Template ring precursors

Another approach towards templated acetylenic ring precursors overcomes this problem. Instead of preparing the bisacetylenes first and attaching them to the template in an additional step, the complete synthesis of the bisacetylenic ring... 

The synthesis of 24-, 30- and 36-membered macrocycles in good yield is presumably facilitated by a two-centre (double) template effect as illustrated schematically by formation of the complex with ring precursor [K2(L1450)] +(Eq. 6.55). 

The ligand reaction step may occur either with the template metal still intact or may take place after removal of the metal ion from the ring. As already mentioned, many of the Schiff-base macrocycles are unstable in the absence of a coordinated metal ion. However, for such systems, it has often been possible to hydrogenate the coordinated imine functions directly. The resulting saturated ligands will not be subject to the hydrolytic degradation which occurs for the imine precursors in the absence of their metal ion. 

In a recent study, some mechanistic aspects of this templated process have been determined quantitatively [28]. Using UV-Vis spectroscopy to monitor the kinetics of the macrocyclization reaction, it has been established that the rate of ring closure of the cationic precursor to the [ljimidazoliophane (4) is increased up to ten times in the presence of 0.04 mol/1 solution of a chloride source. The chloride stabilizes the transition state (i.e. a kinetic template) favouring the macrocyclization through hydrogen bonding. 

The hexahydropyrimidine (58), formed from l-phenylpropane-l,2-dione and propane-1,3-diamine, is an excellent precursor for the a-diimine macrocyclic complexes (60), presumably via the amino ketone (59) (Scheme 36).126 In this case, intramolecular cyclization of (59) to (58) is reversible, so that the metal ion can exert a thermodynamic template effect in formation of the complex (60). This represents a further example of a long-known phenomenon in which a metal ion can stabilize an a-diimine structure by virtue of the formation of stable five-membered chelate rings. Many 2-hydroxy- or 2-mercapto-amines undergo reaction with a-dicarbonyl compounds to yield heterocyclic compounds rather than a-diimines. However, in the presence of suitable metal... 

For this latter system, the synthetic approach is based on the template effect of an octahedral ruthenium(II) centre [97-100]. Two 1,10-phenanthroline ligands are incorporated in a ring, affording the precursor to the catenane. The... 

Tripincer template-directed cyclization of a dienic pyridine precursor leads to the formation of the 69-membered ring trimeric heterocycle 112 (Figure 2.6). Decomplexation of the tripincer from the product is realized with NaCl [72]. The synthesis of a [2]rotaxane by M-RCM using a similar pincer-pyridine complex for pre-assembly was also reported by Leigh et al. [73]. 

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