Template effect positive

An approach to isobacteriochlorins1 ln-e makes use of Pd(II) or metal-free bilatrienes 1 as starting materials. Cyclization of the corresponding bilatriene derivatives is induced by base in the presence of palladium(II) or zinc(II) which exercise a template effect. Zinc can be readily removed from the cyclized macrotetracycles by acid whereas palladium forms very stable complexes which cannot be demetalated. Prior to the cyclization reaction, an enamine is formed by elimination of hydrogen cyanide from the 1-position. The nucleophilic enamine then attacks the electrophilic 19-position with loss of the leaving group present at the terminal pyrrole ring. 

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. 

In the use of templates, Sanders and co-workers [47] differentiate between two modes of action that are complementary a positive template directs the approach of two reactive end groups in a molecule, thereby generally supporting the desired intramolecular cyclization. In contrast, a negative template effect holds the reactive groups apart, thereby suppressing the intramolecular cyclization and leading preferentially to the (undesired) intermolecular reaction. 

Aluminosilicate zeolites are normally synthesized under basic conditions. The introduction of OH- ions to the synthetic system will necessarily lead to the introduction of correlated cations. These positively charged cations play an important role in the polymerization of polysilicates and aluminates by affecting the polymeric state and their distribution, and have an important effect on the colloidal chemistry of aluminosilicate as well. In addition, cations existing in the synthetic system also have important effects on the formation of the framework structure of zeolites. For example, plenty of synthetic data indicated that a tight correlation between the formation of the SBU cages of zeolites and the charge and size of the cations existed, and this was named the templating effect of cations by R.M. Barrer.[191... 

The question about the physical origin of the template effect of the alumina film is not easy to answer, moreover since not only a unique site seems to function as template but rather two different sites are active depending on the deposited metal. Indeed the strong preference of Pd for nucleation on the so-called dot structure of the alumina film was attributed to the presence of a localized electronic state in the band gap of the oxide at this particular position [68,186]. This view was lately however challenged by high-resolution STM investigations, which suggest the presence of a hole in the oxide film at that very position. It was claimed that Pd - as opposed to other metals -... 

A different picture emerges when this template effect is extended to flexible substrates such as all-tran.v-farnesol and all-/rani-geranylgeraniol. Thus the ester 4 under the conditions of Sharpless epoxidation is epoxidized to the 6,7-epoxide and the 10,11-epoxide in the ratio 40 60, with only negligible reaction at the allylic position. Similarly the ester 5 is converted into the 6,7-epoxide, the... 

The synthesis of a macrocyclic ligand in the presence of a metal cation has been used since the mid-1960s. N.F. Curtis and D.H. Busch should be mentioned as pioneer workers in the field. The so-called "template effect" is simple to understand. The presence of a metal cation leads to positioning of the reactants (by virtue of the presence of coordinating sites) such that macrocyclization is highly favored (Fig. 20). ... 

The key step in this approach is the formation of the precatenate 1 from the diphenolic derivative of 1,10-diphenylphenanthroline in presence of Cu(I). It can be seen that in structure l,the four phenol groups are in good position for subsequent attack and hence cyclization by the diiodo compound giving the catenate 2 (Fig. 21b). The Cu(I) ion can be easily removed by treatment with potassium cyanide, affording the catenane. The tridimensional template effect induced by Cu(I) was extended to the formation of a trefoil knot (Fig. 21c,d). 

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