Site-differentiated polyboron arenes prepared by direct C=H borylation and their highly selective Suzuki-Miyaura cross-coupling reactions

Article abstract of DOI:10.1002/anie.201309546

Di- and polyboron (hetero)arenes, site-differentiated with MIDA boronyl (MIDA=N-methyliminodiacetic acid) and pinacolato boronyl (Bpin), were prepared by an iridium-catalyzed direct C=H borylation of readily available (hetero)aryl MIDA boronates. The exce

Full text of DOI:10.1002/anie.201309546

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Communications
DOI: 10.1002/anie.201309546
Chemoselective Coupling
À
Site-Differentiated Polyboron Arenes Prepared by Direct C H
Borylation and Their Highly Selective Suzuki–Miyaura Cross-Coupling
Reactions**
Liang Xu, Siyi Ding, and Pengfei Li*
Abstract: Di- and polyboron (hetero)arenes, site-differentiated
with MIDA boronyl (MIDA = N-methyliminodiacetic acid)
and pinacolato boronyl (Bpin), were prepared by an iridium-
bis(boronyl) (Bpin/Bdan) arenes by Miyaura borylation of
Bdan-incorporated bromoarenes. Subsequent SMC reaction
was achieved exclusively on the Bpin moiety while the Bdan
group was inactive under usual coupling conditions.[9]
Molander and Sandrock found that terminal alkenes
equipped with an aryl trifluoroborate group underwent
hydroboration with 9-BBN to produce corresponding alkyl
boranes which were useful in subsequent efficient and
selective SMC reactions.[10] Gillis and Burke developed
olefinic compounds functionalized with a N-methyliminodi-
acetic acid (MIDA)[11] boronyl and a Bpin group, and applied
them successfully in the synthesis of complex polyene natural
products.[12] Very recently, Hall and co-workers,[13] and Yun
and co-workers[14] reported the synthesis of gem diboronyl
compounds by enantioselective borylation of borylalkenes,
and demonstrated their uses in stereoselective alkyl SMC
reactions.
Despite the significant advancements highlighted above,
current methods for the synthesis and selective transforma-
tion of differentiated di- or polyboron compounds suffer from
numerous drawbacks. For example, current synthetic
approaches start from monoboron molecules containing
a (pseudo)halide or alkene group, which themselves require
multistep preparations, and sometimes may be inaccessible.
Furthermore, Suginomeꢀs modular synthesis of oligo- and
À
catalyzed direct C H borylation of readily available (hetero)-
aryl MIDA boronates. The excellent synthetic uses of these
multisite nucleophiles were demonstrated by the high-yield
production of
a
variety of multifunctionalized poly-
(hetero)arenes with the highly chemoselective Suzuki–Miyaura
coupling (SMC) of the Bpin moiety being an essential step.
M
odular assembly of small polyfunctionalized building
blocks into complex molecules is an inherently efficient and
flexible approach in organic synthesis, therefore, methods for
the facile synthesis and chemoselective transformation of
polyfunctionalized compounds are highly desirable. In this
context, poly(pseudo)halogen-substituted arenes and hetero-
arenes have been well studied and used as multisite electro-
philes in chemoselective cross-coupling reactions.[1] Excellent
examples of haloboronic acids derivatives have also been
harnessed for iterative electrophilic/nucleophilic cou-
plings.[2,3] In contrast, polymetallic arenes, which may
behave as multisite nucleophiles for this purpose, have been
much less developed.
To date, a few reports have described the successful
synthesis of compounds bearing the combination of boron/
tin,[4] boron/silicon,[5] or silicon/silicon[6] moieties and their
applications in selective cross-coupling reactions. Because of
the broad availability of boronic acid derivatives, and general
efficiency and mildness of Suzuki–Miyaura coupling (SMC),[7]
differentiated polyboron compounds would in principle
represent a class of valuable polyvalent nucleophiles in such
reactions. Despite their potential utility, few examples of
differentiated diboron compounds have been described until
recently (Scheme 1a).[8] Suginome and co-workers first
reported the synthesis of pinacolato/naphthalene-1,8-diamino
[*] L. Xu,[+] S. Ding,[+] Prof. Dr. P. Li
Center for Organic Chemistry, Frontier Institute of Science and
Technology (FIST), Xi’an Jiaotong University
99 Yanxiang Road, Xi’an, Shaanxi, 710054 (China)
E-mail: lipengfei@mail.xjtu.edu.cn
[+] These authors contributed equally to this work.
[**] We are grateful for financial support by the NSFC (No. 21202129),
the MOST (No. 2014CB548200), and Xi’an Jiaotong University. We
thank Prof. Xiaoxing Wu for helpful discussions, Prof. Zhiping
Zheng and Dr. Andrew T. Parsons for proofreading the manuscript,
and Guojun Zhou and Lei Qin for their help on X-ray diffraction
analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309546.
Scheme 1. a) Previous diboron compounds in selective Suzuki–
Miyaura coupling. b) The outline of this work.
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polyarenes, a very useful class of organic molecules, remains
the sole demonstration of the use of site-differentiated
diboron arenes.[9a] In the Suginome method, the Bdan group
must be hydrolyzed to the boronic acid prior to the second
coupling, thus an additional step involving relatively harsh
reaction conditions (e.g. 5m hydrochloric acid) is reqiured.
Finally, to the best of our knowledge, site-differentiated
polyboron arenes have never been documented.
To overcome these limitations, we herein report an
alternative and straightforward approach to the preparation
of polyboron arenes and their subsequent application in SMC
reactions (Scheme 1b). Starting from simple and readily
available aryl MIDA boronates, we proposed that differ-
entiated di- or polyborylated arenes might be obtained by
À
direct C H borylation. This idea was inspired by two
important research areas. Recent work by Gillis and
Burke[15] and Yudin and co-workers[16] showed that the
MIDA boronyl group could be carried through an array of
transformations, thus rendering these substrates an appropri-
ate starting point for our studies. Additionally, iridium-
[17]
À
catalyzed C H borylation, pioneered by Smith et al. and
Hartwig et al.,[18] has proven to be an effective means for
rapid functionalization of a variety of (hetero)arenes.[19]
À
Consequently, we envisioned developing a direct C H
borylation of aryl MIDA boronates[20] and utilizing the
resulting products as polyvalent reagents for modular con-
struction of complex aromatic molecules by consecutive
chemoselective SMC.
À
Scheme 3. Substrate scope of the C H borylation. Thermal ellipsoids
shown at 50% probability.[31] Boc=tert-butoxycarbonyl.
The 3-chlorophenyl MIDA boronate 1a was used as
a model substrate and subjected to various reaction con-
(2 f, 2m and 2n), thus providing multifunctionalized fluo-
roarenes in a step-economic fashion. Heterocyclic boronic
acid derivatives are important building blocks in medicinal
and materials chemistry, but their preparations are challeng-
À
ditions for iridium-catalyzed C H borylation (for details see
Table S1 in the Supporting Information; Scheme 2). To our
ing because of their instability.[22] We found that C H
À
borylation of readily available monoborylated pyridine,
thiophene, furan, and pyrrole derivatives directly provided
differentially diborylated heteroarenes (2g–l) in high
yields.[23] Polyboron compounds are interesting building
blocks for construction of multifunctionalized structures.
When excess (3.3 equivalents) bis(pinacolato)diboron was
used, double borylation occurred to afford high yields of 2l–n.
These compounds represent the first examples of (hetero)-
arenes which are substituted by differently masked polybor-
onyl groups. For the cases of 2e and 2l, 1,10-phenanthroline
was found to be superior to dtbpy as the ligand. Notably, all of
these di- or polyborylated (hetero)arenes were crystalline
solids, and in most cases, could be isolated in good purity by
a simple workup procedure including solvent removal,
precipitation, and washing with diethyl ether, although we
still purified them by flash chromatography for general
considerations.
The utility of the site-differentiated polyboron arenes in
chemoselective SMC reactions was subsequently assessed.
The 2,5-diborylated thiophene 2h and 3-bromoanisole were
chosen as the model substrates (Table 1). After extensive
variation of reaction parameters, we found that a system using
[Pd2(dba)3] as the catalyst and SPhos[24] as the ligand in
anhydrous acetonitrile as the solvent worked best in terms of
reactivity and selectivity. Thus, under the optimal reaction
À
Scheme 2. The model reaction of the C H borylation. cod=cyclo-1,5-
octadiene.
delight, the meta-borylation product 2a could be isolated in
95% yield under our optimized reaction conditions: 1 mol%
[{Ir(OMe)(cod)}2], 4 mol% 4,4’-di-tert-butyl-2,2’-bipyridine
(dtbpy), and 1.1 equivalents of bis(pinacolato)diboron
(B2pin2) as the borylating reagent in 1,4-dioxane at 808C for
15 hours. The result indicated that the MIDA boronyl group
was well tolerated under the reaction conditions. As pre-
viously demonstrated by Smith and co-workers and Hartwig
et al., the borylation occurred at the least sterically hindered
position.[17–19,21]
With the optimized reaction conditions in hand, we
explored the substrate scope for this direct C H borylation
À
of aryl MIDA boronates as shown in Scheme 3. Various
substituents at the 3-position (2b–e) on the phenyl ring were
well tolerated, thus affording the corresponding borylated
products in high to excellent yields. Borylation also occurred
efficiently at the ortho-position of fluorine-substituted arenes
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Table 1: Reaction conditions for the selective mono SMC.
obtained in good to excellent yields. The only exception is the
compound 3n, which was obtained in acceptable yield when
the less stable diboronyl pyridine 2g was used. It is worth
noting that when aryl bromide functionalized with BMIDA
reacted with a differentiated di- or triboron compound, the
same chemoselectivity in SMC was achieved with the
production of polyaryls equipped with two or three MIDA
boronyl groups. These compounds represent a new type of
multisite nucleophile (3o–q) which are expected to be useful
for further chemical elaborations.
Taking the advantage that reactivity of the MIDA boronyl
group could be switched by simply adding water, we
envisioned a two-step consecutive sequence for convergent
and flexible SMC reactions. In comparison with Suginomeꢀs
Bdan-based method,[2a,9a,b] this sequence eliminated the extra
step for activation with strongly acidic conditions. We took 2h
as an example, since thiophene is among the most important
subunits in organic dyes,[25] semiconducting[26] and electro-
optical molecules, and polymers.[27] Thus, the advanced
intermediate 4 was synthesized in two high-yielding steps
involving sequential anhydrous and aqueous SMC reactions
of 2h (Scheme 5). The compound 4 could be elaborated into
the high-performance donor–acceptor dye 5 as previously
reported in the literature.[28] Therefore, doubly nucleophilic
heterocycles such as 2h may serve as versatile building blocks
in relevant areas.[4a,b]
Entry
Solvent
Yield [%][b]
Yield [%][a]
3aa
3a
1
2
3
4
5
6
7
MeCN
1,4-dioxane
toluene
THF
96
29
63
26
80
46
94[c]
<5
<5
<5
<5
<5
28
DMF
MeCN[b]
MeCN
<5
[a] Yields of analytically pure isolated compounds. [b] 5.0 equiv of water
added. [c] 2.1 equiv of aryl bromide used. DMF=N,N-dimethylform-
amide, THF=tetrahydrofuran.
conditions (508C, 10 h), the Bpin group could be selectively
transformed into the aryl group while the BMIDA group
remained intact.[12a,b] After workup and purification, the
desired product 3a was isolated in 96% yield (entry 1).
Solvent choice was critical for the success of this reaction. For
example, solvents such as 1,4-dioxane, toluene, or THF led to
much lower conversions (entry 2–4), while DMF gave full
conversion but diminished yield (entry 5). Addition of water
to the reaction mixture was detrimental to the selectivity,
presumably because both pinacol and MIDA boronates were
hydrolyzed to boronic acids (entry 6).[2b] It is worthy to
mention that the chemoselectivity was remarkably high even
in the presence of excess aryl bromide (entry 7). These results
suggest that the BMIDA group was nearly inert in anhydrous
acetonitrile but could be activated by hydrolysis under
aqueous conditions.
The generality of this selective SMC was subsequently
examined using various combinations of (hetero)aryl bro-
mides and (hetero)arenes site-differentiated with Bpin and
BMIDA groups. Collected in Scheme 4 are the products, all
Scheme 5. Sequential anhydrous and aqueous SMC reactions of 2h.
dba=dibenzylideneacetone.
The flexibility and efficiency of this successive and
selective coupling scheme was further demonstrated with
the high-yield synthesis of the multisubstituted arene 6
(Scheme 6). Multiply arylated or vinylated arenes are impor-
tant motifs in materials[29] and medicinal research.[30] Follow-
Scheme 6. Successive and selective SMC reactions for the synthesis of
the multisubstituted arene 6.
Scheme 4. Substrate scope of the selective mono SMC.
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In conclusion, an efficient regioselective iridium-cata-
À
lyzed C H borylation has been developed for the synthesis of
differentiated di- and polyboron-substituted (hetero)arenes
from readily available aryl and heteroaryl MIDA boronates.
Further investigations culminated in a highly chemoselective
Suzuki–Miyaura cross-coupling of these products. Thus they
may serve as polyvalent nucleophiles for modular construc-
tion of multifunctionalized poly(hetero)arenes by consecutive
cross-coupling reactions. The mode of selective transforma-
tions of these reagents is complementary to that of poly-
(pseudo)halogen and haloboronic acid derivatives.[1,2]
Together, these methods enable rapid, efficient, and flexible
synthesis of complex molecules from simple building blocks.
We anticipate that the protocol described herein will find
wide use in organic synthesis, materials science, and medicinal
chemistry.
Received: November 3, 2013
Published online: January 21, 2014
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À
Keywords: boron · C H functionalization · chemoselectivity ·
homogeneous catalysis · synthetic methods
.
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