The m-phenylene-bridged bis-oxazole 4,4′,5,5′-tetraphenyl-2, 2′-m-phenyl-enedi-1,3-oxazole and its bulkily substituted analogue 4,4′,5,5′-tetra-kis(4-tert-butylphenyl)-2,2′-m-phenyl-enedi-1, 3-oxazole

Article abstract of DOI:10.1107/S0108270110045725

The title m-phenyl-ene-bridged bis-oxazoles, C₃₆H ₂₄N₂O₂, (I), and C₅₂H ₅₆N₂O₂, (II), feature different aryl substituents in the 4- and 5-positions of the oxazole unit

Full text of DOI:10.1107/S0108270110045725

organic compounds
Acta Crystallographica Section C
Crystal Structure
between the ring fragments (Table 1; ring definitions in
Fig. 1).
Communications
ISSN 0108-2701
The m-phenylene-bridged bis-oxazole
0
0
0
4
,4 ,5,5 -tetraphenyl-2,2 -m-phenyl-
enedi-1,3-oxazole and its bulkily
0
0
substituted analogue 4,4 ,5,5 -tetra-
0
kis(4-tert-butylphenyl)-2,2 -m-phenyl-
enedi-1,3-oxazole
The crystal structure of bis-oxazole (I) shows bond
distances within the oxazole rings which permit exact
J o¨ rg H u¨ bscher, Marika Felsmann, Wilhelm Seichter and
Edwin Weber*
˚
distinction between N and O [C1—N1 = 1.303 (2) A and
˚
˚
C1—O1 = 1.337 (2) A for ring A; C22—N2 = 1.297 (2) A and
˚
0
Institut f u¨ r Organische Chemie, TU Bergakademie Freiberg, Leipziger Strasse 29,
D-09596 Freiberg/Sachsen, Germany
C22—O2 = 1.350 (2) A for ring A ], so that the molecule
adopts a conformation with an anti arrangement of identical
heteroatoms (Fig. 1a). The central tricyclic part of the mol-
ecule is not perfectly planar; the planes of the oxazole rings
are inclined slightly to that of the phenylene unit (Table 1).
Except for phenyl ring C, the other peripheral phenyl rings are
considerably twisted with reference to the oxazole unit to
which they are bound (Table 1).
Correspondence e-mail: edwin.weber@chemie.tu-freiberg.de
Received 25 August 2010
Accepted 7 November 2010
Online 23 November 2010
The title m-phenylene-bridged bis-oxazoles, C H N O , (I),
2
36
24
2
and C H N O , (II), feature different aryl substituents in the
5
2
56
2
2
The crystal structure of (I) is characterized by a columnar
packing of molecules in the direction of the a axis. As shown in
Fig. 2, extensive ꢀ–ꢀ interactions (Dance, 2004; Janiak, 2000)
4- and 5-positions of the oxazole units. In the solid state, aside
from the different twist of the peripheral aryl rings, the
molecules show distinctly different conformations, with anti
and syn orientations of the O and N atoms for (I) and (II),
respectively. Connected with this property, in the crystal
structure of (I), extensive ꢀ-stacking is found between the
molecules, while the crystal structure of (II) only involves
dimer formation as the prominent packing motif.
0
between phenyl ring C and oxazole ring A occur along the
stacking axis of molecules. The closest distances between the
˚
centroids of interacting rings vary from 3.558 (3) A at (x + , y,
1
2
1
2
˚
1
2
1
2
ꢀ
z + ) to 3.597 (3) A at (x ꢀ , y, ꢀz + ). The molecular
stacking following from that seems to be stabilized as well by
1
additional ꢀ–ꢀ interactions between A and D at (x ꢀ , y,
2
1
˚
z + ), with a centroid–centroid distance of 3.615 (3) A. Only
2
ꢀ
one of the N atoms participates in hydrogen bonding
ꢂ
Comment
˚
(
H5ꢁ ꢁ ꢁN1 = 2.56 A and C5—H5ꢁ ꢁ ꢁN1 = 168 ). Further details
Owing to the highly fluorescent behaviour emanating from
their structure, aryl-substituted oxazoles are an interesting
class of compounds (Wiley, 1945). Derived from this property,
some 2,5-diaryloxazoles have found commercial application as
solutes in liquid scintillators (Bell & Hayes, 1958). A well
known representative of this compound type is 1,4-bis(5-
phenyloxazol-2-yl)benzene, usually termed POPOP (Hayes et
al., 1955). Other arylene- and heteroarylene-bridged bis-
oxazoles or bis-benzoxazoles behave as brightening agents
of the C—Hꢁ ꢁ ꢁN, C—Hꢁ ꢁ ꢁO and C—Hꢁ ꢁ ꢁꢀ interactions are
given in Table 2.
In bis-oxazole (II), the molecular structure of which is
illustrated in Fig. 1(b), the presence of the bulky tert-butyl
residues markedly changes the molecular conformation and
the packing behaviour of the molecules in the crystal structure
compared with (I). The m-phenylene bis-oxazole fragment of
(II) is approximately planar, with the largest deviation of any
˚
atom of the fragment from the mean plane being 0.055 (2) A
(
Schinzel et al., 1987) or as efficient ligands for metal-ion
for atom C23. Without exception, and thus differing most
markedly from (I), all the peripheral aryl rings show a distinct
twist with reference to the oxazole unit to which they are
bound (Table 1). Moreover, unlike in (I), corresponding
heteroatoms of the heterocyclic rings adopt a syn arrange-
ment, so that in the solid state the molecule has pseudo-mirror
complexation (Singh et al., 2008; Drew et al., 2004; R u¨ ttimann
et al., 1992). In view of these potential applications, the new
title compounds, (I) and (II), featuring m-phenylene-bridged
bis-oxazole derivatives with different aryl substituents in the
4- and 5-positions of the oxazole units, have been synthesized
and their crystal structures are reported here.
symmetry (point group C ), with atoms C18 and C21 lying in
s
Perspective views of the molecular structures of (I) and (II)
are shown in Fig. 1. The conformations of the molecules of (I)
and (II) may be described by the relevant interplanar angles
the pseudo-symmetry plane.
The steric requirement of the tert-butyl substituents dras-
tically reduces the extent of intermolecular interaction, which
Acta Cryst. (2010). C66, o623–o626
doi:10.1107/S0108270110045725
# 2010 International Union of Crystallography o623

organic compounds
Figure 1
Perspective views of the molecular structures of (a) compound (I) and (b) compound (II), showing the atom-numbering schemes and the ring
specifications. Displacement ellipsoids are drawn at the 50% probability level.
Figure 2
The crystal packing of (I), viewed down the crystallographic b axis. O
atoms are displayed as dark-grey and N atoms as light-grey spheres.
Dashed lines represent hydrogen bonds and double-dashed lines
represent arene stacking interactions. H atoms not involved in hydrogen
bonding have been omitted for clarity.
is restricted to the few C—Hꢁ ꢁ ꢁꢀ contacts (Nishio, 2004) listed
in Table 3. Hence, the crystal structure is stabilized by van der
Figure 3
The crystal packing of (II). O atoms are displayed as dark-grey and N
atoms as light-grey spherees. H atoms have been omitted for clarity.
Waals forces rather than directed noncovalent bonding. As
shown in the crystal packing diagram (Fig. 3), the crystal
structure of (II) is composed of weakly bound dimers centred
about an inversion centre.
to the lack of fluorescence of bis-oxazoles (I) and (II) in the
solid state, while POPOP is highly fluorescent.
In summary, the most remarkable distinguishing feature
between the crystal structures of (I) and (II) is the different
molecular conformations with respect to the orientation of the
O and N atoms of the oxazole units: anti in (I) and syn in (II).
This might be a result of the long-range ꢀ-stacking interaction
between the molecules in the structure of (I), whereas the
bulky tert-butyl groups of (II) prevent extensive ꢀ-stacking
but cause the molecules to associate into weakly bound dimers
centred about inversion centres, with the hetero atoms in a syn
conformation. Moreover, compared with the almost flat
structure of crystalline POPOP (Schindler et al., 2010), which
makes efficient conjugation of the ꢀ-systems very likely, the
molecular conformations of (I) and (II) are much more
twisted, in particular with respect to the peripheral aryl–
oxazole bonds. This could be one of the reasons contributing
Experimental
The starting benzoins (IVa) (Breuer & Zincke, 1879) and (IVb)
(Hahn et al., 1981) were synthesized via benzoin condensation from
the corresponding benzaldehydes following the described proce-
dures.
For the preparation of the bis(keto esters) (IIIa) and (IIIb),
triethylamine (15 ml, 0.11 mol, dried over potassium hydroxide) was
added slowly over a period of 3 h to a refluxing solution of the
corresponding benzoin (IVa) or (IVb) (20 mmol) and isophthaloyl
dichloride (V) (2.03 g, 10 mmol) in dry toluene (20 ml, dried over
sodium and freshly distilled before use). After evaporation of the
solvent, ethanol (10 ml) was added to the oily residue and the mixture
stirred for 2 h. The solid which formed was collected and crystallized.
ꢃ
o624 H u¨ bscher et al.
C H
36 24
N O
2 2
and C52
H N O
56 2 2
Acta Cryst. (2010). C66, o623–o626

organic compounds
For (IIIa), benzoin (IVa) (4.24 g, 20 mmol) was reacted; crystal-
lization from 1,4-dioxane yielded a colourless powder (yield 40%;
m.p. 440 K). For (IIIb), benzoin (IVb) (6.44 g, 20 mmol) was reacted;
crystallization from 1,4-dioxane yielded a colourless powder (yield
Table 1
Interplanar angles ( ) in (I) and (II).
ꢂ
Ring definitions are given in Fig. 1.
96%; m.p. 421 K).
Planes
(I)
(II)
A/B
0
46.1 (1)
40.5 (1)
5.5 (1)
25.1 (1)
2.3 (1)
8.2 (1)
39.6 (1)
42.6 (1)
31.3 (1)
27.1 (1)
2.3 (2)
3.5 (2)
54.4 (1)
54.2 (1)
0
A /B
A/C
0
0
A /C
A/D
0
0
A /D
B/C
50.3 (1)
51.3 (1)
0
0
B /C
Table 2
˚
ꢂ
C—Hꢁ ꢁ ꢁX and C—Hꢁ ꢁ ꢁꢀ interactions for (I) (A, ).
Ring definitions are given in Fig. 1 and Cg denotes a ring centroid.
Symmetry code
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D—Hꢁ ꢁ ꢁA
1
2
1
2
C5—H5ꢁ ꢁ ꢁN1
C33—H33ꢁ ꢁ ꢁCgB0
(ꢀ + x, y, ꢀ z)
2.56
2.76
2.81
3.489 (1)
3.486 (1)
3.550 (1)
168
134
136
0
C29—H29ꢁ ꢁ ꢁCgC
(1 ꢀ x, ꢀy, ꢀz)
(ꢀx, ꢀy, ꢀz)
Table 3
˚
ꢂ
C—Hꢁ ꢁ ꢁꢀ interactions for (II) (A, ).
Ring definitions are given in Fig. 1 and Cg denotes a ring centroid.
Symmetry code
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D—Hꢁ ꢁ ꢁA
C51—H51Aꢁ ꢁ ꢁCgB
C26—H26ꢁ ꢁ ꢁCgC
C43—H43Aꢁ ꢁ ꢁCgB
(2 ꢀ x, 2 ꢀ y, 2 ꢀ z)
(2 ꢀ x, 2 ꢀ y, 2 ꢀ z)
(2 ꢀ x, 2 ꢀ y, 2 ꢀ z)
2.88
2.87
2.89
3.691 (2)
3.470 (2)
3.810 (2)
141
122
157
For the preparation of the title bis-oxazoles (I) and (II), the
corresponding bis(keto ester) (IIIa) or (IIIb) (7.5 mmol) and
ammonium acetate (3.46 g, 45 mmol) were dissolved in concentrated
acetic acid (55.0 ml) and refluxed for 3.5 h. After cooling of the
reaction mixture to room temperature, the precipitate which formed
was collected, neutralized with aqueous sodium hydrogen carbonate,
washed several times with water, dried and crystallized. For (I),
compound (IIIa) (4.16 g) was used for the reaction; crystallization
from 1,4-dioxane–acetonitrile (1:1, v/v) yielded colourless crystals
Refinement
2
2
R[F > 2ꢃ(F )] = 0.052
wR(F ) = 0.156
361 parameters
H-atom parameters constrained
Áꢄmax = 0.41 e A
Áꢄmin = ꢀ0.31 e A˚
2
˚
ꢀ3
S = 0.99
10121 reflections
ꢀ3
(yield 88%; m.p. 476 K). For (II), compound (IIIb) (5.84 g) was used
for the reaction; crystallization from 1,4-dioxane yielded a colourless
powder (yield 93%; m.p. 585 K) and X-ray quality crystals were
obtained by recrystallization from chloroform.
Compound (II)
Crystal data
ꢂ
C
52
H
56
N
2
O
2
ꢆ = 108.463 (2)
V = 2140.25 (11) A
Spectroscopic and other synthetic details for compounds (I) and
II) are available in the archived CIF.
˚
3
M
r
= 740.99
Triclinic, P1
(
Z = 2
Mo Kꢁ radiation
˚
˚
a = 10.3121 (3) A
b = 11.1215 (3) A
˚
c = 19.8676 (6) A
ꢀ1
ꢂ = 0.07 mm
T = 153 K
Compound (I)
ꢂ
ꢂ
ꢁ
= 91.593 (2)
0.32 ꢄ 0.18 ꢄ 0.17 mm
ꢅ = 96.918 (2)
Crystal data
˚
V = 5277.5 (2) A
3
C
36
24
H N
O
2 2
Data collection
M
r
= 516.57
Orthorhombic, Pbca
Z = 8
Mo Kꢁ radiation
ꢂ = 0.08 mm
T = 153 K
Nonius Kappa APEXII CCD area-
detector diffractometer
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)
33601 measured reflections
9309 independent reflections
5281 reflections with I > 2ꢃ(I)
˚
a = 7.1523 (2) A
b = 20.0145 (5) A
˚
c = 36.8669 (8) A
ꢀ1
˚
0.48 ꢄ 0.34 ꢄ 0.17 mm
Rint = 0.044
T
min = 0.978, Tmax = 0.988
Data collection
Refinement
2
Nonius Kappa APEXII CCD area-
detector diffractometer
Absorption correction: multi-scan
80848 measured reflections
10121 independent reflections
5826 reflections with I > 2ꢃ(I)
2
R[F > 2ꢃ(F )] = 0.057
wR(F ) = 0.180
517 parameters
H-atom parameters constrained
2
˚
ꢀ3
Áꢄmax = 0.36 e A
(SADABS; Sheldrick, 1996)
min = 0.962, Tmax = 0.986
R
int = 0.062
S = 1.02
9309 reflections
Áꢄmin = ꢀ0.33 e A˚
ꢀ3
T
ꢃ
Acta Cryst. (2010). C66, o623–o626
H u¨ bscher et al.
36 24 2
C H N O
2
and C52H N
56 2
O
2
o625

organic compounds
H atoms were positioned geometrically and treated as riding, with
˚
C—H = 0.95 (aromatic) or 0.98 A (aliphatic) and Uiso(H) = kUeq(C),
Breuer, A. & Zincke, T. (1879). Liebigs Ann. Chem. 198, 141–190.
Bruker (2007). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin,
USA.
Dance, I. A. (2004). Encyclopedia of Supramolecular Chemistry, edited by J. L.
Atwood & J. W. Steed, pp. 1076–1092. Boca Raton: CRC Press.
Drew, M. G. B., Hill, C., Hudson, M. J., Iveson, P. B., Madic, C., Vaillant, L. &
Youngs, T. G. A. (2004). New J. Chem. 28, 462–470.
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565.
where k = 1.2 for aromatic H or k = 1.5 for aliphatic H atoms.
For both compounds, data collection: APEX2 (Bruker, 2007); cell
refinement: SAINT (Bruker, 2007); data reduction: SAINT;
program(s) used to solve structure: SHELXS97 (Sheldrick, 2008);
program(s) used to refine structure: SHELXL97 (Sheldrick, 2008);
molecular graphics: ORTEP-3 (Farrugia, 1997); software used to
prepare material for publication: SHELXTL (Sheldrick, 2008).
Hahn, B., K o¨ pke, B. & Voss, J. (1981). Liebigs Ann. Chem. pp. 10–19.
Hayes, F. N., Rogers, B. S. & Ott, D. G. (1955). J. Am. Chem. Soc. 77, 1850–
1852.
Janiak, C. (2000). J. Chem. Soc. Dalton Trans. pp. 3885–3896.
Nishio, M. (2004). CrystEngComm, 6, 130–158.
Ruttimann, S., Moreau, C. M., Williams, A. F., Bernardelli, G. & Addison,
¨
A. W. (1992). Polyhedron, 11, 635–646.
Schindler, D., Felsmann, M. & Weber, E. (2010). Acta Cryst. C66, o361–
o363.
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: SU3052). Services for accessing these data are
described at the back of the journal.
Schinzel, E., Frischkorn, H. & Martini, T. (1987). Eur. Patent EP 0106916.
Sheldrick, G. M. (1996). SADABS. University of G o¨ ttingen, Germany.
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.
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References
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York: Pergamon.
ꢃ
o626 H u¨ bscher et al.
C H
36 24
N O
2 2
and C52
H N O
56 2 2
Acta Cryst. (2010). C66, o623–o626