Catalytic B-C Coupling by Si/B Exchange: A Versatile Route to π-Conjugated Organoborane Molecules, Oligomers, and Polymers

Article abstract of DOI:10.1021/jacs.7b01835

Conjugated organoboranes have emerged as attractive hybrid materials for optoelectronic applications. Herein, a highly efficient, environmentally benign catalytic B-C bond formation method is presented that uses organosilicon compounds, dibromoboranes, and the metal-free organocatalyst Me₃SiNTf₂. This Si/B exchange approach has been successfully applied to the synthesis of arylborane molecules 4a-c, oligomers 8a,b, and polymers 8a′,b′. Photophysical investigations, supported by TD-DFT calculations, reveal highly effective π-conjugation in thienyl- and furylborane species; the latter are also highly emissive.

Full text of DOI:10.1021/jacs.7b01835

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Communication
Catalytic B-C Coupling by Si/B Exchange: A Versatile Route to #-
Conjugated Organoborane Molecules, Oligomers, and Polymers
Artur Lik, Lars Fritze, Lars Müller, and Holger Helten
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01835 • Publication Date (Web): 10 Apr 2017
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Catalytic B–C Coupling by Si/B Exchange: A Versatile Route to π-
Conjugated Organoborane Molecules, Oligomers, and Polymers
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Artur Lik, Lars Fritze, Lars Müller, and Holger Helten*
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany
Supporting Information Placeholder
The types a and b utilize transition metalꢀcatalyzed C–C
coupling reactions of organoborane (co)monomers.5 For
this, the boron centers of the reactants must be particularly
kinetically stabilized to survive the crossꢀcoupling process.
Recently, Liu, Jäkle, and coꢀworkers applied an AB type
procedure (type b; i.e., using a single monomer containing
both complementary functional groups required for conꢀ
densation) to synthesize an azaborine polymer.5 Routes c–
e, on the other hand, make use of B–C bond forming proꢀ
cesses.6 The currently most sophisticated method availaꢀ
ble, developed by Jäkle and coꢀworkers, uses distannyl
ABSTRACT: Conjugated organoboranes have emerged as
attractive hybrid materials for optoelectronic applications.
Herein, a highly efficient, environmentally benign catalytic
B–C bond formation method is presented that uses organoꢀ
silicon compounds, dibromoboranes, and the metalꢀfree
,11
3 2
organocatalyst Me SiNTf . This Si/B exchange approach has
been successfully applied to the synthesis of arylborane
d
′
molecules 4aꢀc, oligomers 8a,b, and polymers 8a,b . Phoꢀ
tophysical investigations, supported by TDꢀDFT calculaꢀ
tions, reveal highly effective πꢀconjugation in thienylꢀ and
furylborane species; the latter are also highly emissive.
ꢀ8
3
compounds (X = SnMe ) in combination with diboryl speꢀ
7
cies (c) or boranes (d) with Y = Br. Polycondensations via
such Sn/B exchange reactions generally proceed under mild
conditions, which is advantageous for controlled polymeriꢀ
zations.
πꢀConjugated organoboranes have emerged as an imꢀ
portant class of organic–inorganic hybrid compounds in
materials science.1,2 Intriguing optical and electronic feaꢀ
A major drawback of the use of organotin compounds,
however, is their pronounced toxicity. This prompted us to
seek for a suitable alternative. We considered organosilicon
compounds as the ideal candidates. Si/B exchange reactions
are generally slower than analogous Sn/B processes. For
tures result from the incorporation of the vacant p orbital of
boron into extended πꢀsystems, thus often leading to highly
emissive and/or effectively electronꢀtransporting materiꢀ
1
,2
als.
This has enabled applications in organic
instance, PhSiMe
perature to yield PhBBr
of a second bromine by a phenyl group using PhSiMe
give Ph BBr requires harsh reaction conditions, i.e., heating
of the neat reactants at 180 °C in a sealed ampule for 24 h.
3
and BBr
. However, subsequent replacement
to
3
react smoothly at room temꢀ
(
opto)electronic devices (OFETs, OLEDs, photovoltaic
1
1,3
2
cells) or as chemosensors for certain anions or amines. In
recent years, donor–acceptor type conjugated oligomers
and polymers comprising trivalent boron centers alternating
with electronꢀrich πꢀsystems came into the focus of intense
3
2
12
1a,dꢀf,4ꢀ11
Certainly, these conditions are not suitable for controlled
polymerizations, but we speculated that it might be possible
to initiate such processes under milder conditions by a
strong reagent.
research activity.
Rational synthetic routes to cycloꢀ
linear extended species that feature B–Caryl linkages follow
one of the general polycondensation strategies a–e (Figure
4
1).
Herein, we demonstrate the use of Si/B exchange as a facꢀ
ile route to conjugated organoborane molecules, oligomers,
13
and polymers. Si/B exchange condensations are effectively
initiated at ambient temperature with the electrophilic silyl
reagents Me SiOTf and Me SiNTf (Tf = SO CF ). This can
3 3 2 2 3
be performed catalytically, and in some cases, AB type monꢀ
omers can be generated and subsequently oligomerized
through this approach in a controlled manner.
We started our investigations with the reaction between
ꢀ(trimethylsilyl)thiophene (1a) and 2ꢀ(dibromoboryl)thioꢀ
2
5c
phene (2a) to give bromoꢀbis(thienꢀ2ꢀyl)borane (3a; Figꢀ
ure 2). In fact, some condensation was observed by H NMR
spectroscopy at ambient temperature in CH Cl already
2 2
1
without an initiator added.14 However, the reaction did not
go to completion within 12 d (Figure 3; Supporting Inforꢀ
mation, Figure S3). We anticipated that it could be accelerꢀ
ated through the presence of a halogenꢀabstracting reagent.
Figure 1. Strategies for the synthesis of arylborane oligoꢀ
mers or polymers (Ar, Ar = arenes; R = organic substituent;
X/Y = complementary leaving groups).
′
3 3 2
For this, we chose Me SiOTf and Me SiNTf , which we apꢀ
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Page 2 of 13
plied in substoichiometric amounts (5 mol%). Me
yielded significant rate acceleration with 95 % conversion
after d. Finally, the stronger electrophilic species
Me SiNTf proved to be a highly efficient catalyst: complete
and selective conversion to 3a was observed within 72 min.
The volatile condensation byꢀproduct, Me SiBr, was easily
removed at reduced pressure. Subsequent derivatization of
a with TipLi (Tip = 2,4,6ꢀtriisopropylphenyl) afforded the
3
SiOTf
The twist angle between the phenyl groups in 4c, on the
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other hand, amounts to 43.8°. The furan derivative 4b also
shows the shortest B–C bond lengths [d(B–Cfuryl): 1.527(6),
1.529(6), 1.542(5), 1.547(5) Å]. The B–Cphenyl bonds of 4c are
significantly longer (1.579(3), 1.581(3) Å), i.e. in the same
5
3
2
3
range with the B–Cthienyl bonds of Mes*B(2ꢀthienyl)
2
5c
(
1.589(6), 1.568(4) Å).
3
airꢀ and moistureꢀstable triarylborane 4a.
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Figure 2. Synthesis of 4aꢀc via catalytic B–C coupling.
Figure 4. Molecular structures of 4b,c in the solid state
Hꢀatoms omitted).
(
Figure 5. UV–vis absorption spectra of 4a/8a′ and
4b/8b′ in THF.
1
Figure 3. Consumption of 1a (by H NMR) during the
reaction with 2a, without catalyst (red), with 5 mol% of
Me SiOTf (blue), and with 5 mol% of Me SiNTf (green).
3 3 2
As furanꢀcontaining organoborane oligomers and polyꢀ
mers were unknown, so far, we carried out the reaction of
1
3 2
b with 2b in the presence of Me SiNTf (5 mol%). This
yielded full conversion to 3b within less than 2 h at ambient
temperature. After reaction with Mes*Li (Mes* = 2,4,6ꢀtriꢀ
tertꢀbutylphenyl), 4b was obtained, which was purified via
column chromatography. The bulky Mes* substituent was
chosen in this case because it was also effective in stabilizing
′
a furylborane polymer (8b ; see below).
We further considered as reactants the less activated
phenyl species 1c and 2c. Condensation thereof proceeded
significantly slower and required increased substrate conꢀ
centration (4 M) and higher catalyst loadings (25 mol%).
Nevertheless, the reaction was fully selective with 95 %
conversion to 3c after 3 d at room temperature. In this case,
the final Mes*ꢀsubstituted product, 4c, turned out to be airꢀ
sensitive, but we succeeded in isolating it via crystallization.
The constitution of 4aꢀc was unambiguously ascertained by
NMR spectroscopy and mass spectrometry.
Figure 6. Energy levels and plots of the LUMO and the
Tip
HOMO−2 (isovalue: 0.04 a.u.) of 4a and 4b
.
The UV–vis spectra of 4a,b (Figure 5; Table 1) display a
major band, which is, according to our TDꢀDFT calculations
16
(
B3LYPꢀD3(BJ)/def2ꢀSV(P)), assigned to a
π–
π* transiꢀ
tion from the HOMO−2 to the LUMO (Figure 6; the
HOMOꢀLUMO process is of low probability). The
HOMO−2 is delocalized over the hetaryl rings, while the
The solid state structures of 4b,c were determined by
singleꢀcrystal Xꢀray diffraction (Figure 4). The asymmetric
unit of 4b contains two independent molecules with similar
metrical data. In both 4b,c the Mes* group is nearly perꢀ
LUMO shows significant contribution from the p orbital on
π
boron. Hence, the excitation process is associated with inꢀ
tramolecular charge transfer (ICT) from the hetaryl groups
to the boron center. When going from the thiophene (4a) to
3
pendicular oriented to the respective BC plane [dihedral
angles: 83.0°/87.0° (4b), 83.4° (4c)]. The furan rings of 4b
are almost perfectly coplanar, adopting antiꢀconformation.
The interplanar angle between these rings is only 4.6° and
Tip
the furan system (4b ), the energy levels of the relevant
orbitals are increased to approximately the same degree,
resulting in similar excitation energies. This correlates with
the more electronꢀrich nature of furan vs. thiophene. The
absorption band of compound 4c appears at significantly
higher energy. While 4c is basically nonꢀemissive, comꢀ
7
.7°, respectively, which is even smaller than that between
the thiophene rings in Mes*B(2ꢀthienyl)
2
(19.0°).5c This
parallels observations made for oligofurans, which also
show greater planarity than analogous oligothiophenes.15
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pound 4a shows weak and the difurylborane 4b shows
intense fluorescence.
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a
Table 1. Photophysical data for 4aꢀc, 8a,b, and
′
b
′
8
a,b , and GPC data for 8a,b and 8a,b
Compd.
λ
abs,max
λ
em,max
Φ
/ %
3.1
f
M
n
M
w
DP
n
/
nm
/ nm
410
400
–
4a
325
315
271
409
412
407
411
–
–
–
–
–
–
–
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0
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4
b
24.0
–
4
8
c
–
–
a
421
455
422
426
0.6
0.8
70.3
71.1
1,550
3,270
1,760
4,100
8a′
8b
10
5
1,880 2,890
3,260 5,220
b
8
b′
9
aIn THF, except for 4c (in CH
2
2
Cl ). In THF, vs. polystyꢀ
Figure 7. Synthesis of oligomers 8a,b and polymers 8a,b′.
In the UV–vis spectra of 8a,b/8a,b , the * absorption
rene standards.
We then attempted polycondensations using our catalytic
approach (Figure 7, Route A). Careful addition of BBr (1
′
π–π
3
band appears significantly redꢀshifted from that of their
equiv.) to 5a,b in nꢀpentane at −78 °C and subsequent
warming to 0 °C selectively yielded the AB monomers 6a,b,
which were obtained in considerable purity after removing
the volatiles in vacuo (see Figures S35–S40). In the case of
respective small molecule congeners 4a,b (Figure 5; Table
1). The shift is in a similar range with that of a recently reꢀ
ported Mes*ꢀsubstituted poly(thienylborane)5f with respect
5c
to Mes*B(2ꢀthienyl)
2
,
and evidences highly effective πꢀ
6
°
a, minor byꢀproducts were removed by filtration at −40
C. Catalytic polycondensation of 6a,b was then performed
in CH Cl at ambient temperature. Subsequent derivatizaꢀ
′
conjugation. The furylborane species 8b,8b showed inꢀ
tense blue fluorescence.
2
2
In conclusion, we have developed a highly efficient, enviꢀ
ronmentally benign B–C bond formation method and
demonstrated its application for the synthesis of conjugated
organoborane molecules, oligomers, and polymers. Thereꢀ
by, we applied a catalytic B–C coupling17 process for polyꢀ
condensation for the first time. Our new approach offers
several advantages over previous routes. Silyl groups are
chemically robust and can be introduced into organic subꢀ
strates by facile methods. The starting materials are less
toxic than those employed previously. Notably, the condenꢀ
tion of 7a with TipLi gave 8a, which was purified by preꢀ
cipitation with ethanol. As the product obtained upon treatꢀ
ing 7b with TipLi was found to be unstable towards ethanol,
we employed Mes*Li for derivatization in this case. After
workup this yielded 8b, which proved to be perfectly airꢀ
and moistureꢀstable. The NMR spectra of 8a,b are conꢀ
3
sistent with the proposed structures. The SiMe end groups,
1
13
detected by H and C NMR spectroscopy, were partially
cleaved off, giving rise to a new signal in the aromatic reꢀ
gion. Analysis of 8a,b by GPC revealed that these species
were oligomeric and almost monodisperse, featuring on
average 4 and 5 repeat units, respectively (Table 1). A plauꢀ
sible reason for this is the observed precipitation of 7a,b at
some point during the polycondensation process. The prodꢀ
ucts 8a,b, obtained after incorporation of the aryl side
groups, however, were readily soluble in solvents such as
3
sation byꢀproduct, Me SiBr, is less harmful and easily reꢀ
moved. As the formation of B–C bonds is fundamental for
the construction of organoboron compounds in general, we
believe that our new method will be of use in various fields
of chemical research.
2 2
CH Cl , THF, and nꢀpentane. In view of this solubilizing
effect of the Tip and Mes* substituents, we decided to exꢀ
plore polymerizations starting from 9a,b, in which the
growing polymer chain, 10a,b, carries one of these groups
on every second boron atom and should, therefore, show
increased solubility (Route B). For the reaction of 9a we
also changed the solvent to the more polar oꢀdichlorobenꢀ
zene (oꢀDCB), from which we anticipated further enhanceꢀ
ment of the solubility of the macromolecular intermediate
ASSOCIATED CONTENT
Supporting Information
Full experimental and computational details and crystalloꢀ
graphic data (CIF) for 4b,c. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
1
0a. Precipitation of 10a was observed after 2 d. It was
subjected to centrifugation/decantation prior to derivatizaꢀ
tion in toluene with TipLi to 8a . In an attempt to adopt
analogous conditions for the synthesis of 8b , the intermeꢀ
Corresponding Author
′
*holger.helten@ac.rwthꢀaachen.de
′
diate 10b underwent decomposition during the centrifugaꢀ
Notes
tion process. Therefore, the polymerization was repeated in
′
The authors declare no competing financial interests.
CH
2
Cl
2
, which gave 8b after derivatization without a prior
′
centrifugation step. The GPC analyses revealed that 8a,b
ACKNOWLEDGMENT
were of significantly increased molecular weight (Table 1).
Support by the DFG (Emmy Noether Programme) and the
COST action CM1302 (SIPs) is gratefully acknowledged. We
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Page 4 of 13
thank Q. Guo and K.ꢀN. Truong for Xꢀray diffraction data
collection, Dr. K. Beckerle for GPC measurements, and Prof.
Dr. J. Okuda for generous support and helpful discussions.
Soc. 2011, 133, 4632. (d) Cheng, F.; Bonder, E. M.; Jäkle, F. J. Am.
Chem. Soc. 2013, 135, 17286. (e) Wan, W.ꢀM.; Cheng, F.; Jäkle, F.
Angew. Chem. Int. Ed. 2014, 53, 8934.
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(11) For polymers with tetraꢀcoordinate boron, see, e.g.: (a)
Meng, G.; Velayudham, S.; Smith, A.; Luck, R.; Liu, H. Macromole-
cules 2009, 42, 1995. (b) Nagai, A.; Chujo, Y. Macromolecules
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H.ꢀW.; Wagner, M. Angew. Chem. Int. Ed. 2015, 54, 8800. (l)
Brown, A. N.; Li, B.; Liu, S.ꢀY. J. Am. Chem. Soc. 2015, 137, 8932.
(15) (a) Gidron, O.; DiskinꢀPosner, Y.; Bendikov, M. J. Am.
Chem. Soc. 2010, 132, 2148. (b) Bunz, U. H. F. Angew. Chem. Int.
Ed. 2010, 49, 5037.
(
16) For better comparison with 4a, the furylborane congener
Tip
was also calculated with Ar′ = Tip (i.e., 4b ).
(17) For catalytic B–C coupling reactions yielding molecular
compounds, see, e.g.: (a) Mkhalid, I. A. I.; Barnard, J. H.; Marder,
T. B.; Murphy, J. M.; Hartwig, J. F. Chem. Rev. 2010, 110, 890. (b)
Del Grosso, A.; Pritchard, R. G.; Muryn, C. A.; Ingleson, M. J. Or-
ganometallics 2010, 29, 241. (c) Farràs, P.; OlidꢀBritos, D.; Viñas,
C.; Teixidor, F. Eur. J. Inorg. Chem. 2011, 2011, 2525. (d) Chow,
W. K.; Yuen, O. Y.; So, C. M.; Wong, W. T.; Kwong, F. Y. J. Org.
Chem. 2012, 77, 3543. (e) Zhao, D.; Xie, Z. Angew. Chem. Int. Ed.
2
016, 55, 3166.
(
2
m) Wang, X.ꢀY.; Narita, A.; Feng, X.; Müllen, K. J. Am. Chem. Soc.
015, 137, 7668. (n) Wang, S.; Yang, D.ꢀT.; Lu, J.; Shimogawa, H.;
Gong, S.; Wang, X.; Mellerup, S. K.; Wakamiya, A.; Chang, Y.ꢀL.;
Yang, C.; Lu, Z.ꢀH. Angew. Chem. Int. Ed. 2015, 54, 15074.
(3) (a) Thomas, S. W., III.; Joly, G. D.; Swager, T. M. Chem. Rev.
2
007, 107, 1339. (b) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge,
S.; Gabbaï, F. P. Chem. Rev. 2010, 110, 3958. (c) Zhao, H.; Leamer,
L. A.; Gabbaï, F. P. Dalton Trans. 2013, 42, 8164.
(4) Priegert, A. M.; Rawe, B. W.; Serin, S. C.; Gates, D. P. Chem.
Soc. Rev. 2016, 45, 922.
(
5) (a) Pammer, F.; Lalancette, R. A.; Jäkle, F. Chem. Eur. J.
2
011, 17, 11280. (b) Reus, C.; Guo, F.; John, A.; Winhold, M.; Lerꢀ
ner, H.ꢀW.; Jäkle, F.; Wagner, M. Macromolecules 2014, 47, 3727.
(c) Yin, X.; Chen, J.; Lalancette, R. A.; Marder, T. B.; Jäkle, F.
Angew. Chem. Int. Ed. 2014, 53, 9761. (d) Baggett, A. W.; Guo, F.;
Li, B.; Liu, S.ꢀY.; Jäkle, F. Angew. Chem. Int. Ed. 2015, 54, 11191.
(e) Wang, X.ꢀY.; Zhuang, F.ꢀD.; Wang, J.ꢀY.; Pei, J. Chem. Com-
mun. 2015, 51, 17532. (f) Yin, X.; Guo, F.; Lalancette, R. A.; Jäkle,
F. Macromolecules 2016, 49, 537.
(
6) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998,
1
20, 10776.
(7) (a) Sundararaman, A.; Victor, M.; Varughese, R.; Jäkle, F. J.
Am. Chem. Soc. 2005, 127, 13748. (b) Li, H.; Jäkle, F. Angew.
Chem. Int. Ed. 2009, 48, 2313. (c) Chen, P.; Lalancette, R. A.;
Jäkle, F. J. Am. Chem. Soc. 2011, 133, 8802.
(8) For other B–C coupling polymerization types, see: (a) Heilꢀ
mann, J. B.; Scheibitz, M.; Qin, Y.; Sundararaman, A.; Jäkle, F.;
Kretz, T.; Bolte, M.; Lerner, H.ꢀW.; Holthausen, M. C.; Wagner, M.
Angew. Chem. Int. Ed. 2006, 45, 920. (b) Hübner, A.; Qu, Z.ꢀW.;
Englert, U.; Bolte, M.; Lerner, H.ꢀW.; Holthausen, M. C.; Wagner,
M. J. Am. Chem. Soc. 2011, 133, 4596.
(9) Hydroboration polymerization leads to polymers with Bꢀvinyl
linkages. See, e.g., (a) Matsumi, N.; Naka, K.; Chujo, Y. J. Am.
Chem. Soc. 1998, 120, 5112. (b) Lorbach, A.; Bolte, M.; Li, H.;
Lerner, H.ꢀW.; Holthausen, M. C.; Jäkle, F.; Wagner, M. Angew.
Chem. Int. Ed. 2009, 48, 4584.
(10) For side chain boronꢀfunctionalized oligomers and polyꢀ
mers, see, e.g.: (a) Zhao, C.ꢀH.; Wakamiya, A.; Inukai, Y.; Yamaguꢀ
chi, S. J. Am. Chem. Soc. 2006, 128, 15934. (b) Li, H.;
Sundararaman, A.; Venkatasubbaiah, K.; Jäkle, F. J. Am. Chem.
Soc. 2007, 129, 5792. (c) Welch, G. C.; Bazan, G. C. J. Am. Chem.
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Figure 1. General strategies for the synthesis of arylborane oligomers and polymers (Ar, Ar′ = arenes; R =
organic substituent; X, Y = complementary leaving groups).
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Synthesis of 4a-c via catalytic B–C coupling.
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Figure 4. Molecular structures of 4b and 4c in the solid state (hydrogen atoms omitted for clarity).
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Tip
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