2-[(2Z)-Azepan-2-ylidene]-1-(4-nitrophenyl)ethanone

The title compound, C14H16N2O3, is an NH-vinylogous amide (enaminone) produced by the reaction of 4-nitrophenacyl bromide with azepane-2-thione. The conformation about the C=C bond [1.3927 (14) Å] is Z, which allows for the formation of an intramolecular N—H⋯O hydrogen bond that leads to an S(6) loop. Inversion-related molecules associate via N—H⋯O hydrogen bonds to form a 12-membered {⋯OC3NH}2 synthon.


Comment
The title compound was prepared as part of an ongoing methodological investigation into the use of enaminones in the synthesis of azabicyclic alkaloids (Michael et al., 1999). For example, its reaction with acryloyl chloride, according to the method of Paulvannan & Stille (1994), produced a 2,3,6,7-tetrahydro-5(1H)-indolizinone related to numerous natural products. The crystal structures of analogous 4-bromophenyl enaminones with 5-, 6-and 7-membered rings have been reported (Balderson et al., 2007).
The hydrogen bonding consists of an intramolecular N-H···O═C hydrogen bond, and an intermolecular N-H···O═C hydrogen bond (Table 1). The combination of these two hydrogen bonds results in an R 2 2 (4) ring ( Fig. 2) as described by graph set notation (Bernstein et al., 1995).

Experimental
The employed synthesis followed the Eschenmoser procedure (Roth et al., 1971). p-Nitrophenacyl bromide (995 mg, 4.08 mmol) was added to a solution of azepane-2-thione (502 mg, 3.88 mmol) in dry acetonitrile (30 ml). The resulting solution was stirred at room temperature for 4 h, after which S-alkylation was complete as shown by the precipitation of the thioiminium salt. This was then followed by the addition of triphenylphosphine (1.069 g, 4.08 mmol) and triethylamine (413 mg, 4.08 mmol) to induce sulfur extrusion. The reaction mixture was poured into water and the organic components were extracted with diethyl ether (3 × 30 ml). The resulting organic layer was dried over MgSO 4 , filtered and the solvent removed in vacuo. The resulting residue was purified by column chromatography on silica gel with hexane:ethyl acetate (19:1 v/v) as eluent to yield 2-[(2Z)-azepan-2-ylidene]-1-(4-nitrophenyl)ethan-1-one (829 mg, 82%) as yellow crystals, m.p. 398-400 K.

Refinement
The C-bound H atoms were geometrically placed [C-H = 0.95 Å (alkenyl-and aromatic-H) and 0.99 Å (methylene-H)] and refined as riding with U iso (H) = 1.2U eq (C). The N-bound H atom was located in a difference Fourier map and refined freely.  The molecular structure of (I) showing the atomic numbering scheme. Displacement ellipsoids are shown at the 50% probability level.

Figure 2
Hydrogen bonding diagram of the compound. Intermolecular and intramolecular N-H···O hydrogen bonds (shown as dashed red lines) form a four-membered ring. Non-participating H atoms have been omitted for clarity.