5884
M. Tomar et al. / Tetrahedron Letters 54 (2013) 5883–5885
S
S
OH
B
S
4
OH
N
N
H
O
B
5
O
N
N
6
Pd(PPh3)4,
Na2CO3
S
N
3a
N
N
N
S
N
N
N
S
100 °C
H
O
O
B
7
8
N
H
9
Scheme 2. Synthesis of model compounds based on 3,9-dibromo-6-aryl-5H-
dibenzo[d,f][1,3]diazepine.
79%, and 47% yields, respectively (Scheme 2). The model com-
pounds were characterized by 1H and 13C NMR, mass spectrometry,
UV–vis absorption, and emission spectroscopy.
Single crystals of the dibromide 3a (DCM/ethanol) and the mod-
el compounds 5, 7, and 9 (DCM/methanol) were studied by X-ray
diffraction, allowing the solid-state molecular conformation and
packing to be probed.23 In all structures the phenyl-5H-
dibenzo[d,f][1,3]diazepine unit adopts the same boat conformation
with the pendant 6-phenyl ring close to parallel with the C1–C6
benzo ring (Fig. 1); the intervening imine group shows a C2–N1–
C13 bond angle between 121.7° and 123.7°, suggesting a degree
of conjugation between the phenyl ring and benzo ring of the
monomer backbone. The twist angle between the two benzo rings
spans the range 30.7° (3a)–40.57° (9), increasing as the size of the
substituent increases. Monomer 5 exhibits the most planar confor-
mation overall (the thienyl groups are rotated 18.9/3.7° with re-
spect to the benzo groups to which they are attached).
The structures of 5, 7, and 9 all contain methanol molecules
hydrogen bonded to the diazepine ring, linking to neighbor mono-
mers, most notable in 5ꢀ(CH3OH)4 where a chain of four methanol
molecules bridge the diazepine N–H and N groups of adjacent mol-
ecules. When solvents other than methanol were used in monomer
crystallizations, powders rather than single crystals were obtained,
indicating the importance of methanol in forming H-bonded net-
works as part of the bulkcrystallinestructure. In addition to H-bond-
ing, the structure of benzothiadiazole-substituted 7 also shows
chains of close Sꢀ ꢀ ꢀN contacts (3.168/3.322 Å) that run along the
a-axis (see Electronic Supplementary data for packing diagrams).
To date only five dibenzo[d,f][1,3]diazepines have been structurally
characterized,24–28 none of them 3,9-substituted. The dibenzo
twist angle for these structures ranges between 30.8° and 35.5°
when the rings are unsubstituted; a 1,11-dimethyl-substituted
dibenzo[d,f][1,3]diazepine is forced into a more twisted conforma-
tion (57.4°) due to steric hindrance between the methyl groups.
The optical properties of the model compounds were measured
in dilute THF solutions (10ꢁ5 mol/L) at room temperature. A plot of
the extinction coefficient versus wavelength for 5, 7, and 9 is shown
in Figure 2. 3,9-Dibromo-6-aryl-5H-dibenzo[d,f][1,3]diazepine has
its absorption maximum at 278 nm. When the diazepine moiety is
end-capped at the 3,9-positions with thiophene and fluorene units,
the absorbancemaxima is observed at 304 and 314 nm, respectively,
Figure 1. ORTEP representations of crystal structures of 3a, 5, 7, and 9. Methanol/
dichloromethane solvate molecules have been omitted for clarity. Displacement
ellipsoids are depicted at 50% probability.
larger red shift and the absorbance maximum shifts to 356 nm.
The observed red shift in the absorption maximum on coupling of
the diazepine unit with other aromatic moieties can be attributed
to extended
p-conjugation. Despite the more extended p-system
of the fluorene group over the thiophene, the absorbance maximum
for the fluorene model compound 9 is only marginally greater than
the thiophene-substituted 5. The extent of conjugation will depend
on the twist angle between the diazepine core and the neighboring
units; the solid-state structures above reveal larger twist angles
(35.5/41.5°) for 9 compared with 5 (3.7/18.9°). If solution behavior
mirrors that seen in the solid-state then the larger size of the fluo-
rene may be partially negated by a larger twist angle and poorer
p-conjugation with the dibenzodiazepine core.
The photoluminescence spectra for the coupled compounds
are depicted in Figure 2. Compounds 5 and 9 emit in the blue
region of the spectrum with emission maxima centered at 441
and 436 nm, respectively. Dibenzodiazepine end capped with
which is attributed to the
dibenzodiazepine molecule with 2,1,3-benzothiadiazole induces a
p–
p⁄ transition. End-capping the
Tomar, Manisha
