Crystal structure of racemic cis-2-amino-1,2-diphenylethanol (ADE)

In the crystal of the title racemic compound, enantiomers aggregate with each other and are linked by O—H⋯N hydrogen bonds to form chiral 21-helical columnar structures from C(5) chains along the b-axis direction.


Chemical context
The production of chiral compounds has great importance in the pharmaceutical industry, and diastereomer salt separation is still widely applied in the process. An optical resolving agent, chiral 2-amino-1,2-diphenylethanol (ADE) (Read & Steele, 1927), has been widely tried and used in diastereomer salt separation methods; for example, chiral discrimination of 2-arylalkanoic acids by (1R,2S)-ADE (cis-isomer) (Kinbara et al., 1998). The ADE molecule with two adjacent stereogenic centers exists as diastereoisomers (and more, enantiomers of cis-and trans-forms), and can be purchased without difficulty. It was considered that cis-and trans-ADE have different properties and play different roles in diastereomer salt separations. In fact, co-crystal structures with cis-ADE enantiomers have been found in previous reports. The racemic structure of trans-ADE has been reported (Bari et al., 2012), but that of cis-ADE has not. The crystal structure of racemic cis-ADE is reported on herein.

Figure 1
A view of the molecular structure of cis-(1S,2R)-ADE, with atom and ring labelling. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
A partial view of the crystal packing of the title compound. Dashed lines indicate the hydrogen bonds, and C-HÁ Á Á and N-HÁ Á Á interactions (see Table 2).

Synthesis and crystallization
cis-Enantiomers of 2-amino-1,2-diphenylethanol (ADE) were purchased from Sigma-Aldrich Co. Ltd. Equivalent weights were mixed in a bottle. Plate-like colourless crystals of the title racemic compound were obtained by vapour-phase diffusion of an aqueous ethanol solution at 297 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were located in difference Fourier maps. The NH 2 and OH H atoms were freely refined. The C-bound H atoms were included in calculated positions and treated as riding atoms: C-H = 0.93-0.98 Å with U iso (H) = 1.2U eq (C).

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The observed criterion of F 2 > σ(F 2 ) is used only for calculating -R-factor-obs etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.