rac-(2R,3S)-2-Phenyl-3-(3-phenyl-1,2,3,4-tetrahydroquinoxalin-2-yl)quinoxaline

The title compound, C28H22N4, is the unexpected by-product of the reaction of 2-hydroxyacetophenone and 1,2-diaminobenzene under iodine catalysis, during which a carbon–carbon σ-bond between two quinoxaline units was formed. Although a fully oxidized title compound should sterically be possible, only one quinoxaline ring is fully oxidized while the second ring remains in the reduced form. As expected, the tetrahydroquinoxaline unit is not planar; it adopts a sofa conformation, whereby the atom joining the two heterocyclic systems lies out of the plane of the other atoms. The quinoxaline ring system makes a dihedral angle of 53.61 (4)° with its phenyl ring substituent. The crystal packing is determined by pairs of N—H⋯N, N—H⋯π and weak C—H⋯N hydrogen bonds, forming a chain parallel to the a axis.

The title compound, C 28 H 22 N 4 , is the unexpected by-product of the reaction of 2-hydroxyacetophenone and 1,2-diaminobenzene under iodine catalysis, during which a carbon-carbon -bond between two quinoxaline units was formed. Although a fully oxidized title compound should sterically be possible, only one quinoxaline ring is fully oxidized while the second ring remains in the reduced form. As expected, the tetrahydroquinoxaline unit is not planar; it adopts a sofa conformation, whereby the atom joining the two heterocyclic systems lies out of the plane of the other atoms. The quinoxaline ring system makes a dihedral angle of 53.61 (4) with its phenyl ring substituent. The crystal packing is determined by pairs of N-HÁ Á ÁN, N-HÁ Á Á and weak C-HÁ Á ÁN hydrogen bonds, forming a chain parallel to the a axis.

Comment
Quinoxalines are a versatile class of heterocyclic compounds. This moiety is found in pharmaceutically and biologically active molecules e.g. as potential antibiotics (Kim et al., 2004), DNA cleavage agents (More et al., 2005) and for inhibition of tumor activity (Gazit et al., 1996). The electron-withdrawing property of quinoxalines leads to their use in electroluminescent devices as electron transporters (Shirota & Kageyama, 2007). Often these transporters are designed as a dipolar unit consisting of an acceptor (quinoxaline) and a donor (e.g. triarylamines) Kulkarni et al., 2006). Lately quinoxalines have been used as ligands for metal complexes (Jones et al., 2006) that show efficient electroluminescence (Hwang et al., 2005) in organic light-emitting diodes (OLEDs).
These catalysts are necessary to oxidize the alcohol from the a-hydroxy ketones. In the present study a synthesis for 2phenylquinoxaline was planned under similiar conditions to those used by More et al. (2005) from 2-hydroxyacetophenone, 1,2-diaminobenzene and iodine without the use of an oxidization catalyst. As expected the yield of the reaction was low, but beside the anticipated product (I) the title compound (II) was formed. To the best of our knowledge neither the structure itself nor this type of formation have been described in the literature. Raw et al. (2004) describe the formation of an azobenzene derivative as the by-product. The most striking feature is the formation of a carbon-carbon σ-bond [C1-C15 1.536 (2) Å] between two quinoxaline moieties. Assuming that compound (I) is formed in the first place, either the attack on the C═ N bond or the reaction of (I) as a nucleophile with the starting materials could lead to the formation of a dimer. Subsequent reduction with 2 equivalents of hydrogen would form the title compound (II). Barik et al. (1999) demonstrated that dimeric structures starting from imines can be formed via a samarium-induced iodine-catalyzed reduction. The authors postulate a one electron transfer mechanism across the C═ N bond resulting in two carbon radicals merging in a pinacol type reaction.
Even though these conditions cannot be found in our case, it is this reference that is most relevant to the formation of a dimer.
The red color of compound (II) is remarkable and the origin is unclear, because the UV/VIS-spectrum shows no significant absorption above a maximum of 320 nm (ε = 9300 in CH 3 CN). In comparison McGovern et al. (2005) have shown that an intramolecular charge-transfer causes the red color of 9,14-dihydrodipyridophenazine, which possesses a moiety like the 1',2',3',4'-tetrahydroquinoxaline in the present study. The title compound potentially exists as two different diastereomers, but one of them is formed exclusively, as shown by spectroscopic evidence. We surmise that the other diastereomer is suppressed for steric reasons.
The molecular structure of compound (II) is illustrated in Fig. 1. Bond lengths and angles in the two phenyl rings and in the quinoxaline unit are normal. As expected the tetrahydroquinoxaline unit is not planar; it adopts a sofa conformation, whereby the atom joining the two heterocyclic systems lies out of the plane of the other atoms. In the crystal structure of (II) the packing of the molecules is determined by weak N4-H02···N2 and C15-H15···N3 hydrogen bonds (Fig 2 and Table 1). Pairs of alternating C-H···N and N-H···N hydrogen bonds are formed across inversion centres. Additionally, there is an N-H···π contact from N3-H01 to the centroid of the phenyl ring [C23-C28]. The overall effect is to form a chain parallel to the a axis.
630 mg (14%) of 2-phenylquinoxaline, the expected product, and 390 mg (9%) of the unexpected title compound, (II), were obtained. Red crystals of (II) grew overnight from the eluted fractions of the flash column chromatography.