Absolute Configuration and Polymorphism of 2‑Phenylbutyramide and α‑Methyl-α-phenylsuccinimide

Crystal structures of racemic and
homochiral forms of 2-phenylbutyramide
(1) and 3-methyl-3-phenylpyrrolidine-2,5-dione (2) were investigated in detail by a single crystal X-ray diffraction
study. Absolute configurations of the homochiral forms of 1 and 2, obtained by chromatographic separation of racemates,
were determined. It was revealed that racemate and homochiral forms
of 1 are very similar in terms of supramolecular organization
(H-bonded ribbons) in crystal, infrared (IR) spectral characteristics,
and melting points. The presence of two different molecular conformations
in homochiral forms of 1 allowed mimicking of crystal
packing of the H-bonded ribbons in racemate 1. Two polymorph
modifications (monoclinic and orthorhombic) comprising very similar
H-bonded zigzag-like chains were found for the homochiral
forms of compound 2 that were significantly different
in terms of crystal structure, IR spectra, and melting points from
the racemic form of 2. Unlike compound 1, homochiral forms of compound 2 have a higher density
than the corresponding racemate which contradicts the Wallach rule
and indicates that, in this case, homochiral forms are more stable
than racemate forms.