Crystal structure of 4-[(adamantan-1-yl)amino]naphthalene-1,2-dione

The title compound, an example of a stable 1,2-naphthoquinone, illustrates steric buttressing of the adamantanyl group.


Chemical context
The formation of 4-amino-1,2-naphthoquinones is important in the colorimetric analysis (Folin analysis) of amines (Folin, 1922). However, the isolation and characterization of these aminoquinones is not common (Asahi et al., 1984). In the literature, it is reported that the yields for the formation of 1,2-naphthoquinones with a primary amino group in the 4-position are greatly inferior to those of secondary amino groups (Bullock et al., 1970). These inferior yields may be due to the equilibrium of amine/imine tautomeric forms (Yano et al., 1980;Fragoso et al., 2010), which would complicate the identification of 4-primary amino-1,2-naphthoquinones (Hartke & Lohmann, 1983). As part of our work on the synthesis and properties of naphthoquinones (Lamoureux et al., 2008), we were interested to prepare and analyze the structure of the title compound 4-[(adamantan-1-yl)amino]naphthalene-1,2-dione, also known as 4-(1-adamantanylamino)-1,2-naphthoquinone). To the best of our knowledge, the hybrid of a naphthoquinone core with an adamantanyl substituent is not known in the literature (Lamoureux & Artavia, 2010). ISSN 2056-9890

Structural commentary
In the molecule of the title compound ( Fig. 1), the C O bond length of the carbonyl group at the 1-position [C11 O2 = 1.216 (2) Å ] is shorter than the other at the 2-position [C2 O1 = 1.241 (2) Å ], suggesting strong delocalization from the trigonal-planar nitrogen at the 4-position, causing a decrease of the double-bond character at the C2 carbonyl (vinylogous amide), whereas the C1 carbonyl atom is unaffected. Further evidence of this delocalization is shown by a short N1-C4 bond distance [1.346 (2) Å ], which is intermediate between the C-N and C N bond distances observed in a related quinone amine/imine structure (Lamoureux et al., 2018). The aliphatic bond distance [N1-C12 = 1.482 (2) Å ] is longer than expected, but may be caused by the bulky adamantanyl group. Further evidence of the steric effect of the adamantanyl group is shown by the large angle at the planar nitrogen atom [C4-N1-C12 = 131.1 (2) ] compared to the ideal value of 120 . Most strikingly, the compression on one side of the adamantane ring causes another through-space compression between the NH group and the aromatic ring of the naphthoquinone ring system (H1Á Á ÁH6 = 1.82 Å ; H1Á Á ÁC6 = 2.37 Å ). The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. The steric compression is shown as a dotted line.

Figure 2
Partial crystal packing of the title compound. C-HÁ Á ÁO interactions are shown as dashed lines. The C-HÁ Á Á interaction is shown as a green dashed line between the orange centroid of the aromatic ring and the hydrogen atom H15B. Table 1 Hydrogen-bond geometry (Å , ).

Supramolecular features
In the crystal structure of the title compound (Fig. 2), molecules are linked into a three-dimensional network by C-HÁ Á ÁO hydrogen bonds (Table 1) involving as donors the C-H groups of both the adamantanyl system and the benzene ring. The crystal packing is further consolidated by C-HÁ Á Á interactions. There are nointeractions, the aromatic rings being separated by more than 6 Å .

Database survey
A search of the Cambridge Structural Database (Version 5.39, update February 2018; Groom et al., 2016) for the substructure 4-amino-1,2-naphthoquinone yielded seven hits. However, only one structure (refcode ZARNOY; Hatfield et al., 2017) contains a primary amine (aniline) in the 4-position. The distance between the N-H group and the coplanar aromatic hydrogen atom [1.93 (4) Å ] in this structure is longer than in the title compound, probably due to the smaller size of the nitrogen substituent. Surprisingly, the carbonyl groups in ZARNOY are almost coplanar [torsion angle of 0.2 (5) ]. In the same reference (Hatfield et al., 2017), another structure is reported (refcode ZARPAM) with a secondary amine (Nmethylaniline), which has a completely different structure from ZARNOY: the nitrogen is not planar, the amino moiety is twisted with respect to the naphthoquinone plane and the C4-N bond distance is greater in the case of the secondary amine. The authors summarize the differences between the structures and rationalize these differences using the concept of tautomerization (more accurately greater delocalization) in the structure with the primary amine.
Of the other structures in the database, four structures contain a secondary amine connected at the 4-position. Two structures (refcodes DMANPQ10 and EANAPQ10; Bechtel et al., 1976), involve the simple aliphatic amines dimethylamine and diethylamine. One structure (SEJZIQ; Ukhin et al., 1997) combines the cyclic morpholine with 1,2-naphthoquinone. The structure of XANRUB (Singh et al., 2011) contains a carbazole moiety at the 4-position of the 1,2naphthoquinone unit.
Finally, one structure AMNPQH10 (Aime et al., 1970) is anomalous since it contains an -NH 2 group at the 4-position, yet has bond and angle parameters completely different from the other molecules. Based on our analysis, this structure from 1970 should be re-analyzed to determine whether it could be best refined as an iminoquinone.

Synthesis and crystallization
The synthesis of 4-[(adamantan-1-yl)amino]naphthalene-1,2dione is based on a new procedure (complete publication in progress). In a reaction tube were mixed 740 mg (2.00 mmol) of 1,2-naphthoquinone-4-sulfonic acid cesium salt, 76 mg (0.50 mmol, 1 equiv) of adamantan-1-amine, and 302 mg (1.00 mmol, 2 equiv) of tetrabutylammonium acetate. The solids were dissolved in tert-amyl alcohol (5.0 mL). A cellulose extraction thimble with Li 2 CO 3 was placed above the reaction mixture. This solution was stirred at 393 K under a nitrogen atmosphere for 5 h. After being allowed to cool to room temperature, the dark-brown solution was diluted with toluene (30 mL), filtered and concentrated under reduced pressure. A brownish-red solid (503 mg) of the crude product was obtained. The crude product was further purified by column chromatography using silica gel with a gradient solvent elution [100% dichloromethane (CH 2 Cl 2 ) and then dichloromethane/2-propanol (CH 2 Cl 2 /C 3 H 8 O, 9:1 v/v)]; the fractions were dried under vacuum to yield 72 mg of a darkorange solid product (47% yield), determined pure by NMR analysis. Part of the purified product was re-dissolved in heptane and cooled to 203 K for crystallization. Red crystalline blocks suitable for X-ray analysis were obtained, m.p. 522 K (decomposition) determined using a Fisher-Johns melting-point apparatus with calibrated thermometer.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The N-bound H atom was located in a difference-Fourier map and refined as riding, with N-H = 0.88 Å , and with U iso (H) = 1.2 U eq (N). All other H atoms were placed geometrically and refined using a riding-atom approximation, with C-H = 0.95-1.00 Å , and with U iso (H) = 1.2U eq (C).

Funding information
The Centro de Investigaciones en Productos Naturales (CIPRONA), the Centro de Electroquímica y Energía Química (CELEQ) and the Escuela de Química, Universidad de Costa Rica (UCR) provided support.

Computing details
Data collection: APEX3 (Bruker, 2015); cell refinement: SAINT (Bruker, 2015); data reduction: SAINT (Bruker, 2015); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: shelXle (Hübschle et al., 2011); software used to prepare material for publication: SHELXL (Sheldrick, 2015b). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.