Synthesis of macrobicyclic tris-dioximates

In the majority of cases, the formation of macrobicyclic metal tris-dioximates is stipulated by the interaction of the reactive oxime groups in tris-dioximate complexes with Lewis acids. The most efficient capping agents have proved to be trigonal organic and inorganic boron compounds. 

Cobalt complexes of this type were initially isolated by Boston and Rose [2] and independently by Umland and coworkers [38]. Boron-capped iron(II) and cobalt(III) tris-dioximates have been obtained by two main routes a direct template reaction on the metal ion and the 

The first route proved to be the most universal one. Owing to a shift in the equilibrium brought about by the formation of a clathrochelate complex, it permits one to prepare a number of compounds in relatively high summary yields. The second pathway, which is carried out in aprotic media, offers higher yields than the first one. However, a low yield of the nonmacrocyclic tris-complex at the first stage imposes restrictions on the scope of this method. 

The yields of desired products have been improved by the azeotropic distillation of water from the reaction mixture and the resultant H ion neutralization. 

The synthesis of boron-capped iron(II) tris-dioximates has also been implemented using two other methods the rearrangement of the Tchugaev type Fe(HD)2Am2 bis-dioximates in the presence of boron-containing Lewis acids  

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The proposed schemes for the synthesis of macrobicyclic tris-dioximates have most readily been realized in high yields for alicyclic dioximes, having a cis-conformation both in crystals and in solutions. The change of the acyclic dioxime conformation from trans to cis during complexation decreases the stability of the compounds formed. 

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