Photochemical Intramolecular C?H Addition of Dimesityl(hetero)arylboranes through a [1,6]-Sigmatropic Rearrangement

Article abstract of DOI:10.1002/anie.201706929

A new reaction mode for triarylboranes under photochemical conditions was discovered. Photoirradiation of dimesitylboryl-substituted (hetero)arenes produced spirocyclic boraindanes, where one of the C?H bonds in the ortho-methyl groups of the mesityl subs

Full text of DOI:10.1002/anie.201706929

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Accepted Article
Title: Photochemical Intramolecular C-H Addition of Dimesityl-
(hetero)arylboranes via a [1,6]-Sigmatropic Rearrangement
Authors: Naoki Ando, Aiko Fukazawa, Tomokatsu Kushida, Yoshihito
Shiota, Shuhei Itoyama, Kazunari Yoshizawa, Yasunori
Matsui, Yutaro Kuramoto,, Hiroshi Ikeda, and Shigehiro
Yamaguchi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201706929
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10.1002/anie.201706929
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COMMUNICATION
Photochemical Intramolecular C–H Addition of Dimesityl-
(hetero)arylboranes via a [1,6]-Sigmatropic Rearrangement
Naoki Ando,[a] Aiko Fukazawa,[a] Tomokatsu Kushida,[a] Yoshihito Shiota,[b] Shuhei Itoyama,[b] Kazunari
Yoshizawa,*[b] Yasunori Matsui,[c] Yutaro Kuramoto,[c] Hiroshi Ikeda,*[c] and Shigehiro Yamaguchi*[a,d]
Dedication ((optional))
Abstract:
A
new reaction mode for triarylboranes under
a)
c)
b)
C
C
photochemical conditions was discovered. Photoirradiation of
dimesitylboryl-substituted (hetero)arenes produced spirocyclic
boraindanes, where one of the C–H bonds in the ortho-methyl
groups of the mesityl substituents was formally added in a syn
fashion to a C–C double bond of the (hetero)aryl group. Quantum
chemical calculations and laser flash photolysis measurements
B
C
B
O
Mes
Mes
B
B
hν
H
Δ
indicated that the reaction proceeds via
a [1,6]-sigmatropic
B
B
rearrangement. This behavior is reminiscent of the photochemical
reaction mode of arylalkenylketones, demonstrating the isosteric
relation between tricoordinate organoboron compounds and the
corresponding pseudo-carbocationic species in terms of pericyclic
reactions. Despite the disrupted π-conjugation, the resulting
spirocyclic boraindanes exhibited a characteristic absorption band at
relatively long wavelengths (370–400 nm).
Mes
1
Mes
2
R
R
R
H
–H+
R
R
R
R
A+
hν
O
O
A
O
A
O
R
d)
hν
B
B
Photochemical reactions of organic compounds containing
boron have attracted continued attention due to the fundamental
interest in their reactivity and their synthetic utility.[1] A crucial
point for the understanding of the reactivity of organoboranes is
the isosterism with their carbon congeners (Figure 1a,b). The
similar photochemical behavior for anionic tetracoordinate boron
compounds and neutral tetracoordinate carbon compounds has
long been recognized. For instance, tetraorganoborates or base-
coordinated triorganoboranes undergo a photoinduced di-π-
borate rearrangement, which is the analogous reaction of the di-
π-methane rearrangement of the corresponding carbon
isosteres.[2–4] Wang and coworkers have recently applied this
type of reaction to develop a photochromic system.[5] In contrast,
photochemical reactions of neutral tricoordinate organoboranes
have been limited to ligand-coupling reactions that involve B–C
π
π
R
H
R
3
4
O
O
hν
Figure 1. Isosterism in the photochemical reactions of organoboranes and
their carbon congeners: isosteric relationships of (a) B–/C and (b) B/C+, as well
as comparisons in (c) the Nazarov cyclization and (d) the formal intramolecular
C-H addition reaction.
bond cleavage.[6–8] We have recently reported isosteric reactivity
for
a triorganoborane and its cationic carbon congener.
Specifically, 9-dimesitylboryl-substituted dibenzoborepin
1
underwent an isomerization to a borata-allyl product 2 upon
photoirradiation,[9] a reaction that can be regarded as a bora-
Nazarov cyclization (Figure 1c). In order to evaluate the
generality of this reaction mode, we examined various simplified
substrates. However, we discovered that dimesitylboryl-
substituted (hetero)arenes 3 undergo a different mode of
photoisomerization, namely a formal intramolecular C–H bond
addition that affords spirocyclic boraindane 4 (Figure 1d). This
[a]
N. Ando, Prof. Dr. A. Fukazawa, Dr. T. Kushida, and Prof. Dr. S.
Yamaguchi
Department of Chemistry, Graduate School of Science, and
Integrated Research Consortium on Chemical Sciences (IRCCS),
Nagoya University
Furo, Chikusa, Nagoya 464-8602 (Japan)
Fax: (+81) 52-789-5947
E-mail: yamaguchi@chem.nagoya-u.ac.jp
Prof. Dr. Y. Shiota, S. Itoyama, and Prof. Dr. K. Yoshizawa
Institute for Materials Chemistry and Engineering, Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
Dr. Y. Matsui, Y. Kuramoto, and Prof. Dr. H. Ikeda
Department of Applied Chemistry, Graduate School of Engineering,
Osaka Prefecture University
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531 (Japan)
Prof. Dr. S. Yamaguchi
Institute of Transformative Bio-Molecules (WPI-ITbM),
Nagoya University
[b]
[c]
reaction likely involves
rearrangement, which should result from
a
[1,6]-sigmatropic hydrogen
significant
a
contribution of the vacant p-orbital of the boron atom. In light of
the fact that the analogous reaction proceeds in
arylalkenylketones,[10] this pericyclic reaction should be another
example of the reaction modes of triorganoboranes that is
characterized by the B/C+ isosterism. Herein, we disclose the
details of this new photochemical reaction.
[d]
Furo, Chikusa, Nagoya 464-8602
Initially, we conducted a photochemical reaction using 3-
dimesitylboryl-2,5-dimethylthiophene (3a) as a starting material.
Supporting information for this article is given via a link at the end of
the document.
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Upon irradiation of a dilute solution of 3a (4.3 × 10–5 M) in THF
with UV light (λ = 320 nm), the solution readily turned to pale
yellow. This color change stands in sharp contrast to the deep
blue color of the previously observed bora-Nazarov product 2.[9]
The color change was monitored by UV-vis absorption
spectroscopy, which revealed that absorbance at λmax = 322 nm
for 3a decreased and new broad bands emerged at λmax = 269
and 375 nm with isosbestic points at λ = 284 and 354 nm
(Figure 2). An NMR study of the reaction mixture demonstrated
that 3a was cleanly converted into a single product 4a (Figure
S1 in the Supporting Information). A synthesis on a preparative
scale using a high-pressure mercury lamp furnished 4a in 80%
isolated yield. Although 4a gradually decomposed in air, it was
sufficiently stable to be purified by column chromatography on
silica gel.
C6
C10
B1
C5
C1
C22
S1
C4
C21
C19
C20
C2
C3
C7
H4
C23
H3
H22
Figure 3. Molecular structure of 4a (atomic displacement parameters set to
50% probability). Only one of the enantiomers is shown. Hydrogen atoms,
except for H3, H4, and H22, are omitted for clarity. Selected bond lengths [Å]
and angles [°]: B1–C1 1.564(2), B1–C10 1.572(2), C1–C2 1.4124(19), C2–C3
1.390(2), C3–C4 1.394(2), C4–C5 1.400(2), C5–C6 1.394(2), C6–C1
1.4076(19), C2–C7 1.508(2), C7–C20 1.552(2), C19–C20 1.543(2), C20–C21
1.519(2), C21–C22 1.332(2), C1–B1–C10 123.21(13), C10–B1–C20
133.28(13), C1–B1–C20 103.06(12), S1–C19–C20 106.84(10), C20–C19–C23
116.17(13), C23–C19–S1 110.61(12).
1
0.8
B
0.6
S
3a
corresponding spirocyclic boraindanes 4a-d in good isolated
yields (79–88%). The reaction of a phenanthrene precursor 3e in
benzene also produced 4e in comparable yield, although a
prolonged reaction time (6 h) was required for completion. In
contrast, trimesitylborane was not susceptible to this reaction.
These results demonstrate that low levels of aromaticity in the
substrates are crucial for this reaction to proceed. In all these
examples, the formal C–H addition products were obtained
exclusively, and bora-Nazarov products were not observed. In
addition, dimesitylboryl-substituted nonaromatic indene 3f also
afforded the cyclized product 4f. This result implies that 5- or 6-
membered ring structures in the substrates may be another
important factor in order to dominantly promote the cyclization
over the bora-Nazarov reaction, which is observed for the 7-
membered borepin.
0.4
0.2
0
250
300
350
400
450
Wavelength / nm
Figure 2. UV-vis absorption spectra of 3a upon irradiation with UV light (λ =
320 nm) in THF (0–100 min).
The molecular structure of 4a was unequivocally determined
by single-crystal X-ray diffraction analysis (Figure 3), and the
unit cell contains a pair of enantiomers. The molecular structure
of 4a is characterized by a spirocyclic boraindane framework
with a chiral quaternary carbon atom, which arises from the
formation of a new C–C bond between the ortho-methyl group of
one of the mesityl substituents and the ipso-carbon atom of the
thiophene moiety. The thiophene ring is transformed into a
nonaromatic dihydrothiophene ring that exhibits substantially
different C–C bond lengths [C19–C20: 1.543(2) Å; C21–C22:
1.332(2) Å] and an sp3-hybridized C19 atom [angle sum C–C–C
and C–C–S: 333.62°]. Thus, 4a can be regarded as the product
of a formal intramolecular C–H bond addition of the ortho-methyl
C–H bond of a mesityl substituent to one of the C=C bonds of
the thiophene ring. The C7–C20 and C19–H22 bonds adopt a
syn configuration, demonstrating that the formal C–H bond
addition proceeds in a highly stereoselective manner (vide infra).
The boron atom in the five-membered ring retains a trigonal
planar geometry [angle sum C–B–C: 359.55°].
This reaction likely proceeds via
a
[1,6]-sigmatropic
rearrangement of the ortho-methyl hydrogen atom (Scheme 1).
To elucidate the mechanism, DFT calculations on both the
ground state (S0) and the lowest (S1) and second lowest excited
singlet states (S2) were conducted at the B3LYP/6-31G(d) level
of theory. The geometry optimization of S0 at the UB3LYP/6-
31G(d) level of theory afforded two electronic structures for the
transition state (TS) of the [1,6]H-shift, i.e., an open-shell
TS(OS) and a closed-shell singlet state TS(CS), which lead to
the H-shifted biradical (5a••) and zwitterionic (5a+–) intermediates,
respectively, prior to the formation of the spirocyclic product 4a
(Figures 4a and S3). The energies of the geometries of these
intermediates relative to the optimized geometry of 3a in S0 are
+56.1 kcal mol–1 [TS(OS)], +68.0 kcal mol–1 [TS(CS)], +44.3 kcal
mol–1 (5a••), +53.8 kcal mol–1 (5a+–), and +12.5 kcal mol–1 (4a).
The high activation energy of at least 56.1 kcal mol–1 indicates
that the [1,6]H-shift unlikely occurs under thermal conditions.
Intermediate 5a••, optimized for the open-shell form, is by 9.5
kcal mol–1 more stable than that for the closed-shell form (5a+–),
The substrate scope of the reaction was investigated by
employing a series of dimesitylboryl-substituted heteroarenes
and related compounds (Table 1). The replacement of the
dimethylthiophene moiety in 3a with other five-membered
heteroarenes, such as benzothiophene (3b), benzofuran (3c),
and N-methylindole (3d), resulted in complete photocyclizations
within an hour in hexane, and successfully furnished the
indicating that the H-shifted intermediate 5a contains
a
significant contribution of the resonance form 5a•• in S0
compared to the zwitterionic form 5a+–. For the transformation of
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Table 1. Substrate scope for the photochemical cyclization of 3a–f.[a]
hν
or
hν
B
H
B
B
[1,6]
(high-pressure
Hg lamp)
H-shift
B
B
H
H
S
S
S
solvent (2 mM)
20 °C, time
π
π
5a+–
5a••
R
3a
H
R
4a–f
C–C bond
formation
(fast)
3a–f
Entry
1
Substrate
Solvent, time
Product [%][b]
B
hexane, 1 h
hexane, 1 h
hexane, 1 h
hexane, 1 h
B
B
S
H
S
H
4a
S
3a
4a 80%
Scheme 1. A plausible reaction mechanism for the photoinduced formation of
4a.
2
3
4
B
B
S
S
3b
4b 82%
B
B
O
O
3c
4c 79%
B
B
N
N
3d
4d 88%
Figure 4. Energy profiles of S0, S1, and S2 for the [1,6]H-shift of 3a, calculated
at the B3LYP/6-31G(d) level of theory. All energy values are relative to
dimesitylborylthiophene 3a. For the energy profiles of the transformation of 3a
into 5a•• (S0), the relative energies from 3a to TS(CS) and TS(OS) to 5a••
correspond to the closed-shell and open-shell singlet states, respectively.
B
5
benzene, 6 h
B
3e
4e 70%
likely proceeds via a [1,6]H-shift in the S1, followed by a
nonadiabatic transition from S1 to S0 to produce 5a+–, and a
spontaneous C–C bond formation via either 5a+– or 5a•• in S0 to
afford the spirocyclic product 4a (Scheme 1).
6
hexane, 1 h
B
B
3f
4f 81%
[a] The reactions were conducted on 2.0 mM solutions using the irradiation
UV light from a high-pressure Hg lamp at 20 °C. [b] Isolated yield.
To gain experimental insight into the mechanism, we carried
out nanosecond laser flash photolysis (LFP) studies. Upon LFP
of a 1 mM solution of 3a at 355 nm in dichloromethane or
toluene at 235 K, no transient absorption band was observed. In
contrast, a similar LFP of 3a (0.2 mM) in acetonitrile afforded a
broad and weak transient absorption band in the 600–900 nm
region (Figure S5),[12] suggesting that the transient intermediate
exhibits at least partially ionic character and can thus be
stabilized in polar solvents. The transient intermediate decays
with rate constants of kDECAY = 0.86 (235 K), 1.06 (245 K), and
1.18 × 107 s–1 (254 K), which were determined by analysis
tracing of ΔOD at 720 nm (Figure S6). A kinetic analysis based
on conventional transition-state theory provided the activation
enthalpy (ΔH‡ = 1.8 kcal mol–1), activation entropy (ΔS‡ = –18.7
cal K–1), and the Gibbs energies of activation (ΔG‡ = 7.4 kcal
mol–1) at 298 K (Figure S7). In particular, the small ΔH‡ and
large negative ΔS‡ values imply that the observed transient
intermediate should be highly reactive to undergo such a bond
5a into 4a, no transition state was found due to a shallow
potential energy surface. These findings suggest the
spontaneous formation of spirocyclic boraindane 4a even in the
ground state, once 5a is produced.[11]
Along the reaction coordinate from 3a to 5a•• (S0), the
respective S1 and S2 energies were estimated by time-
dependent density functional theory (TD-DFT) calculations. In
the early reaction stage, near the Franck–Condon state for 3a,
S1 and S2 are energetically comparable. In contrast, the S2
potential energy surface for the H-shift from 3a to TS is
significantly less favorable (ca. 30 kcal mol–1), while that of S1 is
rather flat, exhibiting a slight increase in energy (ca. 10 kcal mol–
1). Moreover, S1 and S0 seem to form a weakly avoided crossing
in the vicinity of TS(CS) due to the nonadiabatic coupling
between the two states. Overall, the photoinduced cyclization
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formation, which would significantly decrease its degrees of
freedom. These results indicate that the transient intermediate
observed by LFP is most likely H-shifted 5a+–, which contains a
metastable borata-ortho-quinodimethane moiety. The observed
ionic character of the transient intermediate is also consistent
with the zwitterionic character of 5a+–. Considering that the
transient intermediate could not be observed in toluene, the C–C
bond formation from 5a to 4a is likely a very fast process under
the reaction conditions. This is consistent with the fact that the
spirocyclic products 4 were exclusively obtained as the syn-
adducts.
boraindane moiety. Importantly, the HOMO of 4a consists of a σ-
π interaction between the B–C and C–C bond σ-orbital in the
boraindane moiety and the π-orbital of the dihydrothiophene
moiety. This orbital interaction increases the energy of the
HOMO, which is partially responsible for the bathochromically
shifted absorption.
In summary, we have disclosed a photoinduced cyclization
to produce unprecedented spirocyclic boraindanes as a new
reaction mode for dimesityl(hetero)arylboranes. A combined
experimental and theoretical study has supported the hypothesis
that this reaction mode proceeds via
a [1,6]-sigmatropic
rearrangement, followed by an intramolecular C–C bond
formation. This reaction is of particular interest as it is based on
a significant contribution from the vacant p-orbital of the boron
atom. This study indicates that the boron atom can function as
an isostere of the carbocation in pericyclic reactions, which is
one of the most fundamentally important reaction modes in
organic chemistry. The obtained spirocyclic boraindanes
represent a new class of boron-containing π-electron systems
with characteristic absorption properties.
a)
b)
3
4
B
H
B
B
5
6
1
C
2
H
H
H
H
S
TS
S
S
HOMO
LUMO
c)
B
B
Acknowledgements
H
H
H
in-phase
out-of-phase
S
S
in-phase
out-of-phase
thermally forbidden
photochemically allowed
This work is partly supported by Grants-in-Aid for Scientific
Research on Innovative Areas “Stimuli-responsive Chemical
Species for the Creation of Functional Molecules” [JP24109007
and JP15H02163 (S.Y.), JP24109014 and JP15K13710 (K.Y.),
as well as JP24109009 (H.I.)] from MEXT (Japan) and the
MEXT Project of Integrated Research on Chemical Synthesis
from Japan (A.F. and Y.S.).
Figure 5. Frontier molecular orbitals analysis of the transition state for the
[1,6]-sigmatropic rearrangement of 3a: (a) structural formula of a transition
state, (b) pictorial representation of the HOMO and LUMO for TS(OS), and (c)
the corresponding illustrations for the MO analysis.
In the mechanism of this cyclization, the initial [1,6]-
sigmatropic rearrangement is highly intriguing. To elucidate the
role of the boron atom on the orbital symmetry requirements of
this sigmatropic reaction, we analyzed the frontier molecular
orbitals of the transition state TS(OS) for 3a (Figure 5).[13–15] In
the propeller-like arrangement of the mesityl and thienyl groups
on the boron center, only the antarafacial mode should be
possible for this [1,6]-sigmatropic rearrangement. However, in
the HOMO of TS(OS), the p-orbital on C6 and the s-orbital on
the migrating H atom are out-of-phase, indicating that the
antarafacial [1,6]-sigmatropic hydrogen rearrangement in 3a is
thermally forbidden. In contrast, the corresponding orbitals in the
LUMO are in-phase. These results are consistent with the
experimental observations that 3a undergoes the isomerization
only under photochemical conditions. Notably, the vacant p-
orbital of the boron atom contributes to the LUMO, inducing an
effective delocalization of the molecular orbital over the boron-
containing seven-membered cyclic structure.
Keywords: boron • photochemical reaction • sigmatropic
rearrangement • pericyclic reaction • π-conjugation
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It should be noted that the UV-vis absorption spectrum of
spiroboraindane 4a in THF exhibited an absorption band at
longer maximum wavelength (λmax = 375 nm) relative to 3-
(dimesitylboryl)thiophene 3a (λmax = 322 nm), despite the
disruption of the π-conjugation. Based on the results of the TD-
DFT calculations (Figure S4), the longest-wavelength absorption
band of 4a can be assigned to an intramolecular charge-transfer
transition from the HOMO, which is localized on the
dihydrothiophene, to the LUMO, which is localized on the
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[11] The reaction profiles from 3a to TS and from TS to 5a-OS were
calculated for the closed-shell and open-shell singlet states,
respectively. For details, see the Supplementary Information.
[12] The LFP of 3a in acetonitrile was conducted on 0.2 mM solution, as a 1
mM solution of 3a could not be prepared due to its low solubility. The
weak peak in the transient absorption spectrum may be attributed to the
low concentration of 3a in acetonitrile, and/or the small molar
absorption coefficient of the transient species.
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[15] Since the geometries of transition states in S0, i.e., TS(OS) and TS(CS),
and in S1 are almost identical, the MOs of TS(OS) in S0 were used for
the current discussion. For details, see the Supporting Information.
[9]
A. Iida, S. Saito, T. Sasamori, S. Yamaguchi, Angew. Chem. 2013, 125,
3848; Angew. Chem. Int. Ed. 2013, 52, 3760.
[10] Photocyclizations of 1-cyclopentenyl ketones to afford the
corresponding spirocyclic products have already been reported: a) A. B.
Smith, III, W. C. Agosta, J. Am. Chem. Soc. 1973, 95, 1961; b) B.
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10.1002/anie.201706929
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Naoki Ando, Aiko Fukazawa, Tomokatsu
Kushida, Yoshihito Shiota, Shuhei
Itoyama, Kazunari Yoshizawa,* Yasunori
Matsui, Yutaro Kuramoto, Hiroshi
hν
[1,6]H-shift
B
B
&
Ikeda,* and Shigehiro Yamaguchi*
Ar
C–C bond
formation
Ar
R
H
H
R
Page No. – Page No.
The photoinduced isomerization of dimesitylboryl-substituted (hetero)arenes to
afford spirocyclic boraindanes is disclosed. This reaction proceeds via a [1,6]-
sigmatropic rearrangement, followed by an intramolecular C–C coupling. The
former process is characterized by the participation of the vacant p-orbital on boron,
and thus constitutes a new reaction mode for triarylboranes. The obtained
spirocyclic boraindanes showed characteristic absorption properties.
Photochemical Intramolecular C–H
Addition of Dimesityl-
(hetero)arylboranes via a [1,6]-
Sigmatropic Rearrangement
This article is protected by copyright. All rights reserved.