Accessing Two-Stage Regioselective Photoisomerization in Unsymmetrical N,C-Chelate Organoboron Compounds: Reactivity of B(ppz)(Mes)Ar

Article abstract of DOI:10.1021/acs.organomet.8b00598

A new family of unsymmetrical, N,C-chelate organoboron compounds B(ppz)(Mes)Ar have been synthesized and found to undergo a rare, regioselective two-stage photoisomerization, involving the Ar group only. The initial transformation is a Zimmerman rearrangement to afford yellow azaboratabisnorcaradiene isomers that are subsequently converted to unprecedented 14aH-diazaborepins via a photochemical walk rearrangement. Spectroscopic and computational studies provide insight into the formation and properties of these unique systems.

Full text of DOI:10.1021/acs.organomet.8b00598

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Accessing Two-Stage Regioselective Photoisomerization in
Unsymmetrical N,C-Chelate Organoboron Compounds: Reactivity of
B(ppz)(Mes)Ar
Cally Li, Soren K. Mellerup, Xiang Wang, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
S
* Supporting Information
ABSTRACT: A new family of unsymmetrical, N,C-chelate
organoboron compounds B(ppz)(Mes)Ar have been synthe-
sized and found to undergo a rare, regioselective two-stage
photoisomerization, involving the Ar group only. The initial
transformation is a Zimmerman rearrangement to afford
yellow azaboratabisnorcaradiene isomers that are subse-
quently converted to unprecedented 14aH-diazaborepins via
a photochemical “walk” rearrangement. Spectroscopic and
computational studies provide insight into the formation and properties of these unique systems.
Scheme 1. Photochemical and Thermal Reactivity of
B(ppy)(Mes)Ar/B(ppz)(Mes)Ar Compounds
INTRODUCTION
■
Establishing the underlying mechanisms within chemical
systems is important for both their practical implementation,
as well as the discovery of new and useful processes/
transformations. In this regard, the chemistry of organoboron
compounds has received significant attention due to the rich
optoelectronic properties of boron-doped π-systems1 and their
amenability toward numerous different fields in materials
science such as chemical sensors,2 organic light-emitting diodes
(OLEDS),3 and memory devices.4 Our longstanding interest in
photoresponsive boron compounds stems from the previously
discovered N,C-chelate organoboron species (B(ppy)(Mes)2;
ppy = 2-phenylpyridine, Mes = mesityl),5 which displays
thermally reversible photochromic switching to generate a
“dark” azaboratabisnorcornadiene isomer upon exposure to
UV light. Following this initial discovery, many studies have
since been devoted to examining the effects of altering the
phenyl-pyridyl chelating backbone.6 It is only recently that our
focus has shifted toward studying and understanding the
influence of the aryl groups on boron, following the successful
development of a one-pot approach for synthesizing various
unsymmetrical derivatives bearing two different aryl sub-
stituents (B(ppy)(Mes)Ar; Ar = ph or substituted ph; Scheme
1).7 While these unsymmetrical systems rely on the presence of
one bulky Mes group to initiate the photoisomerization
process, the formation of the borirane ring occurs regiose-
lectivity on the less bulky Ar group (I; Scheme 1). Unlike their
symmetric counterparts, these H-borirane dark isomers
undergo a further thermal transformation into 4bH-azabor-
epins (II; Scheme 1) by means of an H-atom migration from
the borirane to the pyridyl moiety.7
in materials science. Given the propensity for the H atom in I
to migrate to the electron deficient C2 position on the py ring,
we postulated that photochromism may be achievable in these
unsymmetrical derivatives if this kinetically favored pathway
were blocked. We therefore designed a new series of chelate
boron compounds (1−7), in which the electron deficient C2
position of py has been replaced by an electron-rich nitrogen
atom of pyrazole (pz). Although 1−7 undergo the expected
In spite of this unique regioselective reactivity available to
the unsymmetrical boron chelate compounds, their inability to
thermally revert back to the original state means that these
compounds are not photochromic, limiting their applicability
Received: August 18, 2018
© XXXX American Chemical Society
A
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regioselective photoisomerization into their respective azabor-
atabisnorcaradienes (a), the resulting isomers are all thermally
inert. In lieu of this, prolonged irradiation of certain derivatives
facilitated their conversion into unprecedented 14aH-diazabor-
epins or BNN-benzotropilidene (e.g., 1b−4b). Although the
B(ppz)(Mes)Ar (ppz = 1-phenylpyrazole) compounds do not
possess the anticipated reversible photochromism, their two-
stage photoreactivity indicates that this type of rare isomer-
ism6a is a general phenomenon for N,C-chelate organoborates
with diazazole donors.
bond lengths in these two molecules are 1.630(5) and
1.648(3) Å, respectively, similar to those with ppy and
derivatives as the chelate units.6 In compound 4, the carbazole
ring and the phenyl ring are not coplanar with a dihedral angle
of 73.3(2)°.
Photophysical Properties of Compounds 1−7. The
absorption spectra and fluorescence spectra of 1−7 are shown
in Figure 2, and the data are summarized in Table 1.
RESULTS AND DISCUSSION
■
Syntheses and Structures of Compounds 1−7.
Compounds 1−7 were prepared in low to moderate yields
via a one-step lithiation of 1-(2-bromo-phenyl)-1H-pyrazole,
followed by the addition of the appropriate B(Mes)(OMe)-
(Ar) reagent or B(F)Mes2 (Scheme 2). B(Mes)(OMe)(Ar)
Scheme 2. Synthesis of B(ppz)(Mes)(Ar) and B(ppz)Mes2
Figure 2. UV/vis and normalized fluorescence (dashed lines) spectra
of 1−7 in toluene at 10−4 M.
Table 1. Photophysical Properties of Compounds 1−7
a
compd
λabs (nm) (ε, M−1 cm−1
)
λem (nm)
Φfl
1
2
3
4
5
6
7
300 (7.11 × 103)
298 (5.11 × 103)
296 (6.10 × 103)
392
384
391
352, 368
374
0.19
0.11
0.08
0.33
0.10
0.12
0.12
296, 333, 347 (2.10 × 104 104)
294 (6.53 × 103)
297 (1.33 × 104)
305 (3.22 × 103)
383
398
a
10−4 M in toluene at 298 K
reagents were prepared using modified literature procedures,7
which involves the addition of 1.05 equiv of the corresponding
Grignard/lithiated reagent (Ar−Nu; Scheme 2) to B(Mes)-
(OMe)2 (see Experimental Section and Supporting Informa-
tion (SI) for details). All compounds were fully characterized
Compounds 1−7 have intense absorption in the mid-UV
region, with λabs < 300 nm in all cases except 4. Compared to
previously reported N,C-chelate organoborates, these values
are similar to those observed for systems with N-heterocyclic
carbene (NHC) donors6c and hypsochromically shifted by
∼60 nm relative to species with pyridine as the donor.5,7,8 This
is due to HOMO stabilization induced by the electron-
accepting ability of pyrazole. TD-DFT calculation data (at the
cam-B3LYP15/SVP16 level of theory; see the Supporting
Information) indicate that the primary absorption bands of
1−2 and 4−7 have different contributions depending on the
substituent. Compounds 1, 6, and 7 have an S1 transition that
corresponds to a HOMO (π-Mes/Ar) → LUMO (π*-ppz)
charge transfer (CT), which is characteristic of these
systems.6−8 Conversely, 2 and 5 possess HOMO-2 →
LUMO as the main contribution to their S1, where HOMO-
2 is delocalized across both the aryl substituents and ppz
backbone. The predicted S1 transition for compound 4 has
HOMO → LUMO+1 as its primary contributor, which is a π
→ π* transition centered on the carbazole unit. This is in
agreement with experimental data, as compound 4 is the only
species to have well resolved vibrational features in its
absorption spectra, which are a combination of S1 and S2
(CT transitions; HOMO → LUMO, 38%; HOMO-4 →
LUMO, 46%; see SI). The fluorescence spectra of 1−7 exhibit
a hypsochromic shift relative to the related N,C-chelate boron
1
by H, 13C, 11B NMR and HRMS spectroscopic methods.
The crystal structures of 4 and 6 were determined by single-
crystal X-ray diffraction analysis (see the Supporting
Information) and are shown in Figure 1. Unsurprisingly, the
steric congestion imposed by the bulky Mes group induces B−
CAr/Mes bonds of 1.62−1.64 Å, which is a feature known to
be essential for instigating photoisomerization.7,8 The B−N
Figure 1. Crystal structures of 4 (left) and 6 (right). Selected bond
lengths (Å) for 4: B1−C1 1.611(5), B1−N1 1.630(5), B1−C10
1.621(5), B1−C16 1.627(5); for 6: B1−C1 1.618(4), B1−N1
1.648(3), B1−C10 1.638(4), B1−C20 1.626(4).
B
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analogues,5,7,8 such as B(ppy)(Mes)Ar with λem = 360−390
nm and relatively low quantum yields (Φfl = 0.10−0.33). In
contrast to the broad featureless CT emission bands of the
other five compounds, compound 4 exhibits a structured
emission spectrum which is most hypsochromically shifted and
has the highest Φfl (0.33), attributable to the carbazole unit.9
Photoisomerization. The response of compound 1
toward UV light was examined first. Upon irradiation at 300
nm in C6D6, 1 undergoes two-stage sequential color changes:
colorless to yellow, then to deep-orange (Figure 3).
new set of peaks belonging to 1b appear. The structure of 1b
was characterized by comprehensive 1D and 2D NMR analysis
(Figure 5 and SI). The 1H−1H COSY spectrum shows
Figure 3. Partial 1H NMR spectra in C6D6 showing the conversion of
1 → 1a → 1b with diagnostic chemical shifts highlighted in blue (1a)
and red (1b). Inset: photographs showing the solution colors of the
three different species at 10−2 M in benzene.
Monitoring the reaction by NMR spectroscopy revealed the
initial transformation of 1 into its dark borirane isomer 1a, as
evidenced by diagnostic 1H and 11B chemical shifts at δ ≈ 1.11
(H atom on borirane; 3J = 6.1 Hz) and −15 ppm, respectively.
From 2D NMR data, it is clear that 1 goes through the same
regioselective photoisomerization as the B(ppy)(Mes)(Ar)
molecules,7 in which the borirane forms exclusively on the less
bulky tolyl group. The formation of 1a and its characteristic
yellow color is further corroborated by UV/vis spectroscopy, as
the gradual irradiation of 1 causes a steady growth of a new low
energy absorption band at λabs ≈ 439 nm (Figure 4 and SI).
Akin to what has been observed for I in Scheme 1, this broad
band emerging in the visible region is a result of a CT
transition from HOMO (σ-borirane + π-cyclohexadienyl ring)
to LUMO (π*-ppz). Extended irradiation for several hours
leads to a second stage color change from yellow to orange, as
Figure 5. 1H−1H COSY spectrum (top) and 1H−13C HSQC
spectrum (bottom) of 1b in C6D6 with diagnostic correlations.
correlations between the three olefin protons and a CH3 group
(tolyl), indicating the preservation of a cyclohexadiene
bonding arrangement. More noteworthy is the coupling of
one olefin proton to the upfield-shifted H atom (δ ≈ 4.26 ppm,
3J = 4.7 Hz; doublet) on the same ring, which, according to
1H−13C HSQC data, is attached to a quaternary carbon center
(δ 13C ≈ 33.5 ppm). A new 11B resonance at ∼29 ppm is
observed, comparable to the chemical shift values of previously
reported R2BC(R′)(R′′) systems10 and the closely related
1,2-azaborabenzotropilidene (BN-BTPD) molecule6a (Figure
6). All of these data led us to ascribe a similar seven-membered
BNN-BTPD structure for 1b. Due to the structural similarities
between BN-BTPD and 1b, a similar two-stage photochemical
reaction pathway via “walk” rearrangement established
previously for BN-BTPD formed from photoisomerization of
B(Me-imidazolyl-ph)Mes2 is likely responsible for the
formation of 1b. The key difference between BN-BTPD and
1b is that the former can thermally revert back to its parent
chelate species, whereas 1b is thermally inactive. Hence, it
should be noted that the thermally accessible, ground-state
“walk” rearrangement observed for BN-BTPD and its
1
the H and 11B peaks of 1a vanish in the NMR spectra, and a
Figure 4. UV/vis absorption spectra of 1a and 1b in toluene. Inset:
photographs showing their colors in solution.
C
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naphthyl ring in 6a. Continued irradiation causes 2a and 3a to
1
undergo the same isomerism as 1a, although their final H
NMR spectra reveal a mixture of products comprising the
corresponding isomer 2b/3b, analogues of 1b, and a minor
unidentifiable species 2b′/3b′ (see Scheme 3 and the
Scheme 3. Summary of Photochemical Reactivity of
B(ppz)(Mes)(Ar) Compounds
Figure 6. Diagrams showing the structures of 1b and BN-BTPD
(right) and the DFT-optimized structure of 1b (left) obtained with
the cam-B3LYP/SVP level of theory. Key bond lengths (Å) for 1b:
B1−C1 1.584, B1−C2 1.455, B1−N1 1.543, N1−N2 1.357, N2−C9
1.434, C2−C7 1.533, C2−C3 1.411, C3−C4 1.354, C4−C5 1.461,
C5−C6 1.344, C6−C7 1.509, C7−C8 1.520, C8−C9 1.400.
respective isomers cannot be applied to the molecules in this
study.
Upon exposure to air, 1b decomposes rapidly, a common
feature of molecules bearing a BC bond.10 Unfortunately,
attempts to obtain single crystals of 1b for X-ray diffraction
analysis were unsuccessful due to the formation of a small
amount of unknown side product during the generation of 1b
and the highly air-sensitive nature of 1b. Insight into the
structural configuration of 1b is gained by its DFT-optimized
structure shown in Figure 6. The calculated BC bond length
of 1.45 Å is comparable to that of the BN-BTPD system (1.47
10c
Å) and to values reported for [Mes2BCH2]− as well as
Lewis acidic methyleneboranes.11 Accompanying the con-
version of 1a → 1b is a color change from yellow to orange,
which corresponds to an ∼40 nm bathochromic shift in the
absorption band of 1b relative to that of 1a (Figure 4).
According to TD-DFT calculations, the HOMO of 1b is
localized on the conjugated BC bond and the cyclo-
hexadienyl moiety while the LUMO is localized on the ppz
unit. On the basis of what has been observed for BN-BTPD,
the destabilized HOMO orbital of 1b compared to that of 1a is
likely responsible for its increased λabs value. From TD-DFT
data, the primary absorption band of 1b was assigned to a
HOMO → LUMO CT transition. The calculated and
experimental UV/vis spectra of 1b match very well, further
validating the formation of the proposed structure for 1b (see
the Supporting Information).
In light of the interesting reactivity of 1, the influence of
electronic effects on the two-stage photoisomerization process
was examined by substituting the boron center with various
aryl groups. As evidenced by their NMR and UV/vis spectra
(see the Supporting Information), compounds 2−6 undergo
the initial regioselective photoisomerization into their
respective borirane isomer “a”. Irradiation at 300 nm induces
the growth of a broad CT (HOMO → LUMO) absorption
band between 370 and 440 nm for all compounds, giving a
pale-yellow color to each of the dark isomers. Given that the
LUMO of the borirane in all cases is composed of the π*-
orbitals on the ppz chelate, the difference in absorption λmax
must be dictated by the substituents undergoing photo-
isomerization.
Supporting Information). Due to the overlapping NMR signals
and the instability of the minor products, 2b′ and 3b′ could
1
not be fully characterized. The five H resonances of the
1
cyclohexadienyl protons in 2b are absent in the H NMR
spectrum of 3b (see the Supporting Information), further
confirming the assignment of the BNN-BTPD structure. The
isomerization of 4a → 4b was also observed, albeit it proceeds
with the formation of minor degradation products. Corre-
spondingly, the characteristic orange colors of 2b−4b are
consistent with their red-shifted absorption bands emerging
between 470−510 nm.
Interestingly, extended irradiation of 5a and 6a results in no
1
spectral change according to H and 11B NMR spectral data,
confirming that their “b” isomers are not formed. The lack of
photoreactivity of 5a is attributable to the CF3 groups, which
stabilizes the HOMO orbital of the molecule (borirane/
cyclohexadienyl). This stabilized HOMO level prevents any
further photochemical transformations that would otherwise
occur through the excited state. In the case of 6a, the increased
steric bulk imposed by the naphthyl substituent is likely
restricting further intramolecular bond rearrangement around
the boron center. Although having one Mes group in these
systems is crucial for instigating photoisomerization, it seems
that excessive crowding by two bulky aryl groups is not
conducive to transformations beyond the dark isomer. For
example, the symmetric derivative B(ppz)(Mes)2 (compound
7, Scheme 3) was synthesized in an effort to probe the full
range of photoreactivity in this series of molecules. When
irradiated at 300 nm, B(ppz)(Mes)2 mirrors the reactivity of 6
Notably, the λabs values of 5a and 6a are hypsochromically
shifted (∼30 to 60 nm) relative to those of 1a−4a, signifying a
stabilization of their HOMO orbitals, which is likely a result of
the electron-withdrawing properties of the 3,5-(CF3)C6H3
substituent in 5a and the extended π-conjugation of the
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temperature overnight. The solvent was removed in vaccuo, and the
residue was extracted with dry/degassed hexanes (200 mL). The
crude mixture was filtered through Celite under N2 and concentrated
under reduced pressure. A separate 50 mL Schlenk was charged with
1-(2-bromophenyl)-1H-pyrazole (0.736 g, 3.3 mmol) and Et2O (15
mL). The solution was cooled to −78 °C for 30 min, after which n-
BuLi (1.5 mL, 3.63 mmol, 2.5 M in hexanes) was added dropwise.
After the lithiated ligand solution was allowed to stir for 1 h at −78
°C, B(OMe)(Mes)(p-tolyl) was added quickly and the completed
reaction mixture was slowly warmed to ambient temperature
overnight. The solvents were removed under reduced pressure, and
the crude product was purified with flash column chromatography
using a gradient elution (10:1 → 4:1 hexanes:ethyl acetate). 1 was
obtained as a white solid in 45% yield (0.31 g). 1H NMR (400 MHz,
C6D6): δ 7.79 (d, J = 7.2 Hz, 1H, Ph−H), 7.40 (d, J = 7.7 Hz, 2H, p-
tolyl−H), 7.13−7.05 (m, 4H, Pz−H, p-tolyl−H, and Ph−H), 7.01 (t,
J = 7.3 Hz, 1H, Ph−H), 6.90 (s, 2H, Mes−H), 6.74 (d, J = 7.8 Hz,
1H, Ph−H), 6.65 (d, J = 2.5 Hz, 1H, Pz−H), 5.52 (t, J = 2.4 Hz, 1H,
Pz−H), 2.28 (s, 3H, p-Mes−CH3), 2.18 (s, 3H, p-tolyl−CH3), 1.94
(s, 6H, o-Mes−CH3) ppm; 13C NMR (100 MHz, C6D6) δ 142.5,
138.3, 134.7, 134.4, 133.6, 132.7, 132.1, 130.1, 129.0, 125.9, 121.3,
110.2, 109.5, 25.3, 21.3, 20.9 ppm; 11B NMR (128 MHz, C6D6) δ 1.8
ppm; HRMS (EI), calcd for C25H25BN2 [M]+: 364.2111, found
364.2101.
Synthesis of 2. Compound 2 was synthesized using the same
procedure as that for compound 1, where bromobenzene was used to
prepare the second Grignard reagent in the synthesis of B(OMe)-
(Mes)(Ph). 2 was obtained as a white powder in 30% yield (0.21 g).
1H NMR (400 MHz, C6D6): δ 7.77 (dd, J = 7.2, 1.3 Hz, 1H, Ph−H),
7.46 (m, 2H, B−Ph−H), 7.21 (dd, J = 8.0, 6.7 Hz, 2H, B−Ph−H),
7.14−7.08 (m, 2H, Ph−H and B−Ph−H), 7.07 (d, J = 2.4 Hz, 1H,
Pz−H), 7.00 (td, J = 7.6, 1.3 Hz, 1H, Ph−H), 6.89 (s, 2H, Mes−H),
6.73 (d, J = 7.8 Hz, 1H, Ph−H), 6.62 (d, J = 2.6 Hz, 1H, Pz−H), 5.49
(t, J = 2.5 Hz, 1H, Pz−H), 2.27 (s, 3H, p-Mes−CH3), 1.91 (s, 6H, o-
Mes−CH3) ppm; 13C NMR (100 MHz, C6D6) δ 142.55, 138.3, 134.8,
133.6, 133.3, 132.7, 132.1, 130.1, 125.9, 125.62, 121.4, 110.3, 109.6,
25.3, 20.9 ppm; 11B NMR (128 MHz, C6D6) δ 1.6 ppm; HRMS (EI),
calcd for C24H23BN2 [M]+: 350.1954, found 350.1969.
Synthesis of 3. Compound 3 was synthesized using the same
procedure as that for compound 1, where bromobenzene-d5 was used
to prepare the corresponding Grignard and B(OMe)(Mes)(Ph)
reagents. 3 was obtained as an orange solid in 19% yield (0.07 g). 1H
NMR (400 MHz, C6D6): δ (d, J = 7.2 Hz, 1H, Ph−H), 7.10 (t, J = 7.3
Hz, 1H, Ph−H), 7.07 (d, J = 2.5 Hz, 1H, Pz−H), 7.01 (td, J = 7.5, 1.1
Hz, 1H, Ph−H), 6.89 (s, 2H, Mes−H), 6.73 (d, J = 7.8 Hz, 1H, Ph−
H), 6.63 (d, J = 2.7 Hz, 1H, Pz−H), 5.50 (t, J = 2.5 Hz, 1H, Pz−H),
2.27 (s, 3H, p-Mes−CH3), 1.91 (s, 6H, o-Mes−CH3) ppm; 13C NMR
(100 MHz, C6D6) δ 142.5, 138.3, 134.8, 133.6, 132.7, 130.1, 128.3,
128.0, 127.8, 127.3, 125.9, 121.3, 110.3, 109.6, 25.31, 20.9 ppm; 11B
NMR (128 MHz, C6D6) δ 1.6 ppm; HRMS (EI), calcd for
C24H18D5BN2 [M]+: 355.2268, found 355.2265.
Synthesis of 4. Compound 4 was synthesized using a similar
procedure as that for compound 1. To prepare the (4-(9H-carbazol-9-
yl)phenyl)lithium solution, 9-(4-bromophenyl)-9H-carbazole (1.61 g,
5 mmol) was dissolved in THF (∼15 mL) and cooled to −78 °C
using a dry ice/acetone bath. After 30 min, n-BuLi (2.3 mL, 5.8 mmol,
2.5 M in hexanes) was added dropwise and the mixture was stirred for
1 h to afford the lithiated reagent. The lithiated solution was used as is
in the next step to prepare B(OMe)(Mes)(p-Ph−Cbz), following the
same procedures described above. 4 was obtained as an off-white
powder in 34% yield (0.30 g). 1H NMR (400 MHz, C6D6): δ 8.07 (d,
J = 7.5 Hz, 2H, Cbz−H), 7.79 (d, J = 7.3 Hz, 1H, Ph−H), 7.52 (d, J =
7.9 Hz, 2H, Cbz−H), 7.37 (d, J = 8.0 Hz, 2H, B−Ph−H), 7.31−7.18
(m, 6H, B−Ph−H and Cbz−H), 7.13 (m, 2H, Ph−H and Pz-H), 7.09
(d, J = 2.4 Hz, 1H, Pz−H), 7.03 (t, J = 7.5 Hz, 1H, Ph−H), 6.94 (s,
1H, Mes−H), 6.77 (d, J = 7.8 Hz, 1H, Ph−H), 6.68 (d, J = 2.7 Hz,
1H, Pz−H), 5.57 (t, J = 2.6 Hz, 1H, Pz−H), 2.29 (s, 3H, p-Mes−
CH3), 1.93 (s, 6H, o-Mes−CH3) ppm; 13C NMR (100 MHz, C6D6) δ
142.6, 141.9, 133.7, 133.4, 132.7, 130.26, 128.3, 128.0, 127.8, 127.5,
126.2, 126.1, 123.8, 121.6, 120.6, 119.9, 110.4, 109.8, 25.3, 20.9 ppm;
and is only able to undergo one-step isomerization into its dark
isomer (7a). To the same extent, the steric congestion inflicted
by bulky o-CH3 groups on Mes renders a restricted
conformation around boron, such that further isomerization
of the dark isomer is not feasible. Compared to the dark isomer
formed from B(Me-imidazolyl-ph)Mes2, 7a is clearly stable
toward “walk” rearrangement to form 7b. These findings
suggest that the nature of the chelate backbone has a great
impact on the photoisomerization pathways, and furthermore,
the photochemical generation of the azabora-BTPD molecules
not only favors the more electron-rich substituent, but also
requires controlled steric bulk around the B atom.
CONCLUSIONS
■
In summary, six new unsymmetrical organoboron molecules
have been prepared and studied. All compounds are photo-
responsive and undergo regioselective isomerization into their
respective azaboratabisnorcaradiene isomers. With suitable
functionalization and steric control around the boron center,
these species can further transform into rare 14aH-diazabor-
epins/BN-tropilidenes. This two-stage photoisomerization is
mechanistically similar to that of our previously reported N-
methyl-2-phenylimidazolyl-chelated B(Mes)2 systems, as im-
plicated by the structural and photophysical resemblances
observed between the diazaborepins and BN-BTPD. Never-
theless, this work marks the first of such a reaction pathway
observed for chiral N,C-chelate organoboron compounds and
illustrates the diverse range of excited-state pathways that can
be accessed by altering their electronic structure.
EXPERIMENTAL SECTION
■
General Methods and Procedures. All experiments were
carried out under an inert atmosphere of N2 using standard Schlenk
techniques. All starting materials were purchased from Sigma-Aldrich
and used without further purification. 1-(2-Bromophenyl)-1H-
pyrazole,12 9-(4-bromophenyl)-9H-carbazole,13 and B(OMe)2-
(Mes)7b were prepared according to literature procedures. All
solvents were dried over Na and degassed. H, 11B, and 13C NMR
1
spectra were recorded on a Bruker Avance 400, 600, or 700 MHz
spectrometer. Deuterated solvents were purchased from Cambridge
Isotopes and dried/degassed prior to use. Photochemical reactions
were performed in J-Young NMR tubes and photochemical reactions
were carried out using a Rayonet Photochemical Reactor. High
resolution mass spectra (HRMS) were obtained using a Micromass
GCT TOF-EI mass spectrometer. Excitation and emission spectra
were recorded using a Photon Technologies International Quanta-
Master Model 2 spectrometer. UV−visible spectra were recorded on a
Varian Cary 50 spectrometer. Photoluminescent quantum yields were
measured using a Hamamatsu QY spectrometer (C11347-11). The
purity of compounds 1−7 was established by 1H and 13C NMR
spectroscopic analyses.
DFT/TD-DFT Calculation Details. DFT and TD-DFT calcu-
lations were performed using the Gaussian 09 suite of programs14 on
the High Performance Computing Virtual Laboratory (HPCVL) at
Queen’s University. Geometry optimizations and vertical excitations
of all compounds were obtained at the cam-B3LYP15/SVP16 level of
theory, with the resulting structures confirmed to be stationary points
through vibrational frequency analysis.
Synthesis of 1. In a 50 mL oven-dried Schlenk flask, p-
tolylmagnesium bromide was prepared from 4-bromotoluene (0.856
g, 5 mmol), magnesium turnings (0.221 g, 5 mmol), and a small
iodine crystal in THF (15 mL). The mixture was refluxed for 2 h at 80
°C until all the magnesium disappeared and then slowly cooled to
−78 °C using a dry ice/acetone bath. After 30 min of cooling, the
flask was charged with B(OMe)2(Mes) (0.914 g, 4.76 mmol) and the
resulting B(OMe)(Mes)(p-tolyl) solution was warmed to ambient
E
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Article
11B NMR (128 MHz, C6D6) δ 1.4 ppm; HRMS (EI), calcd for
C36H30BN3 [M]+: 515.2533, found 515.2518.
carried out using a Rayonet Photochemical Reactor (300 nm), and
the photoisomerization processes of 1−7 were monitored periodically
1
by H, 11B, and 19F NMR (where applicable) until all compounds
Synthesis of 5. Compound 5 was synthesized using a similar
procedure as that for compound 1, where the second Grignard
reagent was prepared with the low-temperature halogen-magnesium
exchange described by Knochel (i.e., 1,3-bis(trifluoromethyl)-5-
bromobenzene).17 5 was obtained as a white powder in 8% yield
(0.15 g). 1H NMR (400 MHz, C6D6): δ 7.92 (d, J = 1.8 Hz, 2H, 3,5-
bisCF3−Ph−H), 7.71 (s, 1H, 3,5-bisCF3−Ph−H), 7.53 (dd, J = 7.2,
1.4 Hz, 1H, Ph−H), 6.98 (td, J = 7.3, 1.1 Hz, 1H, Ph−H), 6.91 (td, J
= 7.7, 1.3 Hz, 1H, Ph−H), 6.83 (s, 2H, Mes−H), 6.80 (d, J = 2.5 Hz,
1H, Pz−H), 6.62 (dd, J = 7.6, 1.6 Hz, 1H, Ph−H), 6.54 (t, J = 3.0 Hz,
1H, Pz−H), 5.46 (q, J = 2.5 Hz, 1H, Pz−H), 2.21 (s, 3H, p-Mes−
CH3), 1.72 (s, 6H, o-Mes−CH3) ppm; 13C NMR (100 MHz, C6D6) δ
142.3, 137.8, 135.8, 133.6, 132.2, 131.7, 131.4, 131.1, 130.8, 130.4,
127.5, 126.6, 126.1, 123.4, 122.1, 119.4, 110.6, 110.1, 25.5, 20.9 ppm;
11B NMR (128 MHz, C6D6) δ 0.7 ppm; 19F NMR (376 MHz,
CD2Cl2) δ −63.04 (s, 6F, −CF3) ppm; HRMS (EI), calcd for
C26H21BN2F6 [M]+: 486.1702, found 486.1717.
Synthesis of 6. Compound 6 was synthesized using the same
procedure as that for compound 1, where 1-bromonaphthalene was
used to prepare the second Grignard reagent in the synthesis of
B(OMe)(Mes)(napthalen-1-yl). 6 was obtained as a tan powder in
19% yield (0.19 g). 1H NMR (400 MHz, C6D6): δ 8.72 (d, J = 8.4 Hz,
1H, naphthyl−H), 7.88 (d, J = 7.1 Hz, 1H, Ph−H), 7.74 (d, J = 8.1
Hz, 1H, naphthyl−H), 7.58 (d, J = 8.0 Hz, 1H, naphthyl−H), 7.50 (d,
J = 2.4 Hz, 1H, Pz−H), 7.35−7.17 (m, 4H, naphthyl−H), 7.04 (t, J =
7.3 Hz, 1H, Ph−H), 6.99 (t, J = 7.5 Hz, 1H, Ph−H), 6.86 (s, 1H,
Mes−H), 6.83 (s, 1H, Mes−H), 6.74 (d, J = 7.6 Hz, 1H), 6.66 (d, J =
2.6 Hz, 1H, Ph−H), 5.40 (d, J = 2.6 Hz, 1H, Pz−H), 2.23 (s, 3H, o-
Mes−CH3), 2.14 (s, 3H, o-Mes−CH3), 1.69 (s, 3H, p-Mes−CH3)
ppm; 13C NMR (100 MHz, C6D6) δ 177.3, 142.3, 140.5, 137.8, 137.0,
134.9, 134.8, 134.3, 132.8, 130.5, 129.4, 128.9, 126.8, 126.4, 126.0,
125.2, 124.9, 121.5, 110.3, 109.5, 26.5, 23.6, 21.0 ppm; 11B NMR
(128 MHz, C6D6) δ 2.4 ppm; HRMS (EI), calcd for C28H25BN2
[M]+: 400.2116, found 400.2110.
were converted to their diazaborepin isomers (1b−4b) or no
additional spectral change was observed (5a−7a). The former were
fully characterized by NMR, while the latter were identified based on
their characteristic 1H and 11B resonances (see SI). Attempts to
collect HRMS data of 1b−4b were unsuccessful due to their
instability under air.
1
Data for 1b: H NMR (400 MHz, C6D6): δ 8.01 (dd, J = 8.5, 1.4
Hz, 1H, Ph−H), 7.63 (d, J = 11.5 Hz, 1H, p-tolyl−H), 7.29−7.20 (m,
2H, Pz−H and Ph−H), 7.10 (d, J = 2.9 Hz, 1H, Pz−H), 6.93 (dd, J =
8.4, 1.3 Hz, 1H, Ph−H), 6.87 (s, 2H, Mes−H), 6.80 (ddd, J = 8.4, 6.9,
1.4 Hz, 1H, Ph−H), 6.35 (dd, J = 11.4, 1.1 Hz, 1H, p-tolyl−H), 5.55
(d, J = 5.3 Hz, 1H, p-tolyl−H), 5.48 (t, J = 2.9 Hz, 1H, Pz−H), 4.26
(d, J = 4.7 Hz, 1H, p-tolyl−H), 2.42 (s, 6H, o-Mes−CH3), 2.21 (s,
3H, p-Mes−CH3), 2.08 (s, 3H, p-tolyl−CH3) ppm; 13C NMR (100
MHz, C6D6) δ 141.2, 136.7, 134.2, 132.9, 132.1, 131.3, 129.9, 127.0,
126.6, 123.5, 123.5, 122.8, 120.4, 118.9, 114.9, 107.2, 33.5, 24.5, 21.4,
20.6 ppm; 11B NMR (128 MHz, C6D6) δ 29.2 ppm.
Data for 2b: 1H NMR (400 MHz, C6D6): δ 8.01 (d, J = 8.4 Hz, 1H,
Ph−H), 7.63 (d, J = 11.5 Hz, 1H, B−Ph−H), 7.29−7.20 (m, 2H, Pz−
H and Ph−H), 7.09 (d, J = 2.8 Hz, 1H, Pz−H), 6.93 (dd, J = 8.6, 1.3
Hz, 1H, Ph−H), 6.84 (s, 2H, Mes−H), 6.80 (ddd, J = 8.3, 6.7, 1.3 Hz,
1H, Ph−H), 6.36−6.32 (m, 1H, B−Ph−H), 6.23 (ddd, J = 10.7, 6.3,
1.8 Hz, 1H, B−Ph−H), 5.77 (dd, J = 10.6, 5.6 Hz, 1H, B−Ph−H),
5.48 (t, J = 2.9 Hz, 1H, Pz−H), 4.40 (dd, J = 5.7, 1.9 Hz, 1H, B−Ph−
H), 2.45 (s, 6H, o-Mes−CH3), 2.18 (s, 3H, p-Mes−CH3) ppm; 13C
NMR (100 MHz, C6D6) δ 140.6, 136.7, 134.4, 133.1, 132.7, 130.9,
130.1, 129.2, 126.9, 126.4, 124.9, 124.7, 122.7, 119.9, 117.7, 115.2,
114.9, 107.2, 34.4, 21.2, 20.5 ppm; 11B NMR (128 MHz, C6D6) δ
30.7 ppm.
1
Data for 3b: H NMR (400 MHz, C6D6): δ 8.00 (dd, J = 8.4, 1.4
Synthesis of 7. In a 50 mL oven-dried Schlenk flask, mesityl
magnesium bromide (35.1 mmol, 2.05 equiv) was prepared from 2-
bromomesitylene (6.98 g, 35.1 mmol), magnesium turnings (0.86 g,
35.1 mmol), and a small iodine crystal in 30 mL of dry/degassed
THF. The mixture was refluxed for 2 h at 80 °C until all the
magnesium disappeared and then slowly cooled to 0 °C using an ice
bath. After 30 min of cooling, the flask was charged with BF3·Et2O
(∼2.11 mL, 17.1 mmol, 1 equiv) and the solution was warmed to
ambient temperature overnight. The solvent was removed in vaccuo
and the residue was extracted with dry/degassed hexanes (200 mL).
The crude mixture was filtered through Celite under N2 and
concentrated under reduced pressure to afford B(F)Mes2 as an off-
white solid. A separate 50 mL Schlenk flask was charged with 1-(2-
bromophenyl)-1H-pyrazole (0.38 g, 1.7 mmol) and 15 mL of dry/
degassed Et2O. The solution was cooled to −78 °C for 30 min, after
which n-BuLi (0.72 mL, 1.8 mmol, 2.5 M in hexanes) was added
dropwise. After stirring the lithiated ligand for 1 h at −78 °C,
B(F)Mes2 (0.53 g, 2.0 mmol, 1.2 equiv) was added to the flask and
the mixture was allowed to stir overnight. The solvents were removed
under reduced pressure and the crude product was purified with flash
column chromatography using a gradient elution (10:1 → 4:1
hexanes:CH2Cl2). 7 was obtained as a pale-yellow powder in 69%
yield (0.44 g). 1H NMR (400 MHz, C6D6): δ 7.83 (d, J = 6.6 Hz, 1H,
Ph−H), 7.37 (d, J = 2.2 Hz, 1H, Pz−H), 6.99−6.89 (m, 2H, Ph−H),
6.82 (s, 4H, Mes−H), 6.76−6.72 (m, 2H, Ph−H and Pz−H), 5.55 (d,
J = 2.0 Hz, 1H, Pz−H), 2.21 (s, 6H, p-Mes−CH3), 2.07 (s, 12H, o-
Mes−CH3) ppm; 13C NMR (100 MHz, C6D6) δ 139.5, 136.8, 135.2,
134.1, 133.6, 130.3, 128.3, 128.0, 127.8, 127.6, 127.4, 125.5, 121.2,
110.1, 109.7, 25.3, 21.0 ppm; 11B NMR (128 MHz, C6D6) δ 2.2 ppm;
HRMS (EI), calcd for C27H28BN2 [M]+: 392.2424, found 392.2421.
Photoisomerization Details. In a N2 filled glovebox, 1−7 were
added to J-Young NMR tubes to obtain concentrations of 10−2 M in
C6D6 (∼0.4 mL). The NMR tubes were sealed with their Teflon caps
and removed from the glovebox. Photochemical experiments were
Hz, 1H, Ph−H), 7.29 (d, J = 3.0 Hz, 1H, Pz−H), 7.24 (m, 1H, Ph−
H), 7.12 (d, J = 3.1 Hz, 1H, Pz−H), 6.95 (d, J = 8.6 Hz, 1H, Ph−H),
6.84 (s, 2H, Mes−H), 6.83−6.78 (m, 1H, Ph−H), 5.50 (t, J = 2.9 Hz,
1H, Pz−H), 2.46 (s, 6H, o-Mes−CH3), 2.18 (s, 3H, p-Mes−CH3)
ppm; 13C NMR (100 MHz, C6D6) δ 135.84, 134.48, 130.17, 129.10,
128.45, 128.33, 128.24, 128.05, 127.81, 127.57, 127.15, 127.07,
126.47, 124.82, 122.76, 120.31, 119.07, 115.08, 107.47, 21.22, 20.63
ppm; 11B NMR (128 MHz, C6D6) δ 30.8 ppm.
Data for 4b: 1H NMR (400 MHz, C6D6): δ 8.09 (d, J = 7.2 Hz, 2H,
Cbz−H), 7.96 (m, 1H, Cbz−H), 7.94 (d, J = 8.8 Hz, 1H, Ph−H),
7.69 (d, J = 11.6 Hz, 1H, B−Ph−H), 7.65 (d, J = 8.0 Hz, 1H, Cbz−
H), 7.30−7.20 (m, 6H, Cbz−H and Ph−H), 7.07 (d, J = 3.0 Hz, 1H,
Pz−H), 6.93 (dd, J = 8.4, 1.0 Hz, 1H, Ph−H), 6.84 (m, 1H, Ph−H),
6.82 (s, 2H, Mes−H), 6.18 (dd, J = 11.6, 1.3 Hz, 1H, B−Ph−H), 6.13
(d, J = 6.3 Hz, 1H, B−Ph−H), 5.45 (t, J = 2.9 Hz, 1H, Pz−H), 4.41
(d, J = 6.3 Hz, 1H, B−Ph−H), 2.38 (bs, 6H, o-Mes−CH3), 2.17 (s,
3H, p-Mes−CH3) ppm; 13C NMR (100 MHz, C6D6) δ 139.5, 137.8,
136.4, 135.5, 135.1, 132.5, 132.3, 130.6, 130.5, 128.8, 127.4, 127.0,
126.1, 125.8, 123.1, 121.4, 120.5, 120.0, 119.5, 119.3, 115.4, 111.0,
110.9, 107.9, 32.8, 21.5, 20.8 ppm; 11B NMR (128 MHz, C6D6) δ
30.0 ppm.
X-ray Crystallographic Analysis. Colorless crystals of 4 and off-
white crystals of 6 were grown by layering CH2Cl2 solutions of each
with hexanes. The crystal data of compounds 4 and 6 were collected
on a Bruker D8-Venture diffractometer with Mo Kα radiation at 180
K. Data were processed using the Bruker APEX III software and
SHELXTL software package (SHELXTL-2014/7)18 and corrected for
absorption effects. All non-hydrogen atoms were refined anisotropi-
cally. The crystal data of 4 and 6 have been deposited at the
Cambridge Crystallographic Data Center (CCDC Nos. 1858646 and
1858647).
F
DOI: 10.1021/acs.organomet.8b00598
Organometallics XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00598.
■
S
Synthesis and characterization data, X-ray, TD-DFT
calculation, and photoisomerization experiment data
(PDF)
Cartesian coordinates for DFT optimized structures
(XYZ)
Accession Codes
CCDC 1858646 and 1858647 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wangs@chem.queensu.ca.
ORCID
Suning Wang: 0000-0003-0889-251X
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
The authors thank the Natural Science and Engineering
Research Council of Canada (RGPIN1193993-2013) for
support.
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Chelate in (2-ph-py)BMes2 and (5-BMes2-2-ph-py)BMes2. J. Am.
Chem. Soc. 2008, 130, 12898−12900.
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DOI: 10.1021/acs.organomet.8b00598
Organometallics XXXX, XXX, XXX−XXX