Bidentate cyclohexanediamine

The same distortion, however, is not required for chelation of transA, -cyclohexanediamine (chxn) molecular models show that the two NH2 groups are now ideally situated for bidentate formation to a metal ion (Fig. 8). 7rart -l,2-cyclohexanediamine differs from the ethylenediamine system in that, when it is coordinated, only one chair conformation of the cyclohexane ring is possible. If the other conformer is assumed, the NH2... 

A similar situation occurs with the Co(III)-/ra>w-l,2-cyclohexanediamine-tetraacetic acid anion. Here only one conformation of the cyclohexane ring will allow the CDTA anion to coordinate as a sexadentate. If the other chair conformation is adopted, the two N atoms are now in trans axial positions and cannot coordinate bidentate to the Co(III) atom (Sec. III). 

For the bidentate chelating agents, x is S for the tridentate, x is 2. fchxn = trans-1,2-cyclohexanediamine. 

Douthwaite et al. have published two types of chiral bidentate NHC ligands [39]. The first of these contains an imidazolylidene and an imine linked by a frans-cyclohexanediamine core according to the synthetic strategy outlined in Scheme 14. The facile modular variation of the peripheral substituents in this class of ligands principally allows for the rapid screening of large ligand libraries. 

Finally, Shi and Qian reported the synthesis of the racemic di-benzimida-zolium 18 obtained in 4 steps from the (+/-) frans-cyclohexanediamine (Fig. 7) [42], The reaction with Pd(OAc)2 gave a dimeric bidentate NHC-Pd complex which was found to be moderately active in Suzuki and Heck reactions. 

This general method was then extended not only to complexes with other bidentate ligands but also to those with a terdentate or a sexadentate ligand, e.g. bidentates, (+)-hydroxymethylenecamphor (-+-)-3-acetylcamphor and (—)-cyclohexanediamine terdentates, 2,3-diaminopropionic add 2,4-diamino-butyric acid l,3-diamino-2-propanol and l-methyl-2,4,6-triaminocyclohexane... 

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