(2R,5S)-5-Benzyl-2,3-dimethyl-4-oxo-2-phenylimidazolidin-1-ium chloride

The title hydrochloride salt, C18H21N2O+·Cl−, is an imidazolidinone catalyst, which was derived from L-phenylalanine through cyclization with acetophenone. The imidazolidinone compound has a five-membered heterocyclic ring including two chiral centres. The imidazolidinone ring displays an envelope conformation, with the flap protonated N atom lying 0.497 (3) Å above the mean plane of the remaining four atoms. In the crystal structure, one-dimensional supramolecular chains parallel to the crystallographic 21 screw axis are formed by N—H⋯Cl hydrogen bonds involving the NH2 + and Cl− groups. Intramolecular N—H⋯Cl interactions are also present.

The title hydrochloride salt, C 18 H 21 N 2 O + ÁCl À , is an imidazolidinone catalyst, which was derived from L-phenylalanine through cyclization with acetophenone. The imidazolidinone compound has a five-membered heterocyclic ring including two chiral centres. The imidazolidinone ring displays an envelope conformation, with the flap protonated N atom lying 0.497 (3) Å above the mean plane of the remaining four atoms. In the crystal structure, one-dimensional supramolecular chains parallel to the crystallographic 2 1 screw axis are formed by N-HÁ Á ÁCl hydrogen bonds involving the NH 2 + and Cl À groups. Intramolecular N-HÁ Á ÁCl interactions are also present.

Related literature
For chiral secondary amine catalysts based on the imidazolidinone architecture, see: Ouellet et al. (2007). For Michael additions of aldehydes to enones with a MacMillan imidazolidinone catalyst, see: Hechavarria Fonseca & List (2004  Hydrogen-bond geometry (Å , ).  (2R,5S)-5-Benzyl-2,3-dimethyl-4-oxo-2-phenylimidazolidin-1-ium chloride S. Zhang, Y. Wang, B. Li, G. Zhang and S. Luo Comment Ten years ago, MacMillan and his laboratory developed chiral secondary amine catalysts based on the imidazolidinone architecture, which has led to the development of over 30 different enantioselective transformations for asymmetric synthesis (Ouellet et al., 2007). In recent years, Michael additions of aldehydes to enones with a MacMillan imidazolidinone catalyst have been reported (Hechavarria Fonseca & List, 2004). The title compound, prepared as a kind of organocatalyst for use in the asymmetric Michael addition of aldehydes to enones, was synthesized from L-phenylalanine. The crystal structure and absolute configuration of the title compound are reported in this article.
The compound consists of an ionic pair, a protonated ammonium cation and a Clanion ( Fig. 1). The chiral atom C1 has the expected S configuration, while the other chiral atom C3 was determined to be in a R configuration. The C1/C2/C3/N1 atoms of the imidazolidinone ring are almost coplanar. The distance of atom N2 to the C1/C2/C3/N1 mean plane is 0.497 (3) Å, while the distance of atom C12 of the benzyl group to the plane is 0.920 (4) Å. In the crystal structure of the title salt, one-dimensional supramolecular chains are formed, by intra-and inter-molecular N-H···Cl hydrogen bonds (Fig. 2).

Experimental
To a stirred solution of L-phenylalanine (3.87 g, 24 mmol) in dry methanol (50 ml) was added thionyl chloride (2.72 ml, 37.5 mmol). After refluxed for 48 h., the solution was concentrated under vacuum and L-phenylalanine methyl ester dichloride crystallized from methanol to give white crystals. To an ethanolic MeNH 2 solution (8 M, 6 ml) was added L-phenylalanine methyl ester dihydrochloride (2.41 g, 10 mmol). The solution was stirred at room temperature for 24 h, and then the solvent removed under reduced pressure. To this residue was added MeOH (80 ml), acetophenone (3 g, 25 mmol), and a catalytic amount of p-toluenesulfonic acid (30 mg, 0.16 mmol). The resulting solution was refluxed for 24 h and further stirred at room temperature for 2 h. The mixture was subsequently concentrated under reduced pressure, giving the crude product.
The resolution of the racemic compounds was by means of column chromatography with methyl and petroleum ether (1:4).
The residue was taken up in ethylether and a solution of HCl-dioxane (4.0 M) was added to the precipitate. Suitable crystals were obtained by slow evaporation of a methanol solution of the crude at room temperature.