Crystal structure of (2R)-1-[(methylsulfonyl)oxy]propan-2-aminium chloride: a chiral molecular salt

In the title chiral molecular salt, C4H12NO3S+·Cl−, the cation is protonated at the N atom, producing [RNH3]+, where R is CH3SO2OCH2C(H)CH3. The N atom in the cation is sp 3-hybridized. In the crystal, cations and anions are connected by strong N—H⋯Cl hydrogen bonds to generate edge-shared 12-membered rings of the form {⋯Cl⋯HNH}3. This pattern of hydrogen bonding gives rise to zigzag supramolecular layers in the ab plane. The layers are connected into a three-dimensional architecture by C—H⋯O hydrogen bonds. The structure was refined as an inversion twin.


S1. Chemical context
The chiral 2-amino-2-(alkyl/aryl/aralkyl)ethyl methanesulfonate hydrochlorides are useful starting materials for the preparation of amines, benzoates, thiobenzoates, sulfonic acids, etc., as methanesulfonate is a very good leaving group in nucleophilic substitution reactions. The chiral 2-(alkyl/aryl/aralkyl)ethanesulfonic acid derivatives and sulfonopeptides (Higashiura et al., 1989) occur in high concentrations in many mammalian tissues. These compounds are involved in various important physiological processes and are used as enzyme inhibitors and heptans in the development of catalytic anti-bodies (Braghiroli & Di Bella, 1996). The enantiomers of chiral 2-(alkyl/aryl/aralkyl)ethanesulfonic acid derivatives mimic the hypotensive effect of taurine (2-aminoethanesulfonic acid), one of the most abundant amino acids in mammals that seems to exhibit a special affinity for excitable tissues, such as brain, nerve and muscle (Xu et al., 2002;Pollack et al., 1989;Morgan et al., 1991). In particular, the title compound was used in the synthesis of chiral amines by our group and as a part of our on-going research the structure of the title compound was determined.

S2. Structural commentary
In the title chiral molecular salt, C 4 H 12 NO 3 S + .Cl -, the N atom is protonated resulting the cation [RNH 3 ] + where R is CH 3 SO 2 OCH 2 CH(CH 3 )-and the anion is chloride ion [Cl] -. The N atom in the cation is sp 3 hybridized and the bond angles represents that the cation has tetrahedral structure around N ( Fig. 1). In the crystal packing N-H···Cl hydrogen bonds connect ions into a supramolecular assembly in the ab plane ( Fig. 2 and Table 1). Further, there exist C-H···O hydrogen bonds that connect the layers into a three-dimensional architecture.

S3. Synthesis and crystallization
The title chiral molecular salt was synthesised as per the literature procedure (Higashiura et al., 1989). An aqueous solution of HCl (4 M, 12 ml) was added to a stirred solution of (2R)-2-[(tert-butoxycarbonyl)amino] propyl methanesulfonate (2.53 g, 10 mmol ) in dioxane (15 ml). The resulting mixture was stirred for a further 1 h. The solution was then concentrated under reduced pressure and the residue obtained was recrystallized from hot ethanol to afford colourless single crystals suitable for single crystal X-ray diffraction.

S4. Refinement details
The H atom of the NH 3 group was located in a difference map but refined with N-H = 0.89, and with U iso (H) = Molecular structure of the title molecular salt showing displacement ellipsoids drawn at the 50% probability level.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refined as a 2-component inversion twin.