Enantioselective Borylation of Aromatic C?H Bonds with Chiral Dinitrogen Ligands

Article abstract of DOI:10.1002/anie.201702628

The borylation of C?H bonds catalyzed by transition metals has been investigated extensively in the past two decades, but no iridium-catalyzed enantioselective borylation of C?H bonds has been reported. We report a set of iridium-catalyzed enantioselective borylations of aromatic C?H bonds. This reaction relies on a set of newly developed chiral quinolyl oxazoline ligands. This process proceeds under mild conditions with good to excellent enantioselectivity, and the borylated products can be converted to enantioenriched derivatives containing new C?O, C?C, C?Cl, or C?Br bonds.

Full text of DOI:10.1002/anie.201702628

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201702628
German Edition: DOI: 10.1002/ange.201702628
Asymmetric Catalysis
À
Enantioselective Borylation of Aromatic C H Bonds with Chiral
Dinitrogen Ligands
Bo Su, Tai-Gang Zhou, Pei-Lin Xu, Zhang-Jie Shi,* and John F. Hartwig*
À
Abstract: The borylation of C H bonds catalyzed by tran-
sition metals has been investigated extensively in the past two
decades, but no iridium-catalyzed enantioselective borylation
À
of C H bonds has been reported. We report a set of iridium-
À
catalyzed enantioselective borylations of aromatic C H bonds.
This reaction relies on a set of newly developed chiral
quinolinyl oxazoline ligands. This process proceeds under
mild conditions with good to excellent enantioselectivity, and
the borylated products can be converted to enantioenriched
À
À
À
À
derivatives containing new C O, C C, C Cl, or C Br bonds.
A
rylboron compounds are important and versatile synthetic
intermediates in organic synthesis that are widely used in drug
discovery and material science.[1] Among various reactions
À
À
that form C B bonds, the direct borylation of aromatic C H
bonds catalyzed by transition metals is particularly valua-
ble.[2,3] The borylation occurs under mild conditions with high
functional group compatibility. The reactions of arenes occur
À
at the least sterically hindered aromatic C H bond (Sche-
me 1a).[2d]
To achieve selectivity that complements that of undirected
À
Scheme 1. Transition metal-catalyzed borylation of aromatic C H
bonds.
À
borylation reactions, the borylations of C H bonds have been
conducted with arenes containing directing groups that
coordinate to the transition metal catalyst (Scheme 1b).[2e,4]
However, most directing groups are difficult to remove after
the introduction of the boryl group. To address this issue, our
reported.[6] The absence of such an enantioselective process
can be traced to the type of catalyst typically used for the
À
À
À
group developed silyl-directed borylations of C H bonds
borylation of C H bonds. The borylation of C H bonds
occurs under the mildest conditions with iridium catalysts
containing planar bipyridine and phenanthroline ligands,
which are achiral. The lack of steric demand and strong
electron donation of these ligands are two properties that
have been proposed to be crucial to the activity of these
catalysts,[2d] so a chiral analog of such ligands should fit these
requirements to achieve both reactivity and selectivity.
Rhodium complexes ligated by chiral bisphosphines catalyze
(Scheme 1c).[5] The silyl directing group can be easily
installed and removed or it can be used to create additional
functionality at the position of the silyl substituent.
Although these undirected and directed borylations of
C H bonds are now commonly used, no iridium-catalyzed,
enantioselective borylation of C H bonds has been
À
À
[*] Dr. B. Su, Dr. T.-G. Zhou, P.-L. Xu, Prof. Dr. Z.-J. Shi
Beijing National Laboratory of Molecular Sciences (BNLMS) and Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education College of Chemistry and Molecular Engi-
neering, Peking University
À
the silylation of aryl C H bonds and could be an alternative
platform on which to develop enantioselective borylation,[7]
but rhodium complexes with such bisphosphines have not
À
been reported to catalyze the borylation of C H bonds.
Beijing (China)
E-mail: zshi@pku.edu.cn
Recently, we described the preparation of a series of chiral
pyridyl and quinolinyl oxazoline ligands.[8] Specific ligands in
this class and an iridium precursor were shown to form
complexes that catalyze the enantioselective silylation of
Dr. B. Su, Prof. Dr. J. F. Hartwig
Department of Chemistry, University of California
Berkeley, CA 94720 (USA)
C H bonds.[9] Here, we show that a complex generated from
À
E-mail: jhartwig@berkeley.edu
an iridium precursor and a specific chiral quinolinyl oxazoline
ligand catalyzes the enantioselective borylation of aromatic
C H bonds (Scheme 1d) at or below room temperature
with good to excellent enantioselectivity. The process occurs
with the highest yield and enantioselectivity with a ligand
related to, but distinct from, that for the enantioselective
Prof. Dr. Z.-J. Shi
Department of Chemistry, Fudan University, Shanghai 200433
(China)
[10]
À
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201702628.
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silylation, underscoring the importance of creating a modular
library of ligands for this class of enantioselective reaction.
To identify a chiral catalyst for the enantioselective
enantioselectivity. Reactions conducted with L8 and L11
containing tert-butyl and cyclohexyl groups in the position of
an isopropyl group in L7 afforded 2a with slightly higher
enantioselectivity than those conducted with L7. However,
the reactions catalyzed by complexes of ligands (L9, L10, and
L12) containing smaller benzyl, phenyl, and even larger
diphenylmethyl groups and with L13 containing geminal
dimethyl substituents buttressing the isopropyl group oc-
curred with lower enantioselectivity. The steric effect at the
C8 position of the quinoline was also investigated. L14
containing a methyl group at the C8 position formed a catalyst
that was less reactive and selective than L7. Thus, an increase
in steric hindrance at the oxazoline ring and the C8-position
of the quinoline ring without further changes to the structure
did not significantly increase the enantioselectivity.
À
borylation of aromatic C H bonds, the reaction of diaryl-
methylsilane 1a was studied (Table 1). The silyl group in this
À
substrate should direct the borylation to an ortho C H bond
Table 1: Evaluation of chiral ligands for the enantioselective borylation of
a diarylmethylsilane.[a]
Reactions catalyzed by complexes of L15 containing an
electron-donating OMe group and of L16 containing an
electron-withdrawing trifluoromethyl group revealed the
effect of the electronic properties of the quinoline ring.
Complexes of these ligands catalyzed the borylation in yields
that were comparable and enantioselectivity that was slightly
lower than those of reactions catalyzed by complexes of L7.
Thus, varying the electronic properties of the ligand also was
not a path to higher enantioselectivity.
In contrast to varying the steric and electronic properties
of the nitrogen heterocycles, fusion of an indane skeleton to
the oxazoline ring (L17) led to catalysts that formed product
2a with a higher er (85:15) and acceptable yield (62%).
Although ligand L18 generated a more reactive and stereo-
selective catalyst than L17 during our recent study on iridium-
À
catalyzed enantioselective silylation of aromatic C H bonds,
the yield and enantioselectivity of the borylation catalyzed by
the Ir-L18 system were lower than those of the borylation
catalyzed by the Ir-L17 system. This result pointed to the
importance of the quinoline ring in this class of ligand and
illustrates the value of a library of ligands for which a small
change in structure can lead to significant changes in
reactivity and stereoselectivity.
1
[a] The yields refer to values obtained by H NMR spectroscopy with
CH2Br2 as internal standard, and the er values were determined by chiral
HPLC.
After identifying a chiral ligand that generates an active
and enantioselective catalyst, additional reaction parameters,
including temperature, solvent, ratio of substrate 1a to B2pin2,
and catalyst loading, were examined (see the Supporting
Information, Table S1). The enantioselectivity was higher
(87:13 er) when the reaction was conducted at 08C than when
conducted at room temperature (85:15 er) (Table S1, entry 1).
The enantioselectivity was even higher (93:7 to 96:4 er) when
the reaction was run in alkane or less polar ether solvents, but
the yields were lower (35 to 62%) in these solvents than they
were in THF (66%) (Table S1, entries 1–6). Thus, the
reactions were run with a small excess of B2Pin2 (1.2 vs.
1.0 equiv) and a slightly higher catalyst loading (3 mol% vs.
2 mol%, Table 2, entries 7 and 8). With this stoichiometry in
methyl tert-butyl ether (MTBE) solvent, the product 2a was
obtained in 81% isolated yield with 96:4 er and less than 10%
diborylated product.
À
on the arene, and the borylated product would contain a C B
À
bond and a C Si bond that can be converted individually to
distinct functionalities under the appropriate conditions. For
this reaction, we first examined the combination of [Ir-
(cod)OMe]2 and a series of commercially available, C2-
symmetric ligands, including bisphosphine ligand L1, triden-
tate nitrogen PyBOX ligand L2, bipyridine ligand L3, and
BOX ligands L4 and L5. Complexes generated from these
ligands were inactive as catalysts for the borylation reaction.
In contrast, when C1-symmetric ligands containing two
nitrogen donors were used in the reaction, the reactivity was
higher, and the enantioselectivity was measurable. For
example, reactions conducted with the pyridinyl oxazoline
ligand L6 formed the functionalized product in 37% yield,
albeit with a low 54:46 er. The same reaction catalyzed by
a combination of quinolinyl oxazoline L7 and [Ir(cod)OMe]2
formed the borylated product 2a in 64% yield with a higher
72:28 er.
Because rhodium complexes containing phosphines also
catalyze the borylation of aromatic C H bonds,[3c,4d] we tested
À
Reactions conducted with ligands L8–L14 revealed the
effect of the steric properties of the oxazoline ligand on
various combinations of rhodium precursors and chiral
phosphines as catalyst for the enantioselective borylation
2
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À
Table 2: Scope of the enantioselective borylation of aromatic C H
However, the reaction of naphthyl-substituted 1r formed
product 2r with moderate enantioselectivity.
bonds.[a]
The enantioselective borylation occurs on larger scale as
well as on small scale. The reaction of 1a on a 5 mmol scale
[Eq. (1)] formed 2a in 71% yield and 95:5 er with just 0.75%
mol% catalyst instead of the 3 mol% catalyst in reactions on
an 0.25 mmol scale.
To illustrate the utility of the reaction, we investigated
transformations of the silyl and the boryl groups in compound
2a (Scheme 2). Silane 2a was transformed to silyl ether 3 in
75% yield in the presence of an iridium catalyst. Treatment of
[a] Yields of isolated products for reactions conducted on a 0.25 mmol or
0.10 mmol scale. The er values were determined by chiral HPLC. The
absolute configuration was assigned by analogy (details see the
Supporting Information).
Scheme 2. Versatile transformations of the borylated products.
(for details, see the Supporting Information). However, no
conversion was observed for reactions conducted with any of
the rhodium catalysts we generated from [Rh(cod)Cl]2 and
chiral bisphosphine ligands.
compound 3 with H2O2 and KHCO3, converted the silyl and
boryl groups to hydroxyl groups. Other processes led to
À
reaction exclusively at the C B bond. Under Suzuki coupling
The scope of the borylation of diarylmethylsilane 1 under
the conditions described above is shown in Table 2. The
reaction occurred with high enantioselectivity with alkyl, aryl
and alkoxy substituents on the aryl ring. For example, the
reactions of unsubstituted 1a and the meta-substituted 1b–1 f
formed the corresponding products 2a–2 f with excellent
enantioselectivity (95:5 to 98:2 er). The reaction of meta-
chloro 1g led to the corresponding product 2g in slightly
lower enantioselectivity (85:15 er). For reasons we do not
understand, the reaction of para-fluoro 1h formed product 2h
with significantly low er (61:39 er). The reactions of substrates
(1i–1l) containing alkyl, aryl, and silyl groups at the para
position proceeded with moderate to good enantioselectivity.
The presence of an ortho-substituent had a significant neg-
ative influence on the yield and enantioselectivity, suggesting
that this substituent causes the substrate to adopt an
unfavorable conformation. The enantioselectivity of the
reaction of ortho-methyl-substituted 1n was lower than that
of ortho-fluoro substituted 1m. Disubstituted substrates (1o–
1q) also underwent borylation with high enantioselectivity.
conditions, the reaction of 3 formed product 5 from coupling
À
at the C B bond in 65% yield. Reaction of compound 3 with
À
À
CuCl2 or CuBr2 led to conversion of the C B bond to a C Cl
À
or C Br bond, respectively. Under these conditions, the silyl
ether hydrolyzed to the corresponding silanol, forming
products 6 and 7 in 75% and 89% yields, respectively.
Silanols have been studied in medicinal chemistry because the
hydrogen-bonding abilities and acidity of silanols are higher
than those of their carbon analogues.[11] No significant erosion
of the enantiopurity of the products was observed during any
of the transformations.
In summary, we have developed the first Ir-catalyzed
enantioselective borylation of aromatic C H bonds through
À
a silyl-directed desymmetrization. The success of the reaction
relies on the development of a catalyst derived from an
iridium precursor and a chiral quinolinyl oxazoline ligand.
The borylated products can be converted to a series of final
products containing a series of different functional groups at
the position of the aryl–boron bond. Further studies to
elucidate the mechanism of the reaction and to expand the
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Acknowledgements
B.S. thanks the MSD Scholarship. B.S. and Z.J.S thank the
MOST (2015CB856600 and 2013CB228102) and NSFC (Nos.
21332001 and 21431008) for support of work at Peking
University. B.S. and J.F.H. thank the NIH (GM-11581201) for
support of work at Berkeley.
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Conflict of interest
The authors declare no conflict of interest.
À
Keywords: asymmetric catalysis · borylation · C H activation ·
chiral dinitrogen ligand · iridium
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Manuscript received: March 13, 2017
Revised: April 7, 2017
Final Article published: && &&, &&&&
4
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Communications
Asymmetric Catalysis
B. Su, T.-G. Zhou, P.-L. Xu, Z.-J. Shi,*
J. F. Hartwig*
&&&& — &&&&
Enantioselective Borylation of Aromatic
À
À
C H Bonds with Chiral Dinitrogen
Ligands
Enantioselective C H borylation: The
first Ir-catalyzed enantioselective boryla-
this reaction relies on a newly developed
catalyst that derives from an iridium
precursor and a chiral quinolinyl oxazo-
line ligand.
À
tion of aromatic C H bonds was devel-
oped with up to 98:2 er. The success of
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www.angewandte.org
5
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