Functionalized dibenzoborepins as components of small molecule and polymeric π-conjugated electronic materials

Article abstract of DOI:10.1021/jo2001726

We present the synthesis and characterization of dibenzo[b,f]borepins (DBBs) functionalized at the para and meta position with respect to the boron center in order to understand how regiochemical issues influence photophysical and electrochemical properti

Full text of DOI:10.1021/jo2001726

ARTICLE
pubs.acs.org/joc
Functionalized Dibenzoborepins as Components of
Small Molecule and Polymeric π-Conjugated Electronic Materials
Anthony Caruso, Jr.† and John D. Tovar*,†,‡
†Department of Chemistry and ‡Department of Materials Science and Engineering, Johns Hopkins University, Baltimore,
Maryland 21218, United States
S Supporting Information
b
ABSTRACT: We present the synthesis and characterization of
dibenzo[b,f]borepins (DBBs) functionalized at the para and meta
position with respect to the boron center in order to understand
how regiochemical issues influence photophysical and electroche-
mical properties. An expanded synthetic repertoire is presented,
using palladium catalysis (to perform Stille, Suzuki, Buchwald-
Hartwig, and Sonogashira cross-coupling reactions) and lithium-
halogen exchange to synthesize a series of extended π-conjugated
DBBs. These chemistries are enabled by the use of a sterically bulky
Mes* (2,4,6-tri-tert-butylphenyl) group on boron and the inclusion of reactive bromide handles on the DBB core. Photophysical,
electrochemical, and computational analyses of these compounds indicate that relative to the protio-DBB the installation of groups
at the meta positions decreases the optical band gap while para substitution raises the electron affinity of the system. Thus, both the
HOMO-LUMO gap and specific frontier molecular orbital levels can be tuned by the installation of different conjugated
substituents.
’ INTRODUCTION
Lateral substitutions have been achieved by tin/silicon-boron
exchange or by addition reactions between aryl lithiates and
dimesitylboron fluoride to form both small molecules and
side-borylated polymers such as Yamaguchi’s oligophenylene
ethynylenes12 that feature dimesitylborane side groups and
J€akle’s oligo- and polythiophenes bearing the same.9 End-capped
molecules have been prepared through similar chemistry to
incorporate boron-boron or boron-donor electronic interac-
tions such as Shirota’s dimesitylborane-capped oligothiophenes13
and M€ullen’s donor-acceptor ladder-type oligofluorenes.3b
In addition to incorporation through triaryl borane connectiv-
ities, boron has also been included as part of polycyclic aromatic
frameworks such as azabora-,14 borata-,15 and bora-annulenes16
(Figure 2). Azabora-annulenes are annulenes where one pair of
adjacent π-conjugated carbon centers is replaced with a boron-
nitrogen double bond with the most basic example being azaborine
(I), which is isoelectronic with benzene. Dewar initially synthe-
sized derivatives of azaborine in 1962;17 however, sufficient char-
acterization was difficult to obtain until the experimental studies
of Ashe18 and more recently Liu19 investigated the aromaticity,
reactivity, and optoelectronic properties. In recent years, strong
synthetic efforts out of the Piers laboratory led to the construc-
tion of a variety of more complex polycyclic aromatics such as
BdN variants of phenanthrene, pyrene, and triphenylene.14b-d
Borata-annulenes are anionic molecules in which one sp2 hybri-
dized carbon is replaced by an anionic boron center; such is the
Boron-containing π-conjugated materials have received con-
siderable attention in recent years due to their unique optical and
structural properties that result from the vacant p-orbital of the
boron center.1 The tricoordinate organoborane center is inher-
ently electron-deficient, which makes these boron-containing
materials amenable for use as electron-accepting units for
donor-acceptor nonlinear optical (NLO) materials.2 The Lewis
acidity of the vacant p-orbital makes boron an ideal material for
sensor applications;3 in fact, Gabbaï and M€ullen have used
organoboranes to selectively bind fluoride and cyanide anions.
These properties, along with the planar geometry of the boron
center, make organoboranes desirable components for construct-
ing tunable π-conjugated materials4 such as components for a
diverse array of colored organic light-emitting diodes (OLEDs).5
The majority of organoborane systems used in these applica-
tions incorporate tricoordinate boron into the π-system formally
as triaryl boranes through either direct main-chain conjugation,6-8
lateral substitution,5a,b,9 or as an end-capping unit3b,10 (Figure 1).
The former includes synthetic methodology employed by Chujo
such as hydroboration polymerization between a dihydroborane
and a suitable diyne forming poly(p-arylene-borane)s11 where
the boron center is directly incorporated into the main
π-conjugation pathway. More recently, J€akle has synthesized poly-
(fluorenyl B-bromoborane)s via the highly selective tin-boron
exchange between a bis(trimethylstannyl)fluorene and bis(dibro-
moboryl)fluorene followed by post-polymerization modification
by quenching with a variety of aryl nucleophiles to construct
electronically diverse triaryl boranes within the polymer chain.6
Received: January 24, 2011
Published: February 25, 2011
r
2011 American Chemical Society
2227
dx.doi.org/10.1021/jo2001726 J. Org. Chem. 2011, 76, 2227–2239
|

The Journal of Organic Chemistry
ARTICLE
reported a series of different benzo- and napthoborepins that
utilized mesityl (1,3,5-trimethylbenzene) as the boron substitu-
ent, which imparted enough stability to enable purification and
exposure to ambient environments. These molecules exhibited
blue photoluminescence with rather high quantum yields in
some cases suggesting potential application as light-emitting
components in OLEDs. In an associated paper,32 they describe
the synthesis and electronic characterization of new silepin
molecules as potential precursors to formation of borepins via
tin-silicon exchanges.
Figure 1. Different tricoordinate triarylborane architectures.
We extended the DBB conjugation pathway through the
vacant boron p-orbital by transition-metal-catalyzed cross-cou-
pling reactions onto chlorides placed para to the boron center.
Due to the rather low reactivity of the chlorides, installation of
π-electron groups onto the DBB core under even highly active
palladium catalysis was unsuccessful; however, nickel-catalyzed
Kumada cross-couplings with aryl Grignards led to aryl-coupled
DBBs. The kinetic protection afforded by the B-Mes* groups
(2,4,6-tri-tert-butylphenyl)33,23c allowed these molecules to with-
stand treatment with aggressive transmetalation reagents neces-
sary for doing Kumada-type cross-coupling chemistry. Despite
these initial advances in elaboration of the DBB scaffold, it will be
important for future materials development to have a more
general and reactive platform through which to do a diverse
array of chemical modifications for electronic property tuning.
Herein, we report the synthesis of brominated DBBs as well as
the design, synthesis, and characterization of functionalized
DBBs via expanded synthetic manipulations now available with
these borepin aryl bromides.
Figure 2. Boron-based heteroaromatics: 1,2-azaborine (I), borataben-
zene (II), 9-borafluorene (dibenzo[b,d]borole) (III), borepin (IV), and
dibenzo[b,f]borepin (V).
case for boratabenzene (II), which was first synthesized by
Herberich in 1970 and shown to be a suitable ligand for transition
metal complexes.20 Since the seminal publication on borata-
annulenes, Bazan, Ashe, and others have synthesized a variety of
larger systems including borataanthracene21 and boratanapthalene22
and explored their application to coordination chemistry. Bora-
annulenes are conjugated heterocycles where one carbon is
replaced with a neutral boron center as exemplified by the 4π-
electron formally anti-aromatic borole ring first synthesized by
Eisch and co-workers in 1969, which is isoelectronic with the
cyclopentadienyl cation. The unsubstituted borole ring is un-
stable; however, the perarylation or incorporation into larger
polycyclic aromatics such as 9-borafluorene (dibenzoborole, III)
allow for further functionalization and manipulation.23 In con-
trast to its carbon and nitrogen analogues (fluorene and carba-
zole, respectively), dibenzoborole has the potential of being an
electron-transporting material due to the electron deficiency of
the boron center.
’ RESULTS AND DISCUSSION
Design Considerations. In designing functionalized DBB
molecules there were two apparent positions where π-electron
groups could be installed that could lead to extended intramo-
lecular delocalization, meta or para to the boron center. The meta
isomer could facilitate the delocalization of π-electrons through
the stilbene-like carbon moiety by essentially isolating the boron
and precluding it from participating in charge delocalization.
However, installation of functional groups para to the boron
could make the boron center a more integral part of the
π-conjugation pathway. In order to better understand these
electronic issues, we synthesized brominated DBBs that allowed
us to prepare a series of π-conjugated para and meta isomers to
compare the effect of electron delocalization through the boron
center or the stilbene-like olefin.
One class of less-utilized boron-based building blocks for
π-electronic materials investigation is the borepins (IV), seven-
membered, 6π-electron neutral aromatic heterocycles that are
isoelectronic with the tropylium cation. Vol’pin initially sug-
gested that borepins would be H€uckel-type aromatic com-
pounds,24 and in the nearly 50 years following, various B-sub-
stituted borepins and benzoborepins were studied theoretically25,26
and experimentally.27-29 Ashe evaluated the aromaticity of the
borepin ring by X-ray crystal analysis, NMR and UV-vis
spectroscopy and determined that they are indeed aromatic as
predicted. Some years later, Schleyer as well as Schulman and
Disch performed extensive theoretical studies of borepin and its
benzo-annulated analogues spurring much interest in their de-
sign and synthesis. Unfortunately, the air and moisture sensitivity
of these molecules due to the unshielded vacant p-orbital on the
boron center hindered the investigation and application of
borepin-based materials, and their preparative chemistry has
been restricted to small molecule analogues rather than for
elaboration into extended π-conjugated systems unlike the other
boron-based examples described above.
Synthesis of para and meta Brominated DBBs. Brominated
DBBs 1a-b were targeted to allow for more facile cross-coupling
reactions due to the anticipated enhanced reativities of aryl
bromides under palladium catalyzed cross-coupling chemistries.
The syntheses of the brominated DBBs are shown in Scheme 1.
The common bromoiodobenzylbromide precursors 2a-b were
used to prepare phosphonium salts 3a-b via Arbuzov chemistry,
and N-methylmorpholine N-oxide mediated oxidations furn-
ished benzaldehydes 4a-b. The stilbenes were assembled by
To overcome these limitations, Piers30 and we31 indepen-
dently described the synthesis and characterization of a variety of
more robust molecules containing different flavors of the
dibenzo[b,f]borepin ring system (DBB, V) in an effort to better
understand the local aromaticity of the central borepin ring and
its involvement in the overall π-conjugated network. Piers
2228
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
Scheme 1. Synthesis of Dibenzo[b,f]borepins with para and
meta Bromides for Further Functionalization
Chart 1. B-Mes* Dibenzo[b,f]borepins Functionalized by
Stille (7a-b,e,f), Suzuki (7g), and Sonogashira (7d,h)
Cross-Coupling Reactions
Wittig reactions between 3 and 4 to form stilbenes 5a-b with a
Z:E selectivity of greater than 82%.34 Subsequently, chemo-
selective lithium-halogen exchange at the iodides of 5 followed
by in situ treatment of the dilithio species with dimethyltin
dichloride resulted in formation of the brominated stannepins
6a-b. Tin-boron exchange proved to be very capricious and
highly sensitive to any trace air and/or moisture present espe-
cially in the toluene. Tin-boron exchange between stannepin 6a
and boron trichloride furnished a B-chloro DBB that was treated
in situ with Mes*Li35 to form the para brominated B-Mes* DBB
1a in 92% yield; however, utilizing this procedure for the
synthesis of the meta-DBB 1b resulted in yields ranging from
6% to 14% with the remainder of the material being B-hydroxy
DBB formed through unavoidable oxidation of the intermediate
B-chloro DBB species. Since a large excess (5 equiv) of Mes*Li
was added, we presumed the B-chloro DBB intermediate formed
from 6b was not as reactive as that from 6a possibly due to the
electronics of the meta-brominated ring system. As a result,
boron tribromide was chosen for the tin-boron exchange with
6b followed by the addition of Mes*Li at room temperature.
These alterations increased the yield of 1b to 32%. Installation of
the Mes* group at the boron center rendered the DBBs stable
against air, moisture, and even nucleophiles such as fluoride, thus
allowing for standard synthetic manipulation and purification by
column chromatography under ambient conditions.
a7c was previously synthesized in our lab by Kumada cross-coupling.31
bYield determined by 1H NMR.
Scheme 2. Synthesis of para (8a) and meta (8b) DBB
polymers via Sonogashira polymerizations
Pd-Mediated Cross-Couplings. The meta and para bromi-
nated DBBs were subjected to a variety of palladium-catalyzed
cross-coupling reactions in order to tune electronic properties
(e.g., Stille, Suzuki, and Sonogashira; see Chart 1). These coupl-
ing methods required a broad range of cross-coupling partners
and reaction conditions, thus showing the robustness of the
borepin core. Stille couplings of stannylated thiophene and
bithiophene with 1a-b (in dimethyl formamide at 80 °C) led
to coupled products 7a,e and 7b,f respectively. The Suzuki
coupling of 4-methoxyphenyl boronic acid with 1b (in biphasic
toluene/aqueous potassium bicarbonate at reflux) led to the
corresponding aryl coupled product 7g, whereas Sonogashira
couplings of 1a-b with phenylacetylene (in toluene and diiso-
propylamine at 75 °C) furnished products 7d,h. The borepin
core survived all of these chemical exposures, and all coupling
products were handled and purified under ambient conditions.
These compounds also exhibited stability toward thermal de-
composition as seen by the heating of 7b to 130 °C for 45 min
1
without noticeable decomposition apparent by H NMR (see
Figure 13 in the Supporting Information).
Sonogashira coupling between the DBBs and trimethylsilyl
acetylene under standard conditions yielded para and meta TMS-
protected diethynyl DBBs in 97% and 64% yields, respectively
2229
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
(Scheme 2). Subsequent TMS deprotection with methanol and
potassium carbonate in THF gave terminal para and meta
acetylenes that were then polymerized under Sonogashira
cross-coupling conditions with 1,4-bis(decyloxy)-2,5-diiodoben-
zene36 to form poly(arylene)ethynylene polymers 8a and 8b,
respectively (Scheme 2). Molecular weight determination of the
polymers by gel permeation chromatography (GPC) revealed
reasonable polydispersity indexes of 2.45 (8a) and 1.93 (8b);
furthermore, both polymers are readily soluble in common
organic solvents (i.e., chloroform, toluene), which could enable
solution processing.
In order to further explore the stability of the DBBs toward
even more aggressive reaction conditions, para-DBB 1a was
subjected to Buchwald-Hartwig amination conditions. This
cross-coupling installed diphenylamine onto the DBB core by
palladium catalysis in the presence of sodium tert-butoxide in
toluene at 100 °C in 60% yield with no sign of borepin
decomposition (Scheme 3). Attempts to synthesize the meta
bis(diphenylamino) DBB from 1b resulted in protio-DBB and
starting materials suggesting that the proximity of the Mes* and
the meta bromides precluded the insertion of the diphenylamine
moiety due to steric restraints. DBB 7i has the general structure
of D-π-A-π-D, where D is an electron donor, π is a π-conjugated
bridge, and A is an acceptor, which has been shown to be an
efficient chromophore motif useful for two-photon absorption
processes.37 It might also be possible to use boron or nitrogen
centered redox activity to control intramolecular charge-transfer
processes within molecules such as 7i and analogues of the same
design.
Lithium-Halogen Exchange. We also demonstrated that
DBBs 1a-b could undergo lithium-halogen exchange with
s-BuLi in THF at -78 °C followed by quenching with an
electrophile, in this case solid carbon dioxide, to form diacid
DBBs 7j-k in 57% and 71% yields respectively (Scheme 3). This
general procedure should be applicable to install other functional
groups including aldehydes, which could then be used to
synthesize polymers or extended systems that incorporate ar-
ylene vinylene motifs through Wittig or Horner-Wadsworth-
Emmons olefinations.
Frontier Molecular Orbtial Calculations. We carried out
density functional theory (B3LYP/6-31G*) calculations for the
highest occupied molecular orbital (HOMO) and lowest un-
occupied molecular orbital (LUMO) of the DBB systems (using
B-2,6-dimethylphenyl rather than B-Mes*, Figure 3) with sub-
stituents attached at the meta and para positions to gain insight
into the effect of site-specific functionalization on the frontier
molecular orbital densities. These calculations revealed a node in
the HOMO located at the boron center for DBBs 7a-c and 7e-
g when functionalization was at the para position, effectively
dividing the HOMO in half, while the HOMO of the meta
substituted DBBs were continuous and fully delocalized onto the
substituents (representative surfaces shown in Figure 3). Only
the phenylacetylene functionalized DBBs 7d, h did not follow
Scheme 3. Exposure of Brominated DBBs to Aggressive
Reaction Conditions: Buchwald-Harwig Amination (leading
to 7i) and Lithium-Halogen Exchange (leading to 7j,k)
Figure 3. Structures (left) and DFT (B3LYP/6-31G*) Calculations Depicting the HOMO (center) and LUMO (right) Wave Functions of (a) para 7a
and meta 7e and (b) para 7d and meta 7h.
2230
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
Figure 4. Normalized UV-vis data for (a) para substituted DBBs 7a-d and (c) meta substituted DBBs 7e-h and normalized photoluminescence data
for (b) para substituted DBBs 7a-d and (d) meta substituted DBBs 7e-h. UV-vis and PL spectra were acquired in CHCl3 at room temperature. The
legends for panels a and b are shown in panel b, and those for panels c and d are shown in panel d.
this trend due to the phenyl group of the substituent being
rotated orthogonal to the DBB core; however, DFT calculations
done on the para 4-ethynylanisole-substituted DBBs exhibited
similar frontier molecular orbital surfaces as 7a-c. This suggests
that electron rich substituents might lead to more planar
molecules to optimize the charge-transfer interaction to the
boron center as evident in the large wave function densities
located on boron in the LUMO.
DBBs had red-shifted absorption and emission profiles with
respect to the protio-DBB. Unlike the Piers silepins, we observed
no significant solvatochromism (that is, no greater than 10 nm
shifts) for the corresponding borepins except for the strongly
donating diphenylamine substituted DBB 7i (see Figure 5 and
Table S1 in the Supporting Information), which showed a
fluorescence λmax red-shift of 65 nm on going from a nonpolar
solvent (cyclohexane) to a polar solvent (acetonitrile). While we
found negligible solvatochromism for the anisole functionalized
DBB 7c, the analogous silepin in the associated paper (p-1-
PhOMe)32 showed a 25 nm fluorescence red-shift in moving to
more polar solvents. This is not unexpected because the wave
functions for the borepin HOMOs and LUMOs are much more
equally distributed throughout the molecular core when com-
pared to the silepins.
The para polymer 8a had an onset of absorption at 449 nm and
a more intense band centered at 397 nm with higher energy
vibrational fine structure, whereas the meta polymer 8b onset was
red-shifted by 24 nm. The photoluminescence spectra for both
polymers were intense with quantum yields near 70% with the
maximum for meta polymer 8b observed at 462 nm being red-
shifted by 20 nm compared to that of the para polymer 8a.
Additionally, para polymer 8a had a <τ> of 0.92 ns, and meta
polymer 8b had a <τ> of 0.89 ns. It was suprising that both
polymers had near identical quantum yields and lifetimes.
Otherwise, these polymers exhibited the same photophysical
trends (Figure 6) as all of the previously discussed substituted
DBBs with the meta isomer having red-shifted absorption and
photoluminescence spectra with respect to the para isomer.
Electrochemical Characterization. Cyclic voltammetry (CV)
performed on the DBBs revealed reversible one-electron reduction
Optical Characterization. Solution-phase absorption, photo-
luminescence (PL), and lifetime data were measured to observe
how different substituents and conjugation paths through the
DBBs affected electronic properties. Figure 4 shows the optical
characterization data for the para and meta functionalized DBBs
7a-h which have also been compiled numerically in Table 1.
Representatively, the para-DBB 7a had a weak low-energy
charge-transfer band at 389 nm and more intense higher energy
π-π* signature centered at 315 nm. In contrast, the absorption
profile of the meta-DBB 7e did not show the weak charge-transfer
band and the π-π* transitions were red-shifted by over 50 nm.
The photoluminescence maximum observed at 401 nm (with a
shoulder at 419 nm) for 7a was sharp and at higher energy than
that of 7e, which exhibited a bimodal emission with maxima
centered at 422 and 441 nm. The para-DBB 7a had an average
weighted photoluminescence lifetime (<τ>) of 5.63 ns compared
to 2.75 ns for the meta-DBB 7e. The rest of the DBBs followed
the same trend (see Table 1) with red-shifted absorption and
emission profiles and shorter <τ> upon going from the para- to
meta-DBBs with a particular substituent; however, there were no
notable trends in the radiative (kr) and nonradiative (knr) rate
processes among the isomeric compounds surveyed. Regardless
of the regiochemistry of the substitution, all of the substituted
2231
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
peaks presumed to originate from the vacant p-orbitals of the boron
centers. Figure 7 shows representative CVs for the reversible
reduction of 7a (E1/2 = -2.15 V) and 7e (E1/2 = -2.26 V), which
shows that the reduction half-wave potential (E1/2) for the para-
substituted DBB was 110 mV less negative than the meta-DBB
(data compiled numerically in Table 2). This difference can be
rationalized by the fact that the para-DBB 7a includes the boron
center in the π-electron delocalization pathway resulting in a more
facile reduction, whereas the meta-DBB 7e extends linear conjuga-
tion through the carbon-carbon double bond moiety. In general,
the reduction potentials for all of the para-DBBs were less negative
than those for the corresponding meta-DBBs (see Table 2) with
exception of the phenyl acetylene substituted DBBs, which showed
identical reduction potentials as a result of the π-conjugating
phenyl groups being orthogonal to the DBB cores. Furthermore,
all substituted DBBs had more positive reduction potentials than
the protio-DBB (E1/2 = -2.54 V) suggesting that any substitution
on the ring regardless of regiochemistry helps to make the system
more reducible.
Table 1. Optical Properties of Functionalized B-Mes*
Dibenzo[b,f]borepinsa
abs λ,
knr,109
compound nm (log ε) PL λ, nm ΦF, % <τ>, ns kr, 109 ns-1
ns-1
With repeated cycles to ca. þ900 mV in the CV experiment,
monomer 7b eventually polymerized due to oxidation of the
bithiophene segment and subsequent follow-up oligomerization.
Under comparably positive applied potentials, monomers 7a, e,
and f appeared to oxidize irreversibly but did not polymerize or
otherwise deposit on the working electrode. The CVs recorded
during the polymerization of 7b and the subsequent polymer CV
of poly(7b) recorded in monomer-free electrolyte are shown in
Figure 8. Poly(7b) showed a reversible reduction with an E1/2
of -2.12 V (vs -2.10 for the monomer) and an anodic peak
potential (Epa) of 0.81 V. This indicates that the borepin cores
remain electronically isolated along the polymer; furthermore,
anodic polymerization is a fairly destructive technique com-
pared to chemical cross-coupling methods, and the borepin
appears to be no more susceptible to oxidative damage than do
typical electropolymerizable monomers. Attempts to chemi-
cally polymerize 7b via iron(III) chloride oxidation were
unsuccessful in our hands; however, it should be noted that
the failed attempts resulted in recovery of the monomer,
testifying to the stability of the DBB core under oxidizing
conditions.
Comparison to Dibenzo[b,f]silepins. Boron and silicon are
emerging as two attractive constituents for main-group func-
tional π-conjugated electronic materials. In an associated paper,
Mercier et al.32 present the synthesis of a variety of para and meta
functionalized dibenzo[b,f]silepins in parallel for comparison to
the DBBs reported here. The syntheses of their silicon-contain-
ing compounds utilized Wittig chemistry to form halogenated cis-
stilbene precursors that could be converted into the silepins by
quenching the appropriate dilithio stilbene with dimethylsilicon
dichloride. Similar to the DBBs, the silepin cores withstood a variety
of palladium-catalyzed cross-coupling conditions. The Piers
protio-
357 (3.56)
384
36
na
na
na
DBB12 340 (3.76)
321 (4.29)
309 (4.36)
350 (5.41)
337 (5.60)
315 (5.64)
388 (4.87)
339 (4.66)
7a
7b
401, 419 35
5.5
0.0636
0.118
434, 454 45
395, 411 38
0.63*
7.2
0.714
0.873
7c12
381 (3.55)
363 (3.84)
331 (4.77)
310 (4.86)
389 (3.46)
371 (3.63)
336 (4.81)
311 (4.77)
402 (4.12)
380 (4.23)
303 (4.01)
273 (4.23)
399 (4.35)
385 (4.32)
361 (4.51)
274 (4.68)
390 (3.79)
367 (3.92)
274 (3.93)
380 (4.42)
319 (3.59)
397 (na)
0.0528
0.0861
7d
403, 423 43
422, 441 39
462, 496 33
7.1
0.0606
0.139
0.0803
0.218
7e
2.8*
7f
7g
0.71*
5.7*
0.465
0.105
0.944
0.0701
425
60
7h
405. 428 99.8
1.6*
0.624
0.00125
7i
437
23
5.1
0.0451
0.750
0.151
0.337
8a
442, 470 69
0.92
339 (na)
304 (na)
410 (na)
8b
462, 491 64
0.89
0.720
0.404
a All lifetime data were fit to single exponential decays except for those
marked with an asterisk (*), which were double exponential.
Figure 5. UV-vis (a) and photoluminescence (b) data for 7i. UV-vis was acquired in CHCl3, and PL spectra were acquired in cyclohexane, CHCl3,
THF, and MeCN at room temperature, with excitation at 379.
2232
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
Figure 6. UV-vis and photoluminescence data for 8a (a) and 8b (b). UV-vis and PL spectra were acquired in CHCl3 at room temperature, with
excitation at 397 nm (a) and 410 nm (b).
Figure 7. Cyclic voltammetry of 7a (a) and 7e (b). CVs were acquired in 2.5 mM solutions of analyte in 0.1 M nBu4NPF6/THF and are reported relative
to Ag/Agþ.
Table 2. Electrochemical Properties of Functionalized
silepins, which is evidenced by the red-shifted absorption pro-
files. This suggests that the tricoordinate boron center is
crucial for complete electron delocalization in the para-DBBs
and imparts a more planar geometry relative to the boat-
shaped silepin cores. In the meta-DBBs and silepins, the boron
and silicon centers are not intimately involved in the conjuga-
tion pathway; however, they act to planarize the stilbene
portion of the molecule, which the trigonal planar boron does
more effectively than the tetrahedral silicon as witnessed by
the red-shifted onsets of absorption. The meta-silepins were
more easily reduced than the corresponding para-silepins;
however, this trend was opposite for the borepins indicat-
ing that π-conjugation to the vacant p-orbital stabilizes
the LUMOs.
B-Mes* Dibenzo[b,f]borepinsa
compound
abs λonset, nm
Eg, eV
E1/2, V
Ered, eV
protio-DBB12
358
400
491
392
400
426
510
416
412
418
449
473
3.46
3.10
2.53
3.16
3.10
2.91
2.43
2.98
3.01
2.97
2.76
2.62
-2.54
-2.15
-2.10
-2.23
-2.10
-2.26
-2.19
-2.40
-2.10
-2.23
na
-3.47
-3.06
-2.99
-3.12
-3.09
-3.14
-2.98
-3.24
-3.09
-3.32
na
7a
7b
7c12
7d
7e
7f
7g
7h
7i
8a
’ CONCLUSION
8b
na
na
a Eg values were calculated from λonset, and Ered values were calculated
from onset of the reduction wave and referenced to the energy level of
Fc/Fcþ taken as -4.8 eV.
We have reported a synthetic approach that enables the
formation of extended DBB systems through a variety of
palladium-catalyzed cross-coupling reactions including Suzuki,
Stille, Buchwald-Hartwig, and Sonogashira chemistries, as well
as the ability to install electrophiles after lithium-halogen
exchange. This has been made possible by the installation of
bromides on robust B-Mes* DBB cores and has led to the rapid
diversification with aryl, heteroaromatic, alkynyl, and carbonyl
substituents from common core scaffolds. These molecules could
strategy was to use these functionalized silepins as substrates for
borepin formation via silicon-boron exchange, and they demon-
strated one example of this transformation. In comparing the
photophysical properties, the para- and meta-DBBs all had
smaller HOMO-LUMO energy gaps than the corresponding
2233
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
Figure 8. Electrochemical polymerization data for poly(7b). (a) Growth curve showing multiple CV traces of monomer 7b and (b) cyclic
voltammagram of the resulting polymer. CVs were obtained in 0.1 M nBu4NPF6/THF with polymer growth done in a 2.5 mM solution of 7b. All
potentials are reported relative to Ag/Agþ.
also be included in π-conjugated polymer architectures through
chemical or even electrochemical polymerization techniques.
These synthetic investigations have allowed us to study how
the substitution patterns influence photophysical and electro-
chemical properties. We found that meta-substitution decreases
the optical band gap, whereas para-substitution raises the elec-
tron affinity of the system. This enables the selective tuning of the
HOMO and LUMO energy levels and wave functions of the
DBB system by altering the substituent and/or its location
around the DBB core. The ability to functionalize the DBB core
from a common dibromo DBB scaffold under a variety of reac-
tion conditions, which leads to a diverse array of functional
groups, makes it an exciting new π-conjugated building block for
application-specific electronic fine-tuning in future studies.
absorption spectra were recorded on a UV-vis spectrophotometer.
Photoluminescence spectra were recorded on a fluorometer with a 75 W
xenon lamp, maintaining optical densities below 0.1 au, and lifetime
data were collected on a generic LED system. Quantum yields were
determined relative to quinine sulfate in 0.1 N H2SO4 (55%).
Electrochemical Considerations. Cyclic voltammetry was per-
formed in a one-chamber, three-electrode cell using a potentiostat. A 2
mm2 Pt button electrode was used as the working electrode with a
platinum wire counter electrode relative to a quasi-internal Ag wire
reference electrode submersed in 0.01 M AgNO3/0.1 M n-Bu4NPF6 in
anhydrous acetonitrile. Measurements were taken on millimolar analyte
concentrations in 0.1 M n-Bu4PF6 (in THF) electrolyte solutions
recorded at a scan rate of 100 mV/s. Potentials are reported relative
to the Ag/Agþ couple with which the Fc/Fcþ couple was found to be
205 mV.
Computational Considerations. Molecular orbital calculations
were performed at the DFT level (B3LYP/6-31G*) on equilibrium
geometries using Spartan '04 (Wave Function Inc., Irvine, CA).
For reference, we used the numbering scheme for dibenzo-
[b,f]borepins as shown below.
’ EXPERIMENTAL SECTION
General. All reaction manipulations were carried out under an
atmosphere of prepurified nitrogen or argon using Schlenk techniques.
Nonaqueous solvents were degassed by sparging with nitrogen for 15
min prior to use, and toluene, diethyl ether, and THF were distilled over
sodium/benzophenone ketyl. Tetrakis(triphenylphosphine) palladium
was obtained from Strem Chemicals, and 32-63 μm silica gel was used.
1H NMR and 13C NMR spectra were obtained in deuterated chloroform
(the signal for residual protio solvent was set at 7.26 ppm for 1H NMR
and 77.16 ppm for 13C NMR), deuterated methylene chloride (the
signal for residual protio solvent was set at 5.32 ppm for 1H NMR and 54
ppm for 13C NMR), or deuterated dimethyl sulfoxide (the signal for
2,8-Dibromo-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,f]borepin (1a). A
solution of stannepin 6a (102 mg, 0.209 mmol) in toluene (2 mL) was
cooled to -78 °C in a dried 25 mL Schlenk tube under nitrogen. A solution
of BCl3 in hexane (1.0 M, 0.23 mL, 0.23 mmol) was added dropwise with
stirring after which the reaction mixture was allowed to slowly warm to
room temperature over 1 h. A solution of Mes*Li (265 mg, 1.05 mmol) in
toluene (3 mL) was added dropwise, and the resulting mixture was left
stirring at room temperature for 18 h. The reaction mix was partitioned
between 1:1 water/diethyl ether, and the organic phase was washed with
brine (2ꢀ). The aqueous layer was removed and extracted with diethyl
ether (2ꢀ), the combined organics were dried with MgSO4 and filtered,
1
residual protio solvent was set at 2.50 ppm for H NMR and 39.52
ppm for 13C NMR) using a 400 MHz FT-NMR spectrometer. All efforts
to obtain 13C NMR were unsuccessful due to the low solubility of the
functionalized DBBs (overnight scans did not result in spectra with
reasonable signal-to-noise ratios) and quadrupolar relaxation of boron.
Mass spectra were obtained with EI and FAB ionization (matrix for FAB
was 3-nitrobenzyl alcohol), and GPC analyses were done by Aurora
Analytics, LLC, Baltimore, MD.
Photophysical Considerations. Spectroscopic measurements
were conducted in CHCl3 solution at room temperature. UV-vis
2234
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
and the solvent was removed under reduced pressure to a yellow oil that was
further purified by column chromatography (SiO2, hexane) to yield 114 mg
(0.193 mmol, 92%) of 1a as an off-white solid. H NMR (400 MHz,
CD2Cl2) δ 7.84 (m, 4H), 7.50 (s, 2H), 7.47 (dd, 2H, J = 8.3, 2.0 Hz), 7.11
(s, 2H), 1.42 (s, 9H), 0.94 (s, 18H); 13C NMR (100 MHz, CD2Cl2)
δ 151.1, 149.1, 143.9, 143.3, 139.2, 135.4, 134.1, 130.7, 130.5, 126.7, 123.5,
38.7, 35.3, 31.5, 30.1; HRMS (FAB) m/z calcd for C32H37BBr2 [Mþ]
590.1355, found 590.1314.
room temperature after which precipitates were filtered off and washed
with hexane. The filtrate was dried with MgSO4 and filtered, and the
solvent was removed under reduced pressure to provide an orange solid
that was then rinsed with cold methanol, yielding 3.27 g (8.75 mmol,
66%) of an off-white solid that was used without further purification.
1H NMR (400 MHz, CDCl3) δ 8.01 (d, 1H, J = 2.0 Hz), 7.47 (dd, 1H, J
= 8.3, 2.0 Hz), 7.33 (d, 1H, J = 8.3 Hz), 4.54 (s, 2H); HRMS (EI) m/z
calcd for C7H5Br2I [Mþ] 377.7762, found 377.7759.
1
3,7-Dibromo-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,f]borepin (1b). A
solution of stannepin 6b (80.4 mg, 0.166 mmol) in toluene (2 mL) was
cooled to -78 °C in a dried 25 mL Schlenk tube under nitrogen. A solution
of BBr3 in hexane (1.0 M, 0.18 mL, 0.18 mmol) was added dropwise with
stirring after which the reaction mixture was allowed to slowly warm to
room temperature over 1 h. A solution of Mes*Li (209 mg, 0.829 mmol) in
toluene (2 mL) was added dropwise, and the resulting mixture was left
stirring at room temperature for 18 h. The reaction mix was partitioned
between 1:1 water/diethyl ether, and the organic phase was washed with
brine (2ꢀ). The aqueous layer was removed and extracted with diethyl
ether (2ꢀ), the combined organics were dried with MgSO4 and filtered,
and the solvent was removed under reduced pressure to an off-white solid
that was further purified by column chromatography (SiO2, hexane) to yield
5-Bromo-2-iodobenzyltriphenylphosphonium Bromide (3a). A so-
lution of benzyl bromide 2a (3.74 g, 9.96 mmol) in DMF (10 mL) was
stirred at room temperature in a dried 25 mL round-bottom under
nitrogen. Triphenyl phosphine (2.9 g, 11 mmol) was added portionwise
at room temperature, and the resulting reaction mixture was allowed to
stir overnight. After 18 h, the reaction mixture was poured into toluene
and the resulting suspension was filtered. The solid was added to diethyl
ether rinsing with a minimal amount of DCM as necessary. The resulting
suspension was filtered yielding 6.18 g (9.68 mmol, 97%) of 3a as an off-
1
white solid. H NMR (400 MHz, CDCl3) δ 7.74 (m, 15H), 7.53 (m,
2H), 7.10 (dt, 1H, J = 8.4, 2.4 Hz), 5.84 (d, 2H, J = 14.5 Hz); HRMS
(FAB) m/z calcd for C25H20BrIP [Mþ-79Br] 556.9531, found 556.9527.
4-Bromo-2-iodobenzyltriphenylphosphonium Bromide (3b). A solu-
tion of benzyl bromide 2b (2.42 g, 6.45 mmol) in DMF (6 mL) was stirred
at room temperature in a dried 25 mL round-bottom under nitrogen.
Triphenyl phosphine (1.9 g, 7.1 mmol) was added portionwise at room
temperature, and the resulting reaction mixture was allowed to stir over-
night. After 18 h, the reaction mixture was poured into toluene, and the
resulting suspension was filtered. The solid was added to diethyl ether,
rinsing with a minimal amount of DCM as necessary. The resulting
suspension was filtered, yielding 3.73 g (5.85 mmol, 91%) of 3b as an
off-white solid. 1H NMR (400 MHz, CD2Cl2) δ 7.87 (m, 4H), 7.69 (m,
12H), 7.37 (m, 2H), 5.55 (d, 2H, J = 14.3 Hz); HRMS (FAB) m/z calcd for
1
31.5 mg (0.053 mmol, 32%) of 1b as an off-white solid. H NMR (400
MHz, CD2Cl2) δ 8.06 (d, 2H, J = 2.3 Hz), 7.70 (dd, 2H, J = 8.3, 2.3 Hz),
7.54 (m, 4H), 7.16 (s, 2H), 1.45 (s, 9H), 0.97 (s, 18H); 13C NMR (100
MHz, CDCl3) δ 150.8, 149.1, 145.7, 145.64, 145.62, 144.1, 140.2, 134.6,
134.5, 133.5, 132.0, 128.2, 123.2, 121.8, 38.6, 35.3, 31.6, 29.9; HRMS (FAB)
m/z calcd for C32H37BBr2 [Mþ] 590.1355, found 590.1363.
4-Bromo-2-iodotoluene. A solution of acetic acid (100 mL) and
acetic anhydride (50 mL) was cooled to 0 °C in a 500 mL 3-neck round-
bottom under nitrogen with stirring. NaIO4 (15.4 g, 71.9 mmol) and I2
(12.2 g, 48.1 mmol) were added portionwise at 0 °C after which H2SO4
(21 mL) was added dropwise while maintaining a temperature of 0-
5 °C. Bromotoluene (23.1 g, 135 mmol) was added portionwise at 0 °C,
and after 2 h the resulting mixture was allowed to warm to room
temperature and stir overnight. The reaction mixture was poured into
10% NaHCO3 (aq) (200 mL) and ice (250 g) and extracted with
methylene chloride (3ꢀ). The combined organics were washed with
water (3ꢀ), dried with MgSO4, and filtered, and the solvent was
removed under reduced pressure to provide a reddish oil that was
further purified by vacuum distillation (92 °C at 1 mmHg) yielding 30 g
(10 mmol, 66%) of 4-bromo-2-iodotoluene as a colorless oil. 1H NMR
(400 MHz, CDCl3) δ 7.92 (s, 2H), 7.34 (d, 2H, J = 8.1 Hz), 7.08 (d, 2H,
J = 8.1 Hz), 2.36 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 140.8, 131.3,
130.8, 119.6, 101.5, 27.8; HRMS (EI) m/z calcd for C7H6BrI [Mþ]
295.8698, found 295.8696
5-Bromo-1-(bromomethyl)-2-iodobenzene (2a). A solution of
5-bromo-2-iodotoluene (6.75 g, 22.7 mmol) and 1,2-dichloroethane
(25 mL) was stirred at room temperature in a 50 mL two neck round-
bottom under nitrogen. Benzoyl peroxide (281 mg, 1.14 mmol) and
NBS (4.5 g, 25 mmol) were added at once, and the resulting mixture was
heated to reflux and irradiated with a 250 W incandescent flood lamp (12
in away from round-bottom) for 5 h. The reaction mixture was allowed
to cool to room temperature after which precipitates were filtered off and
rinsed with hexane. The filtrate was dried with MgSO4 and filtered, and
the solvent was removed under reduced pressure to provide an orange
solid that was then rinsed with cold methanol, yielding 3.99 g (10.6
mmol, 47%) of an off-white solid that was used without further
purification. Characterization data matched that found in literature.38
4-Bromo-1-(bromomethyl)-2-iodobenzene (2b). A solution of
4-bromo-2-iodotoluene (3.94 g, 13.3 mmol) and 1,2-dichloroethane
(19 mL) was stirred at room temperature in a 50 mL two neck round-
bottom under nitrogen. Benzoyl peroxide (164 mg, 0.663 mmol) and
NBS (2.63 g, 14.6 mmol) were added at once, and the resulting mixture
was heated to reflux for 5 h. The reaction mixture was allowed to cool to
C25H20BrIP [Mþ
-
79Br] 556.9531, found 556.9535.
5-Bromo-2-iodobenzaldehyde (4a). A solution of N-methyl mor-
pholine-N-oxide (870 mg, 7.2 mmol) and 4 Å sieves (5.8 g) in
acetonitrile (18 mL) in a dried 250 mL Schlenk flask under nitrogen
was cooled to 0 °C, and benzyl bromide 2a (880 mg, 2.3 mmol) was
added at once. The reaction mixture was held at 0 °C for 2 h before
warming to room temperature. The reaction mixture was then filtered
through a short plug (SiO2, hexane) to yield 761 mg (2.44 mmol, 99%)
of 4a as an off-white solid. Characterization data matches that found in
literature.39
4-Bromo-2-iodobenzaldehyde (4b). A solution of N-methyl mor-
pholine-N-oxide (2.81 g, 23.2 mmol) and 4 Å sieves (19 g) in acetonitrile
(60 mL) in a dried 250 mL Schlenk flask under nitrogen was cooled to
0 °C, and benzyl bromide 2b (2.9 g, 7.7 mmol) was added at once. The
reaction mixture was held at 0 °C for 2 h before warming to room
temperature. The reaction mixture was then filtered through a short plug
(SiO2, hexane) to yield 2.15 g (6.92 mmol, 90%) of 4b as an off-white
solid. 1H NMR (400 MHz, CDCl3) δ 9.99 (s, 1H), 8.13 (d, 1H, J = 0.8
Hz), 7.73 (dd, 1H, J = 8.3, 0.8 Hz), 7.61 (dd, 1H, J = 8.3, 0.8 Hz); 13C
NMR (100 MHz, CDCl3) δ 194.7, 142.8, 134.1, 132.3, 131.1, 130.2,
100.9; HRMS (EI) m/z calcd for C7H4BrIO [Mþ] 309.8490, found
309.8489.
(Z)-1,2-Bis(5-bromo-2-iodophenyl)ethene (5a). A solution of phos-
phonium salt 3a (3.84 g, 6.02 mmol) in THF (60 mL) was cooled to
0 °C in a 250 mL Schlenk flask under nitrogen with stirring. Potassium
tert-butoxide (815 mg, 7.12 mmol) was added portionwise, and the
resulting mixture was held at 0 °C for 30 min. A solution of benzaldehyde
4a (1.7 g, 5.5 mmol) in THF (60 mL) was added dropwise at 0 °C, and
the reaction was allowed to warm to room temperature and left stirring
for 24 h. The reaction mix was partitioned between 1:1 water/diethyl
ether. The aqueous layer was removed and extracted with diethyl ether
(3ꢀ), the organics were dried with MgSO4 and filtered, and the solvent
was removed under reduced pressure to provide an off-white solid that
2235
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
was further purified by a short plug (SiO2, 10% ethyl acetate in hexane)
to yield 2.95 g (4.99 mmol, 91%) 82:18 Z:E of 5a as an off-white solid
that was used without further purification. 1H NMR (400 MHz, CDCl3)
δ 7.70 (d, 2H, J = 8.4 Hz), 7.05 (dd, 2H, J = 8.4, 2.4 Hz), 7.02 (d, 2H, J =
2.4 Hz), 6.58 (s, 2H); HRMS (FAB) m/z calcd for C14H8Br2I2 [Mþ]
587.7082, found 587.7067.
2,8-Di(thiophen-2-yl)-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,f]borepin
(7a). A solution of borepin 1a (28 mg, 0.047 mmol) and Pd(PPh3)4
(3.0 mg, 0.026 mmol) in DMF (1.5 mL) was stirred in a dry 25 mL
Schlenk tube under nitrogen. 2-Tributylstannyl thiophene (55 mg,
0.14 mmol) was added dropwise, and the resulting mixture was heated
to 80 °C for 18 h. Upon cooling, the reaction was diluted with diethyl
ether and stirred vigorously with KF (2ꢀ). The filtered organic layer was
then partitioned between 1:1 water/diethyl ether. The aqueous layer was
removed and extracted with diethyl ether (3ꢀ), and the organics were
washed with brine (2ꢀ). The combined organics were dried with
MgSO4 and filtered, and the solvent was removed under reduced
pressure to provide a yellow oil that was further purified by column
chromatography (SiO2, 1% ethyl acetate in hexane) to yield 15 mg
(Z)-1,2-Bis(4-bromo-2-iodophenyl)ethene (5b). A solution of phos-
phonium salt 3b (2.96 g, 4.64 mmol) in THF (42 mL) was cooled to
0 °C in a 100 mL Schlenk flask under nitrogen with stirring. Potassium
tert-butoxide (628 mg, 5.49 mmol) was added portionwise, and the
resulting mixture was held at 0 °C for 30 min. A solution of benzaldehyde
4b (1.31 g, 4.22 mmol) in THF (40 mL) was added dropwise at 0 °C,
and the reaction was allowed to warm to room temperature and left
stirring for 24 h. The reaction mix was partitioned between 1:1 water/
diethyl ether. The aqueous layer was removed and extracted with diethyl
ether (3ꢀ), the organics were dried with MgSO4 and filtered, and the
solvent was removed under reduced pressure to provide an off-white
solid that was further purified by a short plug (SiO2, 5% ethyl acetate in
hexane) to yield 2.34 g (3.98 mmol, 94%) 85:15 Z:E of 5b as an off-white
1
(0.025 mmol, 53%) of 7a as an off-white solid. H NMR (400 MHz,
CD2Cl2) δ 7.99 (d, 2H, J = 7.9 Hz), 7.92 (d, 2H, J = 1.6 Hz), 7.61 (dd,
2H, J = 8.2, 1.9 Hz), 7.52 (m, 4H), 7.39 (dd, 2H, J = 5.1, 1.0 Hz), 7.27 (s,
2H), 7.14 (m, 2H), 1.45 (s, 9H), 1.00 (s, 18H); MS (FAB) m/z calcd for
C40H43BS2 [Mþ] 598.3, found 598.3.
2,8-Di([2,20-bithiophen]-5-yl)-5-(2,4,6-tri-tert-butylphenyl)dibenzo-
[b,f]borepin (7b). A solution of borepin 1a (32.8 mg, 0.0554 mmol) and
Pd(PPh3)4 (3.2 mg, 0.0028 mmol) in DMF (1.5 mL) was stirred in a
dry 25 mL Schlenk tube under nitrogen. 2-Tributylstannyl thiophene
(73 mg, 0.16 mmol) was added dropwise, and the resulting mixture was
heated to 80 °C for 18 h. Upon cooling, the reaction was diluted with
diethyl ether and stirred vigorously with KF (2ꢀ). The filtered organic
layer was then partitioned between 1:1 water/diethyl ether. The aqueous
layer was removed and extracted with diethyl ether (3ꢀ), and the
organics were washed with brine (2ꢀ). The combined organics were
dried with MgSO4 and filtered, and the solvent was removed under
reduced pressure to provide an orange solid that was further purified by
rinsing with methanol to yield 31 mg (0.041 mmol, 73%) of 7b as an
orange solid. 1H NMR (400 MHz, CDCl3) δ 8.03 (d, 2H, J = 8.1 Hz),
7.87 (d, 2H, J = 1.8 Hz), 7.60 (dd, 2H, J = 8.1, 1.8 Hz), 7.52 (s, 2H), 7.41
(d, 2H, J = 3.8 Hz), 7.25 (m, 4H), 7.21 (d, 2H, J = 3.8 Hz), 7.08 (m, 4H),
1.47 (s, 9H), 1.02 (s, 18H); HRMS (FAB) m/z calcd for C48H47BS4
[Mþ] 762.2654, found 762.2674.
1
solid that was used without further purification. H NMR (400 MHz,
CDCl3) δ 7.99 (d, 2H, J = 2.0 Hz), 7.21 (dd, 2H, J = 8.3, 2.0 Hz), 6.73
(d, 2H, J = 8.3 Hz), 6.55 (s, 2H); HRMS (FAB) m/z calcd for
C14H8Br2I2 [Mþ] 587.7082, found 587.7087.
2,8-Dibromo-5,5-dimethyldibenzo[b,f]stannepin (6a). A solution of
5a (1.51 g, 2.05 mmol) in diethyl ether (70 mL) was cooled to -78 °C in
a 200 mL Schlenk flask under nitrogen with stirring. A solution of n-butyl
lithium in hexanes (1.73 M, 2.61 mL, 4.52 mmol) was added dropwise,
and the resulting mixture was held at -78 °C for 10 min followed by the
addition of TMEDA (525 mg, 4.52 mmol). The reaction mixture was
held at -78 °C for 2 h after which a solution of dimethyltin dichloride
(451 mg, 2.05 mmol) in THF (5 mL) was added dropwise. The resulting
mixture was allowed to warm to room temperature and left to stir for
18 h. The reaction mixture was partitioned between 1:1 water/diethyl
ether. The aqueous layer was removed and extracted with diethyl ether
(3ꢀ), and the organics were washed with 0.2 M HCl (1ꢀ) and water
(1ꢀ). The combined organics were dried with MgSO4 and filtered, and
the solvent was removed under reduced pressure to provide a yellow oil
that was further purified by column chromatography (SiO2, hexane) to
yield 476 mg (0.982 mmol, 60%) of 6a as a white solid. 1H NMR (400
MHz, CDCl3) δ 7.47 (d, 2H, J = 1.7 Hz), 7.38 (dd, 2H, J = 7.8, 1.7 Hz),
7.29 (d, 2H, J = 7.8 Hz), 6.84 (s, 2H), 0.51 (s, 6H); 13C NMR (100 MHz,
CDCl3) δ 144.9, 140.3, 136.1, 134.0, 131.9, 130.3, 123.3, -11.3; HRMS
(FAB) m/z calcd for C16H15Br2Sn [MHþ] 484.8536, found 484.8553.
3,7-Dibromo-5,5-dimethyldibenzo[b,f]stannepin (6b). A solution of
5b (501 mg, 0.848 mmol) in diethyl ether (42 mL) was cooled to -
78 °C in a 100 mL Schlenk flask under nitrogen with stirring. A solution
of n-butyl lithium in hexanes (1.73 M, 1.1 mL, 1.9 mmol) was added
dropwise, and the resulting mixture was held at -78 °C for 10 min
followed by the addition of TMEDA (0.217 mg, 1.86 mmol). The
reaction mixture was held at -78 °C for 2 h after which a solution of
dimethyltin dichloride (187 mg, 0.848 mmol) in THF (5 mL) was added
dropwise. The resulting mixture was allowed to warm to room tem-
perature and left to stir for 18 h. The reaction mixture was partitioned
between 1:1 water/diethyl ether. The aqueous layer was removed and
extracted with diethyl ether (3ꢀ), and the organics were washed with
0.2 M HCl (1ꢀ) and water (1ꢀ). The combined organics were dried
with MgSO4 and filtered, and the solvent was removed under reduced
pressure to provide a yellow oil that was further purified by column
chromatography (SiO2, hexane) to yield 254 mg (0.525 mmol, 73%) of
6b as a white solid. 1H NMR (400 MHz, CD2Cl2) δ 7.56 (d, 2H, J = 2.2
Hz), 7.41 (dd, 2H, J = 8.3, 2.2 Hz), 7.19 (d, 2H, 8.3 Hz), 6.86 (s, 2H),
0.54 (s, 6H); 13C NMR (100 MHz, CD2Cl2) δ 145.0, 142.4, 137.5,
134.1, 131.9, 131.3, 123.1, -11.0; HRMS (FAB) m/z calcd for
C16H15Br2Sn [MHþ] 484.8536, found 484.8526.
2,8-Bis(phenylethynyl)-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,f]-
borepin (7d). A solution of borepin 1a (30 mg, 0.05 mmol), CuI (1 mg,
0.005 mmol) and Pd(PPh3)4 (2.9 mg, 0.0025 mmol) in toluene (1 mL)
and DIPA (0.2 mL) was stirred in a dried 25 mL Schlenk flask under
nitrogen. Phenyl acetylene (14 mg, 0.13 mmol) was added dropwise at
room temperature, and the resulting reaction mixture was heated to
75 °C for 18 h. The reaction mixture was allowed to cool to room
temperature after which it was partitioned between water and diethyl
ether and washed with NH4Cl (2ꢀ) and brine (2ꢀ). The aqueous layer
was removed and extracted with diethyl ether (3ꢀ), the combined
organics were dried with MgSO4 and filtered, and the solvent was
removed under reduced pressure to provide a brown solid that was
further purified by column chromatography (SiO2, 1% ethyl acetate in
hexane) to yield 18.2 mg (0.0287 mmol, 56%) of 7d as an off-white solid.
1H NMR (400 MHz, CD2Cl2) δ 7.99 (d, 2H, J = 8.0 Hz), 7.85 (s, 2H),
7.58 (m, 4H), 7.54 (s, 2H), 7.49 (d, 2H, J = 8.0 Hz), 7.39 (m, 6H), 7.21
(s, 2H), 1.45 (s, 9H), 0.99 (s, 18H); HRMS (FAB) m/z calcd for
C48H47B [Mþ] 634.3771, found 634.3784.
3,7-Di(thiophen-2-yl)-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,f]-
borepin (7e). A solution of borepin 1b (7.8 mg, 0.013 mmol) and
Pd(PPh3)4 (1.0 mg, 0.0009 mmol) in DMF (1 mL) was stirred in a dry
25 mL Schlenk tube under nitrogen. 2-Tributylstannyl thiophene
(21 mg, 0.056 mmol) was added dropwise, and the resulting mixture
was heated to 80 °C for 18 h. Upon cooling, the reaction was diluted with
diethyl ether and stirred vigorously with KF (2ꢀ). The filtered organic
layer was then partitioned between 1:1 water/diethyl ether. The aqueous
layer was removed and extracted with diethyl ether (3ꢀ), and the
organics were washed with brine (2ꢀ). The combined organics were
2236
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
dried with MgSO4 and filtered, and the solvent was removed under
reduced pressure to provide a yellow solid that was further purified by
column chromatography (SiO2, 5% ethyl acetate in hexane) to yield 7.9
mg (0.013 mmol, quant) of 7e as an off-white solid. 1H NMR (400 MHz,
CD2Cl2) δ 8.30 (d, 2H, J = 2.2 Hz), 7.85 (dd, 2H, J = 8.2, 2.2 Hz), 7.68
(d, 2H, J = 8.2 Hz), 7.58 (s, 2H), 7.24 (dd, 2H, J = 5.1, 1.1 Hz), 7.20 (m,
4H), 7.03 (dd, 2H, J = 5.1, 3.6 Hz), 1.48 (s, 9H), 0.97 (s, 18H); HRMS
(FAB) m/z calcd for C40H43BS2 [Mþ] 598.2899, found 598.2906.
3,7-Di([2,20-bithiophen]-5-yl)-5-(2,4,6-tri-tert-butylphenyl)dibenzo-
[b,f]borepin (7f). A solution of borepin 1a (11.2 mg, 0.0189 mmol) and
Pd(PPh3)4 (1.5 mg, 0.051 mmol) in DMF (1 mL) was stirred in a
dry 25 mL Schlenk tube under nitrogen. 2-Tributylstannyl thiophene
(38 mg, 0.083 mmol) was added dropwise, and the resulting mixture was
heated to 80 °C for 18 h. Upon cooling, the reaction was diluted with
diethyl ether and stirred vigorously with KF (2ꢀ). The filtered organic
layer was then partitioned between 1:1 water/diethyl ether. The aqueous
layer was removed and extracted with diethyl ether (3ꢀ), and the
organics were washed with brine (2ꢀ). The combined organics were
dried with MgSO4 and filtered, and the solvent was removed under
reduced pressure to provide an orange solid that was further purified by
rinsing with methanol to yield 8.0 mg (0.01 mmol, 56%) of 7f as an
orange solid. 1H NMR (400 MHz, CD2Cl2) δ 8.29 (d, 2H, J = 2.1 Hz),
7.84 (dd, 2H, J = 8.2, 2.1 Hz), 7.70 (d, 2H, J = 8.2 Hz), 7.61 (s, 2H),
7.152 (m, 12H), 1.51 (s, 9H), 1.01 (s, 18H); HRMS (FAB) m/z calcd
for C48H47BS4 [Mþ] 762.2654, found 762.2670.
3,7-Bis(4-methoxyphenyl)-5-(2,4,6-tri-tert-butylphenyl)dibenzo-
[b,f]borepin (7g). A solution of borepin 1b (13.2 mg, 0.0223 mmol),
4-methoxyphenyl boronic acid (11 mg, 0.072 mmol), Na2CO3 (24.4 mg,
0.231 mmol) and Pd(PPh3)4 (1.6 mg, 0.0014 mmol) in toluene (1 mL),
water (0.3 mL) and ethanol (0.3 mL) was stirred in a 5 mL round-
bottom equipped with a reflux condenser under nitrogen. The resulting
mixture was heated to reflux for 18 h after which it was allowed to cool to
room temperature. The reaction mixture was partitioned between water
and diethyl ether and washed with NH4Cl (2ꢀ) and brine (2ꢀ). The
aqueous layer was removed and extracted with diethyl ether (3ꢀ), the
combined organics were dried with MgSO4 and filtered, and the solvent
was removed under reduced pressure to provide a brown solid that was
further purified by column chromatography (SiO2, 5% ethyl acetate in
hexane) to yield 11.3 mg (0.0175 mmol, 79%) of 7g as an off-white solid.
1H NMR (400 MHz, CD2Cl2) δ 8.25 (d, 2H, J = 2.1 Hz), 7.84 (dd, 2H, J =
8.2, 2.1 Hz), 7.73 (d, 2H, J = 8.2 Hz), 7.55 (s, 2H), 7.41 (d, 4H, J = 8.8 Hz),
7.23 (s, 2H), 6.89 (d, 4H, J = 8.8 Hz), 1.46 (s, 9H), 0.98 (s, 18H); HRMS
(FAB) m/z calcd for C46H51BO2 [Mþ] 646.3982, found 646.4002.
3,7-Bis(phenylethynyl)-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,
(s, 2H), 1.46 (s, 9H), 1.00 (s, 18H); HRMS (FAB) m/z calcd for
C48H47B [Mþ] 634.3771, found 634.3780.
2,8-Bis(diphenylamino)-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,f]-
borepin (7i). A solution of borepin 1a (19.4 mg, 0.0328 mmol),
diphenylamine (13.6 mg, 0.0786 mmol), NaOtBu (9.1 mg, 0.092 mmol),
Pd2(dba)3 (0.95 mg, 0.0016 mmol) and P(o-tolyl)3 (1.1 mg, 0.0033
mmol) in toluene (1 mL) was stirred in a dried 25 mL Schlenk flask
under nitrogen, and the resulting reaction mixture was heated to 100 °C
for 18 h. The reaction mixture was allowed to cool to room temperature
after which it was partitioned between water and diethyl ether and
washed with brine (2ꢀ). The aqueous layer was removed and extracted
with diethyl ether (3ꢀ), the combined organics were dried with MgSO4
and filtered, and the solvent was removed under reduced pressure to
provide a orange solid that was further purified by rinsing with MeOH to
yield 15 mg (0.019 mmol, 60%) of 7i as tan solid. 1H NMR (400 MHz,
CD2Cl2) δ 7.72 (d, 2H, J = 8.6 Hz), 7.45 (s, 2H), 7.30 (t, 8H, J = 7.8 Hz),
7.14 (m, 16H), 6.91 (dd, 2H, J = 8.6, 2.3 Hz), 6.79 (s, 2H), 1.37 (s, 9H),
1.02 (s, 18H); HRMS (FAB) m/z calcd for C56H57BN2 [Mþ] 768.4615,
found 768.4616.
2,8-Dicarboxylic Acid-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,f]borepin
(7j). A solution of 1a (41.9 mg, 0.0601 mmol) in THF (1 mL) was cooled
to -78 °C in a 25 mL Schlenk flask under nitrogen with stirring. A solution
of sec-butyl lithium in hexanes (1.4 M, 0.152 mL, 0.213 mmol) was added
dropwise, and the resulting mixture was held at -78 °C for 30 min after
which excess solid CO2 was added. The resulting mixture was allowed to
warm to room temperature and left to stir for 18 h. The reaction mixture was
diluted with hexane and filtered, rinsing with copious amounts of hexane to
yield 18 mg (0.034 mmol, 57%) of 7j as an off-white solid. 1H NMR (400
MHz, DMSO) δ 13.3 (s, 2H), 8.35 (d, 2H, J = 1.1 Hz), 8.00 (d, 2H, J = 1.1
Hz), 7.93 (dd, 2H, J = 8.1, 1.1 Hz), 7.51 (s, 2H), 7.44 (s, 2H), 1.40 (s, 9H),
0.91 (s, 18H); HRMS (FAB) m/z calcd for C34H39BO4 [Mþ] 522.2941,
found 522.2951.
3,7-Dicarboxylic Acid-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,f]borepin
(7k). A solution of 1b (12.1 mg, 0.0204 mmol) in THF (1 mL) was cooled
to -78 °C in a 25 mL Schlenk flask under nitrogen with stirring. A solution
of sec-butyl lithium in hexanes (1.4 M, 0.05 mL, 0.07 mmol) was added
dropwise and the resulting mixture was held at -78 °C for 30 min after
which excess solid CO2 was added. The resulting mixture was allowed to
warm to room temperature and left to stir for 18 h. The reaction mixture was
diluted with hexane and filtered, rinsing with copious amounts of hexane
to yield 7.6 mg (0.014 mmol, 71%) of 7k as an off-white solid. 1H NMR
(400 MHz, CDCl3) δ 8.52 (d, 2H, J = 1.3 Hz), 8.03 (dd, 2H, J = 7.5,
1.1 Hz), 7.57 (d, 2H, J = 8.5 Hz), 7.44 (s, 2H), 7.22 (s, 2H), 1.40 (s, 9H),
0.96 (s, 18H); MS (FAB) m/z calcd for C34H39BO4 [Mþ] 522.3,
found 522.3.
f]borepin (7h). A solution of borepin 1b (40.2 mg, 0.0679 mmol), CuI
(1.3 mg, 0.0068 mmol) and Pd(PPh3)4 (3.9 mg, 0.0034 mmol) in
toluene (1.5 mL) and DIPA (0.3 mL) was stirred in a dried 25 mL
Schlenk flask under nitrogen. Phenyl acetylene (15.2 mg, 0.149 mmol)
was added dropwise at room temperature, and the resulting reaction
mixture was heated to 75 °C for 18 h. The reaction mixture was allowed
to cool to room temperature after which it was partitioned between
water and diethyl ether and washed with NH4Cl (2ꢀ) and brine (2ꢀ).
The aqueous layer was removed and extracted with diethyl ether (3ꢀ),
the combined organics were dried with MgSO4 and filtered, and the
solvent was removed under reduced pressure to provide a brown oil/
solid that was purified by column chromatography (SiO2, 2% ethyl
acetate in hexane) to yield 7h (0.022 mmol, 33% by NMR) and
stannepin 6b as an off-white solid. The mixture was further purified
by dissolving in toluene (1 mL) and treating with dichlorophenylborane
(0.05 mL) followed by repeating the aqueous workup. The resulting
residue was purified by column chromatography (SiO2, hexane) to yield
5.1 mg of 7h (0.008 mmol, 11%) as an off-white solid. 1H NMR (400
MHz, CDCl3) δ 8.17 (d, 2H, J = 1.7 Hz), 7.69 (dd, 2H, J = 8.1, 1.7 Hz),
7.61 (d, 2H, J = 8.1 Hz), 7.53 (s, 2H), 7.46 (m, 4H), 7.32 (m, 6H), 7.18
2,8-Bis(trimethylsilylethynyl)-5-(2,4,6-tri-tert-butylphenyl)dibenzo-
[b,f]borepin (para bis(TMS acetylene) DBB). A solution of borepin
1a (125 mg, 0.179 mmol), CuI (3.4 mg, 0.018 mmol) and Pd(PPh3)4
(10 mg, 0.009 mmol) in toluene (2 mL) and DIPA (0.4 mL) was stirred
in a dried 25 mL Schlenk flask under nitrogen. TMS acetylene (39.6 mg,
0.395 mmol) was added dropwise at room temperature, and the
resulting reaction mixture was heated to 45 °C for 18 h. The reaction
mixture was allowed to cool to room temperature after which it was
partitioned between water and diethyl ether and washed with NH4Cl
(2ꢀ) and brine (2ꢀ). The aqueous layer was removed and extracted
with diethyl ether (3ꢀ), the combined organics were dried with
MgSO4 and filtered, and the solvent was removed under reduced
pressure to provide a brown oil that was further purified by column
chromatography (SiO2, hexane) to yield 109 mg (0.175 mmol, 56%) of
1
para bis(TMS acetylene) DBB as an off-white solid. H NMR (400
MHz, CDCl3) δ 7.92 (d, 2H, J = 8.0 Hz), 7.73 (s, 2H), 7.47 (s, 2H),
7.38 (d, 2H, J = 8.0 Hz), 7.09 (s, 2H), 1.43 (s, 9H), 0.92 (s, 18H), 0.27
(s, 18H); HRMS (FAB) m/z calcd for C42H55BSi2 [Mþ] 626.3935,
found 626.3927.
2237
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
3,7-Bis(trimethylsilylethynyl)-5-(2,4,6-tri-tert-butylphenyl)dibenzo-
[b,f]borepin (meta bis(TMS acetylene) DBB). A solution of borepin
1a (34 mg, 0.057 mmol), CuI (1.1 mg, 0.0057 mmol) and Pd(PPh3)4
(3.3 mg, 0.0029 mmol) in toluene (1.5 mL) and DIPA (0.3 mL) was
stirred in a dried 25 mL Schlenk flask under nitrogen. TMS acetylene
(12.7 mg, 0.126 mmol) was added dropwise at room temperature, and
the resulting reaction mixture was heated to 45 °C for 18 h. The reaction
mixture was allowed to cool to room temperature after which it was
partitioned between water and diethyl ether and washed with NH4Cl
(2ꢀ) and brine (2ꢀ). The aqueous layer was removed and extracted
with diethyl ether (3ꢀ), the combined organics were dried with MgSO4
and filtered, and the solvent was removed under reduced pressure to
provide a brown oil that was further purified by column chromatography
(SiO2, hexane) to yield 23 mg (0.037 mmol, 64%) of meta bis(TMS
acetylene) DBB as an off-white solid. 1H NMR (400 MHz, CD2Cl2) δ
8.04 (d, 2H, J = 1.6 Hz), 7.59 (m, 6H), 7.19 (s, 2H), 1.47 (s, 9H), 0.98 (s,
18H), 0.19 (s, 18H); HRMS (FAB) m/z calcd for C42H55BSi2 [Mþ]
626.3935, found 626.3927.
resulting reaction mixture was heated to 75 °C for 18 h. The reaction
mixture was allowed to cool to room temperature after which it was
precipitated into 30 mL of stirring methanol. The resulting solid was
collected by vacuum filtration and rinsed with methanol to yield 27 mg
1
(0.031 mmol, 88%) of 8b as an orange solid. H NMR (400 MHz,
CDCl3) δ 8.17 (s, 2H), 7.69 (m, 2H), 7.59 (dd, 2H, J = 8.0, 1.3 Hz), 7.51
(m, 2H), 7.18 (m, 2H), 6.89 (m, 2H), 3.94 (m, 4H), 1.77 (m, 4H), 1.45
(m, 13H), 1.25 (m, 24H), 1.02 (s, 18H), 0.87 (m, 6H). Mn = 4390.
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra for
S
b
all new compounds, UV-vis and PL spectra, CV data and
molecular orbital calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
2,8-Diethynyl-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,f]borepin
(para diethynyl DBB). A solution of borepin para bis(TMS acet-
ylene) DBB (103 mg, 0.165 mmol) and potassium carbonate (91 mg,
0.66 mmol) in 3 mL of a 1:1 methanol/THF solution was stirred at room
temperature for 4 h. The reaction mixture was partitioned between
NH4Cl and diethyl ether and washed with NH4Cl (1ꢀ), brine (2ꢀ).
The aqueous layer was removed and extracted with diethyl ether (2ꢀ),
the combined organics were dried with MgSO4 and filtered, and the
solvent was removed under reduced pressure to yield 79 mg (0.165
mmol, quant) of the para diethynyl DBB as a pale yellow solid that was
used without further purification. 1H NMR (400 MHz, CDCl3) δ 7.96
(d, 2H, J = 8.0 Hz), 7.76 (d, 2H, J = 1.6 Hz), 7.47 (s, 2H), 7.40 (s, 2H),
7.11 (s, 2H), 3.20 (s, 2H), 1.43 (s, 9H), 0.93 (s, 18H).
Corresponding Author
*E-mail: tovar@jhu.edu.
’ ACKNOWLEDGMENT
We thank the Johns Hopkins University and the Petroleum
Research Fund (administered by the American Chemical Society)
for financial support. A.C. is supported by a JHU Chemistry
Alumni Graduate Fellowship. We acknowledge helpful discus-
sions and sharing of experimental results with Professor Warren
E. Piers (University of Calgary).
3,7-Diethynyl-5-(2,4,6-tri-tert-butylphenyl)dibenzo[b,f]borepin
(meta diethynyl DBB). A solution of borepin meta bis(TMS acet-
ylene) DBB (22 mg, 0.035 mmol) and potassium carbonate (19 mg,
0.14 mmol) in 2 mL of a 1:1 methanol: THF solution was stirred at room
temperature for 4 h. The reaction mixture was partitioned between
NH4Cl and diethyl ether and washed with NH4Cl (1ꢀ), brine (2ꢀ).
The aqueous layer was removed and extracted with diethyl ether (2ꢀ),
the combined organics were dried with MgSO4 and filtered, and the
solvent was removed under reduced pressure to yield 17 mg (0.035
mmol, quant) of the meta diethynyl DBB as a pale yellow solid that was
used without further purification. 1H NMR (400 MHz, CDCl3) δ 8.13
(d, 2H, J = 1.7 Hz), 7.65 (dd, 2H, J = 8.0, 1.7 Hz), 7.57 (d, 2H, J = 8.0),
7.48 (s, 2H), 7.16 (s, 2H), 3.06 (s, 2H), 1.44 (s, 9H), 0.97 (s, 18H).
para Polymer (8a). A solution of 1,4-bis(decyloxy)-2,5-diiodoben-
zene (106.5 mg, 0.165 mmol), CuI (3.2 mg, 0.017 mmol) and Pd-
(PPh3)4 (9.6 mg, 0.0083 mmol) in toluene (1 mL) and DIPA (0.4 mL)
was stirred in a dried 25 mL Schlenk flask under nitrogen. The para
diethynyl DBB (prepared above, 79 mg, 0.165 mmol) in toluene (1 mL)
was added dropwise at room temperature and the resulting reaction
mixture was heated to 75 °C for 18 h. The reaction mixture was allowed
to cool to room temperature after which it was precipitated into 30 mL of
stirring methanol. The resulting solid was collected by vacuum filtration
and rinsed with methanol to yield 136 mg (0.156 mmol, 94%) of 8a as an
orange solid. 1H NMR (400 MHz, CD2Cl2) δ 7.98 (m, 2H), 7.80 (m,
2H), 7.50 (d, 2H, J = 1.7 Hz), 7.47 (m, 2H), 7.14 (d, 2H, J = 3.0 Hz), 7.05
(s, 2H), 4.03 (m, 4H), 1.86 (m, 4H), 1.45 (s, 9H), 1.39 (m, 4H), 1.25
(m, 24H), 0.97 (s, 18H), 0.85 (s, 6H). Mn = 7190.
’ REFERENCES
(1) (a) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbaï, F. P.
Chem. Rev. 2010, 110, 3958–84. (b) J€akle, F. Chem. Rev. 2010, 110,
3985–4022.
(2) (a) Matsumi, N.; Chujo, Y. Polym. J. 2008, 40, 77–89. (b) Lin, N.;
Zhao, X.; Cheng, X.; Jiang, M. J. Mol. Struct.: THEOCHEM 2007,
820, 98–106. (c) Entwistle, C. D.; Marder, T. B. Chem. Mater. 2004,
16, 4574–4585.
(3) (a) Hudnall, T. W.; Chiu, C.-W.; Gabbaï, F. P. Acc. Chem. Res.
2009, 42, 388–97. (b) Zhou, G.; Baumgarten, M.; M€ullen, K. J. Am.
Chem. Soc. 2008, 130, 12477–12484. (c) Hudnall, T. W.; Gabbaï, F. P. J.
Am. Chem. Soc. 2007, 129, 11978–11986.
(4) Yamaguchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413–
1424.
(5) (a) Nagai, A.; Kokado, K.; Nagata, Y.; Chujo, Y. Macromolecules
2008, 41, 8295. (b) Wakamiya, A.; Mori, K.; Yamaguchi, S. Angew.
Chem., Int. Ed. 2007, 46, 4273–4276. (c) Qin, Y.; Kiburu, I.; Shah, S.;
J€akle, F. Org. Lett. 2006, 8, 5227–30.
(6) Li, H.; J€akle, F. Angew. Chem., Int. Ed. 2009, 48, 2313–2316.
(7) Lorbach, A.; Bolte, M.; Li, H.; Lerner, H-W; Holthausen, M. C.;
J€akle, F.; Wagner, M. Angew. Chem., Int. Ed. 2009, 121, 4654–4658.
(8) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998, 120,
10776–10777.
(9) Li, H; Sundararaman, A.; Venkatasubbaiah, K; J€akle, F. J. Am.
Chem. Soc. 2007, 129 (18), 5792–5793. (b) J€akle, F. J. Inorg. Organomet.
Polym. Mater. 2005, 15 (3), 293–307.
(10) Liu, X. Y.; Bai, D. R.; Wang, S. Angew. Chem., Int. Ed. 2006, 45,
5475–5478.
meta Polymer (8b). A solution of 1,4-bis(decyloxy)-2,5-diiodoben-
zene (22.6 mg, 0.0352 mmol), CuI (0.7 mg, 0.0035 mmol) and
Pd(PPh3)4 (2.0 mg, 0.002 mmol) in toluene (0.5 mL) and DIPA
(0.3 mL) was stirred in a dried 25 mL Schlenk flask under nitrogen.
The meta diethynyl DBB (prepared above, 17 mg, 0.0352 mmol) in
toluene (1 mL) was added dropwise at room temperature and the
(11) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998, 120,
5112–5113.
(12) Zhao, C. H.; Wakamiya, A.; Inukai, Y.; Yamaguchi, S. J. Am.
Chem. Soc. 2006, 128, 15934–15935.
(13) Noda, T.; Ogawa, H.; Shirota, Y. Adv. Mater. 1999, 11,
283–285.
2238
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239

The Journal of Organic Chemistry
ARTICLE
(14) (a) Lepeltier, M.; Lukoyanova, O.; Jacobson, A.; Jeeva, S.;
Perepichka, D. F. Chem. Commun 2010, 46, 7007–7009. (b) Bosdet,
M. J. D.; Jaska, C. A.; Piers, W. E.; Sorensen, T. S.; Parvez, M. Org. Lett.
2007, 9, 1395–1398. (c) Bosdet, M. J. D.; Piers, W. E.; Sorensen, T. S.;
Parvez, M. Angew. Chem., Int. Ed. 2007, 46 (26), 4940. (d) Jaska, C. A.;
Emslie, D. J. H.; Bosdet, M. J. D.; Piers, W. E.; Sorensen, T. S.; Parvez, M.
J. Am. Chem. Soc. 2006, 128, 10885–96.
(15) (a) Rogers, J. S.; Bu, X.; Bazan, G. C. J. Am. Chem. Soc. 2000,
122, 730–731. (b) Lee, B. Y.; Bazan, G. C. J. Am. Chem. Soc. 2000,
122, 8577–8578. (c) Hoic, D. A.; DiMare, M.; Fu, G. C. J. Am. Chem. Soc.
1997, 119, 7155–7156.
(16) (a) Wood, T. K.; Piers, W. E.; Keay, B. A.; Parvez, M. Chem. Eur.
J. 2010, 16, 12199–12206. (b) Metz, M. V.; Schwartz, D. J.; Stern, C. L.;
Nickias, P. N.; Marks, T. J. Angew. Chem., Int. Ed. 2000, 39 (7),
1312–1316. (c) Ashe, A. J., III; Fang, X.; Kampf, J. W. Organometallics
1999, 18, 466–473.
(17) Dewar, M. J. S.; Marr, P. A. J. Am. Chem. Soc. 1962, 84, 3782.
(18) Ashe, A. J., III; Fang, X.; Fang, X.; Kampf, J. W. Organometallics
2001, 20, 5413–5418.
(19) (a) Daly, A.; Tanjaroon, C.; Marwitz, A. J. V.; Liu, S.-Y.;
Kukolich, S. J. Am. Chem. Soc. 2010, 132, 5501–5506. (b) Abbey,
E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2008, 130, 7250–7252.
(20) Herberich, G. E.; Greiss, G.; Heil, H. F. Angew. Chem., Int. Ed.
1970, 9, 805–806.
(21) Lee, R. A.; Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc. 1998,
120, 6037–6046.
(22) Ashe, A. J., III; Fang, X.; Kampf, J. W. Organometallics 1999,
18, 466–473.
(23) (a) Fan, C.; Piers, W. E.; Parvez, M. Angew. Chem., Int. Ed. 2009,
48, 2955–2958. (b) Braunschweig, H.; Kupfer, T. Chem. Commun
2008, 4487. (c) Wakamiya, A.; Mishima, K.; Ekawa, K.; Yamaguchi, S.
Chem. Commun 2008, 579–581. (d) Yamaguchi, S.; Shirasaka, T.;
Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2002, 124, 8816–8817.
(24) Vol’pin, M. E. Russ. Chem. Rev. 1960, 29, 129–160.
(25) Schulman, J. M.; Disch, R. L. Organometallics. 2000, 19, 2932–
2936.
(26) Subramanian, G.; Schleyer, P. v R.; Jiao, H. Organometallics
1997, 16, 2362–2369.
(27) Tamelen, E. E. v.; Brieger, G.; Untch, K. G. Tetrahedron Lett.
1960, 8, 14–15.
(28) Leusink, A. J.; Drenth, W.; Noltes, J. G.; Kerk, G. J. M. v. d.
Tetetrahedron Lett. 1967, 14, 1263–1266.
(29) (a) Ashe, A. J., III; Kampf, J. W.; Nakadaira, Y.; Pace, J. M.
Angew. Chem., Int. Ed. 1992, 31 (9), 1255–1258. (b) Ashe, A. J., III;
Klein, W.; Rousseau, R. Organometallics 1993, 12, 3224–3231.
(30) Mercier, L. G.; Piers, W. E.; Parvez, M. Angew. Chem., Int. Ed.
2009, 48, 6108–6111.
(31) Caruso, A., Jr.; Siegler, M. A.; Tovar, J. D. Angew. Chem., Int. Ed.
2010, 49 (25), 4213–4217.
(32) Mercier, L. G.; Furukawa, S.; Piers, W. G.; Wakamiya, A.;
Yamaguchi, S.; Parvez, M.; Harrington, R. W.; Clegg, W. Organometallics
2011, DOI is 10.1021/om2000597.
(33) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T.
J. Am. Chem. Soc. 1981, 103, 4587–4589.
(34) Dunne, E. C.; Coyne, E. J.; Crowley, P. B.; Gilheany, D. G.
Tetrahedron Lett. 2002, 43, 2449–2453.
ꢀ
(35) Sterk, D.; Modec, B.; Mohar, B. J. Org. Chem. 2007, 72,
8010–8018.
(36) Swager, T.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995,
99, 4886–4893.
(37) Albota, M.; Beljonne, D.; Brꢁedas, J.-L.; Ehrlich, J. E.; Fu, J.-Y.;
Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.;
McCord-Maughon, D.; Perry, J. W.; Rockel, H.; Rumi, M.; Subramaniam,
G.; Webb, W. W.; W. X-L.; Xu, C. Science 1998, 281, 1653.
(38) Amijs, C. H. M.; Van Klink, G. P. M.; Van Koten, G. Green
Chem. 2003, 5, 470–474.
(39) Zhou, N.; Wang, L.; Thompson, D. W.; Zhao, Y. Org. Lett.
2008, 10, 3001–3004.
2239
dx.doi.org/10.1021/jo2001726 |J. Org. Chem. 2011, 76, 2227–2239