Aryl Insertion vs Aryl-Aryl Coupling in C,C-Chelated Organoborates: The "missing Link" of Tetraarylborate Photochemistry

Article abstract of DOI:10.1021/acs.orglett.8b01534

The photoreactivity of 9-borafluorene-based, C,C-chelated organoborates was investigated. Unlike the related tetraarylborates, the charge-transfer transitions imparted by the biphenyl chelate lead to selective insertion of one aryl substituent into the endocyclic B-C bond of the 9-borafluorene moiety, resulting in the formation of boratanorcaradienes. This photoreaction likely proceeds according to a Zimmerman rearrangement, which is analogous to one of the initially proposed mechanisms for tetraarylborates and provides additional insight into these long-debated photochemical reactions.

Full text of DOI:10.1021/acs.orglett.8b01534

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Aryl Insertion vs Aryl−Aryl Coupling in C,C-Chelated Organoborates:
The “Missing Link” of Tetraarylborate Photochemistry
Julian Radtke,† Soren K. Mellerup,‡ Michael Bolte,† Hans-Wolfram Lerner,† Suning Wang,*,‡
and Matthias Wagner*,†
†Institut fur Anorganische und Analytische Chemie, Goethe-Universitat Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt
̈
̈
(Main), Germany
‡Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
S
* Supporting Information
ABSTRACT: The photoreactivity of 9-borafluorene-based, C,C-
chelated organoborates was investigated. Unlike the related tetraar-
ylborates, the charge-transfer transitions imparted by the biphenyl
chelate lead to selective insertion of one aryl substituent into the
endocyclic B−C bond of the 9-borafluorene moiety, resulting in the
formation of boratanorcaradienes. This photoreaction likely proceeds
according to a Zimmerman rearrangement, which is analogous to one
of the initially proposed mechanisms for tetraarylborates and provides
additional insight into these long-debated photochemical reactions.
Scheme 1. Photoreactivity of Tetraarylborates
etraarylborates represent a class of anionic tetracoordinate
Torganoboron species with great significance in synthetic
and analytical chemistry. Initially prepared by Wittig in 1947,1
such molecules are frequently used as precipitating agents,2 as
weakly coordinating anions,3 and for base-free Suzuki−
Miyaura-type cross-coupling reactions.4 In addition, tetraar-
ylborates are known to undergo photoreactions upon exposure
to strong UV light (254 nm), with the outcome of these
experiments causing vigorous debate in the literature. Williams
et al. were among the first5 to report on the irradiation of
tetraphenylborates I (Scheme 1; R = H, M = Na) in water,
wherein they isolated biphenyl, 1-phenyl-1,4-cyclohexadiene,
and Ph2BONa as major products.6 This distribution differed
depending on whether irradiation was performed under N2 (up
to 97% 1-phenyl-1,4-cyclohexadiene) or air (ca. 59% biphenyl).
Labeling studies revealed that the two benzene rings of the
biphenyl product originated from the same Na[BPh4] and that
both of the carbon atoms linking the two rings were initially
bound to the boron center. This photoreactivity was therefore
rationalized as a variant of the di-π-methane rearrangement
(i.e., Zimmerman rearrangement).7 Eisch et al. continued this
investigation using tetraphenylborates in aprotic solvents (THF
and DME). In their hands, irradiation under inert conditions
afforded biphenyl and, upon the addition of diphenyl acetylene,
a trapped borirene species II (Scheme 1). This finding led them
to propose that photolysis of [BPh4]− generates the
corresponding diphenyl-substituted anion [BPh2]− in solution.8
Schuster et al. reported a different interpretation of this
photoreaction based on their isolation and structural
determination of a borirane anion III by X-ray crystallography
(Scheme 1).9 This deep red species was also characterized by
NMR spectroscopy, with diagnostic chemical shifts at −26.6
(11B) and 37.1 (13C) ppm for the atoms of the three-membered
ring.9 The formation of III was once again explained according
to the Zimmerman rearrangement, where excitation at 254 nm
leads to an excited-state C−C-coupled biradical. The boron
atom subsequently “walks” along the newly generated biphenyl
to give the most stable borirane isomer.
Thus far, the photoreactivity of tetraarylborates has been
heavily focused on compounds with four individual sub-
Received: May 15, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01534
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
stituents, with one important exception being the spiro-
molecule IV which is photochemically inert (Scheme 1).10
Considering these differing reactivities and the debate
surrounding them, it is surprising that an intermediate case
with one biphenyl C,C-chelating unit has not been studied.11
We therefore set out to prepare anionic 9-borafluorene
derivatives A with tetracoordinate boron atoms and various
aryl substituents to examine whether they are also photo-
reactive and, if so, what types of products are generated via
photolysis (e.g., [BAr2]− vs borirane). Irradiation of these new
C,C-chelated compounds leads to structural rearrangement via
selective aryl insertion into one of the endocyclic B−C bonds of
A,11,12 forming boratanorcaradienes B which are related to the
previously reported (aza)boratanorcaradienes (e.g., C, Scheme
1) generated from neutral C,N- or C,(NHC)-chelate systems
(NHC = N-heterocyclic carbene).13,14 Unlike the work of
Williams, Eisch, and Schuster for I, no aryl−aryl C−C coupling
product (D) was observed upon irradiation of A, regardless of
the aryl substituents utilized. Mechanistically, the photo-
chemical reaction of A likely follows a Zimmerman rearrange-
ment, where charge-transfer transitions introduced by the biaryl
chelating unit govern the selectivity of the reaction.15 The
details are presented herein.
Figure 1. (Left) UV−vis spectra of K[1H/tBu]−K[3H/tBu] and K[1N]
(cf. Figure 4) in CH3CN at 10−4 M. (Right) Orbital diagrams of the
calculated S1 and S2 transitions for K[1H].
localized on two of the four phenyl rings, while the LUMO is
delocalized across all four phenyl substituents (π*-Ph).
−
Although compounds [1H/tBu]−−[3H/tBu
]
share a similar
HOMO → LUMO (π-biPh → π*-biPh) transition, the
HOMO-1 resides on the aryl substituents (π-Mes/p-Tol). As
a result, the corresponding absorption bands of [1H/tBu]−−
[3H/tBu]− can be assigned as having mixed π → π* and charge-
transfer (CT) character. This is related to the neutral N,C-
chelate systems, which show dominant CT character for their
S1.14a Bidentate chelation of boron therefore introduces
accessible CT transitions, which are known to contribute to
the unique photoreactivity of these types of systems.18
There exist few examples of tetracoordinate 9-borafluorene
anions in the literature, with each requiring elaborate synthetic
strategies.16 We developed a new two-step protocol starting
from 2,2′-dilithiobiphenyl (for Li[1H]−Li[3H]) or its 4,4′-tert-
butyl-substituted derivative (for Li[1tBu]−Li[3tBu]) and the
appropriate borane, followed by cation exchange with KOtBu.
Compounds K[1H/tBu]−K[3H/tBu] were thereby prepared in
moderate to good yields (see Scheme 2 and Supporting
Irradiation of Li[1H] or Li[1tBu] with UV light (300 nm) in
dried/degassed CD3CN causes the initially colorless solutions
to become dark orange or red. Monitoring this transformation
by NMR spectroscopy reveals the conversion of Li[1H] to a
new species with one 11B resonance at −22.4 ppm (see the SI),
comparable to that of III (−26.6 ppm). Unlike the borirane
anion of Schuster et al. which displays cyclohexadienyl and H-
Scheme 2. Synthesis of 5H-Dibenzo[b,d]borol-5-ides
a
M[1H/tBu]−[3H/tBu
]
1
borirane H NMR signals at 5.55 and 1.39 ppm, respectively,
our photoproduct is characterized by two cyclohexadiene
singlets at 5.41 and 4.81 ppm (1:1) and six signals of equal
intensity for the chemically inequivalent methyl groups. The
most upfield-shifted signal (0.16 ppm) belongs to the methyl
group located on the C(sp3) atom. Although this species is
expected to be of type B (Scheme 1), its tendency to
decompose, even at room temperature (see SI), prevented full
spectroscopic characterization. To circumvent this issue, the Li+
counterion was exchanged for K+. Repeating the photolysis
experiment with K[1H] gives the same NMR-spectroscopic
data (Figure 2 and SI), where the borirane photoproduct
K[1Ha] is now stable indefinitely at room temperature under an
inert atmosphere. 2D NMR analysis confirms the identity of the
photoproduct to be the boratanorcaradiene derivative K[1Ha],
which is related to known (aza)boratanorcaradienes (e.g., C,
Scheme 1)13,14a and therefore most likely generated via a di-π-
methane rearrangement.15 Similar to the phototransformations
of C,(NHC)-chelated organoborates,13 this aryl insertion is not
thermally reversible even up to 120 °C.19 It is worth noting that
K[1H] undergoes exclusive aryl insertion into a B−C bond of
the 9-borafluorene backbone, with no aryl−aryl coupling as
demonstrated for I. This is believed to be a result of the S1 and
S2 transitions, whose π → π* and CT character do not facilitate
aryl−aryl coupling. The analogous transformation was also
observed for K[1tBu] → K[1tBua] (see SI). To investigate the
possibility of trapping a borene anion from the photoreaction of
a
Carbon atoms marked with asterisks bear H or tBu substituents.
Information (SI) for details). All of the compounds were fully
characterized by 1H, 11B{1H}, and 13C{1H} NMR spectroscopy
as well as HRMS. The crystal structures of Li[1H], Li[1tBu],
Li[2H], and Li[3tBu] were determined by single-crystal X-ray
diffraction (see SI). The B−C bond lengths in Li[1H], Li[1tBu],
Li[2H], and Li[3tBu] (1.628(8)−1.678(4) Å) are comparable to
those of other C,C-chelate 5H-dibenzo[b,d]borol-5-ides as well
as the neutral B,N-analogues.14b,16,17
Compounds K[1H/tBu]−K[3H/tBu] all absorb intensely (ε =
6900−26400 M−1cm−1) in the UV region between 300−340
nm as shown in Figure 1. These values are bathochromically
shifted relative to those of tetraarylborates (λmax ≈ 275
nm)9b,13b and blue-shifted compared to the π-extended C,
(NHC)-chelates of Yamaguchi and co-workers.13a TD-DFT
calculations of [1H/tBu]−−[3H/tBu]− and [BPh4]− reveal that the
computed S1 and S2 transitions are close in energy and
correspond to HOMO → LUMO and HOMO-1 → LUMO
respectively (see Figure 1 and the SI). For [BPh4]−, both
HOMO and HOMO-1 are of π-Ph character and mainly
B
DOI: 10.1021/acs.orglett.8b01534
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
Figure 3. Photoreactivity of K[2H/tBu] (top) and K[3H/tBu] (bottom)
upon irradiation at λ = 300 nm in CH3CN under an inert atmosphere;
carbon atoms marked with asterisks bear H or tBu substituents.
Figure 2. (Left top) 1H NMR spectra (400.13 MHz, CD3CN)
showing the conversion of colorless K[1H] (spectrum a) to red
K[1Ha] (spectrum c) via the stage of a clean mixture (spectrum b)
upon irradiation (λ = 300 nm) in a sealed NMR tube over 5 h. (Left
bottom) UV−vis tracking of the conversion of K[1H] (black curve) to
K[1Ha] (red curve) upon irradiation (λ = 300 nm). (Right) crystal
structure of K[1Ha] (H atoms are omitted for clarity).
To further explore the selectivity of these photoreactions,
unsymmetric borates K[2H] and K[2tBu] were examined for
their response to UV irradiation (Figure 3, top). Similar to
K[1H], photolysis of K[2H] in CD3CN with 300 nm light
results in the formation of K[2Ha], which was identified by its
1
3
distinct H and 11B NMR resonances at −0.25 (d, JH−H = 5.9
Hz; borirane-H) and −25.1 ppm, respectively. The absence of
any singlets in the upfield region of the H NMR spectrum
1
K[1H], 1,2-bis(4-tert-butylphenyl)ethyne was added to sol-
utions of K[1H] before photolysis and K[1Ha] once it was
formed. Both attempts failed to yield the desired borirene, even
when K[1Ha] and alkyne were heated to 80 °C, suggesting that
the borirane K[1Ha] is more stable than the photoproduct of
tetraphenylborates as described by Eisch.
indicates that the reaction is completely selective for the p-tolyl
substituent, similar to the related N,C-systems.14b Again, no
aryl−aryl C−C coupling was observed. K[2Ha] possesses a
similar absorption profile as K[1Ha], with a new broad band
centered at λmax = 409 nm (ε = 2400 M−1cm−1). The
photoreactivities of K[2H] and K[2tBu] were found to be the
most efficient of the series, with ΦPI near unity (0.98 for K[2H]
and 0.92 for K[2tBu]). This finding is quite different compared
to the N,C-analogues, which display a reduction in ΦPI with
similar substituents.14b The subsequent H-migration observed
in the unsymmetric N,C-chelate system14b was not detected in
K[2H] and K[2tBu], which may account for their high
photoisomerization quantum efficiency.
Single crystals suitable for X-ray diffraction analysis were
obtained by slow evaporation of a saturated CH3CN solution
containing K[1Ha]. The internal bond angles of the central
three-membered ring are close to ideal (B−C−C: 59.6(2)° and
61.9(2)°; C−B−C: 58.5(2)°), which is in line with previous
reports on borirane structures.20 The K+ ion is positioned
between one Mes and one C6H4 ring of the backbone from a
neighboring molecule with the distance between the K+ ion and
the centers of the aryl rings being 2.884(3) and 3.000(3) Å,
forming an extended 1D-structure. Furthermore, the K+ ion has
short contacts of 3.367(3) and 3.607(3) Å with a CC bond
of the cyclohexadienyl fragment. One acetonitrile molecule is
coordinated to the cation with a K−N distance of 2.818(3) Å.
Tracking the photoreaction of K[1H] by UV−vis spectros-
It is known that diphenyl substituted N,C-chelate boron
compounds are photochemically inert,17 which is in stark
contrast to I. Considering that our biphenyl-chelated borates
possess similarities between both of these systems, we decided
to investigate also the photoreactivity of K[3H] and K[3tBu],
which feature two p-tolyl substituents at boron (Figure 3,
bottom). Following short irradiation times (15 min) at 300 nm,
copy reveals a decrease in its main absorption band at λmax
=
1
324 nm with the concomitant appearance of a new, broad
absorption band belonging to K[1Ha] that ranges from 320−
600 nm. Continued irradiation eventually results in full
conversion to the photoproduct, with λmax ≈ 436 nm and ε =
3400 M−1 cm−1 (for a corresponding study on K[1tBu], see the
SI). TD-DFT data suggest that this low-energy band originates
from CT from HOMO (π-trimethylcyclohexadiene and
borirane) to LUMO (π*-biPh), which is in agreement with
(aza)boratanorcaradienes.18 The absorption profile of K[1Ha]
shares some similarities with C,(NHC)-stabilized boriranes
(λmax ≈ 430 nm),13 which absorb at a much higher energy
compared to the N,C-variants (λmax ≈ 600 nm). This is
attributed to LUMO destabilization by C,C/C,(NHC)-
chelation (see the SI).13b The photoisomerization quantum
efficiencies (ΦPI) of K[1H] and K[1tBu] were determined to be
0.83 and 0.79 respectively, which is comparable to that of the
N,C-chelate analogues.18
the H and 11B NMR spectra of K[3H] and K[3tBu] show a
significant decrease in signal intensity of the starting material
and a large number of overlapping emergent peaks. These C,C-
chelate borates are therefore unstable toward irradiation and
clearly operate according to a different photochemical pathway
compared to tetraarylborates. This finding also supports the
importance of the CT transition in the formation of
boratanorcaradienes, as the CT character of K[3H] and K[3tBu
]
is much less compared to K[1H] and K[1tBu] (see the SI for
TD-DFT oscillator strengths).
Given that the C,C-chelate borates seem to follow analogous
photoreactivity of the N,C-chelate compounds, the role of the
N-atom was probed by preparing K[1N] (Figure 4), which was
also found to be photoresponsive at 300 nm in CH3CN,
changing from colorless to orange. The UV−vis spectrum
shows a new broad band in the region between λ = 350−550
nm with a shoulder at λ = 505 nm. The 11B NMR spectra
C
DOI: 10.1021/acs.orglett.8b01534
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing 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
■
Figure 4. Photoreactivity of K[1N] upon irradiation at λ = 300 nm in
CH3CN under an inert atmosphere.
Corresponding Authors
*E-mail: suning.wang@chem.queensu.ca.
*E-mail: matthias.wagner@chemie.uni-frankfurt.de.
revealed two new resonances at −22.3 and −22.6 ppm,
respectively, which correspond to the two potential photo-
isomerization products (K[1Na] and K[1Na]-iso) that can be
formed upon irradiation of K[1N]. Monitoring the reaction
ORCID
Suning Wang: 0000-0003-0889-251X
Matthias Wagner: 0000-0001-5806-8276
Author Contributions
1
with H NMR spectroscopy reveals a 2:1 ratio of the two
isomers (see the SI for NMR analysis), whereby insertion into
the B−Cpy bond (K[1Na]) is favored, possibly due to greater
stabilization of the excited-state biradical when localized on py
and mesityl vs phenyl and mesityl.15 The photoisomerization
reaction of K[1N] proceeds with a comparable efficiency (ΦPI =
0.76) to other C,C-chelate species.
J.R. did the experimental work; S.K.M. performed calculations;
J.R. and S.K.M. contributed equally to the process of writing;
M.B. conducted the X-ray diffraction, and the structure
determination of Tol2BCl, 2,6-dimethyl-pyrimidin-4-amine,
Li[1H]·(thf)4, Li[1tBu]·(thf)4, Li[2H]·(thf)4, Li[3tBu]·LiCl·
(thf)5, and H[1N]·Et2O; K[1Ha]·CH3CN was contributed by
S.W.; H.-W.L. and M.W. supervised the experiments; S.W. and
M.W. supervised the writing process.
In conclusion, we have prepared a series of organoborates
with a biphenyl chelating unit and two varying aryl substituents.
Examination of their photoreactivity reveals that they do not
behave in accordance with tetraarylborates, likely due to their
differing electronic transitions imparted by chelation. Instead,
selective insertion of one aryl substituent into an endocyclic
B−C bond yields boratanorcaradienes with an embedded
borirane ring. These findings are in line with previous reports
on N,C-chelate organoboron systems, suggesting that a similar
CT-induced Zimmerman rearrangement is responsible for this
unique transformation. Although the photoreactions are not
thermally reversible, the efficiency of the photoisomerizations
was found to be exceptionally high. Mechanistically, it would
appear as though the C,C-chelated organoborates behave in
line with the initial postulations of Williams and Schuster,
thereby filling in the missing photoreactivities of this class of
molecules.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.R. thanks the “Vereinigung von Freunden und Forderern der
Johann Wolfgang Goethe-Universitat” for a travel grant. S.K.M.
thanks the Canadian Government for the Vanier CGS-D. Dr.
̈
̈
Alexander Hubner (Goethe-Universitat Frankfurt) contributed
̈
̈
to this publication by crystallizing compound Li[1H]·(thf)4.
REFERENCES
■
̈
(1) Wittig, G.; Keicher, G. Uber metallorganische Komplexverbin-
dungen. Naturwissenschaften 1947, 34, 216−216.
(2) Geilmann, W.; Gebauhr, W. Zur Fallung der Alkalimetalle als
Tetraphenylborverbindungen. Fresenius Z. Anal. Chem. 1953, 139,
161−181.
̈
ASSOCIATED CONTENT
* Supporting Information
■
S
(3) (a) Brookhart, M.; Grant, B.; Volpe, A. F. [(3,5-
(CF3)2C6H3)4B]−[H(OEt2)2]+: A Convenient Reagent for Generation
and Stabilization of Cationic, Highly Electrophilic Organometallic
Complexes. Organometallics 1992, 11, 3920−3922. (b) Krossing, I.;
Raabe, I. Noncoordinating AnionsFact or Fiction? A Survey of
Likely Candidates. Angew. Chem., Int. Ed. 2004, 43, 2066−2090.
(4) (a) Yamada, Y. M. A.; Watanabe, T.; Beppu, T.; Fukuyama, N.;
Torii, K.; Uozumi, Y. Palladium Membrane-Installed Microchannel
Devices for Instantaneous Suzuki-Miyaura Cross-Coupling. Chem. -
Eur. J. 2010, 16, 11311−11319. (b) Sarkar, S. M.; Uozumi, Y.; Yamada,
Y. M. A. A Highly Active and Reusable Self-Assembled Poly-
(Imidazole/Palladium) Catalyst: Allylic Arylation/Alkenylation.
Angew. Chem., Int. Ed. 2011, 50, 9437−9441.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01534.
Experimental details and NMR data of Li[1H/tBu]−
Li[3H/tBu], Li[1N], K[1H/tBu]−K[3H/tBu], K[1N], K-
[1H/tBua], K[2H/tBua], K[1Na], and K[1Na]-iso; NMR
monitoring of the photoisomerization of K[1H/tBu]−
K[3H/tBu] and K[1N]; photophysical properties of
K[1H/tBu]−K[3H/tBu] and K[1N]; UV−vis photoisomeri-
zation data of K[1H/tBu], K[2H/tBu], and K[1N]; kinetic
data for ΦPI of K[1H/tBu], K[2H/tBu], and K[1N]; X-ray
crystal structure analyses of Li[1H]·(thf)4, Li[1tBu]·
(thf)4, Li[2H]·(thf)4, Li[3tBu]·LiCl·(thf)5, H[1N]·Et2O,
K[1Ha]·CH3CN, Tol2BCl, and 2,6-dimethyl-pyrimidin-4-
amine; computational details for compounds K[1H/tBu]−
K[3H/tBu], K[1N], K[1H/tBua], K[2H/tBua], K[1Na], K-
[1Na]-iso, and [BPh4]− (PDF)
(5) There exists one earlier report by: Razuvaev, G. A.; Brikina, T. G.
Zh. Obs. Khim. 1954, 24, 1415.
(6) (a) Williams, J. L. R.; Doty, J. C.; Grisdale, P. J.; Regan, T. H.;
Borden, D. G. Boron Photochemistry: 1-Phenylcyclohexa-1,4-diene
from Sodium Tetraphenylborate. Chem. Commun. 1967, 3, 109.
(b) 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. (c) Williams, J. L. R.; Doty, J. C.; Grisdale, P. J.;
Regan, T. H.; Happ, G. P.; Maier, D. P. Boron Photochemistry. II.
Irradiation of Sodium Tetraarylborates in Alcohol Solutions. J. Am.
Chem. Soc. 1968, 90, 53−55.
Cartesian coordinates (XYZ)
Accession Codes
CCDC 1843173−1843180 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
D
DOI: 10.1021/acs.orglett.8b01534
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(7) Zimmerman, H. E.; Schissel, D. N. Di-π-Methane Rearrange-
ments of Highly Sterically Congested Molecules: Inhibition of Free-
Rotor Energy Dissipation. Mechanistic and Exploratory Organic
Photochemistry. J. Org. Chem. 1986, 51, 196−207.
(8) (a) Eisch, J. J.; Galle, J. E. Rearrangements of Organometallic
Compounds. XIII. Bora-aromatic Systems. IV. Synthesis of Hepta-
phenylborepin via the Thermal Rearrangement of Heptaphenyl-7-
borabicyclo[2.2.1]heptadiene. J. Am. Chem. Soc. 1975, 97, 4436−4437.
(b) Eisch, J. J.; Boleslawski, M. P.; Tamao, K. Bora-aromatic Systems.
X. Photochemical Generation of the Diphenylborate(I) Anion from
Metal Tetraphenylborates(III) in Aprotic Media: Repudiation of a
Contravening Claim. J. Org. Chem. 1989, 54, 1627−1634.
(9) (a) Wilkey, J. D.; Schuster, G. B. 2,5,7,7-Tetraphenyl-7-
boratabicyclo[4.1.0]hepta-2,4-diene: The First Isolation and Charac-
terization of a Boratanorcaradiene. J. Am. Chem. Soc. 1988, 110, 7569−
7571. (b) Wilkey, J. D.; Schuster, G. B. Irradiation of Tetraphenylbo-
rate Does Not Generate a Borene Anion. J. Org. Chem. 1987, 52,
2117−2122. (c) Wilkey, J. D.; Schuster, G. B. Photochemistry of
Tetraarylborate Salts (Ar4B−): Formation of 2,5,7,7-Tetraphenyl-7-
boratabicyclo[4.1.0]hepta-2,4-diene (a Boratanorcaradiene) by Irradi-
ation of (p-Biphenylyl)triphenyl Borate. J. Am. Chem. Soc. 1991, 113,
2149−2155.
mation and C−H Bond Insertion. J. Am. Chem. Soc. 2012, 134,
11026−11034.
(14) (a) Rao, Y.-L.; Amarne, H.; Zhao, S.-B.; McCormick, T. M.;
́
Martic, S.; Sun, Y.; Wang, R.-Y.; Wang, S. Reversible Intramolecular
C−C Bond Formation/Breaking and Color Switching Mediated by a
N,C-Chelate in (2-Ph-py)BMes2 and (5-BMes2-2-Ph-py)BMes2. J. Am.
Chem. Soc. 2008, 130, 12898−12900. (b) Mellerup, S. K.; Li, C.; Peng,
T.; Wang, S. Regioselective Photoisomerization/C−C Bond For-
mation of Asymmetric B(ppy)(Mes)(Ar): The Role of the Aryl
Groups on Boron. Angew. Chem., Int. Ed. 2017, 56, 6093−6097.
(15) Mellerup, S. K.; Li, C.; Radtke, J.; Wang, X.; Li, Q.-S.; Wang, S.
Angew. Chem., Int. Ed. 2018, DOI: 10.1002/anie.201803760.
(16) (a) Chernichenko, K.; Lindqvist, M.; Kot
́
ai, B.; Nieger, M.;
Sorochkina, K.; Pap
́
ai, I.; Repo, T. Metal-Free Sp2-C−H Borylation as
a Common Reactivity Pattern of Frustrated 2-Aminophenylboranes. J.
Am. Chem. Soc. 2016, 138, 4860−4868. (b) Yu, L.; Li, Y.; Wang, X.;
Wang, X.; Zhou, P.; Jiang, S.; Pan, X. Consecutive Reduction, Radical-
Cyclization, and Oxidative-Dehydrogenation Reaction of Ortho-
Substituted Diboryl Compounds. Chem. Commun. 2017, 53, 9737−
9740.
(17) Amarne, H.; Baik, C.; Murphy, S. K.; Wang, S. Steric and
Electronic Influence on Photochromic Switching of N,C-Chelate Four-
Coordinate Organoboron Compounds. Chem. - Eur. J. 2010, 16,
4750−4761.
(18) Mellerup, S. K.; Wang, S. Photoresponsive Organoboron
Systems. In Main Group Strategies towards Functional Hybrid Materials;
Baumgartner, T., Jaekle, F., Eds.; John Wiley & Sons Ltd.: Hoboken,
2018; pp 47−78.
(10) Eisch, J. J.; Tamao, K.; Wilcsek, R. J. Rearrangements of
Organometallic Compounds. XII. Generation of Boracarbenoid and
Boracyclopropene Intermediates from the Photolysis of Tetraorgano-
borate Salts in Aprotic Media. J. Am. Chem. Soc. 1975, 97, 895−897.
(11) For a recent review article covering new results in 9-
borafluorene chemistry, see: von Grotthuss, E.; John, A.; Kaese, T.;
Wagner, M. Doping Polycyclic Aromatics with Boron for Superior
Performance in Materials Science and Catalysis. Asian J. Org. Chem.
2018, 7, 37−53.
(19) Catalytic CH3CN trimerization with formation of 2,6-
dimethylpyrimidin-4-amine was observed instead (see the SI).
(20) (a) Bissinger, P.; Braunschweig, H.; Kraft, K.; Kupfer, T.
Trapping the Elusive Parent Borylene. Angew. Chem., Int. Ed. 2011, 50,
4704−4707. (b) Braunschweig, H.; Claes, C.; Damme, A.;
(12) (a) Koster, R.; Willemsen, H. Borverbindungen, XXVIII, 1,2-
̈
(2,2′-Biphenylen)diborane(6). Justus Liebigs Ann. Chem. 1974, 1974,
Deißenberger, A.; Dewhurst, R. D.; Horl, C.; Kramer, T. A Facile
̈
1843−1850. (b) Hubner, A.; Qu, Z.-W.; Englert, U.; Bolte, M.; Lerner,
̈
and Selective Route to Remarkably Inert Monocyclic NHC-Stabilized
Boriranes. Chem. Commun. 2015, 51, 1627−1630. (c) McFadden, T.
R.; Fang, C.; Geib, S. J.; Merling, E.; Liu, P.; Curran, D. P. Synthesis of
Boriranes by Double Hydroboration Reactions of N-Heterocyclic
Carbene Boranes and Dimethyl Acetylenedicarboxylate. J. Am. Chem.
H.-W.; Holthausen, M. C.; Wagner, M. Main-Chain Boron-Containing
Oligophenylenes via Ring-Opening Polymerization of 9-H-9-Bora-
fluorene. J. Am. Chem. Soc. 2011, 133, 4596−4609. (c) Hubner, A.;
̈
Diefenbach, M.; Bolte, M.; Lerner, H.-W.; Holthausen, M. C.; Wagner,
M. Confirmation of an Early Postulate: B-C-B Two-Electron-Three-
Center Bonding in Organo(hydro)boranes. Angew. Chem., Int. Ed.
Soc. 2017, 139, 1726−1729. (d) 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 PAH
Substituents. Org. Lett. 2017, 19, 3851−3854.
2012, 51, 12514−12518. (d) Biswas, S.; Maichle-Mossmer, C.;
̈
Bettinger, H. F. Rearrangement from the Heteroantiaromatic Borole
to the Heteroaromatic Azaborine Motif. Chem. Commun. 2012, 48,
4564−4566. (e) Muller, M.; Maichle-Mossmer, C.; Bettinger, H. F.
̈
̈
BN-Phenanthryne: Cyclotetramerization of an 1,2-Azaborine Deriva-
tive. Angew. Chem., Int. Ed. 2014, 53, 9380−9383. (f) Braunschweig,
H.; Horl, C.; Mailander, L.; Radacki, K.; Wahler, J. Antiaromaticity to
̈
̈
Aromaticity: From Boroles to 1,2-Azaborinines by Ring Expansion
with Azides. Chem. - Eur. J. 2014, 20, 9858−9861. (g) Huang, K.;
Martin, C. D. Ring Expansion Reactions of Pentaphenylborole with
Dipolar Molecules as a Route to Seven-Membered Boron Hetero-
cycles. Inorg. Chem. 2015, 54, 1869−1875. (h) Barnard, J. H.; Yruegas,
S.; Huang, K.; Martin, C. D. Ring Expansion Reactions of Anti-
Aromatic Boroles: A Promising Synthetic Avenue to Unsaturated
Boracycles. Chem. Commun. 2016, 52, 9985−9991. (i) Yruegas, S.;
Wilson, C.; Dutton, J. L.; Martin, C. D. Ring Opening of Epoxides
Induced by Pentaphenylborole. Organometallics 2017, 36, 2581−2587.
(j) Kaese, T.; Trageser, T.; Budy, H.; Bolte, M.; Lerner, H.-W.;
Wagner, M. A Redox-Active Diborane Platform Performs C(sp3)−H
Activation and Nucleophilic Substitution Reactions. Chem. Sci. 2018,
9, 3881−3891.
(13) Neutral C,C-chelates based on N-heterocyclic carbenes are
known: (a) Nagura, K.; Saito, S.; Frohlich, R.; Glorius, F.; Yamaguchi,
̈
S. N-Heterocyclic Carbene Boranes as Electron-Donating and
Electron-Accepting Components of π-Conjugated Systems. Angew.
Chem., Int. Ed. 2012, 51, 7762−7766. (b) Rao, Y.-L.; Chen, L. D.;
Mosey, N. J.; Wang, S. Stepwise Intramolecular Photoisomerization of
NHC-Chelate Dimesitylboron Compounds with C−C Bond For-
E
DOI: 10.1021/acs.orglett.8b01534
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