Crystal structure of 4,5-dimethyl-1,3-dioxol-2-one

4,5-Dimethyl-1,3-dioxol-2-one crystallizes on a mirror plane in P21/m and forms layered sheets that comprise antiparallel linear strands. Close O⋯H, C⋯O and C⋯C intermolecular contacts are formed between strands and sheets.

Although a few coordination complexes with the dimethyldioxolene ligand are known, they have been prepared by oxidative addition of the corresponding -diketone to a low-valent metal precursor (Chisholm et al., 1983) or by an obscure route involving the reductive coupling of CO(g) with methyl ligands (Hofmann et al., 1985). This context of demonstrated usefulness and unrealized, but plausible, possibility for 1 persuaded us to undertake a study of its utility as a dioxolene ligand precursor. In an early research stage, serendipitously obtained diffraction-quality crystals of 1 provided an opportunity for characterization by X-ray diffraction, details of which are reported herein.

Structural commentary
Compound 1 crystallizes in the monoclinic space group P2 1 /m upon a crystallographic mirror plane that coincides with the carbonyl bond ( Fig. 1).

Supramolecular features
Molecules of 1 are aligned as one-dimensional strands by simple translation along one of the diagonals of the ac face of the unit cell (Fig. 2). The C O oxygen atom of 1 forms close contacts of 2.53 Å with the hydrogen atoms from each of the methyl groups of the molecule aligned before or behind (Table 1), at a distance that approaches the sum of the van der Waals radii of the elements (Batsanov, 2001). These strands are further organized into two-dimensional sheets through side-by-side placement but with an alternating orientation of the polarized, carbonyl end of the molecules (Fig. 2). The b axis defines the 2nd dimension of these sheets. Between strands within these sheets, interatomic HÁ Á ÁH separations are 2.89 and 3.05 Å , while nearest OÁ Á ÁO distances between rings are 3.3962 (13) Å . Fig. 3 presents a perspective of these sheets that is approximately along the b axis of the cell such that the close stacking between them is visible.
An alternative description of the third dimension of the packing is that the one-dimensional strands noted above translate as a whole along the a axis of the cell with an offset that places the carbonyl oxygen atom of one molecule of one strand near the five-membered ring centroid of a neighboring molecule (Fig. 4). In contrast to the intrasheet strands depicted in Fig Packing diagram for 4,5-dimethyl-1,3-dioxol-2-one (1) illustrating the sheetlike arrangement in the plane defined by the b axis and an ac face diagonal. Displacement ellipsoids are at the 50% level, and closest intermolecular contacts are indicated.

Figure 3
Cell packing diagram of 1 with a view along the b axis of the cell. Displacement ellipsoids are drawn at the 50% level, and all H atoms are omitted for clarity.

Figure 1
Displacement ellipsoid plot (50% probability) of 4,5-dimethyl-1,3-dioxol-2-one (1) with non-H atom labeling. intermolecular CÁ Á ÁC and CÁ Á ÁO contacts between these parallel strands are the C c Á Á ÁC o separation of 3.3413 (16) Å , the O c Á Á ÁC c spacing of 3.3452 (18) Å , and the O r Á Á ÁC o distance of 3.3742 (13) Å (c = carbonyl, o = olefin, r = ring). It is likely that the electrostatic interactions of polarized bonds, e.g., placement of the negative end of the (-) O C (+) carbonyl dipole above the positive end of the same bond in the sheet below, exert a decisive role in guiding the organization and spacing of one molecular plane over another. An end-on view of these sheets in space-filling presentation mode emphasizes the packing efficiency imposed by these cumulative intermolecular interactions (Fig. 5).

Database survey
Of the relatively few vinylene carbonates that have been structurally characterized, only 1 and the parent compound 2a (Cser, 1974) are simple, symmetrically substituted variants. All other structurally identified compounds bearing this moiety are more complex organic molecules that have been prepared and studied as angiotensin II receptor blockers (Yanagisawa et al., 1996;Dams et al., 2015;Zhang et al., 2017). Despite its ostensible similarity to 1, compound 2a crystallizes in a rather different fashion. Although arranged into extended sheets, which also contain the b axis, molecules of 2a are not organized into discernible linear strands but instead are twisted relative to their neighbors so as to accommodate multiple C-HÁ Á ÁO hydrogen-bonding interactions (Fig. 6). A glide plane, rather than simple translation, relates one molecule of 2a to another in the horizontal direction (Fig. 6). As their different space groups would necessitate, the packing arrangements for vinylene trithiocarbonate (2b, CSD refcode LAGMUC; Mereiter & Rosenau, 2005) and vinylene triselenolate (2c, SELOLS; Lyubovskaya et al., 1976) contrast greatly with 2a.
The former reveals linear strands of molecules arranged in sheets with a parallel orientation of all strands. Intermolecular stacking interactions appear to be the decisive packing force between sheets. The latter, when viewed along the b axis of the cell, reveals a herringbone-like pattern in the arrangement of molecules.
Compounds similar to 1 with methyl substituents at the 4 and 5 positions of the ring include 4a, already noted, and the all-sulfur form, 4b (DMTHTN; Smith & Luss, 1980). Compound 4b occurs in the same space group (No. 62, Pnma) as 2c with a qualitatively similar packing arrangement that differs in having the herringbone pattern visible when viewed along the cell's a axis. Compound 4a crystallizes in P2 1 /c on a general position with similar generalities of description pertinent to its packing pattern as found for 2a. However, adjacent strands of 4b that are generated by the glide plane operation are slightly out of plane relative to one another. The selenium analogue (6) of 1 and 4a has not been characterized crystallographically but is a target of current study in our laboratory. View of the cell packing arrangement in 1 depicting the closest intermolecular contacts between linear strands extending in the direction of a diagonal to the ac face. Displacement ellipsoids are at the 50% level. Space-filling plot of the unit-cell packing in 1 as viewed along the length of the linear strands, which are orthogonal to the paper plane.

Figure 6
Ball and stick representation of the sheetlike arrangement in 2a, where molecules (left-to-right) are related by a glide-plane operation rather than simple translation. Closest intermolecular contacts are illustrated. Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) x À 1; y; z þ 1.

Synthesis and crystallization
The sample of 4,5-dimethyl-1,3-dioxol-2-one used in this study was purchased from AK Scientific, Inc. and recrystallized by evaporation of a MeOH solution from a test tube at room temperature.

4,5-Dimethyl-1,3-dioxol-2-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. 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 > 2sigma(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. Hydrogen atoms were added in calculated positions and refined with isotropic displacement parameters that are approximately 1.5 times those of the carbon atom to which they are attached. The C-H distances assumed were 0.98 Å.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )