Benzene-1,3,5-triyl tris(2,2-dimethylpropanoate)

In the title compound, C21H30O6, the three acetoxy groups are essentially planar with their normals rotated by −57.75 (4), −62.36 (4) and 63.36 (4)° from the normal to the mean plane of the C6 ring. The arrangement of carbonyl groups around the ring is of two groups ‘up’ and one ‘down’, and the propeller-style arrangement of substituent groups is spoiled with the plane of the ‘down’ group twisted in the opposite direction; all the Cring—O—C—CMe3 conformations are trans. In the crystal, aromatic π–π stacking occurs [centroid–centroid separation = 3.320 (1) Å]; the other main intermolecular interaction is a C—H⋯π-ring contact on the opposing side from the overlapped ring pairing; there are no short C—H⋯O contacts.


Comment
Structural factors which enhance the solubility of organic compounds in liquid carbon dioxide are difficult to identify, but a knowledge of these is important in view of the possibility of using liquid carbon dioxide as an environmentally acceptable, cheap, safe and readily available alternative to replace organic-based solvents in the development of so-called "green chemistry". Previous studies (Haines et al., 2008) have shown that certain types of acyl group promote the solubilities of per-acylated D-glucopyranose derivatives in liquid carbon dioxide; in particular trimethylacetyl groups promoted solubility, their effect being comparable to acetyl groups and superior to dimethylacetyl groups. In searching for an explanation for solubility differences in this series based on differing intermolecular forces in the solid state, we conducted crystal structure studies on the compounds (Haines & Hughes, 2007), but the results indicated no substantial difference in such intermolecular forces.
Measurement of solubilities in liquid carbon dioxide of 1,3,5-triacetoxybenzene (2) and substituted derivatives, viz 1,3,5tris-(dimethylacetoxy)benzene (3) and 1,3,5-tris-(trimethylacetoxy)benzene (1), chosen in an attempt to separate the effects on solubility of the number and structure of peripheral substituents in compounds of similar overall molecular dimensions to the carbohydrate derivatives, showed no major differences (Haines, et al., 2009, unpublished results) and prompted an investigation of their crystal structures in order to compare intermolecular interactions in these compounds.
The stucture of the title compound, (1), is shown in Figure 1; compound 2 is described in the preceding paper (Haines and Hughes, 2009). Unfortunately, of the crystals of 3 grown under a variety of conditions, none gave acceptable diffraction patterns, all showing diffuse and blurry diffracted beams and refinement of a disordered structure to rather poor R-values. It is noteworthy that the corresponding dimethylacetyl derivative in the D-glucopyranose series also showed some disorder in its crystalline form. It is also of interest that in both the D-glucopyranose and benzene series, we were unable to crystallize the propionyl derivatives.
Compound 1 was prepared by the acylation of 1,3,5-trihydroxybenzene with trimethylacetyl chloride and formed crystals with molecules arranged in interacting pairs by overlap of the arene rings about a centre of symmetry. C(2) lies over the centre of the opposing ring and is 3.320 (1) Å from the mean-plane of that ring, and C(1) overlaps C(3 a ) at a distance of 3.326 (2) Å. H(13Bb) is directed towards the centre of the C 6 ring from the opposite side and is displaced 2.81 Å from the ring plane; the six H···C distances lie in the range 3.06-3.22 Å; supercripts denote symmetry operations. In contrast to 1,3,5-triacetoxybenzene (2) (previous paper, Haines and Hughes, 2009), there are no intermolecular C-H···O contacts with H···O distances less than 2.55 Å. There are also differences in intramolecular structure from that in 2 in the arrangement of carbonyl groups around the ring. Thus, the three acetoxy groups are essentially planar (as in 2) but have normals rotated -57.75 (4), -62.36 (4) and 63.36 (4)° from the normal to the mean-plane of the C 6 ring (in contrast to the three positive angles in 2). Also, the carbonyl O-atoms of two groups here are 'up' and one is 'down' (in contrast to three 'up' in 2), and  The title compound was prepared by conventional acylation of the parent 1,3,5-trihydroxybenzene and gave analytical data (HRMS) and spectral data ( 1 H and 13 C NMR) in full accord with the expected structure.

Refinement
Hydrogen atoms were included in idealized positions and their U iso values were set to ride on the U eq values of the parent carbon atoms. Fig. 1. The molecular structure of (I). Part of a related, neighbouring molecule is also shown; C(2 1 ) is directly under the ring C(1-6). Displacement ellipsoids are drawn at the 50% probability level.