1,2-Di-2-furylethane-1,2-dione

The title compound, C10H6O4, lies across a twofold rotation axis through the midpoint of the C—C bond between the two carbonyl groups. The furan ring plane and the plane through all atoms are inclined at 23.88 (1)°. In the crystal structure, weak C—H⋯O hydrogen bonds form sheets in the bc plane and columns down the c axis.

The title compound, C 10 H 6 O 4 , lies across a twofold rotation axis through the midpoint of the C-C bond between the two carbonyl groups. The furan ring plane and the plane through all atoms are inclined at 23.88 (1) . In the crystal structure, weak C-HÁ Á ÁO hydrogen bonds form sheets in the bc plane and columns down the c axis.

Comment
The structures of vicinal polycarbonyl compounds have been of interest for many years (Rubin & White, 1982). Bond lengths, bond angles, and torsion angles in such molecules can deviate from 'normal' values in order to minimize the repulsive interactions resulting from juxtaposition of dipolar carbonyl groups (Brown & Sadanaga,1965) and the steric interactions of the chain of carbonyl groups with the end groups present (Beddoes et al., 1975). We report here the crystal structure of the title vicinal dione compound (I) , Fig 1. The molecule lies about a twofold rotation axis at the mid-point of the C5-C5A bond (A = −x + 1/2, −y + 1/2, z). Bond lengths and angles are similar to those observed for benzil (Brown & Sadanaga,1965). The molecule is approximately planar with the maximum deviation from the plane through all atoms 0.954 (1)Å for the O1. The furan ring plane (O2, C2···C4) and the plane through all atoms are inclined at 23.88 (1)°. In the crystal structure, weak C-H···O hydrogen bonds, Table   1, form sheets in the bc plane and columns down the c axis.
Experimental Furfural (1.92 g, 20.0 mmol) was added to water (20 ml) together with the N,N-dialkylbenzimidazolium salt (1.14 g, 4.0 mmol) and triethylamine (0.5 ml, 3.6 mmol) and the solution stirred vigorously for 2 h under reflux to afford the title compound (1.71 g, yield 70%). Single crystals suitable for X-ray measurements were obtained by recrystallization from THF at room temperature.

Refinement
In the absence of significant anomalous scattering effects, Friedel pairs were merged. Hydrogen atoms were fixed geometrically and allowed to ride on their parent atoms, with d(C-H) = 0.93 Å, and U iso (H) = 1.2 U eq (C). Fig. 1. The molecular structure and atom-labeling scheme for (I), with displacement ellipsoids drawn at the 30% probability level. Atoms labelled A are related to other atoms by the symmetry operation −x + 1/2, −y + 1/2, z.

Figures
supplementary materials sup-2   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.