engleski

Stereochemistry of 2, 6-Diaminoadamantane Salts: Transannular Interactions

A series of bisprimary, bistertiary, and bisquaternary adamantane- 2, 6-diammonium salts were synthesized from adamantane-2, 6- dione starting material. Conversion of the dione to a dioxime followed by H2/PtO2 hydrogenation afforded 2, 6- diaminoadamantane dihydrochloride 2a. Reductive amination of 2a with aqueous formaldehyde (H2/PtO2) gave rac-N, N, N’, N’- tetramethyl-2, 6-diaminoadamantane dihydrochloride salt 3. A methanolic solution of 3 and acetone diffusion gave 2.5 hydrate crystals whose triclinic system P 1 cell had two molecules of opposite handedness and five water molecules in the asymmetric unit. Diamine 2a with excess CH3I yielded rac-N, N, N, N’, N’, N’- hexamethyladamantane-2, 6-diaminium diiodide quaternary salt 4a. Diiodide 4a with molecular bromine gave the corresponding dibromide 4b. Methanolic solutions of 4a (or 4b) with acetone diffusion gave isostructural monohydrate crystals in orthorhombic system Pnma cells. An enantiomeric pair of 4a, 4b (2RS, 6RS)-diaminium ions is disordered about the mirror plane since chiral cation occupancy of this location requires a symmetry which cannot be fulfilled. In solution all cations have C2 symmetry detected by NMR whereas an approximate C2 symmetry was observed in crystals. Quantification of crystallographic pseudo-symmetry rmS(G) shows high fidelity for intermolecular pseudo-inversion between cations of opposite handedness in crystal 3, and for intramolecular pseudo C2-symmetry in adamantane skeletons of crystals 3, 4a and 4b. Axial NMe3 or NHMe2 groups on adamantane-2-(or -6-) positions suffer cis-1, 3-diaxial repulsion very similar to that involving axial t-Bu or i-Pr groups and H(axial) protons on cyclohexane. These transannular steric interactions tilt the NMe3 units outward by ca. 19° while NHMe2 groups slant outward by a smaller and more usual ca. 7°.

2; 6-Diaminoadamantane / X-ray Crystallography / Continuous Symmetry / CSM / Pseudo-Symmetry Quantification / Stereochemistry / Non-Crystallographic Symmetry / Axial t-Butyl

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