Photochemical synthesis of a ladder diborole: A new boron-containing conjugate material

Article abstract of DOI:10.1002/anie.201204367

Climbing the ladder: Reductive cyclization of alkynyl haloboranes lead to the bis-benzocycloborabutylidene rather than the expected ladder diborole, despite the former being much less thermodynamically favored. Photochemical conversion to the ladder diborole was, however, quite facile upon irradiation at 254?nm.

Full text of DOI:10.1002/anie.201204367

Angewandte
Chemie
DOI: 10.1002/anie.201204367
Boron Heterocycles
Photochemical Synthesis of a Ladder Diborole: A New Boron-
Containing Conjugate Material**
Juan F. Araneda, Benedikt Neue, Warren E. Piers,* and Masood Parvez
Incorporation of main-group elements into organic frame-
works has emerged in the last decade as a promising route to
develop new materials.[1] For instance B,[2] Si,[3] P[4] or S[5]
impart interesting features to conjugated hydrocarbon frame-
works such as effective orbital interactions, change in dipole
moments, and diversity in coordination number. In particular,
“ladder” compounds[6] are attractive because, as acene
analogs, they are planar molecules and possess an extended
p-conjugated system, leading to highly desirable photophys-
ical and electronic properties like intense luminescence[7,8]
and high carrier mobility.[9] In 2003 Tamao and co-workers
reported the synthesis of the bis-silicon-bridged stilbene I by
an intramolecular reductive cyclization reaction.[10] Com-
pared to its carbon analogue, this compound has an intense
blue emission, which is red-shifted by about 60 nm.[10] Several
other examples, such as phosphonium borates[11] and bis-
phosphorus-bridged stilbenes[12] have also been reported.
carbenes,[16] unlike anthracene itself, absorbs in the visible
region because the HOMO–LUMO gap is strongly decreased
compared to its all-carbon analogue. The same effect was
found in the more extended boratetracene and borapenta-
cenes analogues.[17]
Apart from the promising materials that can be prepared
with boron-containing molecules, boron heterocycles are of
fundamental interest and have been used for decades to probe
the basic concepts of aromaticity and antiaromaticity.[18]
A
multitude of interesting borole derivatives have been
reported in the past few years,[19] which include metal-
containing,[20] tetrathienyl-substituted,[21] and the highly
Lewis-acidic perfluoropentaphenyl boroles.[22] In this context,
recent studies by Yamaguchi et al. on heteroarene-fused
boroles have provided more insight into the effect incorpo-
ration of the borole ring into extended p systems has on the
antiaromaticity of boroles.[5a,7] Despite the fact that a lower
antiaromatic character and higher stability was predicted for
compound III based on the electron-donating capability of
the thiophene unit, the fused borole showed a surprisingly
high antiaromatic character and air and moisture sensitivity.[7]
This is in contrast to the observations made for the
dibenzoborole derivative IV,[23] where the presence of the
benzene rings decreased the antiaromaticity of the borole
core.
With the remarkable properties of ladder, main group
element-containing p-systems and boroles in mind, we
focused our attention on the synthesis of hitherto unknown
bis-boron-bridged stilbene derivatives. Transmetalation of
Group 14 element-based heterocycles with RnBX3ꢀn is
a reliable method for making boracycles.[24] We thus first
attempted to synthesize the desired bis-boron-bridged stil-
bene core by treatment of Tamaoꢀs silole derivative I with
neat BBr3 (1008C, 48 h), but this approach failed. Therefore,
reductive routes akin to those pioneered by Tamao[10] were
employed in an effort to assemble the desired ladder diborole
framework. It was envisioned that reduction of the halobor-
ane-functionalized diaryl acetylene 1 would result in a cycli-
zation reaction to yield the targeted diborole.
Within the context of heteroacene chemistry, those based
on boron are an emerging class of materials that have been
used to prepare functional p-conjugated molecules for use as
organic light emitting diodes,[13] field effect transistors,[14] and
photovoltaics.[15] Arguably the most important characteristic
of a three-coordinate boron center situated in a p-conjugated
framework is the empty p-orbital, which allows for effective
delocalization of electrons throughout the whole system. The
transposition of carbon for the more electropositive boron
atom can also alter the HOMO–LUMO characteristics of
a compound significantly. For example, we found that 9-
boraanthracene derivatives II, stabilized by N-heterocyclic
The route to the precursor 1 is depicted in Scheme 1 and
relies on two consecutive boron–metal exchange reactions.
First, 2,2’-di-bromophenyl acetylene was lithiated at low
temperature with tBuLi then quenched with Me3SnCl. The
bis-dichloroborane was synthesized as an orange solid in 85%
yield by treating the distannane with an excess of BCl3.
Incorporation of the bulky aryl group was accomplished by
selective transmetalation of one chloro substituent per boron
with the 2,4,6-triisopropylphenyl (tipp) copper(I) reagent.[25]
Compound 1 was isolated as a colorless solid in 70% yield and
is a moisture-sensitive compound whose 11B NMR spectrum
[*] J. F. Araneda, B. Neue, Prof. W. E. Piers, M. Parvez
Department of Chemistry, University of Calgary
2500 University Drive N.W., Calgary, Alberta, T2N 1N4 (Canada)
E-mail: wpiers@ucalgary.ca
Homepage: http://www.chem.ucalgary.ca/research/groups/
wpiers/
[**] Funding for this work was provided by NSERC of Canada in the form
of a Discovery Grant to W.E.P. B.N. thanks the Deutsche
Forschungsgemeinschaft for financial support in the form of
a Postdoctoral Fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204367.
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cally less favored bis-benzocycloborabutylidene structural
isomer 2 (Figure 1). Despite the fact that the data were
collected at 123 K, the B-aryl groups were disordered;
nonetheless, the central part of the structure was free of any
Figure 1. Thermal ellipsoid (50%) diagrams of the molecular structure
of 2. Selected bond lengths [ꢀ], angles [8], and dihedral angles [8]: C1–
B1 1.575(5), C7–C7’ 1.330(7), C7–C6 1.485(5), C1–C6 1.416(5), C7–B1
1.581(5), C1-B1-C7 86.6(3), C1-B1-C8 138.3(3), C7-B1-C8 134.2(3),
C13-C8-B1-C7 67.6(6), C1-C6-C7-C7’ 179.7(2).
Scheme 1. Synthesis of bis-benzocycloborabutylidene 2 and bis-boron-
bridged stilbene 3.
disorder and the analysis unequivocally establishes the
connectivity in the molecular structure of 2 (more details
are given in the Supporting Information). The X-ray structure
of 2 revealed some interesting features. The dihedral angle
C1-C6-C7-C7’ is 179.7(2)8 and the sum of the angles around
boron is 359.18, suggesting a coplanar structure. The tipp
groups are slightly canted relative to the B2C6 plane with
a dihedral angle C13-C8-B1-C7 of 67.6(6)8. The trigonal
planar geometry about the boron centers is significantly
distorted due to the necessarily small C1-B1-C7 angle of
exhibits a signal at 66.3 ppm, characteristic of a 3-coordinate
boron center.[24a,26] Derivatives of 1 in which a less bulky aryl
group is incorporated (Ar= C6H5, 2,4,6-trimethylphenyl)
could also be prepared, but the subsequent reduction
chemistry was not clean in comparison to that observed for
the tipp-substituted haloborane, so here we only describe the
chemistry of 1.
=
Dihaloborane 1 was reacted with a freshly prepared
solution of potassium naphthalenide in THF at ꢀ788C.
Immediately, the solution changed from colorless to deep
purple. The resulting mixture was allowed to reach room
temperature over the course of two hours; the solution slowly
turned brownish yellow. After evaporation of the solvent and
removal of naphthalene by sublimation, a brown solid was left
in the flask, which was triturated with hexanes, allowing for
isolation of a bright yellow solid in 55% yield (Scheme 1).
This compound displayed green fluorescence when irra-
diated under UV light in both solution and solid state and
initially was presumed to be the desired ladder diborole 3.
Indeed, the spectral characteristics of the compound are fully
consistent with the expected structure. The 11B NMR spec-
trum shows a broad resonance at d = 70.1 ppm (in CD2Cl2)
and the 1H and 13C{1H} spectra are indicative of a C2h
symmetric structure, although only two out of three carbons
attached to boron were observable in the 13C NMR spectrum
due to quadrupole relaxation. However, it was observed that
the compound was both air and moisture sensitive and we
anticipated that 3 would be at least tolerant of ambient
atmosphere due to the steric protection offered by the tipp
groups. X-ray crystallographic analysis of single crystals
surprisingly revealed the structure to be the thermodynami-
86.6(3)8. The two boracycles are linked by a C C double bond
(1.330(7) ꢁ), and the boron atoms are found in a trans
arrangement about this double bond. All carbon atoms are
sp2-hybridized, which causes a planar structure and allows
conjugation throughout the whole system. Compounds with
a boracyclobutene moiety are extremely scarce[27] and the
strain associated with the core four-membered rings likely
contributes to the observed moisture sensitivity of the
compounds, despite the considerable steric protection of the
Lewis-acidic boron. Indeed, we were able to characterize the
product of hydrolysis of 2 by X-ray diffraction (structure is
provided in the Supporting Information), which shows that
ꢀ
the B1 C1 bonds are selectively cleaved when 2 reacts with
H2O.
Compound 2 is a structural isomer of the desired ladder
diborole 3, and DFT computations (see below) suggest that it
is significantly higher in energy than the fused diborole;
therefore, attempts to induce thermal isomerization were
made. No change in the 1H NMR spectrum was observed
after heating a toluene solution of 2 at 808C for 1 h.
Conversely, a facile photoisomerization process was observed
upon irradiation of solutions of 2 with 254 nm light (quartz
tube, CD2Cl2, 0.016 molmLꢀ1); use of higher wavelength light
(> 350 nm) did not effect the isomerization. The progress of
2
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the photoreaction was monitored by H NMR spectroscopy
179.6(3)8, respectively. Unlike 2, the tipp groups are essen-
(Figure 2), which revealed the clean formation of a new C2
symmetrical compound after ca. 10 h during which time the
solution changed from yellow to dark green in color.
tially perpendicular relative to the B2C6 plane with a dihedral
angle C1-B1-C8-C13 of ꢀ90.1(4)8. The bond lengths of the
borole ring in 3 differ only slightly compared to heteroarene-
fused boroles reported by Yamaguchi and co-workers.[5a,7]
Notably, there is a large bond alternation in the butadiene
ꢀ
ꢀ
moiety. The C1 C1’ and C2 C3 bond lengths are 1.367(5) and
ꢀ
1.419(13) ꢁ, respectively, while the C1’ C2 bond length is
1.480(4) ꢁ.
Both 2 and 3 have 14 p electrons and 3 may be expected to
exhibit Hꢂckel aromaticity; however, the borole rings at its
core are classical examples of antiaromatic rings. To probe the
aromatic/antiaromatic character of the p framework in 3, we
carried out NICS[28] calculations at the B3LYP/6-311 +
G(d)[29] level of theory. The NICS values for the five-
membered rings are indicative of antiaromaticity
(NICS(0) = 14.9, NICS(1) = 6.3, and NICS(1)zz = 21.8) and
the flanking benzene rings (NICS(0) = ꢀ3.2, NICS(1) = ꢀ6.2)
are less aromatic than benzene itself (NICS(0) = ꢀ9.7).[28a]
Thus, in this system, which by the Hꢂckel convention is
aromatic as a whole, the annulated aromatic benzene rings
stabilize its antiaromatic core.
The mechanism of the clean photoconversion of 2 to 3 is
not known with precision, but the Vis spectra and TD-DFT
calculations provide valuable insight into its character.
Compound 2 exhibits major absorptions at 263 nm (e =
25400) and 314 nm (e = 16000) and a weaker band at
459 nm (e = 8200); notably, selective irradiation of the lower
energy absorptions does not result in any conversion of 2 to 3.
Thus, it is the excitation at 263 nm that results in productive
photoreaction. TD-DFT calculations (B3LYP/6-31G(d), td =
(nstates = 20)) on 2 reproduce the experimental absorption
spectrum quite accurately and show that the absorptions are
(p!p*) in character. The lower energy transitions are mainly
comprised of the HOMO to LUMO transition; both orbitals
have large contributions from the p orbitals associated with
Figure 2. Photochemical reaction of 2 monitored by 1H NMR spectros-
copy (irradiated in a quartz NMR tube at 254 nm). The top spectrum
corresponds to the 1H NMR spectrum of 2 prior to irradiation; the
middle trace is the spectrum at partial conversion, with the peaks
marked by the * due to the intermediate; the bottom is the spectrum
after full conversion to 3. For full spectra, see the Supporting
Information.
Although the conversion was quantitative by NMR spectros-
copy, during the course of the reaction a third species (also
with C2 symmetry) was observed. The 11B NMR resonance at
68.6 ppm, was not significantly different from that observed
for 2. X-ray crystallographic analysis confirmed the final
product to be the desired bis-boron-bridged stilbene 3
(Figure 3).
=
The boron centers in 3 adopt a distorted trigonal planar
geometry with a C3-B1-C1 angle of 102.1(2)8. The bis-
benzoborole skeleton in 3 is completely planar, which can be
appreciated with the dihedral angles C1-C1’-C2-C3, C1’-C1-
B1-C3 and C1-C1’-C2-C7, equal to 0.1(4)8, 0.4(2)8 and
the central C C double bond (see Supporting Information).
However, the band at 263 nm (calcd 286 nm, oscillator
srength f = 0.2168) corresponds mainly to p!p* transitions
with contributions from HOMOꢀ6 to LUMO, but also
HOMO to LUMO+2 (Figure 4). As can be seen, both of
ꢀ
these populate antibonding p orbitals associated with the B
C bonds of the boracyclobutene rings, suggesting that photo-
induced homolytic bond cleavage is the key step in the
isomerization of 2 to 3.[30]
Scheme 2 summarizes our proposed mechanism and
suggestions for the nature of the observed transient inter-
mediate in the reaction; the numbers in square brackets are
the calculated relative energies of the species in kcalmolꢀ1
.
ꢀ
ꢀ
Not including the B CAr bond, there are two B C bonds
associated with the strained four-membered boracyclic rings
in 2, labeled a and b in Scheme 2. Cleavage of bond a allows
for formation of 3; unlike the product from homolytic
cleavage of bond b, which is presumably related to the
species formed by reduction of 1 (Scheme 1), the formation of
the two five-membered rings in 3 is aided by the presence of
Figure 3. Thermal ellipsoid (50%) diagrams of the molecular structure
of 3. Selected bond lengths [ꢀ], angles [8], and dihedral angles [8]: C1–
B1 1.571(4), C1–C1’ 1.367(5), C1’–C2 1.480(4), C2–C3 1.419(3), C3–B1
1.588(4), C3-B1-C1 102.1(2), C8-B1-C1 128.6(2), C8-B1-C3 129.2(2),
C1-B1-C8-C13 ꢀ90.1(4), C1-C1’-C2-C3 0.1(4).
=
a C C double rather than triple bond. This picture of the
reaction also allows for reasonable speculation as to the
nature of the minor intermediate observed in the reaction
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be expected that V would eventually convert to 3 as the
intermediate does. Another viable possibility for the identity
of this intermediate is the cis isomer of 2, VI in Scheme 2.
Photoisomerization of alkenes is well known and the energy
of VI is only 7 kcalmolꢀ1 higher than that of the isolated trans
isomer. Both of these structures are consistent with the
observed symmetry of the intermediateon the basis of its
1H NMR spectrum and both would be expected to fully
convert to 3 upon further irradiation, since the ladder
diborole isomer is substantially more stable than all three
compounds (2, V or VI).
In summary, we have described the synthesis of the
hitherto unknown bis-boron bridged stilbene 3, which was
prepared from the unexpected bis-boracyclobutylidene 2 by
a light-driven isomerization. Both compounds represent new
classes of boron-containing heterocyclic frameworks, which
were fully characterized. Work aimed at further understand-
ing the photochemistry, expanding the generality of the
methodology and reducing the ladder diboroles to the
dianions isoelectronic to the pentalenes[33] is underway.
Experimental Section
General experimental details, full characterization of all new
compounds and computational details are included in the Supporting
Information. CCDC 885080 (2), 885081 (3), and 885085 (hydrolysis
product of 2), contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Figure 4. Molecular orbitals relevant for the photoreaction: HOMOꢀ6,
HOMO, LUMO, LUMO+2 (isocontour value 0.02). Oscillator strength,
and wavelength.
Received: June 5, 2012
Published online: && &&, &&&&
Keywords: boroles · boron materials · DFT calculations ·
.
ladder compounds · photoisomerization
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Boron Heterocycles
Climbing the ladder: Reductive cycliza-
tion of alkynyl haloboranes lead to the bis-
benzocycloborabutylidene rather than the
expected ladder diborole, despite the
former being much less thermodynami-
cally favored. Photochemical conversion
to the ladder diborole was, however, quite
facile upon irradiation at 254 nm.
J. F. Araneda, B. Neue, W. E. Piers,*
M. Parvez
&&&& — &&&&
Photochemical Synthesis of a Ladder
Diborole: A New Boron-Containing
Conjugate Material
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!