the complex brings the reactants into close proximity with the proper orientation for reaction. In addition, complexation may change the electronic stracture sufficiently to promote the reaction. Both are common to all coordinated ligand reactions, but the geometric factor is 

Dyes and Pigments, in G. Wilkinson, R. D. Gillard, and J. A. McCleverty, eds.. Comprehensive Coordination Chemistry, Vol. 6, Pergamon Press, Oxford, 1987, pp. 88-89. 

FIGURE 12-17 Schiff Base Template Reaction, (a) The Ni(II)-o-aminothiophenol complex reacts with pyridine-1-carboxaldehyde to form the Schiff base complex, (b) In the absence of the metal ion, the product is benzthiazoline very little of the Schiff base is formed. (From L. F. Lindoy and S. E. Livingstone, Inorg. Chem., 1968, 7, 1149.) 

The expression template reaction indicates mostly a reaction in which a complexed me) ion holds reactive groups in the correct orientation to allow selective multi-step reactions. T1 template effect of the metal is twofold (i) polymerization reactions are suppressed, since th local concentration of reactants around the metal ion is very high (ii) multi-step reactions are possible, since the metal holds the reactants together. In the following one-step synthesis eleven molecules (three ethylenediamine — en , six formaldehyde, and two ammonia molecules) react with each other to form one single compound in a reported yield of 95%. It is ob vious that such a reaction is dictated by the organizing power of the metal ion (I.I. Creasei 1977), 

A common disadvantage of many template reactions is that it is often difficult to remove the metal ion. Such syntheses are therefore in situ syntheses of metal complexes and can only occasionally be used for the synthesis of the metal-free ligands. 

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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... 

Template Reactions Maria De Sousa Healy and Anthony J. Rest... 

Fig. 3a,b. Template cyclization reactions of a crown ethers and b CPOs. The coordination bonds are illustrated by black arrows. In the crown ether synthesis, ethylene glycols coordinate toward the metal acting as the template (normal template reaction) however, the template coordinates to the incorporated metals of porphyrin In CPO synthesis (inverse-template reaction)... 

To select the metal to be incorporated into the substrate porphyrin unit, the following basic properties of metalloporphyrins should be considered. The stability constant of MgPor is too small to achieve the usual oligomeric reactions and purification by silica gel chromatography. The starting material (Ru3(CO)i2) for Ru (CO)Por is expensive and the yield of the corresponding metalation reaction is low. Furthermore, the removal of rutheniirm is difficult, and it is likewise difficult to remove the template from the obtained ruthenium CPOs. Therefore, ZnPor is frequently used as a substrate in this template reaction, because of the low prices of zinc sources (zinc acetate and/or zinc chloride), the high yield in the metalation reaction, the sufficient chemical stability of the ZnPor under con-... 

Fig. 4. Yields of CPOs 3 and 4 using inverse-template reactions (%).

Synthesis of CPOs, 3 and 4, using inverse-template reaction. The substituents are omitted for clarity... 

Nickel complexes of this group are of interest in biomimetic work. By means of ligand (320) the complete reaction cycle of acteyl CoA synthase could be executed (Scheme 2). Ligand (320) can also be synthesized by a template reaction. Upon reduction of the Ni11 complex (321) with Na/Hg, the ligand backbone is cleaved, resulting in a thermally stable trinuclear Ni11 alkyl thiolato complex (322). 

By a template reaction of 2-aminothiophenol, NaOEt, l,2-(BrCH2)2C6H4, and [Ni(C104)2] 6H20, complex [Ni(491)Br2] could be obtained. Interestingly, this complex could not be made from preformed (491) and Ni11 salts.1337... 

The square planar Ni11 complex (583) was prepared from the template reaction of 3,3 -(ethylene bis(iminomethylidene)bis(2,4-pentanedionato)nickel(II) with l,3-diamino-2-propanol (Equation (18)). The uncoordinated OH group reacted smoothly with acylating agents, resulting in... 

An unusual template reaction leads to the Ni11 complex of a tetrabenzo-N2S2 macrocyclic ligand (676) via C—Cl bond cleavage and C—S bond formation according to Equation (25).1701... 

Sargeson and co-workers have reported the use of [Pt(en)3]4+ in template reactions to produce the platinum(IV) complexes of the macrobicyclic ligands sep and (N02)2sar (170).477 These reactions are analogous to those that occur around cobalt(III). However, in contrast to the [Co((N02)2sar)]3+ system, reduction of the pendant dinitro groups did not yield amines, but hydroxyamine groups.478... 

The unsymmetrical complexes LnPcPc can also be prepared in a stepwise manner (Scheme 8.1, D, upper part) [87, 112-114]. Treatment of phthalonitrile with excess of Lu(OAc)3- H2Oand DBU gives the half-sandwich compound LuPc(0Ac)(H20)2. The latter complex reacts with Na2Nc in 1-chloronaphthalene leading to the formation of LuPcNc [87]. Similarly, the template reaction of unsubstituted and crown-substituted phthalonitriles with lutetium acetate was carried out in boding w-hexanol in the presence of DBU [115-117]. Bi- and tri-nuclear [118] lutetium complexes could also be obtained following this approach [111, 119]. 

Procedures and hence may involve a composite of both template and on-template reactions it may be difficult to define the precise role(s) of the metal ion in such cases. Template effects have been studied in some depth for a few specific systems however, research directed towards obtaining a more general understanding of such effects has been quite sparse. 

Apart from this one-reaction type, the routine use of metal template procedures for obtaining a wide range of macrocyclic systems stems from 1960 when Curtis discovered a template reaction for obtaining an isomeric pair of Ni(n) macrocyclic complexes (Curtis, 1960). Details of this reaction are discussed later in this chapter. The template synthesis of these complexes marked the beginning of renewed interest in macrocyclic ligand chemistry which continues to the present day. 

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. 

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