(S)-Alanine–(S)-2-phenoxypropionic acid (1/1)

In the title co-crystal, C3H7NO2·C9H10O3, the (S)-alanine molecule exists in the zwitterionic form stabilized by two pairs of N+—H⋯O− hydrogen bonds and an electrostatic interaction between the ammonium center and the carboxylate anion, forming a sheet along the ab plane. The carboxyl group of the (S)-2-phenoxypropionic acid molecule is connected to the top and bottom of the sheet via N+—H⋯O=C and O—H⋯O− [R 2 2(7) graph set] hydrogen bonds, giving an (S,S)-homochiral layer, in which both methyl groups of (S)-alanine and the phenyl rings of (S)-2-phenoxypropionic acid are oriented in the same direction along the b axis.

In the title co-crystal, C 3 H 7 NO 2 ÁC 9 H 10 O 3 , the (S)-alanine molecule exists in the zwitterionic form stabilized by two pairs of N + -HÁ Á ÁO À hydrogen bonds and an electrostatic interaction between the ammonium center and the carboxylate anion, forming a sheet along the ab plane. The carboxyl group of the (S)-2-phenoxypropionic acid molecule is connected to the top and bottom of the sheet via N + -HÁ Á ÁO C and O-HÁ Á ÁO À [R 2 2 (7) graph set] hydrogen bonds, giving an (S,S)homochiral layer, in which both methyl groups of (S)-alanine and the phenyl rings of (S)-2-phenoxypropionic acid are oriented in the same direction along the b axis.

Kiichi Amimoto and Yuma Nishioka
Comment Chiral 2-phenoxypropionic acid (PPA) has been known as a good and accessible optical resolving agent for amines (Hasegawa et al., 1998). The crystal structure of optical pure and racemic PPA has been reported (Sørensen & Larsen, 2003). And the crystal structure of co-crystal of (R)-PPA with (S)-alanine has been known (Takahashi & Fujii, 2004), but no results have ever reported on the details of the chiral discrimination between PPA and (S)-alanine. In this work, we present the crystal structure of the co-crystal of (S)-PPA with (S)-alanine (I) (Fig. 1). The co-crystal I crystallizes in the monoclinic system of space group P2 1 . (S)-Alanine assembles a chiral two-dimensional sheet along the ab plane, in which the ammonium cation is strongly held with the carboxylate anion by two hydrogen bonds and one electrostatic interaction. The N(1) + -H···O(5)hydrogen bonds are 2.822 (2) Å and 2.916 (2) Å. The interatomic distance between ammonium center N(1) + and carboxylate O(4)is 2.960 (2) Å. The carboxyl C=O of PPA is connected to the ammonium N + -H of (S)-alanine, and the O-H of PPA to the carboxylate Oof (S)-alanine. The N(1) + -H···O(2) and O(1)-H···O(4)distances are 2.863 (2) Å and 2.521 (2) Å, respectively. The phenyl ring of PPA is oriented in the same direction of the methyl group of (S)-alanine of chiral two-dimensional sheet, yielding a (S,S)-homochiral layer (Fig. 2). On the basis of this finding, the development of optical resolution of amino acid using PPA as an optical resolving agent is under investigation.

Experimental
All reagents were commercially available from WAKO Co. and used without purification.(S)-2-Phenoxypropionic acid (1.66 g, 10 mmol) and (S)-alanine (0.89 g, 10 mmol) were dissolved in a water/ethanol solution (10 ml, 1:1 v/v). The solution was refluxed for 10 min, cooled to room temperature, and then kept in the refrigerator for three days. Colorless single crystals of I were obtained that were suitable for X-ray diffraction study.

Refinement
All hydrogen atoms were found in a difference Fourier map. The hydrogen atom of carboxylic O(1)-H was refined isotropically. Other hydrogen atoms were refined as riding atoms with C aromatic -H = 0.93 Å, C methyl -H = 0.96 Å, and C methine -H = 0.98 Å, and with U iso (H) = 1.5U eq (C methyl ) and U iso (H) = 1.2U eq (C aromatic , C methine ), respectively.

Computing details
Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008) andORTEP-3 (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999) and publCIF (Westrip, 2010).  The moleculer structure of the title compound I, showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness 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 threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) 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.