Interrogation of fractional crystallization behavior of a newly exploited chiral resolution method for racemic 1-(pyridin-2-yl)ethylamine via DFT-D3 calculations of cohesive energy

A novel chiral resolution method for racemic 1-(pyridin-2-yl)ethylamine (PEA) was developed in this study. Sequential conversion of 1-(pyridin-2-yl)ethylamine to 1-(pyridin-2-yl)-N-((4R)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ylidene)ethylamine (PIC) and to dichloro{(E)-1-(pyridin-2-yl)-N-(1,7,7-trimethylbicyclo[2.2.1]heptan-2-ylidene)ethylamine}zinc (PIC-Zn) allowed for the spontaneous separation of dichloro{(1R,E)-1-(pyridin-2-yl)-N-(1,7,7-trimethylbicyclo[2.2.1]heptan-2-ylidene)ethylamine}zinc (RE-PIC-Zn) and dichloro{(1S,E)-1-(pyridin-2-yl)-N-(1,7,7-trimethylbicyclo[2.2.1]heptan-2-ylidene)ethylamine}zinc (SE-PIC-Zn) by fractional crystallization. The overall yields of both (S)- and (R)-PEA were 24.8% with estimated ee values of 98 and 99%, respectively. The preferred formation of diastereopure crystals over that of diastereomeric mixture crystals was interrogated by comparing the cohesive energies (E_coh) of the respective crystals, as obtained via DFT calculations under periodic boundary conditions with a plane wave basis set based on the experimentally determined X-ray crystallographic structure. The E_coh values of diastereopure SE-PIC-Zn and RE-PIC-Zn were −45.8 and −44.4 kcal mol⁻¹, respectively. The E_coh values of the diastereomeric mixture crystal (RE-PIC-Zn·SE-PIC-Zn·2CHCl₃) and its virtual counterpart lacking CHCl₃ (RE-PIC-Zn·SE-PIC-Zn) were −56.0 and −37.2 kcal mol⁻¹, respectively. These data provided the background underlying the spontaneous segregation of the diastereopure crystals where the energy preference for forming diastereopure crystals over their corresponding diastereomeric mixture crystals is ca. 8 kcal mol⁻¹.