Apical-equatorial" chelation

In the reaction of alcohols with the various types of pentavalent organobismuth compounds, 0-phenylation is usually disfavoured, oxidation taking place preferentially. However, in the case of the reaction of glycols with triphenylbismuth diacetate, an apical-equatorial chelation (93) was suggested to explain the facile mono-O-phenylation which is observed. By contrast, diapical chelation (94) takes place with monofiinctional alcohols, and the ensuing reductive process leads to oxidation. 

The correlation between regioselectivity and chelation mode suggests that complexes containing diequatorial bisphosphines like BISBI and complexes containing apical-equatorial chelating diphosphines like DIPHOS have either significantly different steric or electronic properties. Molecular mechanics calculations were employed to explore whether... 

Tables 49 and 50 present some of the structural parameters pertinent to the geometry and hypervalency at tin for (O—Sn) chelates with hydroxy, alkoxy and acyloxy groups. These include the bond lengths of tin to the two apical ligands, the axial OSnXax angles, the displacement of the tin atom (ASn) from the central plane defined by three equatorial ligands and the bite angles of the chelate ligands. Changes in the latter parameter allow...

Aldehyde oxidoreductase and XO/dehydrogenase share the same approximate square-pyramidal geometry at the Mo atom created by an equatorial dithiolene chelate, an apical sulhdo ligand, an oxo ligand trans to one dithiolene sulfur in the equatorial plane and a hydroxo or water group in the remaining equatorial site. 

The mechanism proposed for the formation of the sepulchrates accounts for the fact that, despite seven chiral centres and six nitrogen-metal bonds, the synthesis of [Co(sep)] + cation yields only one isomer. After the formation of the first heminal diamine and second imine centre, the chiral nitrogen centre must be oriented so that the methylene unit of the amine fragment and the proton are apical and equatorial, respectively, before intramolecular cycli-zation. In this case, six-membered ring closure takes place. The same apical orientation of the six-membered chelate cycle must be retained to complete the formation of a capping group. After the completed encapsulation, the methylene unit and the proton cannot... 

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