Crystal structure of 1-ethyl-3-(2-oxo-1,3-dithiol-4-yl)quinoxalin-2(1H)-one

A new quinoxalin-derived ene-dithiocarbonate was synthesized and structurally analysed. The synthetic procedure and the compound’s spectroscopic and analytical characterization are reported.


Chemical context
Non-innocent dithiolenes and their role as interesting ligand systems were discovered in the early 1960s. As a result of their unusual redox and structural characteristics and those of their metal complexes, they immediately attracted considerable scientific interest (Schrauzer & Mayweg, 1962). Initially, dithiolene systems were studied predominantly in the context of electronic and photonic conductors (Wudl et al., 1972;Ferraris et al., 1973). Later, metal dithiolene complexes found application in the purification and separation of olefins (Wang & Stiefel, 2001). In the early 1980s Rajagopalan and co-workers discovered and characterized the natural molybdopterin ligand (mpt) in the active sites of enzymes. Mpt is present in nearly all molybdenum enzymes and all tungsten enzymes and binds the respective central metal by a dithiolene moiety. As these enzymes are ubiquitous to all kingdoms of life, this brought dithiolene chemistry again to the focus of scientific attention (Johnson et al., 1980;Johnson & Rajagopalan, 1982;Kramer et al., 1987). Quinoxaline constitutes a widely exploited platform in the development of pharmaceuticals (Shi et al., 2018). The title compound is a dithiolene ligand precursor, which can be used for the synthesis of molybdopterin cofactor model complexes bearing quinoxaline substituents. The target dithiolene ligand replicates the pyrazine moiety of mpt in its half-reduced form and in addition contains an oxofunction in the position of the pyran ring of the natural product. By itself it is an interesting example of an extended -system involving (by resonance) three different heteroatoms (N, O and S).

Structural commentary
The title molecule I crystallizes in the monoclinic space group C2/c with Z = 8. The quinoxaline ring system [C4-C11, N1, N2; largest deviation from plane = 0.041 (2) Å for C5] and the dithiolene ring [C1-C3, S1, S2; largest deviation from plane = 0.012 (1) Å for C3)], which are connected by the C3-C4 bond [length = 1.465 (3) Å ], are essentially coplanar, with an angle of only 4.89 (12) between the two planes ( Fig. 1). This planitarity is supported by intramolecular hydrogen bonding between the dithiolene hydrogen atom and the quinoxaline carbonyl oxygen atom [C2-H2Á Á ÁO2 with DÁ Á ÁA = 2.859 (3) Å ; Table 1]. Only the alkyl substituent C12-C13 subtends out of the planar geometry with an N1-C12-C13 torsion angle of 112.78 (18) . While the N1-C12 bond [amine nitrogen and ethyl substituent; 1.475 (3) Å ] is of explicit single-bond character, all other N-C distances are decidedly shorter, ranging from 1.296 (3) Å for the, according to the chemical structure, double bond of imine nitrogen (N2 C4) to 1.392 (3) Å for the amine nitrogen-to-benzene ring formal single bond (N1-C6). The longest C-C bond of the benzene ring is the one that is shared with the N-heterocycle [C6-C11, 1.411 (3) Å ]. This, together with the adjacent N-C bonds [N1-C6, 1.392 (3) Å and C11-N2, 1.374 (3) Å ] being significantly shorter than single bonds indicates resonance throughout the entire quinoxaline substituent. The C O bond [1.212 (3) Å ] of the carbonodithioate moiety is slightly shorter than the carbonyl C O bond [1.232 (3) Å ] of the quinoxaline substituent, suggesting that the latter might be involved to a small extent in resonance effects of the -system, whereas the former is not. The C2 C3 double bond of the ene-dithiocarbonate moiety is at 1.341 (3) Å slightly longer than the average value for C C double bonds of 1.331 (9) Å (Allen et al., 1995), which again may be due to participation in resonance effects throughout the entire molecule. The deviation from the average value of 1.751 (17) Å for S-Csp 2 bonds (Allen et al., 1995) of the S1-C2 bond [1.724 (2) Å ] is substantial enough to suggest that the resonance effects extend up to this bond to which partial double-bond character can be assigned. All other C-S bonds concur with typical single bonds.

D-HÁ
x; Ày þ 1; z þ 1 2 ; (iii) Àx þ 1 2 ; Ày þ 1 of the thiazole in B compared to the ene-dithiocarbonate while the quinoxalin carbonyl and amine functions are embraced to a lesser extent and (ii) an overall weaker resonance in A, in which the benzene ring C-C distances are all very similar (i.e. strongly resonant) whereas all other distances are of more pronounced single-and double-bond character and of less aromatic character.

Supramolecular features
In the crystal, the associated dimers are linked by (partly rather weak) C-HÁ Á ÁO and C-HÁ Á ÁS hydrogen-bonding interactions, forming a three-dimensional network (Fig. 2, Table 1). In the three-dimensional network, each molecule forms hydrogen-bonding interactions to six surrounding molecules. These are donor interactions involving C2 [C2-H2Á Á ÁO2(Àx, Ày + 2, Àz + 1); DÁ Á ÁA = 3.133 (3) Even though there are coplanar alignments of layers, only offsetstacking was observed with centroid-centroid distances of 3.587 (3) Å between the benzene ring of one molecule and the pyrazine ring of a molecule in the layer above or below.

Synthesis and crystallization
The title compound, 1-ethyl-3-(2-oxo-1,3-dithiol-4yl-)quinoxalin-2(1H)-one was synthesized based on a reported literature procedure (Mamedov et al., 2005). The compound was synthesized in five steps starting from o-phenylenediamine. The last step in the synthetical pathway was carried out via an acid-catalysed Tchugaeff ring closure reaction, which led to the formation of the dithiolene ring.
Synthesis of 1-ethyl-3-(2-oxo-1,3-dithiol-4yl-)quinoxalin-2(1H)-one: To a solution of S-2-(4-ethyl-3-oxo-4-dihydroquinoxalin-2-yl)-2-oxo-ethyl o-isopropyl carbonodithioate (11.180 g, 31.9 mmol) in 250 ml DCM/Et 2 O 1:1 at ambient temperature, H 2 SO 4 (25.50 ml) was added. The reaction mixture was stirred at room temperature for 2h. After that, the reaction was quenched by addition of 250 ml of ice and the mixture was stirred for 30 min. The organic phase was washed with brine and water 3 Â 250 ml. The solvent was reduced to 10 ml in vacuo and the greenish precipitate was filtered off and washed on the filter with cold acetone 3 Â 50 ml. The title compound was obtained as a greenish-white powder. Single crystals suitable for X-ray analysis were obtained by slow diffusion of solvents with chloroform and Et 2 O Yield: 1.85g (20%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atom of the dithiolene unit (H2) was refined freely without any constraints or restraints. All other C-bound hydrogen atoms were attached in calculated positions and treated as riding: C-H = 0.98 Å with U iso (H) = 1.5U eq (C) for the methyl group, C-H = 0.99 Å with U iso (H) = 1.2U eq (C) for the methylene group and C-H = 0.95 Å with U iso (H) = 1.2U eq (C) for the aromatic atoms.

1-Ethyl-3-(2-oxo-1,3-dithiol-4-yl)quinoxalin-2(1H)-one
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.