Regioselective Photoisomerization/C?C Bond Formation of Asymmetric B(ppy)(Mes)(Ar): The Role of the Aryl Groups on Boron

Article abstract of DOI:10.1002/anie.201700096

Asymmetric N,C-chelate organoboron compounds bearing two different aryl groups at the boron center undergo photoisomerization reactions that involve exclusively the less bulky aryl group, generating various strongly colored “dark isomers”. These species t

Full text of DOI:10.1002/anie.201700096

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700096
German Edition: DOI: 10.1002/ange.201700096
Photochemistry
ꢀ
Regioselective Photoisomerization/C C Bond Formation of
Asymmetric B(ppy)(Mes)(Ar): The Role of the Aryl Groups on Boron
Soren K. Mellerup, Cally Li, Tai Peng, and Suning Wang*
Abstract: Asymmetric N,C-chelate organoboron compounds
bearing two different aryl groups at the boron center undergo
photoisomerization reactions that involve exclusively the less
bulky aryl group, generating various strongly colored “dark
isomers”. These species thermally isomerize to 4bH-azabor-
epin molecules by direct hydrogen atom transfer from
a borirane cycle to the pyridyl moiety and ring expansion.
Mechanistic insight into these highly regioselective transfor-
mations was obtained from kinetic data and through computa-
tional studies.
H
arnessing the utility of molecular transformations relies
on exploiting the underlying nuances of a given chemical
system, with recent progress in main-group chemistry serving
[1]
as an excellent example of such endeavors. This is partic-
ularly true in organoboron-based systems, with the inherent
electron deficiency of the boron atom affording compounds
[
2]
with broad applications in catalysis/synthesis, polymer
[3]
[4]
science, and optoelectronics. Previously, we discovered
a photochromic phenomenon in four-coordinate organoboron
compounds such as A (Scheme 1), which underwent ther-
mally reversible intramolecular CꢀC bond formation/break-
Scheme 1. Photo-/thermal reactivity of symmetric (top) and asymmet-
ric (bottom) B(ppy)(Mes)(Ar) compounds.
[
5]
ing, generating “dark” isomers (e.g., B). Replacing the
pyridyl moiety by various other donors had a great impact on
Scheme 1). Access to unsymmetrically substituted N,C-che-
late boron compounds is key to fully establish the role of the
aryl substituents in the photoisomerization process and
unlock the full potential of this class of compounds in
[
6]
the switching properties of the boron compounds, while
little is known about the influence of the aryl groups on the
[7]
boron center. Less bulky aryl groups (e.g., phenyl) or those
[8]
[9]
that contain a strongly electron-donating group (e.g., NMe2)
applications such as memory devices and switches. For this
yield molecules that are photochemically inert as a result of
strengthened BꢀCAr bonds and the introduction of electronic
purpose, we developed a new synthetic method for various
asymmetric N,C-chelate boron compounds such as 1–4 and 6–
9 (Scheme 1). The derivatives 1–4 and 6–8 undergo photo-
isomerization as A does. Surprisingly, however, the isomer-
ization was found to occur exclusively on the less bulky aryl
group (e.g., 1a). This highly regioselective CꢀC bond
transitions that disfavor isomerization. All studies to date
indicate that steric congestion around the boron center is
required for the photochromic process shown in Scheme 1.
However, it is not clear if both aryl groups on the boron need
to be bulky for the photoisomerization. Furthermore, it is
unknown if such reactions of a boron molecule bearing two
different aryl groups would proceed regioselectively, and if so,
which photoproduct would be favored (1a vs. 1a’ in
formation was found to be general for all photoactive
asymmetric boron compounds of the form B(L)(Mes)(Ar)
(L = N,C-chelate ligand, Mes = mesityl). Furthermore, the
new dark isomers of these asymmetric boron compounds
undergo a hydrogen atom transfer to produce unprecedented
4bH-azaborepin structures (e.g., 1b). Kinetic data collected
for a variety of different substituents give direct insight into
key electronic and steric contributions to this interesting and
unusual photo-/thermal isomerization phenomenon.
[
*] S. K. Mellerup, C. Li, Prof. Dr. S. Wang
Department of Chemistry, Queen’s University
Kingston, Ontario, K7L 3N6 (Canada)
E-mail: suning.wang@chem.queensu.ca
Dr. T. Peng, Prof. Dr. S. Wang
Beijing Key Laboratory of Photoelectronic/Electrophotonic Conver-
sion Materials, School of Chemistry
Compounds 1–4 and 6–9 were synthesized by reacting
appropriate B(OMe)(Mes)(Ar) reagents with 2-(ortho-lith-
iumphenyl)pyridine or related N-heterocycles while 5 was
Beijing Institute of Technology
obtained using B(OMe)(2,4-Me C H ) (see the Supporting
5
South Zhongguancun Street, Beijing (P.R. China)
2
6
3 2
Information for details). All compounds were fully charac-
terized by NMR spectroscopy and HRMS analysis. The
crystal structures of 1, 4, and 6 were determined by X-ray
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700096.
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[
14]
diffraction. Comparing the crystal data of 1 and 4 to those
proton on the borirane ring and the vinyl hydrogen atoms (d
[
5–8]
of A and B(ppy)Ph (ppy = 2-phenylpyridine) reveals that
ꢁ 5.5–6.5 ppm; see the Supporting Information).
2
they have similar BꢀN/BꢀC bond lengths and C -B-C /
The absence of any singlets in the high-field region of the
H NMR spectrum indicates that the photoisomerization does
ppy
Ar
Ar
1
N-B-C
angles. The key difference between these four
ppy
molecules is that the BꢀC bond length decreases as the
not involve the mesityl substituent, and that this reaction
proceeds with high regioselectivity. This is the first example of
an N,C-chelate organoboron photoisomerization involving an
aryl group without ortho-methyl substituents, indicating
a previously unknown generality for this type of transforma-
tion. The formation of 1a/2a is responsible for the first color
change, with UV/Vis spectroscopy revealing one broad new
absorption peak at l ꢁ 540 nm (Figure 1b). Time-dependent
(TD) DFT data indicate that the electronic transition
responsible for this low-energy band is CT from the HOMO
(p-cyclohexadiene and borirane ring) to the LUMO (p*-ppy)
Ar
ortho-methyl groups are removed from the aryl group on the
boron atom (avg. BꢀC bond lengths of 1.649, 1.638, 1.633,
Ar
and 1.614 ꢀ for A, 4, 1, and B(ppy)Ph , respectively). The
2
DFT-optimized structures of 1, 4, and 6 agree well with the
crystal structural data (see the Supporting Information).
Compounds 1–9 all show blue–green fluorescence with
moderate to low fluorescence quantum yields (7–28%) as
a result of charge transfer (CT) from the HOMO (p-Mes/Ar)
[
5]
to the LUMO (p*-ppy; see the Supporting Information).
Compounds 1 and 2 were examined first for their response
to light. Irradiating either compound in solution (l = 300 or
[
5–8]
as observed for B (see the Supporting Information).
3
65 nm) resulted in two successive color changes from
Continued irradiation led to full conversion into the final
isomers 1b/2b, which were air-stable and fully characterized
by NMR spectroscopy and X-ray crystallography (Figure 1c).
The crystal structure of 2b reveals that the pyridyl ring has
been dearomatized by protonation of the C13 atom, giving
rise to a stereogenic carbon center and CꢀC bond lengths
colorless to bright pink and then orange (Figure 1). Monitor-
consistent with a cyclohexadiene bonding arrangement (see
the Supporting Information). The isomerized tolyl ring is
rearomatized by migration of the H atom originally bound to
C6 in the borirane ring of 2a. To accommodate these changes,
1
1
the BꢀN bond length decreases to 1.422(5) ꢀ with a
chemical shift of d ꢁ 45 ppm, which is consistent with a B=N
bond. The overall structure of 2b adopts a pseudo-boat
B
[
6b,11]
conformation with a C12-C13-N1 bond angle of 108.248.
While the isomerization of 1a/2a to 1b/2b occurs in tandem
with the generation of 1a/2a from 1/2, this second trans-
formation does not proceed through an excited state as
heating a mixture of 1, 1a, and 1b at 908C for several hours
converted all of 1a into 1b (see the Supporting Information).
The activation barrier for the H migration reaction must be
relatively small as at the internal temperature of the
photolysis reaction on NMR scale, 1a/2a is fully converted
into 1b/2b within 24 hours.
1
Figure 1. a) H NMR spectra showing the clean conversion of 2!2a!
To confirm that the migrated H atom indeed originates
from the phenyl/tolyl ring in 1/2 and gain kinetic information
on the two processes, deuterium-labeled compound 3 was
prepared, which underwent a similar isomerization process in
C D upon UV irradiation as confirmed by NMR analysis.
2
b in C D after 24 hours of irradiation at l=300 nm; structures and
6 6
colors of 2, 2a, and 2b. b) UV/Vis spectra for the conversion of 2 into
a in toluene (10 m). c) Crystal structure of 2b. Thermal ellipsoids
set at 35% probability.
ꢀ
4
2
6
6
1
ing the reaction by H NMR spectroscopy revealed two clean
and sequential transformations, giving a mixture of inter-
mediate (1a/2a) and product (1b/2b) after several hours of
irradiation. The intermediates (1a/2a) were identified as dark
Most noteworthy are the missing resonances at d = 0.09 and
1
5.38 ppm in the H NMR spectra of 3a and 3b, which
correspond to the H and H ’ atoms in 1a and 1b, respectively
1
1
1
(Figure 2). These missing H resonances were observed in the
1
1
1
2
isomers akin to B from their diagnostic B and H chemical
shifts at d ꢁ ꢀ13.9 and 0.09 ppm (H atom on the three-
H NMR spectra as broad singlets with the same chemical
1
shifts as in the H NMR spectra of 1a and 1b (see the
membered BC ring), respectively, as well as 2D NMR data
Supporting Information), further supporting the correct
assignment of the photo- and thermal isomerization products.
The NMR data also confirm that the hydrogen atom
responsible for the dearomatization of the pyridyl ring in
the 4bH-azaborepin product b originates from the borirane of
the dark isomer a likely via direct H atom transfer. The photo-
and thermal transformations of 1 and 3 follow first-order
kinetics with kinetic isotope effects (KIEs) of about 0.8 for
the isomerization of 1!1a and 3.2 for the H atom migration
2
(
0
see the Supporting Information). Notably, the resonance at
.09 ppm is split into a doublet ( J = 6.1 Hz), and this
H-H
3
proton is significantly more shielded than the protons of the
methyl group on the borirane ring in B (d ꢁ 0.5 ppm). These
observations for 1a/2a agree with the formation of a saturated
[
10]
borirane, indicating that the phenyl or para-tolyl group has
isomerized. This hypothesis was further corroborated by their
H– H COSY spectra, which show correlations between the
1
1
2
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ring. This feature could potentially be exploited to design and
prepare photochromic organoboron derivatives with superior
performance and higher stability. The overall FPI for the
formation of 4a and 4a’ was determined to be 0.33, indicating
that the extra ortho-methyl group of 4 does not have an
impact on photoisomerization efficiency.
Compound 4a is thermally more stable than 1a/2a, as
only a small amount of 4b was observed after 30 hours of
irradiation at l = 300 nm (see the Supporting Information).
The increased activation barrier for the conversion of 4a into
4
b might be due to stabilization by the extra ortho-methyl
1
Figure 2. Comparison of the H NMR spectra of a mixture of 1, 1a,
group. Heating a solution containing 4, 4a, and 4a’ at 908C
converted 4a into 4b and 4a’ into 4. Consequently, 4b can be
isolated quantitatively after three cycles of irradiation/heating
and is structurally similar to 1b/2b (Scheme 2). Interestingly,
and 1b (bottom) and a mixture of deuterium-labeled 3, 3a, and 3b
(
top).
of 1a!1b (see the Supporting Infor-
mation). The inverse secondary KIE
obtained for 1!1a is consistent with
a change in hybridization of the H
bound carbon atom from sp to sp as
1
2
3
the
approached,
transition
state
(TS)
is
[12]
and is in agreement
with the previous DFT studies on the
isomerization of A in which dearoma-
tization of the mesityl ring during
borirane formation was found to be
[6a,b]
the rate-limiting step.
The normal
primary KIE of H atom migration has
a magnitude within the range typical
for proton transfer reactions involving
[
12]
a non-linear TS, further supporting
a direct H atom transfer pathway for
the isomerization of 1a into 1b. The
Scheme 2. Regioselective photoisomerization and thermal isomerization of 4.
photoisomerization quantum efficiencies (F ) of 1, 2, and 3
the symmetric compound 5 was found to decompose upon
irradiation with UV light at l = 300 nm . This finding indicates
that the two ortho-methyl groups of the mesityl ring are
required to protect/stabilize the unsaturated boron atom
during the isomerization process. Steric considerations are
therefore critical to achieve/maintain this type of photo-
reactivity.
Next, the influence of electronic effects on the photo-
isomerization was investigated by either extending the
p-conjugated backbone (6) or substituting the phenyl ring
of 1 with electron-donating (7) or -withdrawing (8 and 9)
groups. Compounds 6–8 all undergo the same two-step
isomerization as 1–4, with the changes in FPI and the rate of
H atom migration reflecting the influence of their different
electronic properties. While the photoisomerization of 6 is the
PI
into 1a, 2a, and 3a were determined to be 0.39, 0.34, and 0.24
respectively (see the Supporting Information), which corre-
spond to about half the value of A, indicating that the greater
steric bulk in A significantly enhances the efficiency of the
photoisomerization.
The photoisomerization of B(ppy)(Mes)(Ar) is not only
regioselective towards the less bulky aryl ring, but also
regioselective for the less congested carbon atom when
forming the new BꢀC bond with the aryl ring as demonstrated
by compound 4, which bears a 2,4-dimethylphenyl group. As
with compounds 1–3, irradiation of 4 at l = 300 nm induces
a dramatic color change from colorless to deep purple, and
the red-shifted absorption spectrum is consistent with a desta-
bilized HOMO as the number of ortho-methyl groups on the
aryl ring is increased (labs = 528, 567, and 600 nm for 1 (pink),
least efficient among all molecules reported here (labs
=
1
1
4
(purple), and A (blue), respectively). The B NMR
spectrum displayed two new resonances at d ꢁ ꢀ10.1 and
13.6 ppm, which correspond to the two different dark
511 nm for 6a, F = 0.18), its H atom migration (6a!6b) is
PI
about two times faster than 1a!1b. Both of these features
can be explained by the extended p-conjugation and the lower
aromaticity of the N-heterocycle in 6 relative to pyridine,
ꢀ
isomers 4a and 4a’ present in solution. The H atom bearing
isomer 4a is favored over 4a’, which has a Me group on the
borirane ring, in a ratio of 7:3 (Scheme 2; see also the
Supporting Information). This result further supports the
observation that although crowding around the boron center
is required to initiate photoisomerization, this process still
prefers products with less steric hindrance on the borirane
[
7]
which stabilize the excited state of 6 and decrease the
reorganization energy associated with imidazole dearomati-
zation, respectively. The FPI value of electron-rich 7 was
determined to be 0.54 while that of electron-poor 8 is 0.23,
with the latter being the first example of an N,C-chelate
organoboron compound with an electron-deficient aryl ring
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that undergoes photoisomerization. Compared
to 1 (F = 0.39), this change in FPI confirms
PI
that the electron density on the aryl ring plays
a pivotal role in the photoisomerization process.
While the aforementioned CT to the first
excited state was proposed to be partially
responsible for the photoreactivity of this class
[
6–8]
of compounds,
the behavior of 7 and 8
towards UV light provides the first direct
experimental evidence supporting this hypoth-
esis (see the Supporting Information for TD-
DFT calculations). Compound 8a is yellow
owing to the hypsochromic shift of its absorp-
tion spectrum relative to those of the other dark
isomers. The isomerization of 8a into 8b is more
difficult than that of 1a into 1b and only
observed upon heating 8a at 1108C for
Figure 3. DFT-calculated ground-state reaction pathways for phenyl (solid bars) and
mesityl (empty bars) isomerization in 1 at the B3LYP/6-31g* level of theory. The
structures correspond to those for phenyl isomerization.
2
4 hours; this result constitutes an indication
that the H migration is likely to be hydridic in
nature. These findings demonstrate that the
H atom migration can be controlled and even
completely shut down by tuning the electronic
properties of the aryl group. Compound 9 decomposes upon
in energy (see the Supporting Information). Assuming kinetic
control for this reaction, this translates into a 4a/4a’ product
ratio of 5:1, which roughly matches the observed ratio of 7:3.
The thermal barrier for H atom migration was determined to
be 22 kcalmol for 4a (see the Supporting Information for
details).
irradiation as observed previously with other C F -substituted
6
5
[7]
boron compounds.
Considering that C F5 and 3,5-
6
(
CF ) C H groups have similar electron-withdrawing proper-
3
2
6
3
[
13]
ꢀ1
ties, the ortho-fluorine atoms in 9 are likely the cause of its
photoinstability.
To understand the regioselective isomerization of asym-
metric B(ppy)(Mes)(Ar) compounds, the ground-state reac-
tion pathways of phenyl and mesityl isomerization in 1 were
computed at the B3LYP/6-31g(d) level of theory (Figure 3).
The HOMO and LUMO orbitals of 1 are similar to those of
A, indicating that the electronic properties of these two
compounds are comparable and that the differing reactivity is
due to steric effects (see the Supporting Information). All
intermediates and transition states are higher in energy for
the mesityl isomerization than for the phenyl isomerization.
In particular, the rate-limiting step via TS2 is approximately
In summary, the first examples of regioselective photo-
isomerization/CꢀC bond formation in asymmetric N,C-che-
late boron compounds have been demonstrated, illustrating
both the generality of this type of transformation as well as
the critical roles of both steric and electronic factors within
this class of molecules. Furthermore, direct hydrogen atom
migration reactions involving the borirane ring of the dark
isomers have been established for the first time. These new
findings and detailed mechanistic insight provide valuable
guidance for the future design and development of boron-
based photoresponsive materials.
ꢀ1
7
kcalmol lower in energy for phenyl isomerization. There-
fore, the isomerization of the less bulky substituent is favored
owing to the stabilization of all species along the reaction Acknowledgements
pathway by mesityl shielding of the boron atom. Although the
reaction pathway in the excited state has not been computed,
we expect a similar trend based on our detailed computational
work for A in both the ground and excited state. For the
H atom migration, the computed pathway with the lowest
energy barrier involves concerted H atom transfer from the
We thank the Natural Science and Engineering Research
Council of Canada (RGPIN1193993-2013) and the National
Natural Science Foundation of China (21571017) for financial
support. S.K.M. thanks the Canadian Government for
a Vanier Scholarship.
[6a]
borirane ring to the C2 atom of py. The calculated barrier (E )
a
ꢀ1
for this transformation (20 kcalmol ) is in agreement with
the fact that 1b is formed rapidly during photolysis as this Conflict of interest
value is accepted as the threshold below which reactions can
occur readily at room temperature. For comparison, the
The authors declare no conflict of interest.
E value of the H atom migration in 8a was calculated to be
a
ꢀ
1
2
7 kcalmol , which agrees well with the experimentally
Keywords: 4bH-azaborepins · boriranes · CꢀC bond formation ·
ꢀ1
determined value of 26.9 kcalmol (see the Supporting
Information). With respect to the two dark isomers observed
for 4, the two possible pathways were calculated to be very
photo-/thermal transformations · regioselectivity
ꢀ1
close in energy, with TS2 for 4a being only 1 kcalmol lower
4
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Manuscript received: January 5, 2017
Final Article published: && &&, &&&&
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Communications
Photochemistry
S. K. Mellerup, C. Li, T. Peng,
S. Wang*
&&&& — &&&&
Regioselective Photoisomerization/CꢀC
Bond Formation of Asymmetric
B(ppy)(Mes)(Ar): The Role of the Aryl
Groups on Boron
Dark isomers: Asymmetric N,C-chelate
boron compounds undergo a regioselec-
tive photoisomerization at the less bulky
aryl group to intensely colored “dark
isomers” and a subsequent thermal iso-
merization involving hydrogen atom
transfer. This process demonstrates the
definitive roles of steric and electronic
effects in this photoresponsive system.
6
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