Crystal structure and Hirshfeld surface analysis of 2-benzyl-4,5-dibromo-2,3,3a,4,5,6,7,7a-octahydro-3a,6-epoxy-1H-isoindol-1-one

In the crystal, molecules are linked by weak C—H⋯O hydrogen bonds, forming sheets lying parallel to the (002) plane. These sheets are connected only by weak van der Waals interactions.


Chemical context
Halogenation is a chemical reaction that involves the introduction of one or more halogen atoms to an organic molecule. The pathway and stereochemistry of halogenation reactions is dependent on the configuration of the starting olefine and the halogenating agent. The role/behavior of the attached halogen atom in olefines can be classified into the following types: (1) as an electron-withdrawing substituent, (2) as a halogen-bond donor center, and (3) as a non-covalent bond acceptor site. Thus, not only hydrogen bonding Kopylovich et al., 2011) or other types of non-covalent interactions (Afkhami et al., 2017;Asadov et al., 2016;Ma et al., 2017a,b;2020;Mahmudov et al., 2010;2019;2020, but also halogen bonding can be used in the design of olefines. In this work, we proposed and tested inexpensive and readily available bis[N,N-dimethylacetamide] hydrogen dibromobromate [(Me 2 NCOMe) 2 H]Br 3 as a bromine initiator and a source of a positively charged bromine ion (Rodygin et al., 1992;Prokop'eva et al., 1994;Prokop'eva, 2008). The choice of [(Me 2 NCOMe) 2 H]Br 3 , obtained by one-pot synthesis from N,N-dimethylacetamide, hydrobromic acid and bromine, is down to the simplicity of the synthesis and isolation, and the unambiguous direction of the bromination process. In addition, [(Me 2 NCOMe) 2 H]Br 3 is an excellent reagent for functionalization of phenols and anilines (Rodygin et al., 1992;Mikhailov et al., 1993), and is also used in the synthesis of mono-bromo-substituted ketones Burakov et al., 2001) and for the bromination of various alkenes and alkynes Zaytsev et al., 2017). The present work is aimed at accumulating experimental data and establishing the rules of the halogenation in bridged epoxy-isoindolones Zaytsev et al., 2020). The reaction of N-benzyltetrahydroepoxyisoindolone (1) with [(Me 2 NCOMe) 2 H]Br 3 in dry chloroform under reflux leads to the corresponding 2-benzyl-4,5-dibromohexahydro-3a,6-epoxyisoindol-1(4H)-one (2) (Fig. 1).

Structural commentary
The asymmetric unit of the title compound ( Fig. 2) contains two molecules of similar shape, hereafter referred to as molecules A (including atom C1A) and B (including atom C1B). The conformational differences between molecules A and B are highlighted in an overlay diagram shown in Fig. 3. The r.m.s. deviation of the overlay between the molecules A and B is 0.114 Å .

Hirshfeld surface analysis
The intermolecular interactions (Table 2) were investigated quantitatively and visualized with Crystal Explorer 3.1 (Wolff et al., 2012;Spackman et al., 2009). The Hirshfeld surface plotted over d norm in the range À0.0815 to 0.9926 a.u. is shown in Fig. 6. The red spots on the Hirshfeld surface represent C-HÁ Á ÁO contacts. Fig. 7 shows the full two-dimensional fingerprint plot and those delineated into the major contacts: the HÁ Á ÁH (44.6%) interactions are the major factor in the crystal packing with BrÁ Á ÁH/HÁ Á ÁBr (24.1%), OÁ Á ÁH/HÁ Á ÁO (13.5%) and CÁ Á ÁH/HÁ Á ÁC (11.2%) interactions representing the next highest contributions. The percentage contributions of other weak interactions are listed in Table 3. A view along the b axis of the intermolecular C-HÁ Á ÁO interactions in the title compound. Table 2 Summary of short interatomic contacts (Å ) in the title compound.
Contact Distance Symmetry operation

Figure 4
A view along the a axis of the intermolecular C-HÁ Á ÁO interactions in the title compound.
In the crystal of AGONUH, the molecules are linked by C-HÁ Á ÁO hydrogen bonds into zigzag chains running along the b-axis direction. In TIJMIK, two types of C-HÁ Á ÁO hydrogen bonds generate R 2 2 (20) and R 4 4 (26) rings, with adjacent rings running parallel to ac plane. In addition C-HÁ Á ÁO hydrogen bonds form a C(6) chain, linking the molecules in the b-axis direction. In YAXCIL and UPAQEI, molecules are also linked by C-HÁ Á ÁO hydrogen bonds. In the crystal of ERIVIL, weak intermolecular C-HÁ Á ÁO hydrogen bonds link the molecules into R 2 2 (8) and R 2 2 (14) rings along the b-axis direction. In MIGTIG, the molecules are linked only by weak van der Waals interactions.

Synthesis and crystallization
A solution of isoindolone 1 (4 mmol) and the brominating agent (4 mmol) in dry chloroform (15 mL) was heated under reflux for 18 h (TLC control, EtOAc-hexane, 1:1). The reaction mixture was poured into H 2 O (50 mL) and extracted with CHCl 3 (3 Â 20 mL). The combined organic fractions were dried over anhydrous Na 2 SO 4 , the solvent was evaporated under reduced pressure, and the residue was purified by column chromatography (SiO 2 , 15 Â 1.8

Refinement
Crystal data, data collection and structure refinement details are summarized in , and constrained to ride on their parent atoms, with U iso (H) =1.2U eq (C) or 1.5U eq (C-methyl). Ten reflections (101) program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: PLATON (Spek, 2020).  (12) 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.