Catalytic Double Carbon–Boron Bond Formation for the Synthesis of Cyclic Diarylborinic Acids as Versatile Building Blocks for π-Extended Heteroarenes

Article abstract of DOI:10.1002/anie.201612535

The first catalytic synthesis of cyclic diarylborinic acids is developed using a dihydroaminoborane reagent as the boron source. Unlike previously reported methods that use organolithium reagents, this method allows the easy synthesis of cyclic diarylbori

Full text of DOI:10.1002/anie.201612535

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201612535
German Edition: DOI: 10.1002/ange.201612535
Synthetic Methods
Catalytic Double Carbon–Boron Bond Formation for the Synthesis of
Cyclic Diarylborinic Acids as Versatile Building Blocks for p-Extended
Heteroarenes
Takuya Igarashi, Mamoru Tobisu,* and Naoto Chatani*
Abstract: The first catalytic synthesis of cyclic diarylborinic
acids is developed using a dihydroaminoborane reagent as the
boron source. Unlike previously reported methods that use
organolithium reagents, this method allows the easy synthesis
of cyclic diarylborinic acids bearing a range of functionalities
t
including CN, CO2Et, CONEt2 and NMeCO2 Bu. Further-
more, these cyclic diarylborinic acids provide rapid access to
skeletal diversity, in particular they enable the synthesis of six-
to nine-membered p-extended heteroarenes through simple
cross-coupling reactions, which are important synthetic targets
in both advanced materials and pharmaceuticals.
T
he development of synthetic methods for elaborate p-
conjugated molecules is crucial for the creation of new
functional organic materials, such as light emitting diodes,
2
transistors and solar cells.[1] The annulative two-fold C(sp )
ꢀ
C(sp2) cross-coupling reaction between organodimetallic
reagents and dihalides represents a powerful method for
this purpose (Scheme 1a). Among the organodimetallic
reagents reported to date,[2] cyclic diarylborinic acid 1[3] is
particularly useful, because of its stability towards air and
moisture, and the low toxicity and easily removable nature of
the boron-based residue generated after the cross-coupling. It
thus has similar properties to those of the arylboronic acids.
However, the potential utility of 1 remains limited because all
of the reported syntheses of 1 require the use of organo-
lithium reagents 2, thus rendering the introduction of many
common functional groups impossible (Scheme 1b).[3]
Herein, we report the first catalytic synthesis of 1, which
allows access to a range of functionalized cyclic borinic acids,
including a seven-membered derivative (Scheme 1b). The
synthetic utility of 1 as a dianionic building block was also
demonstrated by its elaboration into a diverse array of ring
systems.
Scheme 1. Cyclic diarylborinic acid 1 and methods for its preparation.
1 would be formed if borylation occurs twice between
dihalide 3 and dihydroborane reagent 4 (Scheme 2a). Diiso-
propylaminoborane (R = iPr2N, 4a)[5–8] was chosen as a readily
available dihydroborane reagent. Although several catalytic
borylation reactions of aryl halides using 4a have been
reported,[6] none afforded a diarylated product, even in the
presence of an excess amount of the aryl halide (Sche-
me 2b).[6d] This suggested that it may not be possible for both
On the basis of the widespread utility of palladium-
catalyzed borylation of aryl halides in the synthesis of
functionalized arylboronic acids,[4] we envisioned that
[*] T. Igarashi, Prof. Dr. N. Chatani
Department of Applied Chemistry, Faculty of Engineering
Osaka University
Suita, Osaka 565-0871 (Japan)
E-mail: chatani@chem.eng.osaka-u.ac.jp
Dr. M. Tobisu
Center for Atomic and Molecular Technologies, Graduate School of
Engineering, Osaka University
Suita, Osaka 565-0871 (Japan)
E-mail: tobisu@chem.eng.osaka-u.ac.jp
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201612535.
Scheme 2. Working hypothesis: two-fold borylation.
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ꢀ
of the B H bonds in 4a to react, and thus raised an
uncertainty with our hypothesis. Nevertheless, we expected
that the second borylation would be facilitated in our system
owing to its intramolecular nature (Scheme 2a).
To test our hypothesis, we initially examined the reaction
of ditriflate 3a with 4a (2 equiv) in the presence of a palladium
catalyst at 658C for 15 h (Table 1). The yield of borylated
Table 1: Optimization of the reaction conditions.[a]
Entry
Substrate
Ligand
Additive
NMR yield of 1 [%]
1
2
3a
3b
DPEPhos
CyJohnPhos
none
KI
>99 (76)[b]
97 (75)[b]
[a] Reaction conditions: 3a or 3b (0.50 mmol), 4a (1.0 mmol), Pd(OAc)2
(0.05 mmol), ligand (0.10 mmol), Et3N (2.5 mmol), additive
(0.25 mmol) in THF (2 mL) at 658C for 15 h. [b] Isolated yields are
shown.
product 1a was estimated by 1H NMR spectroscopy after
treatment with methanol and an aqueous solution of NH4Cl.
A brief screening of the ligand revealed that a bisphosphine
with a diphenyl ether backbone (i.e., DPEPhos) displayed the
highest activity, giving 1a in > 99% NMR yield (entry 1). The
product 1a could be isolated in 76% yield by column
chromatography. Unfortunately, however, reactions under
these optimized conditions using DPEPhos failed to promote
the borylation of dibromide 3b. Reoptimization of the
reaction conditions showed that using KI together with
CyJohnPhos,[9] markedly improved the yield of 1b, affording
product 1b in 97% NMR yield (entry 2, see also the
Supporting Information (SI) for details of optimization and
some discussion regarding the effect of KI).[6h]
The reaction was successfully applied to the synthesis of
a diverse array of cyclic diarylborinic acids (Scheme 3). In
addition to diarylborinic acids bearing simple alkyl, aryl and
alkoxy groups (1c–1h), those containing cyano, chloro, ester,
amide, carbamate, and fluoro groups (1i–1o) were all
compatible with the present catalytic conditions. This high-
lights the synthetic advantage of our protocol over previously
reported methods using organolithium reagents.[3] In partic-
ular, the tolerance of an aryl chloride moiety, as shown for 1k,
is notable when considering that the report that the borylation
of aryl chlorides occurs with 4a using a Pd/XPhos catalyst.[6h]
Our protocol also allowed the synthesis of p-extended
analogue 1p as well as diarylborinic acids containing nitrogen
Scheme 3. Pd-catalyzed synthesis of cyclic diarylborinic acids by annu-
lative two-fold borylation.[a] [a] Reaction conditions: 3 (0.50 mmol), 4a
(1.0 mmol), Pd(OAc)2 (0.050 mmol), ligand (0.10 mmol), Et3N
(2.5 mmol) in THF (2 mL) at 658C for 15 h. Ligand: DPEPhos for
ditriflates 3c–3h; CyJohnPhos for dibromides 3i–3o and 3s; XPhos for
3p and 3r; RuPhos for 3q. 1H-NMR yields are shown. Numbers in
parentheses are isolated yields. [b] KI was added. [c] After the reaction,
2-aminoethanol (4.0 equiv) was added. [d] Using 1.0 g of 3s.
(1q) and sulfur (1r) tethers. Notably, seven-membered diaryl-
borinic acid 1s was successfully synthesized.[10] Although we
routinely isolated the product after hydrolysis in the form of
borinic acid 1, chromatographic separation led to a consid-
erable loss of 1 in some cases (numbers in parentheses in
Scheme 3 refer to isolated yields). However, this issue could
be easily addressed by formation of an aminoalcohol adduct,
such as 1j,[8] which could be isolated by simple filtration and
used directly for the subsequent two-fold cross-coupling (see
below).
Our catalytic protocol can be readily applied to the gram
scale production of cyclic diarylborinic acids (Scheme 4, 3i!
1i). The obtained functionalized boracycle can then serve as
a 1,5-dianion equivalent in annulative cross-coupling with
dihalides under palladium catalysis, and thus allows access to
a diverse range of p-extended heteroarenes (Scheme 4).[3e]
2
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Scheme 4. Pd-catalyzed annulative two-fold Suzuki–Miyaura coupling for the synthesis of benzannulated heterocycles. Reaction conditions: 1i
t
(0.25 mmol), 5 (0.50 mmol), Pd2(dba)3 (0.0038 mmol), tBu3P·HBF4 (0.0090 mmol), Cs2CO3 (0.83 mmol), H2O (2.5 mmol) in AmOH (3 mL) at
1008C for 24–48 h. Yields of isolated products are shown. [a] Run on a 1.0 mmol scale.
For example, cross-coupling of 1i with 1,2-dibromo-
(hetero)arenes 5a–5d enabled the modular synthesis of the
tribenzo[b,d,f]oxepine skeleton in 6a–6c or the heteroarene-
fused analogue 6d. This structural motif is found in anti-
depressant drugs[11] and natural products.[12] In addition, cross-
coupling of 1i with 1,2-dibromocyclopentene 5e proceeded
Scheme 5. Pd-catalyzed two-fold Suzuki–Miyaura coupling using cyclic
borinate 1j.
efficiently to afford corresponding dibenzo[b,f]oxepin deriv-
ative 6e. p-Extended xanthene derivative 6 f was also
assembled by cross-coupling of 1i with 1,1-dibromoalkene
5 f, valuable finding with respect to potential application in
the synthesis of molecular probes for aggregation induced
emission.[13] Moreover, a larger ring system can also be
accessible by the reaction of 1i with dibromobiaryl 5g and 5h,
which led to the construction of a tetrabenzo[b,d,f,h]oxonine
skeleton 6g and 6h.[14] Cross-coupling with 1,8-dibromonaph-
thalene 5i yielded dibenzo[b,g]naphtho[1,8-de]oxocine
framework 6i, for which a synthetic method has not
previously been reported. This modular assembly of six- to
nine-membered p systems by simply changing the dihalide
coupling partners highlights the synthetic utility of the cyclic
diarylborinic acids.
the reaction of ditriflate 3a with 4a, followed by addition of
MesLi instead of aqueous work up, led to the formation of 9-
mesityl-9H-boraxanthene 8a, a class of fluorescent com-
pounds that are currently attracting much attention
(Scheme 6).[15]
Importantly, this annulative two-fold cross-coupling can
be performed directly from dihalide 3 and 4a without the
need for chromatographic isolation of borinic acid
1 (Scheme 5). The palladium-catalyzed reaction of 3i and
4a, followed by treatment with 2-aminoethanol, led to the
formation of borinate 1j, which could be isolated by simple
filtration. This material was used directly in a subsequent
annulation with 5a to form 6a in 51% overall yield.
Scheme 6. One-pot synthesis of 9-mesityl-9H-boraxanthene 8a.
In summary, we have developed the first catalytic method
for the synthesis of cyclic diarylborinic acids through a two-
fold borylation of dihalides using 4a as the boron source. This
method allows the synthesis of cyclic diarylborinic acids
bearing a range of functionalities, which could not be
synthesized by previously reported methods using organo-
lithium reagents. The cyclic diarylborinic acids can serve as
In addition to their use as a 1,5-dianion equivalent, cyclic
diarylborinic acid derivatives can also serve as useful pre-
cursors to functional organoboron compounds. For example,
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research from MEXT, Japan and ACT-C (J1210B2576) from
JST, Japan. We also thank the Instrumental Analysis Center,
Faculty of Engineering, Osaka University, for their assistance
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Conflict of interest
The authors declare no conflict of interest.
Keywords: aminoborane · borylation · diarylborinic acid ·
palladium · heteroarene
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Manuscript received: December 26, 2016
Final Article published: && &&, &&&&
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Communications
Synthetic Methods
T. Igarashi, M. Tobisu,*
N. Chatani*
&&& — &&&
Catalytic Double Carbon–Boron Bond
Formation for the Synthesis of Cyclic
Diarylborinic Acids as Versatile Building
Blocks for p-Extended Heteroarenes
The first catalytic synthesis of cyclic
diarylborinic acids is developed using
a dihydroaminoborane reagent as the
boron source. The reaction products
provide rapid access to skeletal diversity,
in particular they enable the synthesis of
six- to nine-membered p-extended
heteroarenes through simple cross-cou-
pling reactions, which are important
synthetic targets in advanced materials
and pharmaceuticals.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!