β–α Photoisomerization of Cobaloxime Complexes in the Solid State. 6. Asymmetric Induction Due to Chiral Crystal Environment

The crystal structure of (S)-s-butylamine(2-cyanoethyl)cobaloxime was determined by X-ray analysis; the crystal is monoclinic, and the space group is C2, Z = 4 with a = 15.656(2), b = 11.699(2), c = 11.553(1) Å, β = 96.979(9)° and V = 2100.5(5) Å3. The structure was refined by a full-matrix least-squares method to the final R value of 0.052 for 1678 observed reflections. The 2-cyanoethyl group takes a perpendicular conformation to the cobaloxime plane and has disordered conformations (A, B, and C) around the Co–C bond. The occupancy factors of A, B, and C are 0.48, 0.34, and 0.18, respectively. The crystalline sample was irradiated with a xenon lamp. The (R)-1-cyanoethyl group is preferentially produced from the (S)-s-butylamine complex. The conversion rates, which were determined by HPLC analysis using the chiral stationary phase, were obtained to be 1.17 × 10−5 and 0.59 × 10−5 s−1 for the (R)- and (S)-1-cyanoethyl groups, respectively. The diastereomeric excess is 27%. Such asymmetric induction is well explained by the shape of the reaction cavity for the 2-cyanoethyl group, and is also in good agreement with a packing-energy calculation by molecular mechanics.