Crystal structure of 2,4,6-tri-methyl-benzoic anhydride.

The title compound, C20H22O3, was formed in the reaction between 2,4,6-tri-methyl-benzoic acid and N,N-diiso-propyl-ethyl-amine in the presence of 1,3-di-chloro-1,3-bis-(di-methyl-amino)-propenium hydrogen dichloride, and was recrystallized from diethyl ether solution. It is the first exclusively alkyl-substituted benzoic anhydride to have been structurally characterized. The asymmetric unit consists of a half mol-ecule, the other half of which is generated by twofold rotation symmetry; the dihedral angle between the symmetry-related aromatic rings is 54.97 (3)°. The geometric parameters of the aromatic ring are typical of those for 2,4,6-tri-methyl-phenyl substituted groups. The C=O and C-O bond lengths are 1.1934 (12) and 1.3958 (11) Å, respectively, and the angle between these three atoms (O=C-O) is 121.24 (9)°. In the crystal, mol-ecules are linked by weak C-H⋯O hydrogen bonds and C-H⋯π inter-actions. The packing features wavy chains that extend parallel to [001].

Chemical context

Benzoic anhydrides have traditionally been used in synthetic organic chemistry for the preparation of aromatic esters, amides and carb­oxy­lic acids. Aromatic anhydrides have also been shown to be effective acyl­ating agents (Shiina, 2004 ▸; Shiina & Nakata, 2007 ▸). The title compound has been used to trap deprotonated 3,4-ep­oxy-2,3,4,5-tetra­hydro­thio­phene 1,1-dioxide, forming 3-(2,4,6-tri­methyl­benzo­yloxy)-2,3-di­hydro­thio­phene 1,1-dioxide (Alonso et al., 2004 ▸, 2005 ▸). The synthesis of the compound we report here, 2,4,6-tri­methyl­benzoic anhydride (common name: mesitoic anhydride), was first published in 1941, where it was formed in the reaction between (2,4,6-tri­methyl­phenyl)sodium and 2,4,6-tri­methyl­benzoic acid in the presence of pyridine (Fuson et al., 1941 ▸). Recently, several new approaches for the syntheses of symmetric acid anhydrides, including the title compound, have been reported (Kazemi et al., 2004 ▸; Li et al., 2012 ▸; McCallum & Barriault, 2015 ▸). The most recent report involves the in situ generation of a Vilsmeier–Haack reagent for the coupling of symmetric carb­oxy­lic acids (McCallum & Barriault, 2015 ▸). Due to the structural similarities between this reagent and the 1,3-di­chloro-1,3-bis­(di­methyl­amino)­propenium salt used here, it is possible that the title compound was formed via a similar method in our reaction. The crystal structure that we report is the first example of a benzoic anhydride where the aryl rings are substituted with only alkyl groups, although several other substituted benzoic anhydrides are known.

Structural commentary

The mol­ecular structure of the title compound is shown in Fig. 1 ▸. It crystallizes in the monoclinic space group C2/c with one half of the mol­ecule uniquely present in the asymmetric unit. The two C—O bond lengths are significantly different, as would be expected for anhydrides, with lengths of 1.1934 (12) (C1—O2) and 1.3958 (11) Å (C1—O1). The C1—C2 distance is normal for an sp 2–sp 2 bond, with a length of 1.4873 (13) Å. The second half of the mol­ecule, which is generated by rotation about the twofold axis passing through O1 (0, y, 0.25), forms a dihedral angle of 54.97 (3)° between the equivalent aromatic rings. If the planes of the two overlapping CO2 groups are chosen instead, the dihedral angle becomes 59.30 (11)°. The C—C bonds in the aromatic ring are not all statistically equivalent. Unsurprisingly, the longest C—C bonds in the ring are adjacent to the electron-withdrawing anhydride group, C2—C3 [1.4032 (13) Å] and C2—C7 [1.4059 (13) Å]. The remaining C—C bonds are statistically equivalent, averaging 1.3942 (8) Å. All of the C—CH3 bond lengths are statistically equivalent with an average length of 1.5102 (8) Å.

Figure 1

The mol­ecular structure of the title compound. Only half of the mol­ecule is crystallographically unique (labelled atoms). Displacement ellipsoids are drawn at the 50% probability level.

Supra­molecular features

The packing of the mol­ecules, when viewed in projection down the a axis, forms wavy chains that run parallel to the c-axis direction (Fig. 2 ▸). Within the chains, the mol­ecules are oriented in a alternating up and down fashion, shifting by along [001] each time, such that they overlap slightly. There are no close stacking inter­actions between the phenyl rings in various planes. However, if the packing is viewed down the b axis, the C5—C6—C7—C10 fragment of one tri­methyl­phenyl group lies directly above/below the same fragment running in the opposite direction, C10—C7—C6—C5, in the plane above/below it.

Figure 2

Packing diagram of the title compound, viewed in projection down [100], showing wavy [001] chains.

There are short intra­molecular contacts between the aromatic H atoms H4 (2.37 Å to H8A and 2.37 to H9C) and H6 (2.39 Å to H10A), and the designated methyl H atoms, which close five-membered rings in the mol­ecule.

In the crystal, mol­ecules are linked by weak inter­molecular C—H⋯O hydrogen bonds and C—H⋯π contacts (Table 1 ▸, Fig. 3 ▸). It is notable that these contacts involve one H atom from each of the three methyl substituents on the phenyl ring. All of these contacts occur between the chains that run parallel to the c axis and not within the individual chains, thus consolidating the overall structure.

Table 1

Hydrogen-bond geometry (Å, °)

Cg1 is the centroid of the C2–C7 ring.

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C8—H8A⋯O2i	0.98	2.48	3.4612 (13)	176
C9—H9A⋯O2ii	0.98	2.66	3.5842 (14)	157
C10—H10B⋯Cg1iii	0.98	2.96	3.5255 (14)	118

Symmetry codes: (i) ; (ii) ; (iii) .

Figure 3

Short inter­molecular contacts (defined in the text and close to the sum of the van der Waals radii; shown as heavy dotted lines), with only donors from the central mol­ecule included (Table 1 ▸).

Database survey

A survey of the Cambridge Structural Database (CSD, Version 5.38; Groom et al., 2016 ▸), performed on 24 July, 2017, located 25 substituted benzoic anhydrides, all of which were symmetric. Inter­estingly, there were no other benzoic an­hydrides identified that were substituted exclusively with alkyl groups. The only other structurally characterized alkyl-substituted benzoic anhydrides are 2-acet­oxy-5-methyl­benzoic anhydride (CSD refcode IBOCOT; Solanko & Bond, 2011 ▸) and 2-methyl-3-nitro­benzoic anhydride (QUFTIW; Moreno-Fuquen et al., 2015 ▸). Most of the examples found in the CSD contain various substitution patterns involving halogens (Cl, Br or I). There are also several structures that contain aromatic activating groups, such as ethers or amines. The parent compound, benzoic anhydride (ZZZQRI; van Alen & Krauze, 1964 ▸), is known, as is the precursor to the title compound, 2,4,6-tri­methyl­benzoic acid (TMBZAC; Florencio & Smith, 1970 ▸), which can be overlaid with the asymmetric unit of the title compound reasonably well.

Synthesis and crystallization

The title compound was isolated from the following reaction mixture, although more convenient synthetic methods are known (Fuson et al., 1941 ▸; Kazemi et al., 2004 ▸). 2,4,6-Tri­methyl­benzoic acid (1.11 g, 6.78 mmol) and N,N-diiso­propyl­ethyl­amine (1.21 ml, 6.94 mmol) were added to a chloroform solution (25 ml) of 1,3-di­chloro-1,3-bis­(di­methyl­amino)­propenium hydrogen dichloride (0.92 g, 3.42 mmol), which had been prepared following the known literature method (Janousek & Viehe, 1971 ▸), in chloro­form (25 ml). The mixture was stirred at reflux for 18 h under nitro­gen. After cooling to room temperature, a saturated KOH (aqueous) solution (∼2 ml) and water (70 ml) were added to the mixture. The organic layer was extracted and the aqueous phase was washed with chloro­form (two 25 ml portions). The combined organic extracts were washed with brine and dried with MgSO4, and the solvent was removed in vacuo. The resulting material was washed with ice-cold water (25 ml) and then isolated via vacuum filtration. The off-white solid was further purified by recrystallization through slow evaporation of a saturated diethyl ether solution. After 3 h, clear and colourless thin plate-like crystals were obtained [yield: 0.98 g, 3.15 mmol, 93%; m.p. 374–375 K (literature = 375–377 K)]. Elemental analysis, calculated for C20H22O3 (%): C 77.39, H 7.14, N 0.00; found (%): C 77.26, H 7.13, N 0.01. The 1H and 13C{1H} NMR and IR spectroscopic data for the title compound are identical to those previously reported (Kazemi et al., 2004 ▸).

Refinement

Crystal data, data collection, and structure refinement details are summarized in Table 2 ▸. H atoms were included in calculated positions (C—H = 0.95–0.98 Å) and refined as riding with U iso(H) = 1.2 or 1.5U eq(C).

Table 2

Experimental details

Crystal data
Chemical formula	C20H22O3
M r	310.37
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	125
a, b, c (Å)	16.080 (2), 7.9997 (12), 14.308 (2)
β (°)	114.094 (2)
V (Å3)	1680.1 (4)
Z	4
Radiation type	Mo Kα
μ (mm−1)	0.08
Crystal size (mm)	0.55 × 0.26 × 0.25
Data collection
Diffractometer	Bruker APEXII CCD area-detector
Absorption correction	Multi-scan (SADABS; Bruker, 2008 ▸)
T min, T max	0.683, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	9832, 2095, 1857
R int	0.019
(sin θ/λ)max (Å−1)	0.679
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.037, 0.107, 1.05
No. of reflections	2095
No. of parameters	108
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.34, −0.19

Computer programs: APEX2 and SAINT (Bruker, 2008 ▸), SHELXT2014 (Sheldrick, 2015a ▸), SHELXL2014 (Sheldrick, 2015b ▸) and Mercury CSD 3.9 (Macrae et al., 2008 ▸).

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S2056989017014670/hb7710sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989017014670/hb7710Isup2.hkl

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989017014670/hb7710Isup3.cml

CCDC reference: 1579203

Additional supporting information: crystallographic information; 3D view; checkCIF report

C20H22O3	F(000) = 664
Mr = 310.37	Dx = 1.227 Mg m−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 16.080 (2) Å	Cell parameters from 6207 reflections
b = 7.9997 (12) Å	θ = 2.8–28.7°
c = 14.308 (2) Å	µ = 0.08 mm−1
β = 114.094 (2)°	T = 125 K
V = 1680.1 (4) Å3	Wedge shaped (cut from a large block), colourless
Z = 4	0.55 × 0.26 × 0.25 mm

Bruker APEXII CCD area-detector diffractometer	2095 independent reflections
Radiation source: sealed tube	1857 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.019
φ and ω scans	θmax = 28.9°, θmin = 2.8°
Absorption correction: multi-scan (SADABS; Bruker, 2008)	h = −21→21
Tmin = 0.683, Tmax = 0.746	k = −10→10
9832 measured reflections	l = −19→19

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.037	H-atom parameters constrained
wR(F2) = 0.107	w = 1/[σ2(Fo2) + (0.0543P)2 + 0.9964P] where P = (Fo2 + 2Fc2)/3
S = 1.05	(Δ/σ)max < 0.001
2095 reflections	Δρmax = 0.34 e Å−3
108 parameters	Δρmin = −0.19 e Å−3
0 restraints

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.

	x	y	z	Uiso*/Ueq
O1	0.0000	0.31743 (12)	0.2500	0.0206 (2)
O2	0.08932 (5)	0.53524 (10)	0.24991 (6)	0.0272 (2)
C1	0.05461 (6)	0.40599 (12)	0.21238 (7)	0.0193 (2)
C2	0.06727 (6)	0.31349 (12)	0.12897 (7)	0.0190 (2)
C3	0.15548 (7)	0.26116 (12)	0.14551 (7)	0.0206 (2)
C4	0.16788 (7)	0.18095 (13)	0.06542 (8)	0.0229 (2)
H4	0.2269	0.1425	0.0759	0.028*
C5	0.09582 (8)	0.15592 (13)	−0.02937 (8)	0.0237 (2)
C6	0.00895 (7)	0.20904 (13)	−0.04303 (8)	0.0245 (2)
H6	−0.0406	0.1912	−0.1073	0.029*
C7	−0.00725 (7)	0.28740 (12)	0.03480 (8)	0.0213 (2)
C8	0.23514 (7)	0.28879 (15)	0.24696 (8)	0.0275 (2)
H8A	0.2859	0.2167	0.2514	0.041*
H8B	0.2170	0.2615	0.3028	0.041*
H8C	0.2542	0.4061	0.2526	0.041*
C9	0.11059 (9)	0.07659 (15)	−0.11716 (9)	0.0310 (3)
H9A	0.1003	0.1600	−0.1709	0.047*
H9B	0.0679	−0.0164	−0.1449	0.047*
H9C	0.1732	0.0347	−0.0926	0.047*
C10	−0.10199 (7)	0.34744 (15)	0.01547 (8)	0.0268 (2)
H10A	−0.1368	0.3649	−0.0581	0.040*
H10B	−0.0980	0.4530	0.0519	0.040*
H10C	−0.1326	0.2634	0.0401	0.040*

	U11	U22	U33	U12	U13	U23
O1	0.0218 (5)	0.0187 (5)	0.0253 (5)	0.000	0.0138 (4)	0.000
O2	0.0251 (4)	0.0256 (4)	0.0340 (4)	−0.0059 (3)	0.0153 (3)	−0.0060 (3)
C1	0.0149 (4)	0.0210 (5)	0.0220 (5)	0.0018 (3)	0.0074 (4)	0.0031 (4)
C2	0.0187 (5)	0.0190 (4)	0.0200 (5)	−0.0001 (3)	0.0086 (4)	0.0023 (3)
C3	0.0200 (5)	0.0211 (5)	0.0212 (5)	0.0013 (4)	0.0089 (4)	0.0026 (4)
C4	0.0242 (5)	0.0219 (5)	0.0257 (5)	0.0023 (4)	0.0133 (4)	0.0025 (4)
C5	0.0328 (6)	0.0182 (5)	0.0232 (5)	−0.0022 (4)	0.0146 (4)	0.0011 (4)
C6	0.0271 (5)	0.0238 (5)	0.0199 (5)	−0.0043 (4)	0.0067 (4)	0.0013 (4)
C7	0.0200 (5)	0.0208 (5)	0.0222 (5)	−0.0014 (4)	0.0076 (4)	0.0037 (4)
C8	0.0194 (5)	0.0369 (6)	0.0243 (5)	0.0060 (4)	0.0068 (4)	−0.0018 (4)
C9	0.0439 (7)	0.0273 (5)	0.0270 (5)	−0.0031 (5)	0.0197 (5)	−0.0032 (4)
C10	0.0188 (5)	0.0311 (6)	0.0267 (5)	−0.0002 (4)	0.0053 (4)	0.0039 (4)

O1—C1i	1.3958 (11)	C6—C7	1.3921 (15)
O1—C1	1.3958 (11)	C6—H6	0.9500
O2—C1	1.1934 (12)	C7—C10	1.5111 (14)
C1—C2	1.4873 (13)	C8—H8A	0.9800
C2—C3	1.4032 (13)	C8—H8B	0.9800
C2—C7	1.4059 (13)	C8—H8C	0.9800
C3—C4	1.3968 (14)	C9—H9A	0.9800
C3—C8	1.5095 (14)	C9—H9B	0.9800
C4—C5	1.3924 (15)	C9—H9C	0.9800
C4—H4	0.9500	C10—H10A	0.9800
C5—C6	1.3953 (16)	C10—H10B	0.9800
C5—C9	1.5101 (14)	C10—H10C	0.9800
C1i—O1—C1	119.00 (11)	C6—C7—C10	120.03 (9)
O2—C1—O1	121.24 (9)	C2—C7—C10	121.97 (9)
O2—C1—C2	126.87 (9)	C3—C8—H8A	109.5
O1—C1—C2	111.76 (8)	C3—C8—H8B	109.5
C3—C2—C7	121.65 (9)	H8A—C8—H8B	109.5
C3—C2—C1	118.24 (8)	C3—C8—H8C	109.5
C7—C2—C1	120.03 (9)	H8A—C8—H8C	109.5
C4—C3—C2	118.14 (9)	H8B—C8—H8C	109.5
C4—C3—C8	120.45 (9)	C5—C9—H9A	109.5
C2—C3—C8	121.40 (9)	C5—C9—H9B	109.5
C5—C4—C3	121.63 (9)	H9A—C9—H9B	109.5
C5—C4—H4	119.2	C5—C9—H9C	109.5
C3—C4—H4	119.2	H9A—C9—H9C	109.5
C4—C5—C6	118.69 (9)	H9B—C9—H9C	109.5
C4—C5—C9	121.31 (10)	C7—C10—H10A	109.5
C6—C5—C9	119.99 (10)	C7—C10—H10B	109.5
C7—C6—C5	121.91 (9)	H10A—C10—H10B	109.5
C7—C6—H6	119.0	C7—C10—H10C	109.5
C5—C6—H6	119.0	H10A—C10—H10C	109.5
C6—C7—C2	117.96 (9)	H10B—C10—H10C	109.5
C1i—O1—C1—O2	35.04 (7)	C8—C3—C4—C5	−179.10 (10)
C1i—O1—C1—C2	−148.96 (8)	C3—C4—C5—C6	−1.62 (15)
O2—C1—C2—C3	59.63 (14)	C3—C4—C5—C9	177.01 (9)
O1—C1—C2—C3	−116.09 (9)	C4—C5—C6—C7	0.66 (15)
O2—C1—C2—C7	−117.38 (11)	C9—C5—C6—C7	−177.99 (9)
O1—C1—C2—C7	66.89 (11)	C5—C6—C7—C2	0.47 (15)
C7—C2—C3—C4	−0.20 (14)	C5—C6—C7—C10	178.17 (9)
C1—C2—C3—C4	−177.17 (9)	C3—C2—C7—C6	−0.70 (14)
C7—C2—C3—C8	−179.70 (9)	C1—C2—C7—C6	176.21 (9)
C1—C2—C3—C8	3.33 (14)	C3—C2—C7—C10	−178.35 (9)
C2—C3—C4—C5	1.39 (15)	C1—C2—C7—C10	−1.44 (14)

D—H···A	D—H	H···A	D···A	D—H···A
C8—H8A···O2ii	0.98	2.48	3.4612 (13)	176
C9—H9A···O2iii	0.98	2.66	3.5842 (14)	157
C10—H10B···Cg1iv	0.98	2.96	3.5255 (14)	118