Borylated dibenzoborepin: Synthesis by skeletal rearrangement and photochromism based on bora-Nazarov cyclization

Article abstract of DOI:10.1002/anie.201210236

It's a boron kind of magic: A complex reaction sequence involving a skeletal rearrangement leads to 10-dimesitylboryl-substituted dibenzoborepin in a one-pot transformation in good yield. The borylborepin shows remarkable photochromic properties and undergoes a bora-Nazarov cyclization to give a deep-blue compound with an allyl cation-like C-B-C substructure. Copyright

Full text of DOI:10.1002/anie.201210236

Angewandte
Chemie
DOI: 10.1002/anie.201210236
Structure Elucidation
Borylated Dibenzoborepin: Synthesis by Skeletal Rearrangement and
Photochromism Based on Bora-Nazarov Cyclization**
Azusa Iida, Shohei Saito, Takahiro Sasamori, and Shigehiro Yamaguchi*
The incorporation of boron atoms into p-conjugated skele-
tons has emerged as a useful strategy to produce new
optoelectronic materials with unusual electronic structures.[1]
Indeed, many fascinating boron-containing p systems have
been recently developed for various applications, such as
organic light-emitting diodes,[2] two-photon absorption mate-
rials,[3] sensors,[4] and photochromic systems.[5] One key reason
for the use of a tricoordinated boron atom from this point of
view is its isosterism with a cationic tricoordinated carbon
atom (Figure 1). Replacement of a cationic carbon atom in
Scheme 1. Retrosynthesis of diborole 1 and transformation from 3 into
the 10-boryl-substituted dibenzoborepin 2.
Figure 1. B/C+ isosterism as a strategy for the development of new
p-conjugated systems.
workers using an elegant photoisomerization of bi(benzobo-
ratacyclobutylidene).[8] Meanwhile, in our efforts to synthe-
a p-conjugated skeleton by a boron atom enables trans-
formation of the highly electron-accepting p skeleton into
a noncharged, isolable form. It is noteworthy that, alterna-
tively, the substitution of a carbon atom next to the cationic
carbon atom by a boron atom also results in an overall
noncharged isosteric form. We now report an example that
these isosterisms lead to a new reaction and produce a unique
p system.
size this skeleton, we unexpectedly obtained the 10-borylated
dibenzoborepin 2 by an anomalous skeletal rearrangement.
To our surprise, this compound showed dramatic photo-
chromic properties. The photoreactivity of 2 is unusual in light
of the inherent high photostability of the dibenzoborepin
skeleton.[9] We have now succeeded in unambiguously
determining the structure of the photoproduct, and have
disclosed its unusual electronic structure.
As a part of our study of the main group element
containing ladder p systems,[6] we have attempted to synthe-
size the borole-fused borole 1 (hereafter, abbreviated as
diborole) as a highly electron-accepting, antiaromatic scaffold
(Scheme 1).[7] During the course of our research, the first
synthesis of this skeleton was achieved by Piers and co-
Our original idea to synthesize the diborole 1 was to
transform a bis(o-bromophenyl)ethene diboronic ester 3 into
the diborate intermediate 4 through a halogen–lithium
exchange reaction with BuLi, with subsequent introduction
of an Ar group onto the boron atoms by treatment with
ArMgX (Scheme 1). We conducted this reaction sequence
using tBuLi in THF and 2 mol equivalents of MesMgBr.
Purification by silica gel column chromatography afforded
a colorless compound. However, X-ray crystal structure
analysis revealed that the product was not the desired 1, but
the 10-dimesitylboryl-substituted dibenzoborepin 2 (Fig-
ure 2a). The yield of this compound was 35%, which was
further improved to 74% by increasing the amount of
MesMgBr to 3 mol equivalents and conducting protonation
with water.
[*] Dr. A. Iida, Dr. S. Saito, Prof. Dr. S. Yamaguchi
Research Center for Materials Science and Institute of Trans-
formative Bio-Molecules (WPI-ITbM), Nagoya University and
CREST (Japan) Science and Technology Agency (JST)
Furo, Chikusa, Nagoya 464-8602 (Japan)
E-mail: yamaguchi@chem.nagoya-u.ac.jp
Prof. Dr. T. Sasamori
Institute for Chemical Research, Kyoto University
Gakasho, Uji, Kyoto 611-0011 (Japan)
It is noteworthy that the anomalous skeletal rearrange-
ment from 3 to 2 proceeds in high yield. To elucidate the
mechanism of this transformation,[10] we isolated and identi-
fied the key intermediates in the reaction sequence. First, the
halogen–lithium exchange reaction of 3 was performed using
4 mol equivalents of tBuLi at À788C in diethyl ether to obtain
a product as an insoluble compound (Scheme 2). The
[**] This work was partly supported by CREST, JST, and a Grant-in-Aid for
Scientific Research on Innovative Areas (Stimuli-responsive Chem-
ical Species, No. 24109007) from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan. A. I. expresses her
gratitude to the JSPS research fellowship for young scientists.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201210236.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
Scheme 2. Transformation from 3 into 5 and its plausible mechanism.
from one of the borate moieties to produce a sp2-hybridized
carbanion, which undergoes inversion to eventually produce
the Z isomer.[12] The driving force of this isomerization should
be the formation of the energetically favorable chelate
complex 5, which binds two Li ions to the two vicinal borate
oxygen atoms. The chelate structure was confirmed by X-ray
structural analysis (Figure 2b).
The treatment of 5 with 3 mol equivalents of MesMgBr in
THF indeed gave the borylated dibenzoborepin 2 in 75%
yield (Scheme 3). When we tried to isolate an intermediate
for this reaction without conducting the aqueous work-up,
direct recrystallization of the mixture from THF gave red
Figure 2. ORTEP drawings of a) 2, b) 5, and c) 6. Thermal ellipsoids
are shown at 50% probability. Hydrogen atoms, THF molecules
coordinating to Li atoms in 5, Li(thf)4+, and solvent molecules in 6 are
omitted for clarity. For 6, only one of two crystallographically indepen-
dent structures is shown.
Scheme 3. Transformation from 5 into 2 and a plausible mechanism
for skeletal rearrangement.
dilithiation cleanly proceeded and spontaneously produced
precipitates. Recrystallization from THF gave a pure product
as colorless needles. X-ray crystal structure analysis revealed
that the compound was the bi(benzoboratacyclobutylidene) 5
(Figure 2b) instead of the expected diborole diborate 4.[11]
The yield of isolated 5 was 47%. This compound is likely
produced through the formation of (E)-bis(benzoboratacy-
clobutylidene), instead of the five-membered borole rings,
with subsequent E–Z isomerization (Scheme 2). The favor-
able formation of the four-membered ring over the five-
membered ring seems rather general, since a similar reaction
was also reported in the synthesis of 1 by Piers and co-
workers.[8] The E–Z olefin isomerization may be based on the
borate/borane equilibrium. An alkenyl group is dissociated
crystals of 6. X-ray crystal structure analysis demonstrated
that this compound has an unprecedented boratacyclopro-
pene-fused borepin skeleton (Figure 2c).[11] The treatment of
6 with water quantitatively gave the product 2, thus confirm-
ing that 6 is an intermediate in the formation of 2. Although
the details of the substitution reaction of the alkoxy groups on
the boron atoms with MesMgBr are unclear, this skeletal
rearrangement might be again based on the borate/borane
equilibrium (Scheme 3). Thus, a phenyl anion is dissociated
from one of the borate moieties and then the boratacyclo-
propene skeleton is produced. The eliminated phenyl anion
2
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
reacts with the other boron atom to form the borepin
skeleton.
Upon irradiation with UV light (l = 320 nm), a colorless
solution of 2 in benzene turned to navy blue, as shown in
Figure 3. In the absorption spectra of 2, the intensity of two
distinct and sharp bands at l = 346 and 387 nm decreased and
Figure 4. ORTEP drawing of 7A. Thermal ellipsoids are shown at 50%
probability. Hydrogen atoms, except H4, are omitted for clarity.
ization. The sums of the C-C-C and C-C-B angles are 360.08
for both the C3 and C7 atoms. The boron atom B2 also retains
a trigonal-planar geometry with the sum of the C-B-C angles
À
À
being 360.08. The bond lengths for the C3 B2 and B2 C7
bonds are 1.527(5) ꢀ and 1.522(5) ꢀ, respectively. These
Figure 3. UV-Vis absorption spectra of 2 upon irradiation by UV light
(l=320 nm) in benzene. Inset: Photographs of a benzene solution of
2 before (left) and after (right) irradiation of UV light.
=
values are between the bond lengths of typical C B double
[13]
=
bonds [e.g., (Me3Si)2C B(tBu) 1.361(5) ꢀ]
and ordinary
[14]
À
Csp2 B single bonds (e.g., Ar3B, 1.57–1.59 ꢀ),
but still
comparable or slightly longer than the shortest examples of
the Csp2 B single bonds (1.48–1.52 ꢀ).[15] Notably, the methyl
À
a new broad band appeared at l = 634 nm with the isosbestic
point at l = 398 nm. This photochromic behavior was also
observed in other common solvents including n-pentane,
cyclohexane, CH2Cl2, CHCl3, acetonitrile, and MeOH. Even
a cast film of 2 prepared from a toluene solution showed the
photochromic behavior (see the Supporting Information),
whereas its crystals did not show the photochromic response,
probably because of the immobility of the molecule in the
crystal lattice.
group at C8 and the hydrogen atom at C4 have a cis
configuration. No other stereoisomer was observed by
1H NMR spectroscopy of the reaction mixture after the
photoirradiation, thus implying that the formation of 7
proceeds in a stereospecific manner (see below).
The most unique structural feature in 7 is the C-B-C
substructure in the five-membered ring. Taking the number of
p electrons assigned to this moiety into consideration, the C-
B-C skeleton is isoelectronic to the allyl cation. To gain
fundamental insights into this skeleton,[16] we first theoret-
1
Monitoring of the reaction by H NMR spectroscopy in
[D8]THF (see the Supporting Information) showed that 2 was
cleanly converted into the single product 7 by the photo-
irradiation. Notably, the photoproduct 7 cleanly reverted to
the initial compound 2 upon storing the solution overnight in
a dark at room temperature. Thus, this chromic reaction is
reversible.
À
+
=
À
ically compared the electronic structures of CH2 BH CH2
(8) and the allyl cation (9) as model systems. According to the
geometry optimization at the B3LYP/6-31G* level of theory,
À
the C B bond length in 8 is 1.49 ꢀ. The natural bond orbital
(NBO) analysis (MP2/6-31G*//B3LYP/6-31G*)[17] showed
À
To characterize the structure of 7, we successfully
obtained single crystals from a cooled n-pentane solution.
X-ray crystal structure analysis (Figure 4)[11] showed that the
crystal structure contains two crystallographically independ-
ent molecules, 7A and 7B. As their structural parameters are
similar to each other (see the Supporting Information), we
that the Wiberg bond index of the C B bond in 8 is 1.36,
À
while that of the C C bond in 9 is 1.51. This difference implies
that 8 has a smaller double bond character compared to that
of 9.
The geometry optimization of 7 at the B3LYP/6-31G*
level of theory nicely reproduced the crystal structure (see the
Supporting Information).[18] In the optimized structure, the
C3 B2 and B2 C7 bonds are 1.52 and 1.53 ꢀ, respectively.
According to the NBO analysis, the Wiberg bond index of the
C B bonds are 1.09 for B2-C7 and 1.22 for B2 C3, thus
demonstrating that the single bond character of the C B C
À
only discuss the structure of 7A. In this structure, a new C C
À
À
bond is formed between the ortho-carbon atom (C8) of one of
the mesityl groups and the olefinic carbon atom (C4) of the
borepin moiety to produce a five-membered ring. As a result,
the mesityl group is transformed into a nonaromatic 1,3-
À
À
À À
=
cyclohexadiene-like structure (1.351–1.380 ꢀ for C C bonds
moiety becomes more pronounced in the extended p-
À
=
and 1.423–1.487 ꢀ for C C bonds). It is important to note
that the carbon atoms, C3 and C7, connected to the boron
conjugated skeleton of 7, compared to the parent CH2
À
+
À
À À
BH CH2 skeleton. However, the C B C moiety in 7 still
maintains the allyl cation-like charge distribution. Thus, the
atom in the five-membered ring, still retain the sp2 hybrid-
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
natural population analysis (NPA) of 7 showed that the
atomic charges of C3, B2, and C7 are À0.19, + 0.48, and
À0.11, respectively. Compared to the charge distribution of
heating of 2 at 808C for 12 h did not give the product at all.
This is probably because of the thermodynamic instability of
the thermally cyclized product compared with 2. It is also
noted that 7 is isoelectronic to a cyclopentenyl cation
intermediate of the Nazarov cyclization. Thus, one can say
that the B/C+ isosterism allows the isolation of the unstable
cationic Nazarov intermediate as an overall noncharged form.
In summary, we have unexpectedly obtained a boryl-
substituted dibenzoborepin in good yield in one pot from
a diarylethene diboronic ester. The mechanistic study showed
that this product is produced by a skeletal rearrangement
through an unprecedented boratacyclopropene-fused borepin
intermediate. The dimesitylboryl borepin showed dramatic
photochromic behavior with a color change from colorless to
deep blue, and the photoproduct was thermally reverted to
the starting borepin. This photoreaction can be rationalized as
a bora-Nazarov cyclization. Notably, the B/C+ isosterism is
responsible for this new reactivity and renders a product
having an unusual electronic structure. This photochromic
behavior may have the potential to be a basis for the
development of new photoresponsive materials. Further
study to explore this possibility is in progress in our
laboratory.
À
ordinary Csp2 B bonds [e.g., C(À0.43)-B(+0.94) in Ph3B], B2
is more negative, while C3 and C7 are more positive.
The blue color of 7 observed in the photochromism can be
understood by considering its molecular orbitals. Both the
HOMO and LUMO of 7 are spread over the polyene skeleton
À À
which consists of butadiene, an allyl cation-like C B C
moiety, and a borylated phenyl group (see the Supporting
Information). This skeleton is isoelectronic to the electron-
deficient decatetraenyl dication, whose LUMO energy level
should be highly stabilized. Indeed, the LUMO energy level
of 7 calculated at the M06/6-31 + G**//B3LYP/6-31G* level
of theory is quite low at À3.01 eV, while the HOMO level is
moderately high at À5.37 eV. The resulting narrow HOMO–
LUMO gap should be responsible for the absorption band
observed at the long wavelength in the visible region. Indeed,
a time-dependent DFT calculation of 7 confirmed that the
longest absorption band observed at l = 634 nm is assignable
to the HOMO–LUMO transition.
How can we rationalize the mechanism of the photo-
reaction from 2 to 7? In this regard, noteworthy is the
similarity of the present reaction to the Nazarov cycliza-
tion,[19] which is a 4p-electrocyclic reaction of a dialkenylke-
tone to give a cyclopentenyl cation (Scheme 4b). The present
photoreaction can be regarded as a bora-Nazarov cyclization
(Scheme 4a) since 2 is isosteric to an intermediate of the
Nazarov cyclization produced by the activation of the
Received: December 22, 2012
Published online: && &&, &&&&
Keywords: boron · cyclization · electrocyclic reactions ·
.
photochromism · structure elucidation
[1] For reviews: a) C. D. Entwistle, T. B. Marder, Angew. Chem.
2002, 114, 3051; Angew. Chem. Int. Ed. 2002, 41, 2927; b) C. D.
Entwistle, T. B. Marder, Chem. Mater. 2004, 16, 4574; c) S.
Yamaguchi, A. Wakamiya, Pure Appl. Chem. 2006, 78, 1413;
d) F. Jꢁkle, Coord. Chem. Rev. 2006, 250, 1107; e) N. Matsumi, Y.
Chujo, Polym. J. 2008, 40, 77; f) Z. M. Hudson, S. Wang, Acc.
Chem. Res. 2009, 42, 1584; g) F. Jꢁkle, Chem. Rev. 2010, 110,
3985.
[2] a) T. Noda, Y. Shirota, J. Am. Chem. Soc. 1998, 120, 9714; b) T.
Noda, H. Ogawa, Y. Shirota, Adv. Mater. 1999, 11, 283; c) Y.
Shirota, M. Kinoshita, T. Noda, K. Okumoto, T. Ohara, J. Am.
Chem. Soc. 2000, 122, 11021; d) W.-L. Jia, D.-R. Bai, T.
McCormick, Q.-D. Liu, M. Motala, R.-Y. Wang, C. Seward, Y.
Tao, S. Wang, Chem. Eur. J. 2004, 10, 994; e) W. L. Jia, X. D.
Feng, D. R. Bai, Z. H. Lu, S. Wang, G. Vamvounis, Chem. Mater.
2005, 17, 164; f) D. Tanaka, T. Takeda, T. Chiba, S. Watanabe, J.
Kido, Chem. Lett. 2007, 36, 262; g) C. Sun, Z. M. Hudson, M. G.
Helander, Z.-H. Lu, S. Wang, Organometallics 2011, 30, 5552.
[3] a) Z. Yuan, N. J. Taylor, T. B. Marder, I. D. Williams, S. K. Kurtz,
L.-T. Cheng, J. Chem. Soc. Chem. Commun. 1990, 1489; b) Z.
Yuan, N. J. Taylor, Y. Sun, T. B. Marder, I. D. Williams, L.-T.
Cheng, J. Organomet. Chem. 1993, 449, 27; c) K. C. Ching, M.
Lequan, R.-M. Lequan, A. Grisard, D. Markovitsi, J. Chem. Soc.
Faraday Trans. 1991, 87, 2225; d) M. Lequan, R. M. Lequan, K.
Chane-Ching, J. Mater. Chem. 1991, 1, 997; e) C. Lambert, S.
Stadler, G. Bourhill, C. Brꢁuchle, Angew. Chem. 1996, 108, 710;
Angew. Chem. Int. Ed. Engl. 1996, 35, 644; f) Z.-Q. Liu, Q. Fang,
D. Wang, G. Xue, W.-T. Yu, Z.-S. Shao, M.-H. Jiang, Chem.
Commun. 2002, 2900; g) Z.-Q. Liu, Q. Fang, D. Wang, D.-X. Cao,
G. Xue, W.-T. Yu, H. Lei, Chem. Eur. J. 2003, 9, 5074.
Scheme 4. Comparison between the transformation from 2 into 7 (a)
and the Nazarov cyclization (b).
carbonyl group with a Brønsted or Lewis acid. The config-
À
uration of the newly formed C C bond in 7 is consistent with
the frontier orbital theory. Namely, the cyclization proceeds in
a disrotatory fashion to produce the cis configuration between
the methyl group at C8 and the hydrogen atom at C4. While
the 6p-electrocyclic reactions of O-, N-, or S-containing
substrates analogous to the Nazarov cyclization are well
known,[20] the photoreactions based on the bora-Nazarov
cyclizaion is unprecedented, to the best of our knowledge.[21]
While the Nazarov cyclization generally proceeds under both
thermal or photoirradiation conditions, the present bora-
Nazarov cyclization requires photoirradiation. Even the
4
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
[4] a) S. Yamaguchi, S. Akiyama, K. Tamao, J. Am. Chem. Soc. 2001,
123, 11372; b) C. R. Wade, A. E. J. Broomsgrove, S. Aldridge,
F. P. Gabbaꢂ, Chem. Rev. 2010, 110, 3958, and references therein.
[5] a) K. Ansorg, H. Braunschweig, C.-W. Chiu, B. Engels, D.
Gamon, M. Hꢃgel, T. Kupfer, K. Radacki, Angew. Chem. 2011,
123, 2885; Angew. Chem. Int. Ed. 2011, 50, 2833; b) Y.-L. Rao, H.
Amarne, S. Wang, Coord. Chem. Rev. 2012, 256, 759, and
references therein; c) Y.-L. Rao, L. D. Chen, N. J. Mosey, S.
Wang, J. Am. Chem. Soc. 2012, 134, 11026; d) K. Nagura, S.
Saito, R. Frçlich, F. Glorius, S. Yamaguchi, Angew. Chem. 2012,
124, 7882; Angew. Chem. Int. Ed. 2012, 51, 7762.
[6] A. Fukazawa, S. Yamaguchi, Chem. Asian J. 2009, 4, 1386.
[7] a) A. Iida, S. Yamaguchi, J. Am. Chem. Soc. 2011, 133, 6952;
b) A. Iida, A. Sekioka, S. Yamaguchi, Chem. Sci. 2012, 3, 1461.
[8] J. F. Araneda, B. Neue, W. E. Piers, M. Parvez, Angew. Chem.
2012, 124, 8674; Angew. Chem. Int. Ed. 2012, 51, 8546.
[9] a) L. G. Mercier, W. E. Piers, M. Parvez, Angew. Chem. 2009,
121, 6224; Angew. Chem. Int. Ed. 2009, 48, 6108; b) A.
Caruso, Jr., M. A. Siegler, J. D. Tovar, Angew. Chem. 2010,
122, 4309; Angew. Chem. Int. Ed. 2010, 49, 4213; c) D. R. Levine,
A. Caruso, Jr., M. A. Siegler, J. D. Tovar, Chem. Commun. 2012,
48, 6256.
[14] a) F. Zettler, H. D. Hausen, H. J. Hess, J. Organomet. Chem.
1974, 72, 157; b) J. F. Blount, P. Finocchiaro, D. Gust, K. Mislow,
J. Am. Chem. Soc. 1973, 95, 7019; c) M. M. Olmstead, P. P.
Power, J. Am. Chem. Soc. 1986, 108, 4235.
[15] a) A. Sundararaman, R. Varughese, H. Li, L. N. Zakharov, A. L.
Rheingold, F. Jꢁkle, Organometallics 2007, 26, 6126; b) C. Fan,
W. E. Piers, M. Parvez, R. McDonald, Organometallics 2010, 29,
5132; c) Z. Zhou, A. Wakamiya, T. Kushida, S. Yamaguchi, J.
Am. Chem. Soc. 2012, 134, 4529.
[16] A theoretical study on the singlet diradical character of CH2-
BH-CH2 with the coordination of donor species has been
reported: K. Kubota, H. Takahashi, H. Fukui, S. Bonness, K.
Yoneda, R. Kishi, T. Kubo, K. Kamada, K. Ohta, B. Champagne,
E. Botek, M. Nakano, Chem. Phys. Lett. 2009, 477, 309.
[17] NBO version 3.1, E. D. Glendening A. E. Reed, J. E. Carpenter,
F. Weinghold.
[18] We also conducted the broken-symmetry UB3LYP/6-31G*
calculation, which demonstrated no singlet biradical character
of 7. See references: a) L. Noodleman, J. Chem. Phys. 1981, 74,
5737; b) L. Noodleman, E. J. Baerends, J. Am. Chem. Soc. 1984,
106, 2316; c) L. Noodleman, C. Y. Peng, D. A. Case, J. M.
Mouesca, Chem. Rev. 1995, 95, 199.
[10] Eisch and co-workers have reported the transformation from
borole precursors to borepin rings: a) J. J. Eisch, J. E. Galle, J.
Am. Chem. Soc. 1975, 97, 4436; b) J. J. Eisch, J. E. Galle, B.
Shafii, A. L. Rheingold, Organometallics 1990, 9, 2342.
[11] CCDC 912372 (2), 912373 (3), 912374 (5), 915855 (6) and 916528
(7) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
[19] L. Kꢃrti, B. Czakꢄ, Strategic Applications of Named Reactions in
Organic Synthesis, Elsevier Academic Press, Amsterdam, 2005,
p. 304.
[20] A. G. Schultz, Acc. Chem. Res. 1983, 16, 210.
[21] Review for photoreactions of boron compounds: a) A. Pelter,
R. T. Pardasani, P. Pardasani, Tetrahedron 2000, 56, 7339. For 6p-
electrocyclic reaction of aminoboranes, see: b) P. J. Grisdale,
J. L. R. Williams, J. Org. Chem. 1969, 34, 1675; c) P. Grisdale, M.
Glogowski, J. L. R. Williams, J. Org. Chem. 1971, 36, 3821;
d) M. E. Glogowski, P. J. Grisdale, J. L. R. Williams, T. H.
Regan, J. Organomet. Chem. 1973, 54, 51; e) M. E. Glogowski,
J. L. R. Williams, J. Organomet. Chem. 1980, 195, 123.
[12] R. Knorr, E. Lattke, E. Rꢁpple, Chem. Ber. 1981, 114, 1581.
[13] R. Boese, P. Paetzold, A. Tapper, R. Ziembinski, Chem. Ber.
1989, 122, 1057.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Structure Elucidation
It’s a boron kind of magic: A complex
reaction sequence involving a skeletal
rearrangement leads to 10-dimesitylboryl-
substituted dibenzoborepin in a one-pot
transformation in good yield. The boryl-
borepin shows remarkable photochromic
properties and undergoes a bora-Nazarov
cyclization to give a deep-blue compound
A. Iida, S. Saito, T. Sasamori,
S. Yamaguchi*
&&&& — &&&&
Borylated Dibenzoborepin: Synthesis by
Skeletal Rearrangement and
Photochromism Based on Bora-Nazarov
Cyclization
À À
with an allyl cation-like C B C substruc-
ture.
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!