Crystal structure of 2-tert-butyl-2,3-dihydro-1H-benzo[c]pyrrol-1-one

The structure of 2-tert-butyl-2,3-dihydro-1H-benzo[c]pyrrol-1-one is compared with those of the related compounds (3R*,1′S*,3′R*)-3-(1′-tert-butylamino-1′H,3′H-benzo[c]furan-3′-yl)-2-tert-butyl-2,3-dihydro-1H-benzo[c]pyrrol-1-one and 2-isopropyl-2,3-dihydro-1H-benzo[c]pyrrol-1-one, with special attention paid to the planarity of the substituted pyrrole rings in these structures.


Chemical context
Orthophthalaldehyde (o-phthalaldehyde, OPA) is an aromatic dialdehyde bearing two electron-withdrawing carbonyl groups in positions 1 and 2. The reaction scheme involving OPA, (I), shown in Fig. 1 comprises the main concurrent as well as consecutive reactions, which are consistent with the results obtained herein. The reactions of OPA with primary amines, which were carried out by Winter (1900) and Thiele & Schneider (1909) for the first time, have been broadly applied for the synthesis of important heterocyclic compounds with biological relevance. A number of such reactions have been investigated recently and several structures of condensation products have been reported (DoMinh et al., 1977;Nan'ya et al., 1985;Takahashi et al., 1996Takahashi et al., , 2004Takahashi et al., , 2005Takahashi & Hatanaka, 1997). However, the reaction mechanism is still not fully understood. Determination of the products which would serve as a confirmation of the suggested reaction scheme (Fig. 1) is the reason for the present as well as for our previous studies (Donkeng Donkeng Dazie, Liška, Ludvík, Fá bry & Dušek, 2016;Donkeng Dazie et al., 2017).
The reason why a full understanding of the reaction mechanism is still lacking is the complexity of the abovementioned reactions, which are dependent on different variables. Our partly published electrochemical experiments have shown that the reaction kinetics, as well as the reaction products, depend on the primary amine which reacts with OPA, the reaction environment (solvent) and the proportion of the reactants (Donkeng Dazie, Liška & Ludvík, 2016). ISSN 2056-9890 Electrochemical monitoring has indicated the presence of side reactions, which result in a mixture of molecules of different molecular weights with different proportions of OPA and primary amine building blocks [cf. the reaction of OPA with 2-aminoethanol (kolamine); see Urban et al. (2007a,b)].
The complexity of the reactions between primary amines and OPA is affected by the environment in which they take place. The reaction of OPA with aliphatic primary amines in aqueous solutions involves competition between the amines and water molecules as nucleophiles. Although water is a weaker nucleophile than primary amines, an enormous excess of water over primary amines may cause significant additional reactions, such as covalent hydration at the double bond of the carbonyl group and the following cyclization (Zuman, 2004)see compounds (II) and (III) in Fig. 1. The reaction of OPA with aliphatic primary amines represents a concurrent process (DoMinh et al., 1977). All attempts to isolate and identify the products of the reaction of OPA with primary amines in aqueous solutions were unsuccessful due to the number of reactions occurring and products, including the oligo-and polymeric ones (checked by thin-layer chromatography). In order to simplify the reaction media, diethyl ether as a nonaqueous organic solvent was used with the hope that some products might be obtained as crystals suitable for X-ray structure analysis.

Figure 2
The atom-numbering scheme for the the two molecules of (V) (A and B) in the asymmetric unit, with anisotropic displacement parameters shown at the 50% probability level. lized and its structure has been determined previously (Donkeng Dazie et al., 2017).
The spectrometric results were confirmed unequivocally by the X-ray structure analysis of compound (V) (Fig. 2), as well as by the structure determination of (VI) (Donkeng Dazie et al., 2017). In addition to the confirmation of the presence of the products in solution after they had been resolved as crystals, the previous crystallographic studies of (VI) and 2-isopropyl-2,3-dihydro-1H-isoindol-1-one (Donkeng Dazie, Liška, Ludvík, Fá bry & Dušek, 2016) were focused on the problem of planarity of the annelated pyrrole and furan rings.
The planarity of the pyrrole rings, which include two atoms close to sp 3 -hybridized, was explained by propitious values of the inner angle in the regular pentagon of 108 , i.e. close to the ideal tetrahedral value of 109.54 . It turned out that the planarity is correlated on the C-N bond lengths in the pyrrole fragment. Specifically, pyrrole rings with longer N-C carbonyl bond lengths which exceed 1.39 Å tend to show better planarity than pyrrole rings with these shorter bond lengths (see Fig. 4 in the article by Donkeng Dazie, Liška, Ludvík, Fá bry & Dušek, 2016).
The structure of (VI) (Donkeng Dazie et al., 2017) contains pyrrole and furan rings as parts of isoindolinone and isobenzofuran rings, respectively. The planarity of the pyrrole ring is extremely distorted in this structure and deviates more from planarity than the furan ring in the same structure. This phenomenon can be explained by steric reasons due to the presence of a voluminous tert-butyl group. The distortion of the pyrrole ring can be provoked by repulsion of the parts of the isoindolinone and isobenzofuran rings which are close to each other. Therefore, the present structure determination is even more interesting because it offers a comparison of the distortion of the planarity of the pyrrole rings in the title structure with those in 2-tert-butyl-2, with respective less and more voluminous substituents.

Synthesis and crystallization
The synthesis of (V) was carried out at laboratory temperature under an argon atmosphere and the isolation procedure was similar to that reported by Takahashi et al. (2004Takahashi et al. ( , 2005. Orthophthalaldehyde (OPA, 0.335 g) was dissolved in diethyl ether (25 ml, 0.1 mol l À1 ) and tert-butylamine (0.183 g, 264 ml of the pure liquid compound) was added to the solution of OPA. The amounts of the reactants correspond to a 1:1 OPAamine stoichiometric ratio. The reaction mixture was stirred for 6 h. The solution was filtered and the ether was evaporated under reduced pressure. Two previously mentioned compounds, i.e. (V) and (VI), were identified in a light-yellow oily solution by 1 H and 13 C NMR analysis, as well as mass spectroscopy. After a few days at room temperature, lightyellow crystals of (VI) of the size of several tenths of mm appeared. After half a year, other crystals appeared in the form of thin light-yellow needles which were as long as 2 cm. Their other dimensions were smaller than 0.1 mm. These crystals corresponded to the expected product, namely the title compound (V).

Structural commentary
The title compound comprises two symmetry-independent molecules (A and B) in the asymmetric unit (Fig. 2), the ring systems of which are approximately coplanar [dihedral angle between the planes = 8.38 (4) ]. The two molecules are conformationally similar but not identical. The function AutoMolFIT in PLATON (Spek, 2009) yielded the weighted and unit-weight r.m.s. fits for the non-H atoms as 1.437 and 0.952 Å , respectively. The main difference between the two independent molecules lies in the conformations within the tert-butyl substituent group (Fig. 3) Table 1 lists the extremal deviations from the fitted planes through the core atoms of the pyrrole rings in the title structure, i.e. without the carbonyl O atoms, which were omitted from considerations. Fig. 4 illustrates the dependence of the maximal deviations from the best plane through the core atoms of the pyrrole rings on the N-C carbonyl distance.  (Table 1), which were retrieved from the Cambridge Structural Database (Version 5.36; Groom et al., 2016). (The retrieved structures contained no disorder and errors, while they were determined below 150 K, with R factors < 0.05; in case the structures contained two carbonyl groups, the retrieved data were collected twice and the variant with the larger N-C8 distance was selected for further consideration.) Fig. 4 also shows that the largest distortion of the pyrrole ring takes place in (VI) (Donkeng Dazie et al., 2017), the distortion being milder in the title molecules and being mildest in 2-isopropyl-2,3-dihydro-1Hisoindol-1-one (Donkeng Dazie, Liška, Ludvík, Fá bry & Dušek, 2016). It indicates that the reasons for the distortion of the pyrrole rings from planarity are steric ones in these cases: tert-butyl as a more voluminous group causes a larger distortion in comparison with the isopropyl group. In (VI), an interaction between the bulky isoindolinone and isobenzofuran ring moieties also takes place.

Database survey
The survey relating particularly to the structural features of the isoindolinone ring system has been covered in x3.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All H atoms were discernible in difference electron-density maps. However, the aryl, methylene and methyl H atoms were constrained, with aryl C-H = 0.95 Å , methylene C-H = 0.99 Å and methyl C-H = 0.98 Å , and with U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) otherwise.