Rethinking Borole Cycloaddition Reactivity

Article abstract of DOI:10.1002/chem.202101290

Boroles are attracting broad interest for their myriad and diverse applications, including in synthesis, small molecule activation and functional materials. Their properties and reactivity are closely linked to the cyclic conjugated diene system, which has been shown to participate in cycloaddition reactions, such as the Diels-Alder reaction with alkynes. The reaction steps leading to boranorbornadienes, borepins and tricyclic boracyclohexenes from the thermal reaction of boroles with alkynes are seemingly well understood as judged from the literature. Herein, we question the long-established mechanistic picture of pericyclic rearrangements by demonstrating that seven-membered borepins (i. e., heptaphenylborepin and two derivatives substituted with a thienyl and chloride substituent on boron) exist in a dynamic equilibrium with the corresponding bicyclic boranorbornadienes, the direct Diels-Alder products, but are not isolable products from the reactions. Heating gradually converts the isomeric mixtures into fluorescent tricyclic boracyclohexenes, the most stable isomers in the series. Results from mechanistic DFT calculations reveal that the tricyclic compounds derive from the boranorbornadienes and not the borepins, which were previously believed to be intermediates in purely pericyclic processes.

Full text of DOI:10.1002/chem.202101290

Accepted Article
Accepted Article
Title: Rethinking Borole Cycloaddition Reactivity
Authors: Felix Lindl, Xueying Guo, Ivo Krummenacher, Florian Rauch,
Anna Rempel, Valerie Paprocki, Theresa Dellermann, Tom
E. Stennett, Anna Lamprecht, Tobias Brückner, Krzysztof
Radacki, Guillaume Bélanger-Chabot, Todd B. Marder,
Zhenyang Lin, and Holger Braunschweig
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202101290
Link to VoR: https://doi.org/10.1002/chem.202101290
01/2020

10.1002/chem.202101290
Chemistry - A European Journal
RESEARCH ARTICLE
Rethinking Borole Cycloaddition Reactivity
Felix Lindl,[a],§ Xueying Guo,[b],§ Ivo Krummenacher,[a] Florian Rauch,[a] Anna Rempel,[a] Valerie
Paprocki,[a] Theresa Dellermann,[a] Tom E. Stennett,[a] Anna Lamprecht,[a] Tobias Brückner,[a] Krzysztof
Radacki,[a] Guillaume Bélanger-Chabot,[a] Todd B. Marder,[a] Zhenyang Lin,*[b] and Holger
Braunschweig*[a]
[a]
F. Lindl, Dr. I. Krummenacher, Dr. F. Rauch, A. Rempel, Dr. V. Paprocki, Dr. T. Dellermann, Dr. T. E. Stennett, A. Lamprecht, T. Brückner, Dr. K. Radacki,
Prof. Dr. G. Bélanger-Chabot, Prof. Dr. T. B. Marder, Prof. Dr. H. Braunschweig
Institute for Inorganic Chemistry and Institute for Sustainable Chemistry & Catalysis with Boron (ICB)
Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
E-mail: h.braunschweig@uni-wuerzburg.de
X. Guo, Prof. Dr. Z. Lin
Department of Chemistry
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong (P.R. China)
E-mail: chzlin@ust.hk
[b]
§
These authors contributed equally.
Supporting information for this article is given via a link at the end of the document.
Abstract: Boroles are attracting broad interest for their myriad and
diverse applications, including in synthesis, small-molecule activation
and functional materials. Their properties and reactivity are closely
linked to the cyclic conjugated diene system, which has been shown
to participate in cycloaddition reactions, such as the Diels-Alder
reaction with alkynes. The reaction steps leading to
boranorbornadienes, borepins and tricyclic boracyclohexenes from
the thermal reaction of boroles with alkynes are seemingly well
understood as judged from the literature. Herein, we question the
long-established mechanistic picture of pericyclic rearrangements by
1-phenyl-2,3,4,5-tetramethylborole,
boroles
react
with
themselves to form bicyclic 7-boranorbornenes in a Diels-Alder
reaction.[2,3,4] Their proclivity to participate in cycloaddition
reactions was first demonstrated by Eisch and coworkers in
seminal studies where they showed that pentaphenylborole
(structural motif A in Scheme 1) undergoes a [4+2] cycloaddition
with
diphenylacetylene
to
the
corresponding
7-
boranorbornadiene (structural motif B).[5] A borepin species with
a seven-membered ring (structural motif C) was also reported to
be a product of this transformation, and was proposed to be
formed from the initial Diels-Alder adduct via a sequence of
pericyclic reactions that proceed through a boranorcaradiene
intermediate (structural motif D).[5] While direct evidence is
missing for formation of a borepin in this reaction – after initial
claims to the contrary – the studies of Piers and coworkers on a
highly fluorinated borole derivative lent some support to the
proposed isomerization reaction (Scheme 1).[6] In the reaction of
perfluoropentaphenylborole with alkynes of different electronic
and steric properties, borepins were clearly identified as products
and, in one case, conversion of an unsymmetrically substituted
boranorbornadiene into a borepin was observed by NMR
spectroscopy.
demonstrating
that
seven-membered
borepins
(i.e.
heptaphenylborepin and two derivatives substituted with a thienyl and
chloride substituent on boron) exist in a dynamic equilibrium with the
corresponding bicyclic boranorbornadienes, the direct Diels-Alder
products, but are not isolable products from the reactions. Heating
gradually converts the isomeric mixtures into fluorescent tricyclic
boracyclohexenes, the most stable isomers in the series. Results from
mechanistic DFT calculations reveal that the tricyclic compounds
derive from the boranorbornadienes and not the borepins, which were
previously believed to be intermediates in purely pericyclic processes.
Besides cycloaddition,[7,8] boroles also undergo insertions with
alkynes, a mode of reactivity that was first revealed by Piers and
Introduction
coworkers
with
the
isolation
of
a
six-membered
Over the last decade, our understanding of the properties and
reactivity of boracyclopentadienes (or simply boroles) has
improved substantially and they are now widely recognized as
potent electron acceptors and platforms for the synthesis of a
diverse range of boron heterocycles.[1]
Among the defining characteristics of boroles is their marked
propensity to undergo addition reactions. Being isoelectronic to
the quintessential antiaromatic cyclo-pentadienyl cation ([C5H5]+),
boroles are inherently unstable and reactive. For example, if their
five-membered ring is insufficiently sterically hindered, such as in
boracyclohexadiene.[6] This alternative reaction pathway can be
viewed as a carboboration reaction in which the endocyclic B−C
bond of the borole is added to the carbon-carbon triple bond of
the alkyne.[9] Depending on the mode of addition, either one or
two carbon atoms are added to the borole ring, resulting in six-
and seven-membered boracycles, respectively. Thus, the
insertion pathway provides an alternative mechanism for the
formation of seven-membered borepins from the reaction of
boroles with alkynes (Scheme 1). It also becomes evident that a
distinction between the two mechanisms − cycloaddition or
1
This article is protected by copyright. All rights reserved.

10.1002/chem.202101290
Chemistry - A European Journal
RESEARCH ARTICLE
insertion pathways − cannot be readily made, if the borepin and Results and Discussion
boranorbornadiene isomers are readily interconverted. According
to DFT calculations from one of our groups,[10] this is exactly the
Our repeated failure to produce the seven-membered
heptaphenylborepin (3a) from the reaction of pentaphenylborole
(1a) with diphenylacetylene[5b,e] prompted us to re-examine this
reaction and collect additional data for two sterically and
electronically different boroles, namely 1-thienyl- (1b)[11] and 1-
chloro-2,3,4,5-tetraphenylborole (1c).[12]
case
for
the
reaction
of
pentaphenylborole
with
diphenylacetylene: the perphenylated boranorbornadiene and
borepin isomers are of similar energy and connected by a low
barrier for isomerization. Moreover, both the cycloaddition and
insertion pathways involve very similar activation barriers that are
energetically accessible under the reaction conditions. Despite
these insights, there still remains uncertainty about the products
formed in the reaction and about possible equilibria between the
isomeric products. A clearer picture exists about the nature of the
most stable isomer in the reaction of pentaphenylborole with
diphenylacetylene, which was identified to be
a tricyclic
compound consisting of a central boracyclohexene ring.[5e] The
latter is believed to be formed from the borepin isomer via an
intramolecular ene reaction.
Scheme 2. Isolable products from the thermal reaction of boroles 1a-c with
diphenylacetylene.
Synthesis of the Boranorbornadienes. As previously described
by Eisch and one of our groups,[5,8] pentaphenylborole (1a) reacts
readily with diphenylacetylene at room temperature to give the
perarylated boranorbornadiene 2a, whose structure is confirmed
by X-ray crystallography and NMR spectroscopy (see Scheme 2).
Similarly, the reactions of boroles 1b and 1c with
diphenylacetylene lead to the respective boranorbornadienes 2b
and 2c, indicated by the rapid fading of the initially purple
solutions to a light yellow. The bicyclic products were obtained in
over 80% yield as colorless solids and characterized in both the
solid and solution state. The most remarkable feature of the mo-
lecular structures of 2b and 2c is the pronounced tilt of the
bridging boron atom towards one of the double bonds (see Figure
1). Consideration of the tilt angles in 2b (β = 84.9°) and 2c (β =
86.7°) suggests that the magnitude of the interaction is somewhat
smaller than those of the phenyl derivative 2a (β = 83.1°)[8] and its
perfluorinated counterpart (β = 83.0°).[6] This is paralleled by the
elongation of the respective C=C double bonds, which is less
pronounced for 2b and 2c. The trigonal planar geometry around
the boron atom is retained, with sums of bond angles of 359.9(1)°
(2b) and 359.8(1)° (2c), respectively, despite the intramolecular
complexation of the boron atom. The characteristic low-frequency
11B NMR spectroscopic chemical shifts in benzene are consistent
with the persistence of the intramolecular alkene-borane
complexation in solution. For 2b and 2c, the chemical shifts are
−5.5 and −2.8 ppm, respectively, which compare well with those
observed for other derivatives (e.g. δ = −4.2 ppm for 2a).[8] In
Scheme 1. Distinct mechanisms of boranorbornadiene (B) and borepin (C)
formation from boroles (A) and alkynes and their mutual isomerization via a
boranorcaradiene intermediate (D). Regioisomers due to different substituents
are not shown.
In this contribution we rationalize the formation of the diverse
isomers accessible by the thermal reaction of boroles with alkynes
by providing new insight into the mechanisms governing their
formation. We demonstrate that borepins are indeed observable
in equilibrium with the Diels-Alder adducts but are not isolable
species from the reactions. We further show that addition of the
Lewis base IMe (IMe = 1,3-dimethylimidazol-2-ylidene) shifts the
equilibrium to the borepin by forming the corresponding Lewis
acid-base adducts and that heating above 100 °C slowly and
cleanly converts the isomeric mixtures of the base-free
boranorbornadienes and borepins into their highly fluorescent
tricyclic isomers. Studies of their formation by DFT calculations
suggest
a mechanistic pathway that originates from the
boranorbornadiene but does not involve a borepin intermediate
as previously suggested.
2
This article is protected by copyright. All rights reserved.

10.1002/chem.202101290
Chemistry - A European Journal
RESEARCH ARTICLE
addition, the bicyclic structures are readily recognized by charac-
teristic low-frequency 13C NMR signals at around δ = 73 ppm for
the bridgehead carbon atoms.[8]
isomer, only 2c proved to be an isolable product. For the
perphenylated derivative, 2a, the borepin derivative 3a is not
readily apparent in the NMR spectra, suggesting that a possible
equilibrium lies strongly on the side of the boranorbornadiene.
However, by addition of the Lewis base IMe (IMe = 1,3-
dimethylimidazol-2-ylidene), the two isomers in equilibrium can be
trapped in the form of their IMe adducts. We are well aware that
base addition might affect the equilibrium, but we have not found
any evidence that the boranorbornadiene adduct gets
transformed into the more stable borepin adduct at room
temperature. The initial ratios between the base adducts remain
virtually unchanged for many hours and only start to change in
favor of the borepin adduct after heating at 80 °C, in line with our
previous studies.[8] After addition of IMe to 2a in benzene solution
at room temperature, the ratio is about 17:1 in favor of the
boranorbornadiene adduct 2a(IMe), thus implying a similar
equilibrium distribution of the uncomplexed isomers.
Figure 1. Molecular structures of compounds 2b (left) and 2c (right) showing
50% probability ellipsoids. Hydrogen atoms and ellipsoids of the phenyl
substituents have been omitted for clarity. The inset defines the tilt angle β.
Equilibria between Boranorbornadienes and Borepins.
Whereas the NMR spectra of 2a are relatively clean, the presence
of additional signals was noted for the chloro (2c) and the thienyl
derivatives (2b). The latter showed an additional, broad 11B NMR
resonance at 59.8 ppm in dichloromethane solution with about
half the intensity of the major signal. By lowering the temperature
to −40 °C, its corresponding 1H NMR intensity decreased to about
a third, suggesting that the two species are in equilibrium with one
another. This is supported by the dissolution of crystalline material
of 2b and the observation of two distinct sets of signals in the 1H
NMR spectrum in the same 2:1 ratio (see Figure 2).
Scheme 3. Observed equilibria between boranorbornadienes (2a-c) and
borepins (3a-c) and conversion to their corresponding IMe adducts.
In addition, this approach allowed us to characterize the IMe
adducts of 2b and 2c spectroscopically, the latter only partially by
11B NMR and characteristic 1H NMR resonances. Their 11B NMR
signals (2b(IMe): δ = 10.1 ppm; 2c(IMe): δ = 10.2 ppm) are
significantly different from those of the corresponding borepin
derivatives (3b(IMe): δ = −13.7 ppm; 3c(IMe): δ = −5.3 ppm) into
which they are converted upon heating at 80 °C. While the
boranorbornadiene adducts 2b(IMe) and 2c(IMe) proved elusive
to isolation, the corresponding borepin adducts could be isolated
as colorless solids in good yields (64% and 68%, respectively). X-
ray diffraction revealed their nonplanar boat-shaped geometry
with alternating C=C double and C−C single bonds (see
Supporting Information for details). The metric parameters
pertaining to the seven-membered rings are very similar to those
in 3a(IMe)[8] and in the pyridine adduct of the perfluorinated
analog.[6] Consistent with the change in coordination of the boron
atom from three to four, the bond lengths around the boron atom
become longer, as illus-trated by the B−Cl bond change of
1.769(2) in 2c to 1.917(4) Å in 3c(IMe).
Figure 2. Excerpt of the 1H NMR spectrum of 2b in CD2Cl2, showing the
thiophene proton resonances. The open circles indicate the resonances of the
borepin isomer 3b, whereas the filled circle indicates a resonance of the bicyclic
isomer 2b.
While crystallization results in only one isomer, namely the
boranorbornadiene 2b, analysis by heteronuclear NMR
spectroscopy in solution revealed the identity of the minor isomer
to be the corresponding borepin derivative, 3b (Scheme 3). The
presence of a borepin isomer (3c) could also be inferred from
analysis of the 13C NMR spectrum for the chloro derivative 2c (see
Supporting Information for more details). Due to overlapping
signals, the two isomers cannot be distinguished in the relatively
crowded 1H NMR spectrum. The characteristic 11B NMR signal for
the borepin derivative 3c is found at δ = 56.9 ppm (in CD2Cl2).
While we were able to assign the NMR signals to each individual
3
This article is protected by copyright. All rights reserved.

10.1002/chem.202101290
Chemistry - A European Journal
RESEARCH ARTICLE
A base-induced transformation of a boranorbornadiene into a
borepin was also observed with the weak Lewis base thf. While
none of the three boranorbornadiene derivatives 2a-c form Lewis
acid-base adducts with thf, the chloro derivative 2c was
nevertheless found to convert quantitatively into the
corresponding borepin adduct 3c(thf). The reaction is likely driven
by adduct formation of the borepin isomer, thus removing it from
the equilibrium with the boranorbornadiene. Thf adducts of the
other borepins were not observed. Borepin 3c(thf) was
characterized by multinuclear NMR spectroscopy, X-ray
crystallography and by its molecular ion peak, with the loss of thf
or the chloride anion, respectively, in the same high-resolution
mass spectrum (see Supporting Information for details). Having
3c(thf) in hand, we treated it with 1-chloro-2,3,4,5-
energies of the isomers. Our calculations at the M062X-D3/6-
31+G(d,p) level indicate that the boranorbornadiene isomer 2a’ is
6.4 kcal/mol lower in energy than the borepin isomer 3a’. Although
at room temperature the barriers for interconversion are slightly
higher (24.3 kcal/mol for 2a’→3a’ and 17.9 kcal/mol for 3a’
→2a’), they are in reasonably good agreement with experi-
mental observations that these isomers exist in equilibrium and
that this equilibrium is strongly on the side of the
boranorbornadiene 2a. This indicates that borepin 3a’, if
preferentially formed through the insertion mechanism, would be
readily converted into the more stable boranorbornadiene 2a’,
thus giving a false appearance of cycloaddition reactivity of the
borole. Our experimental conclusion that heptaphenylborepin 3a
is not isolable from the reaction of pentaphenylborole 1a and
diphenylacetylene, even at elevated temperatures (vide infra), is
therefore consistent with the results from the DFT calculations.
The choice of the appropriate functional proved to be crucial for
these studies as the estimated energies vary greatly. Results from
previous calculations with the M05-2x+D3 functional predicted a
reversed order of stabilities for the isomers, which would thus be
in conflict with the experimental data.[10] A detailed overview with
a brief discussion of the effects of the functionals on the relative
isomer energies is given in section E in the Supporting Information.
tetraphenyborole,
a
strong Lewis acid,[12] to see if the
displacement of thf re-forms the boranorbornadiene isomer 2c,
which was expected to be the more stable isomer of the two. As
anticipated, alongside the formation of the corresponding borole-
thf adduct, the formation of 2c is readily verified by its distinct 11
B
and 13C NMR resonances (see Supporting Information for NMR
spectra), thus further supporting the notion that borepins can
readily isomerize into boranorbornadienes at room temperature
(Scheme 4).
Scheme 4. Conversion of borepin 3c(thf) to boranorbornadiene 2c via thf
abstraction with chloroborole 1c.
Figure 3. Energy profile for the interconversion of 2a’ and 3a’. Relative free
energies and electronic energies (in parentheses) are given in kcal/mol.
DFT Investigations of the Isomerization Pathways. We
rationalized the experimental results using DFT calculations,
including the mechanism of interconversion between the two
isomers and their relative energies. Moreover, we carried out
calculations to study the effect of the IMe base on facilitating the
ring-opening reactions to the borepin adducts. Computed
structures are labelled with a prime symbol throughout.
Our previous quantum chemical investigations into the
mechanisms of formation of 2a’ and 3a’ have shown that the
activation energies of the cycloaddition (for 2a’) and insertion
pathways (for 3a’) are similar (ca. 14−15 kcal/mol) and readily
surmountable at room temperature.[10] These findings suggest
that the ratio of the two isomers is therefore determined by the
barriers for isomerization and their relative stabilities. In line with
previous analysis and assumptions, we found that the two
isomers, 2a’ and 3a’, are connected through a boranorcaradiene
intermediate (b-int1, Figure 3). Two transition states are closely
associated with this intermediate, which resides in a very shallow
While analogous calculations for the chloro derivative also
indicate thermally accessible barriers for isomerization, we found
differences in its mechanism. For 2c’ and 3c’, the interconversion
between the isomers is only a one-step process as the putative
boranorcaradiene intermediate does not correspond to a local
minimum (see Supporting Information for details). Similar to the
phenyl derivative 2a’, the boranorbornadiene 2c’ is found to be
more stable than its corresponding borepin isomer 3c’, consistent
with the experimental observation that only the former can be
isolated.
As was previously reported and seen in the experiments,
addition of the carbene IMe induces the transformation of the
boranorbornadienes into the corresponding borepins.[8] In all
cases, the carbene-borepin adducts were found to be
thermodynamically more stable than the boranorbornadiene
counterparts, in reversed order of stability compared to the
uncomplexed forms. While the reactions follow
a similar
mechanism compared to that of the uncomplexed derivatives, the
energy barriers involved are significantly higher. The conversion
well (less than
4 kcal/mol). The respective barriers for
isomerization are therefore largely governed by the relative
4
This article is protected by copyright. All rights reserved.

10.1002/chem.202101290
Chemistry - A European Journal
RESEARCH ARTICLE
of the carbene-boranorbornadiene adduct 2a(IMe)’ proceeds
through a high-energy transition state with a barrier of 27.3
kcal/mol to the base-stabilized boranorcaradiene intermediate,
indicating that the reaction should not readily occur at room tem-
perature, as experimentally observed. For R = Cl, the barrier for
borepin formation is even higher at 34.4 kcal/mol. The difference
mainly results from greater stabilization of 2c’ vs 2a’ upon adduct
formation.
Overall, the calculated barriers for interconversion and
relative energies between the boranorbornadienes and borepins
are consistent with the two isomers being in an equilibrium lying
at the side of the boranorbornadienes. Similarly, the energetics of
the base-induced reactions are in agreement that heating is
required in order to convert the boranorbornadienes 2a-c(IMe)
into the thermodynamically more stable borepin isomers 3a-
c(IMe). The calculations are also consistent in that the
boranorcaradiene intermediates are not isolable isomers in these
reactions, or in their base-stabilized forms.[13-16]
fraction has thus far eluded us, analysis of its 13C NMR signals
revealed key structural features. Two low-frequency 13C NMR
spectroscopic signals at 69.7 and 80.3 ppm, indicative of
saturated carbon atoms, along with the 11B NMR signal at 68.5
ppm, suggest a tricyclic boracyclohexene structure similar to that
of 4a and 4b. However, due to the absence of a proton signal at
lower frequency (δ ≈ 5 ppm) we assume that an elimination of HCl
has taken place, resulting in a dimeric structure with a central four-
membered C2B2 ring (for further details, see Supporting
Information). The high-resolution mass spectrum of 4c, which
shows a monoisotopic peak at 1106.4825 m/z ([M+H2O]+), would
be consistent with the proposed structure.
In summary, the thermal transformations of the isomeric
mixtures 2a-c/3a-c to the tricyclic isomers 4a-c proceed cleanly,
without detectable intermediates. We can thus conclude that
borepins, including heptaphenylborepin 3a, are not isolable
products from the thermal reaction of boroles 1a-c with
diphenylacetylene (Scheme 2). In addition, they do not appear to
play a role in the formation of the tricyclic isomers since the
decrease in intensity of the boranorbornadiene NMR signals is
directly related to the increase in signal intensity for the tricyclic
products. NMR spectra showing the progress of the reaction for
2a→4a can be seen in Figure S86 in the Supporting Information.
Synthesis of the Tricyclic Boracyclohexene Isomers.
According to the generally accepted picture of transformations,[5]
the initial Diels-Alder adduct 2a converts first to borepin 3a and
eventually to the tricyclic isomer 4a on heating. Thus, to probe
thermal reactions of the isomeric boranorbornadiene and borepin
1
mixtures, we closely monitored their H and 11B NMR spectra as
a function of temperature. For 2a, even prolonged heating at
80 °C in benzene solution leads to no discernable changes in the
spectra (see Supporting Information for details). Only prolonged
heating of a toluene solution in an oil bath at 140 °C gave rise to
a
new species, 4a, which was readily identified by its
1
characteristic singlet resonance at δ = 4.55 ppm in the H NMR
spectrum (Scheme 2). The low frequency signal can be assigned
to the methine proton of the benzo-cyclopentadiene-fused
boracyclohexene 4a, a tricyclic compound previously described
as a “pseudoborepin” by Eisch.[5e] 4a was obtained as pale yellow
solid in 88% yield and fully characterized. The X-ray structural,
NMR and optical data for 4a are consistent with the limited
characterization data available in the literature; its full
characterization data are provided in the Supporting Information,
its photophysical properties are discussed below. In the case of
2b, a structural transformation occurs only after heating in
refluxing toluene. After 60 h, the 1H NMR spectrum showed
quantitative conversion to compound 4b, which was isolated as a
pale yellow solid in 84% yield (Scheme 2). Diagnostic NMR
features include the methine resonance at 4.73 ppm in the 1H
NMR spectrum and the 11B NMR signal at 60.2 ppm, thereby
identifying 4b as being analogous to 4a. Compound 4b was
further unambiguously characterized by X-ray diffraction studies
(see Figure 4). Similar to the structure of 4a,[5e] the central six-
membered boracyclohexene ring adopts an envelope
conformation with the boron atom residing in a virtually trigonal
planar geometry (Σ(C−B−C) = 359.9(4)°). Despite the presence
of two adjacent stereogenic centers, 4b only exists as one
diastereomer as evidenced by both single crystal X-ray diffraction
and solution NMR spectroscopy.
Figure 4. Molecular structures of the tricyclic boracyclohexenes 4a (left) and 4b
(right) showing 50% probability ellipsoids. Hydrogen atoms (expect for the
hydrogen on the methine carbon C1) and ellipsoids for the phenyl substituents
are omitted for clarity.
Mechanistic DFT Investigations into the Formation of the
Tricyclic Isomers. To refine the conclusions drawn from our
synthetic studies, we have conducted DFT calculations on the
mechanism of formation of the tricyclic compound 4a. The
proposed mechanism for the formation of 4a’ is shown in Figure
5. We first looked into the free energy barriers that would be re-
quired for formation of the tricyclic compound 4a’ from borepin 3a’,
in the concerted one-step process originally proposed by Eisch
and coworkers. As the energy barrier for this transformation,
which involves a boratricyclo[3.2.0.0]heptene intermediate (a-
int1) in the initial step, is predicted to be very high (ΔG⧧ = 44.0
kcal/mol, Figure S97), it appears unlikely. Instead, we found that
the same intermediate, a-int1, can be accessed more favorably
by the boranorbornadiene 2a’, with a free-energy barrier of 32.7
kcal/mol. From this intermediate, a retro-Diels-Alder reaction
results in an alkylideneborane intermediate (a-int2) by passing
through a transition state of high potential energy (39.8 kcal/mol).
A similarly high transition state (41.8 kcal/mol) is involved in the
product-forming step. The tricyclic product 4a’ forms via a strongly
exergonic process through intramolecular C−H activation of a
peripheral phenyl ring. The stereochemistry of 4a’ is the result of
a si-attack of the C−H bond on the prochiral C=B double bond of
the alkylideneborane.
In contrast to 2a and 2b, the isomeric mixture of the chloro-
substituted derivative was found to be more reactive, leading to a
thermal rearrangement at much lower temperatures. Heating of
2c/3c at 80 °C for 15 h in benzene results in clean conversion to
a new species (4c) without any detectable intermediates.
Although a determination of its molecular structure by X-ray dif-
While the activation barriers for the transformation of 2a’ to 4a’
are high, they are consistent with the reaction occurring only in
toluene at 140 °C in a closed vessel. Some support for the
5
This article is protected by copyright. All rights reserved.

10.1002/chem.202101290
Chemistry - A European Journal
RESEARCH ARTICLE
proposed reaction pathway is found in the dimeric product 4c
featuring a four-membered C2B2 ring, conceivably formed through
a formal cyclodimerization of two alkylideneboranes in the final
step.[17]
Figure 5. Energy profile for the formation of 4a’ from 2a’. Computed structures and relative energies of minima and transition state
structures are shown. Relative free energies and electronic energies (in parentheses) are given in kcal/mol.
Photophysical Properties of the Tricyclic Isomers. The
fluorescent nature of the tricyclic borane 4a was previously
observed but not further studied,[5e] prompting us to investigate its
Table 1. Photophysical data for 4a and 4b.
Stokes
Com-
Solvent
λabs
λem
f
[ns]
(rel%)
Φf
shift
photophysical properties together with those of 4b. To this end,
we measured their absorption and emission spectra, including
their fluorescence quantum yield and lifetimes, and verified the
results by time-dependent density functional theory (TD-DFT)
calculations. The photophysical data of 4a and 4b are
summarized in Table 1. The absorption spectra of 4a and 4b
exhibit lowest energy absorption maxima at 412 and 422 nm,
respectively, in hexane (see Figure 6). Increasing the solvent
polarity from hexane to toluene does not influence the lowest
energy absorption. In both cases, TD-DFT calculations (see
Supporting Information) suggest a HOMO to LUMO transition for
S1 ← S0, with the largest contributions to the HOMO from
cyclopentadiene and the largest contribution to the LUMO from
the boron atom. The calculated orbital overlap coefficient[18] (Λ =
0.61 (4a) and 0.54 (4b)) suggest a weak to moderate charge
transfer character. Both compounds are highly emissive, with
emission maxima at 457 (4a) and 466 nm (4b), fluorescence
lifetimes on the order of 10 ns and quantum yields of 0.61 (4a)
and 0.67 (4b) in hexane. Both compounds exhibit a distinct
vibrational progression of the emission, which is more
pronounced in hexane than in toluene. The apparent Stokes shifts
in hexane are 2400 and 2200 cm−1, respectively, and the emission
maxima are slightly bathochromically shifted in more polar
toluene (~800 – 900 cm−1). This indicates a weak charge transfer
character, resulting in a larger dipole moment in the excited state
and an increased solvent reorganization energy in the more polar
solvent. Interestingly, the quantum yields increase from hexane to
toluene.
pound
[nm]
[nm]
[cm−1
]

[M−1cm−1])
4a
4a
hexane
toluene
412
457
474
2400
3100
11.8
11.3
0.61
0.75
414
(9000)
4b
4b
hexane
toluene
422
466
482
2200
2900
3.3 (9%)
9.0 (91%)
0.67
0.76
423
(8200)
3.4 (7%)
10.5 (93%)
Figure 6. Combined absorption (solid line) and emission spectra (dashed line)
of 4a and 4b in hexane.
6
This article is protected by copyright. All rights reserved.

10.1002/chem.202101290
Chemistry - A European Journal
RESEARCH ARTICLE
J. E. Galle, J. Organomet. Chem. 1977, 127, C9-C13; e) J. J. Eisch, J.
E. Galle, B. Shafii, A. L. Rheingold, Organometallics 1990, 9, 2342-2349.
C. Fan, W. E. Piers, M. Parvez, R. McDonald, Organometallics 2010, 29,
5132-5139.
Conclusion
[6]
[7]
In conclusion, our combined experimental and theoretical
investigations into the thermal reaction of three different boroles
with diphenylacetylene have revealed that the isomerization
mechanisms between boranorbornadienes, borepins and tricyclic
boracyclohexenes, originally proposed by Eisch, might not be
valid. While the sterically encumbered borepins were found to
exist in equilibrium with the more stable boranorbornadiene
isomers in solution, they are not isolable products and unlikely to
play a role in the formation of the most stable, tricyclic isomers.
Based on computational data, a mechanism proceeding via C−H
activation by an intermediate alkylideneborane is favored over a
purely pericyclic pathway. The herein established equilibrium
between the boranorbornadienes and borepins supports the
presence of an isomerization process that connects the products
resulting from Diels-Alder and insertion reactivity of the borole. If
the barriers for this process are thermally accessible, the product
distribution will be determined by the relative stability of the two
isomers, thereby obscuring the mechanism of their formation.
According to DFT calculations, the interconversion of the isomers
proceeds via a boranorcaradiene intermediate, a well-established
isomer that is consistent with the previously proposed pericyclic
mechanism.
a) F. Ge, G. Kehr, C. G. Daniliuc, G. Erker, J. Am. Chem. Soc. 2014, 136,
68-71; b) F. Ge, G. Kehr, C. G. Daniliuc, G. Erker, Organometallics 2015,
34, 229-235.
[8]
[9]
H. Braunschweig, J. Maier, K. Radacki, J. Wahler, Organometallics 2013,
32, 6353-6359.
Y. Shoji, N. Tanaka, S. Muranaka, N. Shigeno, H. Sugiyama, K.
Takenouchi, F. Hajjaj, T. Fukushima, Nat. Commun. 2016, 7, 12704.
[10] Z. Wang, Y. Zhou, J.-X. Zhang, I. Krummenacher, H. Braunschweig, Z.
Lin, Chem. Eur. J. 2018, 24, 9612-9621.
[11] H. Braunschweig, A. Damme, J. O. C. Jiménez-Halla, C. Hörl, I.
Krummenacher, T. Kupfer, L. Mailänder, K. Radacki, J. Am. Chem. Soc.
2012, 134, 20169-20177.
[12] H. Braunschweig, T. Kupfer, Chem. Commun. 2008, 4487-4489.
[13] M. Shimoi, I. Kevlishvili, T. Watanabe, K. Maeda, S. J. Geib, D. P. Curran,
P. Liu, T. Taniguchi, Angew. Chem. Int. Ed. 2020, 59, 903-909; Angew.
Chem. 2020, 132, 913-919.
[14] a) P. Bissinger, H. Braunschweig, K. Kraft, T. Kupfer, Angew. Chem. Int.
Ed. 2011, 50, 4704-4707; Angew. Chem. 2011, 123, 4801-4804; b) A.
Hermann, M. Arrowsmith, D. E. Trujillo-Gonzalez, J. O. C. Jiménez-Halla,
A. Vargas, H. Braunschweig, J. Am. Chem. Soc. 2020, 142, 5562-5567.
[15] Z.-C. He, S. K. Mellerup, L. Liu, X. Wang, C. Dao, S. Wang, Angew.
Chem. Int. Ed. 2019, 58, 6683-6687; Angew. Chem. 2019, 131, 6755-
6759.
[16] J. Radtke, S. K. Mellerup, M. Bolte, H.-W. Lerner, S. Wang, M. Wagner,
Org. Lett. 2018, 20, 3966-3970.
[17] See, for example: R. Boese, P. Paetzold, A. Tapper, R. Ziembinski,
Chem. Ber. 1989, 122, 1057-1060.
Acknowledgements
[18] M. J. G. Peach, P. Benfield, T. Helgaker, D. J. Tozer, J. Chem. Phys.
2008, 128, 044118.
We thank the Julius-Maximilians-Universität Würzburg and the
Research Grants Council of Hong Kong (HKUST 16300620) for
financial support of this work.
Keywords: Boron • Computational Chemistry • Isomer •
Isomerization • Pericyclic reaction
[1]
For general reviews, see: a) J. J. Eisch, Adv. Organomet. Chem. 1996,
39, 355-391; b) A. Steffen, R. M. Ward, W. D. Jones, T. B. Marder, Coord.
Chem. Rev. 2010, 254, 1950-1976; c) H. Braunschweig, T. Kupfer, Chem.
Commun. 2011, 47, 10903-10914; d) H. Braunschweig, I.
Krummenacher, J. Wahler, Adv. Organomet. Chem. 2013, 61, 1-53; e)
H. Braunschweig, I. Krummenacher in Organic Redox Systems:
Synthesis, Properties, and Applications, John Wiley & Sons, Hoboken
(New Jersey), 2016, pp. 503-519; f) J. H. Barnard, S. Yruegas, K. Huang,
C. D. Martin, Chem. Commun. 2016, 52, 9985-9991; g) B. Su, R. Kinjo,
Synthesis 2017, 49, 2985-3034; h) L. Ji, S. Griesbeck, T. B. Marder,
Chem. Sci. 2017, 8, 846-863; i) A. Wakamiya in Main Group Strategies
towards Functional Hybrid Materials, 1st ed., John Wiley & Sons Ltd.,
Hoboken (New Jersey), 2018, pp. 1-26; j) Y. Su, R. Kinjo, Chem. Soc.
Rev. 2019, 48, 3613-3659; k) J. He, F. Rauch, M. Finze, T. B. Marder,
Chem. Sci. 2021, 12, 128-147.
[2]
[3]
P. J. Fagan, E. G. Burns, J. C. Calabrese, J. Am. Chem. Soc. 1988, 110,
2979-2981.
a) J. J. Baker, K. H. M. Al Furaiji, O. Tara Liyanage, D. J. D. Wilson, J. L.
Dutton, C. D. Martin, Chem. Eur. J. 2019, 25, 1581-1587; b) X. Su, J. J.
Baker, C. D. Martin, Chem. Sci. 2020, 11, 126-131.
(4)
[5]
a) Z. Wang, Y. Zhou, K.-H. Lee, W. H. Lam, R. D. Dewhurst, H.
Braunschweig, T. B. Marder, Z. Lin, Chem. Eur. J. 2017, 23, 11587-
11597. See, also: b) Z. Zhang, Z. Wang, M. Haehnel, A. Eichhorn, R. M.
Edkins, A. Steffen, A. Krueger, Z. Lin, T. B. Marder, Chem. Commun.
2016, 52, 9707-9710.
a) J. J. Eisch, N. K. Hota, S. Kozima, J. Am. Chem. Soc. 1969, 91, 4575-
4577; b) J. J. Eisch, J. E. Galle, J. Am. Chem. Soc. 1975, 97, 4436-4437;
c) J. J. Eisch, Adv. Organomet. Chem. 1977, 16, 67-109; d) J. J. Eisch,
7
This article is protected by copyright. All rights reserved.

10.1002/chem.202101290
Chemistry - A European Journal
RESEARCH ARTICLE
Entry for the Table of Contents
New insights into the reactivity of perarylated boroles toward alkynes have been derived from both experimental and computational
studies. We show that borepins (the formal alkyne insertion products) can readily isomerize to the thermodynamically more favorable
boranorbornadienes (the formal Diels-Alder products), thus obscuring the mechanism of product formation (see Figure). In addition,
the fluorescent nature of the tricyclic boracyclohexenes, the most stable isomers of the Diels-Alder reactions, has been investigated
by absorption and emission spectroscopy.
8
This article is protected by copyright. All rights reserved.