Multistep Photoisomerization of Dimesitylboron-Functionalized Stilbene Analogues

Article abstract of DOI:10.1021/acs.orglett.0c01042

Dimesitylboron-functionalized stilbene derivatives have been found to undergo an unusual regioselective photoisomerization upon irradiation at 365 nm. Using NMR to follow the photoreaction, the structures of key reaction intermediates and the final produc

Full text of DOI:10.1021/acs.orglett.0c01042

pubs.acs.org/OrgLett
Letter
Multistep Photoisomerization of Dimesitylboron-Functionalized
Stilbene Analogues
Yuxin Ge, Naoki Ando, Lijie Liu, Xiang Wang, Franco
and Suning Wang*
̧ ise Sauriol, Shigehiro Yamaguchi,* Gang Wu,*
Cite This: https://dx.doi.org/10.1021/acs.orglett.0c01042
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Dimesitylboron-functionalized stilbene derivatives have been found to undergo an unusual regioselective
photoisomerization upon irradiation at 365 nm. Using NMR to follow the photoreaction, the structures of key reaction
intermediates and the final products were established. This photoisomerization occurs in four steps: trans−cis isomerization, Diels−
Alder reaction, di-π-methane rearrangement, and ring opening with [1,3]-H migration. This results in the formation of a rare
structure with three fused five-membered rings and a six-membered one.
hotochemical reactions have attracted a growing amount
Pof attention because of their potential to produce high-
energy structures that are difficult to achieve thermally.1 One
well-known example is the photoreaction of stilbene and its
analogues. As illustrated in Scheme 1a, stilbene can undergo a
reversible cis−trans photoisomerization and then the cis isomer
can go through an electrocyclic reaction to form highly
unstable 4a,4b-dihydrophenanthrene (DHP), which can be
converted to phenanthrene by oxidation.2 However, isolated
and well-characterized DHPs are rarely reported in the
literature.3 Photoisomerization of boranes and their derivatives
has emerged as a promising way to generate rare boron-doped
polycyclic structures.4,5 Studies of borylated stilbenes have also
been reported.6,7 For example, Shi et al. recently found that
stilbene analogues containing a para-substituted donor−
acceptor (D−A; Mes2B−Ph−p-NMe2) unit can produce rather
rare but stable 4a,4b-DHP intermediates (Scheme 1b).7 To
better understand the effect of the donor and/or acceptor on
the photoreactivity of the stilbene analogues, we decided to
synthesize Mes2B-functionalized stilbene derivatives with
varying aryl substituents, as shown in Scheme 1c. To our
surprise, we found that these analogues exhibit a completely
different multistep photoisomerization process that results in
dearomatization of one of the mesityl rings to form a highly
unusual structure containing three fused five-membered rings.
The details of our investigation are presented herein.
Scheme 1. Photoisomerization of (a) Stilbene,2 (b)
Mes2BPh-p-NMe2 Stilbene Derivatives,6 and (c) Mes2BPh
Stilbene Derivatives (this work)
Compounds 1a−4a were prepared in good yields, according
to the general procedure as outlined in Scheme 2.5,7 Full
characterization details, including crystal structures for 1a−4a,
are provided in the Supporting Information. The 11B NMR
chemical shifts of 1a−4a in benzene are between 74 and 82
ppm, typical of tricoordinate boron centers.7 On the basis of
3
the JHH coupling constants of the olefinic protons, 1a−4a are
all in the trans conformation in solution. Compounds 1a−3a
Received: March 22, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01042
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
isolated as a yellow solid using column chromatography. 2d is
air-stable for ∼1 month in solution as probed by NMR.
However, attempts to isolate 2d from the reaction mixture
were unsuccessful because it decomposed quickly on column.
Numerous attempts to obtain single crystals for 2c and 2d
failed. In the absence of crystallographic data, we have used a
suite of 2D NMR experiments (COSY, NOESY, HSQC, and
HMBC) to determine unambiguously the molecular structures
of the two reaction intermediates (2b and 2c) and the final
product (2d). To prepare the sample for 2b, we used a high
concentration of E-2a and performed the photoreaction at 273
K. Under these conditions, the downstream photoisomeriza-
tion from 2b to 2c is slow enough to allow NMR
characterization. The results of our NMR structural elucidation
are shown in Figure 2. Complete spectral assignments are
Scheme 2. Synthetic Procedures for Compounds 1a−4a
display an intense absorption band in the mid-UV region (λ =
279−305 nm) and moderate fluorescence (λ = 442−465 nm;
Φfl = 0.39−0.55). In contrast, 4a is only weakly emissive (λ =
440 nm; Φfl ∼ 0.04), which can be ascribed to the pyridine
unit acting as a second acceptor.7 TD-DFT computational data
show that the S0 → S1 transition for 1a−4a mainly involves
charge transfer from the HOMO (stilbene backbone) to the
LUMO (BMes2 group).
Upon irradiation at 365 nm, 1a−4a undergo a rather
complex photoisomerization in either benzene or THF under
nitrogen. Because the photoreactions of 1a−4a are similar, we
will use 2a as an example to illustrate the detailed changes over
the course of the photoreaction. The corresponding spectral
data for other compounds are provided in the Supporting
Information. Figure 1 shows the time-lapse 1H NMR spectra of
Figure 2. Molecular structures determined for 2b−2d.
provided in the Supporting Information. Further evidence to
support the structural elucidation comes from HRMS and
DFT-calculated 1H and 13C NMR chemical shifts (also see the
Supporting Information).
The characteristic NMR features of 2b are the two peaks of
H2 and H4 at 5.83 and 5.08 ppm, respectively. These signals
exhibit long-range J couplings (4JHH = 1.4 Hz) to CH3-7 and
CH3-9, respectively. In addition, the 13C chemical shifts of
C10, C11, and C3 are consistent with alkyl carbons. The steric
geometry of 2b was determined via the cross peaks of H4,
H10, and H11 in the NOESY spectra. The bicyclic structure in
2c was proved by the fact that the signal of H4 (at 5.08 ppm)
displays long-range J couplings to CH3-9 and H2. The 13C
NMR signals of C1 and C2 are moved from the olefinic region
in 2b to the alkyl region in 2c. In the HMBC spectra of 2c, one
can see in the methyl region that CH3-7 and CH3-9 are
Figure 1. Expanded 1H NMR spectral regions to illustrate the
photoisomerization of 2a (∼10−2 M in C6D6) with 365 nm irradiation
at ambient temperature under N2. The representative peaks for each
species are color-coded (purple, Z-2a; red, 2b; blue, 2c; green, 2d).
2a. During the initial 2 h of UV irradiation, E-2a is completely
converted to Z-2a. The 3JHH coupling constant decreases from
21 Hz in E-2a to 14 Hz in Z-2a; also, 2D NOESY cross peaks
were found, which agrees with the literature.7 Upon further UV
irradiation, Z-2a is converted to 2b and 2c, reaching nearly
equal amounts after 12 h. Notably, some unidentified
impurities are present at this stage, indicating possible
decomposition from 2b to 2c. After 32 h, 2b is fully consumed
and the final product 2d begins to form at this stage. After 82
h, the conversion of 2c to 2d is ∼60% (at 10−2 M). Further
irradiation leads to higher 2c-to-2d conversion, but the
decomposition of 2d also sets in. The yields of conversion of
1a, 3a, and 4a to 1d, 3d, and 4d are 75%, 71%, and 64%,
respectively.
1
connected to C1 and C5, respectively. Additionally, the JCH
value on C2 was measured to be 175.6 Hz, which is
comparable with that of the carbon in a cyclopropane ring.8,9
The final product 2d with three fused five-membered rings was
also fully characterized by NMR. One interesting feature of 2d
is that the two peaks from the newly formed alkene CH2-7
fragment appear at 4.80 and 4.81 ppm, with the latter showing
a long-range J coupling to H4 through the zigzag pathway.
While the photoreactions for 1a−4a are similar, in the
products, we found that 4d exhibits a distinct structural feature.
In contrast to 1d−3d that have 11B NMR signals in the region
of 75−82 ppm, 4d has a 11B NMR signal at 7 ppm, which
suggests a tetracoordinate boron center. Indeed, the DFT-
optimized structure of 4d exhibits a B−N dative bond between
Reaction intermediate 2b is extremely sensitive to air and
could not be isolated. 2c is air-stable for 3 days and was
B
https://dx.doi.org/10.1021/acs.orglett.0c01042
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
B and the N atom on the pyridine ring. The in situ 11B NMR
data indicate that the B−N coordination occurs during the
final step of the isomerization from 4c to 4d. Figure 3
with the Karplus equation.10 This provides strong evidence to
support the molecular structures of 1d−4d.
After establishing the structures of key reaction intermedi-
ates and the final product, we now can rationalize the
mechanism of the transformation from a to d. As illustrated in
Scheme 3, the first step of the photoisomerization is the well-
known trans−cis isomerization of the olefin backbone.2
Gilmour et al. also reported E−Z isomerizations of olefin
bond and styrenyl boron species.11 The conversion from Z-a to
b can be seen as an inverse electro-demand Diels−Alder
reaction involving the olefin bond that leads to dearomatiza-
tion of one mesityl ring. Similar reactions for tricoordinate
boron compounds have been reported previously.12,13 Differ-
ent from the previously reported Mes2B−Ph−p-NMe2
functionalized compound that has its HOMO mainly on the
NMe2 group and its LUMO delocalized on the whole
molecule,7 Z-2a with only the accepting BMes2 group has its
HOMO mainly on the olefin backbone and its LUMO mainly
on the BMes2 group (see the Supporting Information). Hence,
the olefin bond can interact with one of the mesityl rings
(acting as an electron rich dienophile) to undergo a Diels−
Alder reaction instead of forming trans-4a,4b-dihydrophenan-
threne derivatives. Transformation from b to c is recognized as
a di-π methane rearrangement type reaction between the two π
moieties C2C1−C6−C5C4, in a fashion similar to that
described by Zimmerman et al.14 Unlike previously reported
photoinduced di-π borate rearrangement of tetraorganoborates
and base-coordinated triorganoboranes,15−17 the B atom
geometry does not change throughout the transformation of
1a−4a to 1c−4c, respectively. After the formation of c, the
strained cyclopropane ring in c undergoes a ring opening and a
[1,3]-hydrogen migration from C7 to C6, forming a vinyl
group, producing the final bora-fused heterocycle product d.
This rare transformation is the first example of boron-
containing heterocycles. Only a few examples of this type of
reaction for carbon analogues have been reported in the
literature.18 For 1d−4d, we also found that all of the steps as
outlined in Scheme 3 are regioselective. Thus, all stereogenic
carbon atoms in 1d−4d have the same absolute configuration
at the same position, e.g., C10(R), C11(R), C6(S), C2(R), and
C3(R).
Figure 3. (a) Comparison between the molecular structures of 2d
(left) and 4d (right). The three fused five-membered rings are color-
coded. (b) Karplus plot for torsion angles around the central ring in
3
1d−4d. The Karplus equation is JHH = A + B cos ϕ + C cos 2ϕ,
where A = 6 Hz, B = −1 Hz, and C = 5 Hz.10
highlights the difference between the molecular structures of
2d and 4d. The most pronounced difference between 2d and
4d is centered on the puckering of two of the five-membered
rings. In 2d, all three five-membered rings are close to planar.
In 4d, in contrast, the ring containing the B atom and the
central ring are in the folded envelope conformation. The best
way to describe this difference is to monitor the HCCH
torsion angles around the central ring: H2−C2−C6−H6, H6−
C6−C11−H11, and H11−C11−C10−H10. In addition, these
3
To further understand the influence of aryl substituents on
the multistep photoisomerization as outlined in Scheme 3, we
performed DFT calculations for the ground state energy
changes from a to d (Figure 4). Regardless of the substituted
torsional angles can be correlated with experimental JHH
values. As shown in Figure 3b, 4d exhibits a distinct set of
torsional angles as compared with those of 1d−3d. More
3
importantly, the experimental JHH values can be fitted nicely
Scheme 3. Proposed Four-Step Mechanism for the Conversion from a to d
C
https://dx.doi.org/10.1021/acs.orglett.0c01042
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
aryl groups, the ground state energy changes of the four
compounds are similar at each isomerization stage.
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Suning Wang − Department of Chemistry, Queen’s University,
Kingston, Ontario K7L 3N6, Canada; orcid.org/0000-
0003-0889-251X; Email: sw17@queensu.ca
Gang Wu − Department of Chemistry, Queen’s University,
Kingston, Ontario K7L 3N6, Canada; orcid.org/0000-
0002-0936-9432; Email: gang.wu@chem.queensu.ca
Shigehiro Yamaguchi − Department of Chemistry, Graduate
School of Science, and Integrated Research Consortium on
Chemical Sciences (IRCCS), Nagoya University, Nagoya 464-
8602, Japan; orcid.org/0000-0003-0072-8969;
Email: yamaguchi@chem.nagoya-u.ac.jp
Figure 4. DFT-calculated energy profile from a to d.
Authors
Yuxin Ge − Department of Chemistry, Queen’s University,
Kingston, Ontario K7L 3N6, Canada
Naoki Ando − Department of Chemistry, Graduate School of
Science, and Integrated Research Consortium on Chemical
Sciences (IRCCS), Nagoya University, Nagoya 464-8602,
Japan
Lijie Liu − Beijing Key Laboratory of Photoelectronic/
Electrophotonic Conversion Materials, School of Chemistry,
Beijing Institute of Technology, Beijing 102488, P. R. China
Xiang Wang − Department of Chemistry, Queen’s University,
Kingston, Ontario K7L 3N6, Canada
Intermediate c has the highest energy due to the large strain
induced by the three-membered ring, which also explains the
sharp energy release from c to d as the ring opens. The
calculated total energy change is increased from a to d by ∼100
kJ/mol. Notably, the calculated ground state energy for 4d
without B−N coordination is 21 kJ/mol higher than that of the
structure with B−N coordination, proving that the B−N bond
in 4d helps to stabilize the structure.
In summary, we have discovered a new photoisomerization
process for Mes2B-functionalized stilbene analogues containing
different aromatic substituents. This photoisomerization occurs
in four steps: (1) trans−cis isomerization, (2) Diels−Alder
reaction, (3) di-π methane rearrangement, and (4) cyclo-
propane ring opening with [1,3]-hydrogen migration. The
decoration of the BMes2 group provides a LUMO orbital to
interact with the electron rich olefin bond to enable the Diels−
Alder reaction. The varying aryl rings do not change the
reaction outcome, but the formation of a B−N bond between
the pyridine ring and the BMes2 group helps to stabilize the
structure of the final product. This photoreaction is completely
different from all previously known photoisomerizations of
stilbene analogues and tricoordinate organoboron compounds.
Our study illustrates the fascinating nature of the photo-
chemistry for organoboron compounds. In addition, this new
type of photoreaction may provide a possible synthetic strategy
for compounds that are similar to the final product in which
the three fused five-membered rings may be difficult to make
via conventional synthetic methods.
Franco̧ ise Sauriol − Department of Chemistry, Queen’s
University, Kingston, Ontario K7L 3N6, Canada
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01042
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Natural Sciences and Engineering
Research Council of Canada (RGPIN-2018-03866) for
financial support.
REFERENCES
■
(1) Buzzetti, L.; Crisenza, G. E. M.; Melchiorre, P. Mechanistic
studies in Photocatalysis. Angew. Chem., Int. Ed. 2019, 58, 3730−
3747.
(2) Mallory, F. B.; Wood, C. S.; Gordon, J. T. Photochemistry of
Stilbenes. III. Some Aspects of the Mechanism of Photocyclization to
Phenanthrenes. J. Am. Chem. Soc. 1964, 86, 3094−3102.
ASSOCIATED CONTENT
* Supporting Information
■
(3) (a) Somers, J. B. M.; Couture, A.; Lablache-Combier, A.;
Laarhoven, W. H. Base-Induced Proton Tautomerism in the Primary
Photocyclization Product of Stilbenes. J. Am. Chem. Soc. 1985, 107,
1387−1394. (b) Doyle, T. D.; Benson, W. R.; Filipescu, N.
Photocyclization of Diethylstilbestrol. Isolation of a Stable, Self-
Trapping Dihydrophenanthrene Intermediate. J. Am. Chem. Soc. 1976,
98, 3262−3267. (c) Doyle, T. D.; Filipescu, N.; Benson, W. R.; Banes,
D. Photocyclization of α,α’-Diethyk-4,4’-stilbenediol. Isolation of a
Stable Tautomer of the Elusive Dihydrophenanthrenes. J. Am. Chem.
Soc. 1970, 92, 6371−6372.
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01042.
Experimental procedures, analytical data (1H, 13C, 11B,
and 2D NMR data; HRMS data; and crystal structure
data), and computational data for characterized products
and intermediates (PDF)
Accession Codes
(4) (a) He, Z. C.; Mellerup, S. K.; Liu, L.; Wang, X.; Dao, C.; Wang,
S. Reversible Photoisomerization from Borepin to Boratanorcaradiene
and Doubly Aryl Migration from Boron to Carbon. Angew. Chem., Int.
Ed. 2019, 58, 6683−6687. (b) Radtke, J.; Mellerup, S. K.; Bolte, M.;
Lerner, H. W.; Wang, S.; Wagner, M. Aryl Insertion vs Aryl−Aryl
CCDC 1985711−1985714 contain the supplementary crys-
tallographic 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
D
https://dx.doi.org/10.1021/acs.orglett.0c01042
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
coupling in C,C-Chelated Organoborates: The “Missing Link” of
Tetraarylborates Photochemistry. Org. Lett. 2018, 20, 3966−3970.
(16) Kropp, M. A.; Baillargeon, M.; Park, K. M.; Bhamidapaty, K.;
Schuster, G. B. Photochemistry of Alkynyl-, Aljenyl-, and Cyclo-
propyl-Substituted Borate Salts: The Di-π- and Cyclopropyl-π-borate
Rearrangements. J. Am. Chem. Soc. 1991, 113, 2155−2163.
(17) Williams, J. L. R.; Doty, J. C.; Grisdale, P. J.; Searle, R.; Regan,
T. H.; Happ, G. P.; Maier, D. P. Boron Photochemistry. I. Irradiation
of Sodium Tetraarylborates in Aqueous Solution. J. Am. Chem. Soc.
1967, 89, 5153−5157.
(18) Hixson, S. S.; Gallucci, C. R. Arylcyclopropane Photochemistry.
Substituent Effects on the Photochemical 1,3-Hydrogen Migration of
1,1-Dimethyl-2-phenylcyclopropane. J. Org. Chem. 1988, 53, 2711−
2713.
̈
(c) Mellerup, S. K.; Hafele, L.; Lorbach, A.; Wang, X.; Wang, S.
Triplet Energy and π-Conjugation Effects on Photoisomerization of
Chiral N,C-Chelate Organoborons with PAHs Substitutes. Org. Lett.
2017, 19, 3851−3854.
(5) (a) Ando, N.; Fukazawa, A.; Kushida, T.; Shiota, Y.; Itoyama, S.;
Yoshizawa, K.; Matsui, Y.; Kuramoto, Y.; Ikeda, H.; Yamaguchi, S.
Photochemical Intramolecular C−H Addition of Dimesityl(hetero)-
arylboranes through a [1,6]-Sigmatropic Rearrangement. Angew.
Chem., Int. Ed. 2017, 56, 12210−12214. (b) Iida, A.; Saito, S.;
Sasamori, T.; Yamaguchi, S. Borylated Dibenzoborepin: Synthtesis by
Skeletal Rearrangement and Photochromism Based on Bora-Nazarov
Cyclization. Angew. Chem., Int. Ed. 2013, 52, 3760−3764.
(6) (a) Hertz, M. V.; Massoth, J. G.; Bolte, M.; Lerner, H. W.;
Wagner, M. En Route to Stimuli-Responsive Boron-, Nitrogen-, and
Sulfur-Doped Polycyclic Aromatic Hydrocarbons. Chem. - Eur. J.
2016, 22, 13181−13188. (b) Brown, A. N.; Zakharov, L. N.; Mikulas,
T.; Dixon, D. A.; Liu, S. Y. Rhodium-Catalyzed B−H Activation of
1,2-Azaborines: Synthesis and Characterization of BN Isosteres of
Stilbenes. Org. Lett. 2014, 16, 3340−3343. (c) Yan, Y. Q.; Li, Y. B.;
Wang, J. W.; Zhao, C. H. Effect of the Substitution Pattern on the
Intramolecular Charge-Transfer Emissions in Organoboron-Based
Biphenyls, Diphenylacetylenes, and Stilbenes. Chem. - Asian J. 2013, 8,
3164−3176. (d) Fukazawa, A.; Yamada, H.; Yamaguchi, S.
Phosphonium- and Borate-Bridged Zwitterionic Ladder Stilbene and
Its Extended Analogues. Angew. Chem., Int. Ed. 2008, 47, 5582−5585.
(e) Murata, M.; Watanabe, S.; Masuda, Y. Rhodium-Catalyzed
Dehydrogenative Coupling Reaction of Vinylarenes with Pinacolbor-
ane to Vinylboronates. Tetrahedron Lett. 1999, 40, 2585−2588.
(7) Shi, Y.; Mellerup, S. K.; Yuan, K.; Hu, G.; Sauriol, F.; Peng, T.;
Wang, N.; Chen, P.; Wang, S. Stabilising Feeting Intermediates of
Stilbene Photocyclization with Amino-Borane Functionalisation: The
Rare Isolation of Persistent Dihydrophenanthrenes and Their [1,5]
H-Shift Isomers. Chem. Sci. 2018, 9, 3844−3855.
(8) Ando, T.; Koseki, N.; Toia, R. F.; Casida, J. E. Three-Bond
13C−1H Coupling Constants for Chrysanthemic Acid and Phenothrin
Metabolites: Detection by Two-Dimensional Long-Range 13C−1H J-
Resolution Spectroscopy. Magn. Reson. Chem. 1993, 31, 90−93.
(9) Weiner, P. H.; Malinowski, E. R. Substituent Effects. VII.
Carbon-13 Nuclear Magnetic Resonance of Substituted Cyclo-
propanes. J. Phys. Chem. 1967, 71, 2791−2795.
(10) Cung, M. T.; Marraud, M.; Neel, J. Experimental Calibration of
a Karplus Relationship in Order to Study the Conformations of
Peptides by Nuclear Magnetic Resonance. Macromolecules 1974, 7,
606−613.
(11) (a) Livingstone, K.; Tenberge, M.; Pape, F.; Daniliuc, C. G.;
Jamieson, C.; Gilmour, R. Photocatalytic E→Z Isomerization of β-
Ionyl Derivatives. Org. Lett. 2019, 21, 9677−9680. (b) Molloy, J. J.;
Metternich, J. B.; Daniliuc, C. G.; Watson, A. J. B.; Gilmour, R.
Contra-Thermodynamic, Photocatalytic E→Z Isomerization of
Styrenyl Boron Species: Vectors to Facilitate Exploration of Two-
Dimensional Chemical Space. Angew. Chem., Int. Ed. 2018, 57, 3168−
3172.
(12) Hilt, G.; Hess, W.; Harms, K. Asymmetric Cobalt-Catalyzed
Diels−Alder Reactions of a Boron-Functionalized 1,3-Diene with
Alkynes and Allylboration with Aldehydes. Org. Lett. 2006, 8, 3287−
3290.
(13) Deng, B.; Wang, X.; Wang, S. Dearomatization and Derivatizing
a Mesityl Group on Boron by One-Pot Photoisomerization and [4 +
2] Diels−Alder Addition. Chem. - Eur. J. 2019, 25, 14694−14700.
(14) Zimmerman, H. E.; Grunewald, G. L. The Chemistry of
Barrelene. III. A Unique Photoisomerization to Semibullvalene. J. Am.
Chem. Soc. 1966, 88, 183−184.
(15) Eisch, J. J.; Shafii, B.; Odom, J. D.; Rheingold, A. L. Aromatic
Stabilization of the Triarylborirene Ring System by Tricoordinate
Boron and Facile Ring Opening with Tetracoordinate Boron. J. Am.
Chem. Soc. 1990, 112, 1847−18853.
E
https://dx.doi.org/10.1021/acs.orglett.0c01042
Org. Lett. XXXX, XXX, XXX−XXX