Full Paper
Chemistry—A European Journal
doi.org/10.1002/chem.202005246
suspension of d-tert-leucine (0.200 g, 1.52 mmol, 2.0 equiv) and sal-
icylaldehyde (0.139 g, 1.14 mmol, 1.5 equiv) in distilled water
Conclusions
(
(
2.0 mL) was stirred at 908C for 1 h after which phenylboronic acid
0.092 g, 0.762 mmol, 1 equiv) was added and the mixture was
This investigation reports on a simple synthetic strategy that
introduces chirality into the molecular backbone of a rigid bor-
on(III) chelate. Both N,N,O and N,O,O bonding motifs have
been realized and a key feature of the synthetic procedure is
that the resultant diastereoisomers are resolved prior to con-
densation. Mild reaction conditions are used for coupling of
the starting materials, without the need for a large excess of
one reagent, and it is anticipated that the one-pot protocol
could be adapted to generate a wide range of functionalized
derivatives. The final compounds have reasonably high dipole
moments and are soluble in most organic solvents, but not
water. They can be crystallized readily from solution, retaining
the imported chirality, to give interesting 3-dimensional struc-
tures. The N,O,O species, in particular, has high thermal and
photochemical stability. As a new family of chromophores,
these tert-leucine complexes are only weakly fluorescent in so-
lution, even in non-polar solvents. The N,N,O derivative is sig-
nificantly more fluorescent when prepared in the form of a
then stirred at 908C for 20 h. The reaction mixture, which appeared
as a biphasic composition of a precipitate and a supernatant
liquid, was filtered and the solid retained in the filter was then
washed with water followed by hexane. The desired compound
was recovered from the filter with dichloromethane, which was
subsequently removed under reduced pressure to afford d-syn-
LB(O) (0.237 g, 97%) as a greenish solid, m.p. 274- 2818C. H NMR
(
2
1
(
1
500 MHz, CDCl ) d 8.34 (s, 1H), 7.57–7.48 (m, 2H), 7.39–7.35 (m,
3
H), 7.24–7.17 (m, 3H), 7.03–6.97 (m, 2H), 4.04 (d, J=1.0 Hz, 1H),
11
13
.02 ppm (s, 9H). B NMR (128 MHz, CDCl ) d 6.80 ppm. C NMR
3
126 MHz, CDCl ) d 169.92, 162.89, 160.08, 139.19, 131.24, 130.29,
3
27.90, 127.57, 120.70, 120.35, 119.03, 74.70, 35.60, 27.21 ppm. IR
1
(
neat): n˜ max =2968, 1733, 1633, 1556, 1453, 1293, 1197, 1133, 992,
À1
+
980, 766, 702, 455 cm . HRMS (ESI): m/z found: [M+H] 322.1615,
C H20BNO requires [M+H] 322.1618.
19 3
+
(3R,11R)-3-(tert-butyl)-10-methyl-11-phenyl-10,11-dihydro-
4l4,11l4-benzo[d][1,3,2]oxazaborolo[3,2-a][1,3,2]diazaborinin-
2
0
(3H)-one d-syn-LB(N).
.739 mmol, 1.0 equiv), 2-(methylamino)benzaldehyde (0.100 g,
0.739 mmol, 1 equiv) and phenylboronic acid (0.090 g, 0.739 mmol,
equiv) in toluene (10 mL) was stirred at 908C for 24 h during
A mixture of d-tert-leucine (0.097 g,
thin film, cast from CHCl solution. This is a further example of
3
[
58]
what has become known as aggregation-induced emission.
1
It has to be mentioned that a somewhat related N,O,O
which the colour changed from yellow to dark red. The resulting
solid product was filtered, washing three times with water and pe-
troleum ether. Purification by flash column chromatography on
silica, eluting with ethyl acetate/petroleum ether 1:1 gave the
boron chelate d-syn-LB(N) (0.185 g, 74%) as a red solid, m.p. 197–
2
boron-fused azomethine complex having a stereogenic boron
[13]
centre was reported recently. In this case, chiral resolution of
the racemic mixture was achieved by high-performance liquid
chromatography using an amylose-based stationary phase. As
found for the tert-leucine complexes, these compounds are
only weakly fluorescent in solution but are more strongly emis-
1
018C. H NMR (300 MHz, CDCl ) d 7.94 (s, 1H), 7.33–7.23 (m, 3H),
3
7
.19 (dd, J=8.0, 1.6 Hz, 1H), 7.13–7.07 (m, 3H), 6.56 (d, J=8.9 Hz,
[13]
1H), 6.50 (ddd, J=7.9, 6.9, 0.9 Hz, 1H), 3.84 (d, J=0.8 Hz, 1H), 2.71
(
1
1
sive in the crystalline phase.
There is, therefore, a high
13
s, 3H), 0.88 ppm (s, 9H). C NMR (75 MHz, CDCl ) d 171.68, 160.12,
3
degree of similarity with LB(O), although no analogue of LB(N)
exists. The main difference between LB(O) and the boron-fused
azomethine complex relates to the ease of obtaining resolved
enantiomers.
51.15, 138.32, 132.93, 129.89, 127.38, 127.33, 115.09, 115.00,
11
13.47, 75.11, 35.25, 31.20, 27.43 ppm. B NMR (96 MHz, CDCl ) d
3
5
.27 ppm. IR (neat): n˜ =2958, 1742, 1625, 1472, 1365, 1255, 978,
max
À1
+
747, 702, 607 cm . HRMS (ESI): m/z found: [M+H] 335.1932,
C H23BN O requires [M+H] 335.1935.
20 2 2
+
In common with the BORANIL family of boron(III) che-
[
9,43]
lates
and these new N,O,O boron-fused azomethine com-
[
13]
plex, the tert-leucine derivatives have little propensity for the
development of fluorescent labels or sensors, at least in solu-
tion. The problem relates to a radiationless channel that rapid-
ly deactivates the excited-singlet state. This channel involves
Crystallography
[43]
Single crystal diffraction data were collected at 150 K on an Xcali-
bur, Atlas, Gemini ultra-diffractometer equipped with an Oxford
Cryosystems CryostreamPlus open-flow N2 cooling device using
copper radiation (lCuKa =1.54184 ). The intensities were corrected
for absorption using a multifaceted crystal model created by index-
intramolecular charge-transfer processes which, in the first
instance, promote translation of the Franck–Condon state to a
relaxed excited-singlet state. This internal relaxation is hin-
dered in the thin film, thereby stabilising the Franck–Condon
state and favouring fluorescence. Unlike the situation reported
[59]
ing the faces of the crystal for which data were collected. Cell re-
[
9]
for the BORANILS, fluorescence is only partially recovered in
non-polar solvents for the tert-leucine complexes. The same is
true at low temperature where fluorescence from the Franck–
Condon state has to compete with an important radiationless
channel.
finement, data collection and data reduction were undertaken via
[60]
[61]
the software CrysAlisPro. All structures were solved using XT
[62]
[63]
and refined by XL using the Olex2 interface. All non-hydrogen
atoms were refined anisotropically and hydrogen atoms were posi-
tioned with idealised geometry. The displacement parameters of
the hydrogen atoms were constrained using a riding model with
U(H) set to be an appropriate multiple of the Ueq value of the
parent atom.
Experimental Section
contain the supplementary crystallographic data for this paper.
These data are provided free of charge by the joint Cambridge
Synthesis and characterisation
(
3R,11R)-3-(tert-butyl)-11-phenyl-11H-4l4,11l4-benzo[e][1,3,2]ox-
azaborolo[2,3-b][1,3,2]oxazaborinin-2(3H)-one d-syn-LB(O).
A
Chem. Eur. J. 2021, 27, 5246 – 5258
5255
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