1,3-Diphenylisobenzofuran

The structure of the title compound, 1,3-diphenyl-2-benzofuran, C20H14O, exhibits a distinct alternation of short [mean 1.361 (3) Å] and long [mean 1.431 (3) Å] C—C bonds around the benzofuran ring system, indicating a predominantly polyene character. Over 60 Diels–Alder adducts of this commercially available furan have been structurally characterized, but this is the first report of the structure of the parent compound.

The structure of the title compound, 1,3-diphenyl-2-benzofuran, C 20 H 14 O, exhibits a distinct alternation of short [mean 1.361 (3) Å ] and long [mean 1.431 (3) Å ] C-C bonds around the benzofuran ring system, indicating a predominantly polyene character. Over 60 Diels-Alder adducts of this commercially available furan have been structurally characterized, but this is the first report of the structure of the parent compound.

Comment
Isobenzofuran is a ten π-electron system exhibiting very high reactivity in Diels-Alder reactions (Wege, 1998;Friedrichsen, 1999). The commercially available 1,3-diphenyl-2-benzofuran, (I), is a molecule with many interesting features (Friedrichsen, 1980) and unlike the parent compound is stable in the solid state. It is brightly fluorescent, electroluminescent and, despite its stability relative to isobenzofuran is still highly reactive in Diels-Alder reactions. Its reactivity is exploited in the quantitative kinetic investigations of biological singlet oxygen generation and in the in situ trapping of transient olefin intermediates. With respect to this latter application, the high reactivity of (I) and the crystallinity of its adducts account for the sixty-eight X-ray structures of diphenylisobenzofuran adducts that appear in the Cambridge Crystallographic Database (Allen, 2002). Surprisingly, an X-ray structure of (I) has not appeared in the literature.
Calculations have suggested that isobenzofuran has a low resonance energy (Yang & Duan, 1991) and mainly polyene character. The structures of isobenzofurans are of interest since the degree of bond length alternation serves to indicate the balance between aromatic and polyene character. Only three structures of isobenzofurans have been published previously: 1-cyano-4,5-methylenedioxyisobenzofuran (Rodrigo et al., 1986); 3,6-dimethoxyisobenzofuran (Lynch et al., 1995) and the highly strained 9,10,12,13-tetraphenyl-11-oxacyclopenta[b]triphenylene (Lu et al., 2006). All three structures show that the isobenzofuran core is essentially polyene in character with the structure of the furan ring very similar to that of furan itself.
The structure of (I) (Fig. 1) is very closely comparable to those of the three previously published examples. There is no significant evidence of bond length averaging indicating that it has predominantly polyene character. Of the three published structures, the bond lengths of (I) are closest to those calculated for the parent compound using the MP2/6-31G* basis set (Friedrichsen, 1980). The only noticeable difference is in the C1-C2, C7-C87 bonds which are slightly longer, perhaps the effect of conjugation to the phenyl substitutents. While the most recent structure (Lu et al., 2006) contains the diphenylisobenzofuran substructure, extensive peri interactions throughout the molecule lead to significant deviations from planarity making direct structural comparisons less meaningfull.
Strong steric interactions between the phenyl substituents of (I) and the peri H atoms (H3 and H6) are indicated by a 25° torsional twist of the phenyl rings out of the plane of the isobenzofuran ring and wide 135.3 (2)° and 135.5 (2)° bond angles for C2-C1-C9, C7-C8-C15.

Refinement
The space group was determined by trial and error and confirmed by a sucessful refinement. The alternative choice, Pc, gave unrealistic geometrical parameters. H-atoms were included at geometrically idealized positions with C-H distance supplementary materials sup-2 0.95 Å and U iso = 1.2 times U eq of the C-atoms to which they were bonded. The model was refined to convergence, and there were no chemically significant features on the final difference Fourier map. Fig. 1. Thermal ellipsoid plot of the structure of (I) drawn at 50% probability level showing the atom numbering scheme.