Donor chair conformation

Similarly (— )-trans-1,2-cyclohexanediaminetetraacetic acid (CDTA) also coordinates stereospecifically (25). In diaminocyclohexane, both amino groups must be equatorial (Fig. 8) since with the donor groups in the axial positions the ligand cannot function as a bidentate and trans- 1,2-cyclo-hexanediaminetetraacetic acid could not coordinate as a sexadentate. If the cyclohexane ring adopts the boat conformation ( 5 kcal/mole less stable than the chair conformer), the l structure (Fig. 8) is still the only one possible, as the ligand now will not coordinate at all six positions in the d configuration. 

Two recent crystal structures suggest that this situation should be rectified. The adduct TICL-L (L = 4-pyridinecarbonitrile V-oxide) has a trigonal bipyramidal structure, with axial oxygen donors,401 while in TlBr3-diox the same stereochemistry arises from planar TlBr3 units bridged by 1,4-dioxane molecules in the chair conformation.402... 

With an octahedral metal center there are two orientations possible for an isolated tn ring in the chair conformation (25) and these become non-equivalent if some of the other four donor atoms are different. There is solid state evidence for such alternative conformations.520... 

Trithiane readily forms complexes to both soft and hard metal ions and borderline cases. For complexes where X-ray structural information is available, it appears that utilization of S-donor sites in normal tridentate chelation is never attained often the ligand is involved in the form of a bridging mode linking two or more separate metal centers. In addition, the 1,3,5-trithiane ring retains the chair conformer which it possesses in the free state. 

Figure 11.26 Forcing a glycoside into an all-axial conformation by steric or structural constraints dramatically Increases reactivity towards hydrolysis (note the chair conformation of "super "-armed donor is a simplification - steric hindrance leads to considerable distortions).

This methodology was optimised further by protecting the C-4 and C-6 hydroxyl groups with a cyclohexylidene acetal [66]. It was envisaged that the rigid chair conformation of the cyclohexylidene protected thiomethyl mannoside donor 83 would favour an Sn2 type pathway during the lAD reaction. This in turn would suppress any side reactions occurring from the formation of an anomeric carbonium ion. 

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