3-Nitroso-2,4,6,8-tetraphenyl-3,7-diazabicyclo[3.3.1]nonan-9-one

In the title compound, C31H27N3O2, the two piperidine rings fused to each other each adopt a slightly distorted chair conformation. The phenyl rings on the N-unsubstituted piperidine ring occupy an equatorial position, while those on the N-nitroso-substituted piperidine ring are in axial positions. The NO group is approximately coplanar with the piperidine ring with a maximum deviation of 0.048 (4) Å. The dihedral angles between the mean planes of the axially and equatorially oriented phenyl rings are 27.7 (1) and 31.9 (1)°, respectively. Molecular packing is stabilized by weak intermolecular C—H⋯O and C—H⋯π interactions.

In the title compound, C 31 H 27 N 3 O 2 , the two piperidine rings fused to each other each adopt a slightly distorted chair conformation. The phenyl rings on the N-unsubstituted piperidine ring occupy an equatorial position, while those on the N-nitroso-substituted piperidine ring are in axial positions. The NO group is approximately coplanar with the piperidine ring with a maximum deviation of 0.048 (4) Å . The dihedral angles between the mean planes of the axially and equatorially oriented phenyl rings are 27.7 (1) and 31.9 (1) , respectively. Molecular packing is stabilized by weak intermolecular C-HÁ Á ÁO and C-HÁ Á Á interactions.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: JJ2089).

S. Natarajan and R. Mathews
Comment The piperidine ring system offers a wide variety of conformational flexibility such as chair, boat and twisted boat conformations (Hofer, 1976). However, both the chair and slightly distorted chair conformations are found to be the most favored (Ramalingam et al., 1979;Mulekar & Berlin, 1989). N-nitroso piperidine compounds have been shown to occupy both axial and equatorial positions with the mean plane of the N-NO 2 group being coplanar to the mean plane of the piperidine ring (Hemalatha & Nagarajan, 2010;Sampath et al., 2005). The phenyl rings tend to occupy equatorial positions when the N-NO 2 group orients itself perpendicular to the piperidine ring to avoid steric hindrance. π-electron delocalization on the N-N-O group, which restricts the free rotation of N-N bond, results in orientations that are planar (syn; Pandiarajan et al., 1991) or perpendicular (anti; Rogers & Woodbrey, 1962) with respect to the piperidine ring. In 2,6-diphenyl-3-methyl-N-nitrosopiperidin-4-one (Hemalatha & Nagarajan, 2010) the nitroso group shows both syn and anti conformations while the piperidine ring displays a boat conformation which may influence the phenyl rings to occupy axial and equitorial positions with respect to the piperidine ring.
In the title compound both piperidine rings adopt a slightly distorted chair conformation (Cremer & Pople, 1975)

Experimental
Noller & Baliah (1948) developed a novel method to synthesize piperidin-4-one derivatives by the Mannich condensation reaction using respective aldehydes and ketones with ammonium acetate in the ratio of [2:1:1], respectively. The title compound was synthesized using benzaldehyde (0.2 M), acetone (0.1) and ammonium acetate (0.1M) added to pure ethanol and heated on a hot plate up to the boiling range. The resulting product of diazabicyclic[3.3.1]nonan-9-one was separated out supplementary materials sup-2 and treated with an equimolar (1:1) quantity of NaNO 2 /HCl/80% ethanol and kept at 80° C for 4 h with vigorous stirring.
The resuling title compound was separated out and crystals were grown using acetonitrile as the solvent.

Refinement
H atoms were positioned geometrically and refined using a riding model with C-H = 0.93 Å for aromatic H, 0.97 Å for methylene, 0.96 Å for methyl H atoms and N-H = 0.86 Å. The U iso parameters for H atoms were constraned to be 1.5U eq of the carrier atom for the methyl H atoms and 1.2U eq of the carrier atom for the remaining H atoms.