Crystal structure of 2-oxo-1,2-diphenylethyl diisopropylcarbamate

The title compound, C21H25NO3, crystallizes as a racemic twin in the chiral space group P21. Both R- and S-enantiomers are connected into infinite helical chains by weak C—H⋯O hydrogen bonds between the phenyl ring of the benzoyl group and the carbamate carbonyl group.


Chemical context
Phenacyl and desyl compounds may act as photoremovable protecting groups (PPGs) and have been a subject of interest for many years (Givens et al., 2012;Kammari et al., 2007;Klá n et al., 2013;Sheehan & Umezawa, 1973). In addition to the protection of carboxylic acids, they have also been shown to act as suitable groups for the protection and deprotection of amines (Speckmeier et al., 2018). Besides several carbamate compounds, Lange and co-workers also synthesized the title compound via a Cu I -catalysed stereospecific coupling reaction using -stannylated benzyl carbamates (Lange et al., 2008). We chose a different procedure to synthesize the title compound, according to a synthetic route that has already been reported by Speckmeier et al. (2018). Recently, we reported on the crystal structure of the highly related achiral derivative 2-oxo-2-phenylethyl diisopropylcarbamate (Martens et al., 2021).

Supramolecular features
In the crystal structure, molecules of both enantiomers show infinite helical arrangements parallel to the b axis formed by weak C-HÁ Á ÁO hydrogen bonds (Desiraju & Steiner, 2001;Figs. 2 and 3) between the phenyl ring of the benzoyl group and the carbamate carbonyl group (S-enantiomer: C12A-H12AÁ Á ÁO3A, R-enantiomer: C14B-H14BÁ Á ÁO3B; Table 1). In each of the helices, only one enantiomer is present. Nevertheless, the helices do not act as mirror images because the arrangement of the molecules relative to each other is different. In the case of the R-enantiomer (Fig. 3), the supramolecular helix is additionally stabilized by a bifurcated hydrogen bond between the carbonyl function of the benzoyl group towards both phenyl groups of the molecule (C11B-H12BÁ Á ÁO1B and C12B-H12BÁ Á ÁO1B; Table 1).

Database survey
In the Cambridge Structural Database (CSD; ConQuest Version 2020.3.0; Groom et al., 2016)  Crystal structure of the S-enantiomer of the title compound showing the helical arrangement of molecules parallel to the b axis built up by C-HÁ Á ÁO hydrogen bonds.

Figure 1
Molecular structures of both enantiomers of the title compound with displacement ellipsoids drawn at the 50% probability level (R left; S right).
compound shows a diethylamino group and a p-chlorophenyl substituent instead of the diisopropylamino group and the non-substituted phenyl group as in the title compound. Contrary to the title compound, the carbamate plane and the benzoyl plane are almost coplanar. The carbonyl oxygen atoms show numerous short contacts towards different C-H groups of neighbouring molecules, leading to a dense threedimensional network. In addition, we recently reported a structure, in which there also is a CH 2 -C(O)-Ph group instead of the CH(Ph)-C(O)-Ph unit in the title compound (Martens et al., 2021). In this structure, a layered arrangement is realized by all three oxygen atoms acting as hydrogen-bond acceptor sites. Moreover, there is one structure reported in the literature that is identical to the title compound with the exception of one bromine substituent at the 4-position of the phenyl ring attached to the C1 O1 carbonyl group (DOKMAS; Lange et al., 2008). In the latter case, the enantiopure S-enantiomer was crystallized. The supramolecular structure of this compound shows the same bifurcated hydrogen bond as is observed for the R-enantiomer of the title compound. On the other hand, the analogue of O3 is not engaged in a C-HÁ Á ÁO interaction but shows a short oxygen-bromine contact (3.139 Å ). These two interactions lead to a double-strand arrangement of molecules parallel to the a axis.

Synthesis and crystallization
Diisopropylamine (0.05 mol, 5.05 g) and one equivalent of caesium carbonate (0.05 mol, 16.55 g) were placed in a Schlenk tube and dissolved in anhydrous DMSO (150 ml). The tube was sealed with a septum, and two balloons filled with CO 2 were bubbled through the reaction mixture within one h while stirring. After the addition of CO 2 , 1.1 equivalents of 2bromo-1,2-diphenylethan-1-one (0.055 mol, 15.13 g) dissolved in a small amount of DMSO were added in one portion. The consumption of the 2-bromo-1,2-diphenylethan-1-one was monitored by TLC, and after 30 min the reaction mixture was poured onto ice to quench the reaction. After extraction with dichloromethane (3 Â 40 ml), the combined organic phases were washed with brine, separated and dried over Na 2 SO 4 . The solvent was removed in vacuo and the crude product was recrystallized from n-hexane/ethylacetate

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were placed in idealized positions (C-H = 0.95-0.98 Å ) and refined using a riding model with isotropic displacement parameters calcu-lated as U iso (H) = 1.2(C) for methine and hydrogen atoms of the phenyl group or 1.5ÂU eq (C) for methyl groups. The crystal studied was refined as a two-component twin with fractions of 29% vs 71%.

Funding information
Funding for this research was provided by: the Open Access Fund of the University of Koblenz-Landau. Financial support of the PhD project of VM by Lohmann GmbH & Co. KG, Neuwied, Germany, is gratefully acknowledged.  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. Refinement. Refined as a two-component inversion twin.