Trans-cyclohexane-1,2-diphosphonates synthesis, structural and NMR spectroscopic investigations.
A stereospecific addn. of sodium diethylphosphite to cyclohexene-1-phosphonic acid di-Et ester 1 was used to synthesize trans-cyclohexane-1,2-diphosphonic acid tetra-Et ester 2b. Acidolysis of the ester yields the corresponding diphosphonic acid 2a (shown as structure I). Dissocn. consts. pK1-pK4 of 2a were obtained in aq. solns. by precision potentiometric titrn. The mol. structure of solid 2a detd. by x-ray diffraction shows a slightly disturbed chair form of the cyclohexane ring and a bisaxial conformation of the phosphonate groups with a P-C-C-P dihedral angle of 143 Deg. Results of mol. modeling of 2a as well as of its anions (with and without consideration of Na+ cations) obtained by semi-empirical MNDO methods and by empirical force field calcns. including mol. dynamics are reported. The investigation of org. solns. of 2b and of aq. solns. of 2a by NMR spectroscopy allows conclusions concerning the conformative state via a Karplus-type relation for the vicinal coupling const. 3JPP on the P-C-C-P dihedral angle. In 2b the preferred conformation is that with a bisaxial arrangement of the phosphonate groups, while in aq. solns. of 2a the bisequatorial conformation dominates. By titrn. of 2a with a strong base the portion of the bis-equatorial conformation rises to a max. value at a degree of titrn. t ~ 3. Continuation of the titrn. leads to stabilization of the bisaxial conformation.
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