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Amphidynamic Crystals of a Steroidal Bicyclo[2.2.2]octane Rotor: A High Symmetry Group That Rotates Faster than Smaller Methyl and Methoxy Groups

2013; American Chemical Society; Volume: 135; Issue: 28 Linguagem: Inglês

10.1021/ja4024463

1943-2984

Braulio Rodríguez‐Molina, Salvador Pérez‐Estrada, Miguel A. Garcı́a-Garibay,

Crystal structures of chemical compounds

The synthesis, crystallization, single crystal X-ray structure, and solid state dynamics of molecular rotor 3 provided with a high symmetry order and relatively cylindrical bicyclo[2.2.2]octane (BCO) rotator linked to mestranol fragments were investigated in this work. By use of solid state (13)C NMR, three rotating fragments were identified within the molecule: the BCO, the C19 methoxy and the C18 methyl groups. To determine the dynamics of the BCO group in crystals of 3 by variable temperature (1)H spin-lattice relaxation (VT (1)H T1), we determined the (1)H T1 contributions from the methoxy group C19 by carrying out measurements with the methoxy-deuterated isotopologue rotor 3-d6. The contributions from the quaternary methyl group C18 were estimated by considering the differences between the VT (1)H T1 of mestranol 8 and methoxy-deuterated mestranol 8-d3. From these studies it was determined that the BCO rotator in 3 has an activation energy of only 1.15 kcal mol(-1), with a barrier for site exchange that is smaller than those of methyl (E(a) = 1.35 kcal mol(-1)) and methoxy groups (E(a) = 1.92 kcal mol(-1)), despite their smaller moments of inertia and surface areas.