Ligand-Promoted Borylation of C(sp3)-H Bonds with Palladium(II) Catalysts

Article abstract of DOI:10.1002/anie.201509996

A quinoline-based ligand effectively promotes the palladium-catalyzed borylation of C(sp³)-H bonds. Primary β-C(sp³)-H bonds in carboxylic acid derivatives as well as secondary C(sp³)-H bonds in a variety of carbocyclic rings, including cyclopropanes, cyclobutanes, cyclopentanes, cyclohexanes, and cycloheptanes, can thus be borylated. This directed borylation method complements existing iridium(I)- and rhodium(I)-catalyzed C-H borylation reactions in terms of scope and operational conditions.

Full text of DOI:10.1002/anie.201509996

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201509996
German Edition: DOI: 10.1002/ange.201509996
CÀH Activation
3
À
Ligand-Promoted Borylation of C(sp ) H Bonds with Palladium(II)
Catalysts
Jian He, Heng Jiang, Ryosuke Takise, Ru-Yi Zhu, Gang Chen, Hui-Xiong Dai,
T. G. Murali Dhar, Jun Shi, Hao Zhang, Peter T. W. Cheng, and Jin-Quan Yu*
Abstract: A quinoline-based ligand effectively promotes the
3
palladium-catalyzed borylation of C(sp )ÀH bonds. Primary
3
b-C(sp )ÀH bonds in carboxylic acid derivatives as well as
3
secondary C(sp )ÀH bonds in a variety of carbocyclic rings,
including cyclopropanes, cyclobutanes, cyclopentanes, cyclo-
hexanes, and cycloheptanes, can thus be borylated. This
directed borylation method complements existing iridium(I)-
and rhodium(I)-catalyzed CÀH borylation reactions in terms
of scope and operational conditions.
[
1]
T
he borylation of CÀH bonds catalyzed by iridium,
[2] [3–5]
rhodium, and other metals
is an important research
topic in the field of CÀH activation, as the newly formed
carbon–boron bonds can be converted into a variety of
[6]
carbon–carbon and carbon–heteroatom bonds. In contrast,
Scheme 1. Palladium-catalyzed directed CÀH borylation.
the development of CÀH borylation reactions with Pd
[7]
catalysts has met with limited success. Building on the
II
Pd -catalyzed cross-coupling of CÀH bonds with organo-
[
8]
[1k–m,2d]
II
3
boron reagents, we and others developed rare examples of
catalytic systems.
Importantly, Pd -catalyzed C(sp )ÀH
directed ortho borylation reactions of arenes with Pd catalysts
borylation proceeds through a reaction mechanism and redox
processes that are fundamentally different to those of the Ir-
and Rh-catalyzed CÀH borylation reactions. Thus the suc-
[
9]
(
Scheme 1a). Recently, Daugulisꢀ bidentate directing group
3
has been applied to the C(sp )ÀH borylation of amines with
2
0 mol% of a Pd catalyst (Scheme 1b), but this method was
cessful development of an effective quinoline-based ligand
[10]
limited to methyl CÀH bonds. Difficulties encountered in
paves the way for the development of further CÀH borylation
3
these efforts to develop a broadly useful C(sp )ÀH borylation
reactions using a new class of catalysts, which may emerge as
an important and complementary approach to existing ones.
reaction pointed to the need for a new ligand that can
3
II
0
drastically promote C(sp )ÀH activation. Herein, we report
We chose the Pd /Pd catalytic cycle as a promising
an efficient Pd-catalyzed b-borylation of carboxylic acid
platform owing to our experience with this system in CÀH
[8]
II
derived amides with bis(pinacolato)diboron through ligand
cross-coupling, as well as the precedents of Pd -catalyzed
3
[9]
acceleration (Scheme 1c). This C(sp )ÀH borylation reaction
CÀH borylation. However, significant challenges have to be
II
is compatible with a-methyl CÀH bonds as well as methylene
CÀH bonds in a wide range of cyclic amide substrates,
including cyclopropanes, cyclobutanes, cyclopentanes, cyclo-
overcome for Pd -catalyzed CÀH borylation. First, reductive
elimination from a PdÀB species is known to be inefficient in
the absence of a suitable ligand, as shown in the palladium-
3
[11]
hexanes, and cycloheptanes. The C(sp )ÀH borylation of this
catalyzed borylation of aryl halides. A second potentially
class of substrates has not been demonstrated using other
problematic issue is that the borylated products could also
II
react with Pd by transmetalation, resulting in deborylation
or b-hydride elimination reactions. These potential hurdles
[
*] J. He, H. Jiang, R. Takise, R.-Y. Zhu, Dr. G. Chen, Prof. Dr. H.-X. Dai,
Prof. Dr. J.-Q. Yu
Department of Chemistry
[9a,10]
could account for the low efficiency of previous methods
3
3
for C(sp )ÀH borylation. For example, C(sp )ÀH borylation
The Scripps Research Institute (TSRI)
according to our highly optimized conditions initially devel-
1
0550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
oped for the ortho CÀH borylation of benzamides resulted in
E-mail: yu200@scripps.edu
low yields (see the Supporting Information). These early
results suggested that it is crucial to identify a ligand that can
drastically accelerate CÀH activation under relatively mild
Dr. T. G. M. Dhar
Discovery Chemistry, Bristol-Myers Squibb Company
Route 206 and Provinceline Road, Princeton, NJ 08543 (USA)
conditions so that decomposition of the borylated products
Dr. J. Shi, Dr. H. Zhang, Dr. P. T. W. Cheng
Discovery Chemistry, Bristol-Myers Squibb Company
II
may be avoided. Our recently reported ligand-enabled Pd -
3
catalyzed cross-coupling of C(sp )ÀH bonds with organo-
3
11 Pennington Rocky Hill Road, Pennington, NJ 08534 (USA)
II
0
[12]
silicon coupling partners through a Pd /Pd manifold
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201509996.
prompted us to focus on developing pyridine- or quinoline-
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based ligands that might promote C(sp )ÀH borylation
group (L9) drastically decreased the yield to 30%, revealing
that the steric properties of ligands have a strong effect on the
reaction. Gratifyingly, further increasing the electron density
on the quinoline ligand improved the yield to 67% (L10).
Various bulkier alkoxy groups at the 2- and 4-position of
quinoline consistently led to a decrease in yield (L11–L14). To
further improve the turnover of this reaction, we screened
a number of ammonium salts that are known to prevent the
[
9a]
reactions. Based on our previous method,
we screened
further palladium catalysts, bases, oxidants, and solvents using
alanine-derived amide substrate 1, which resulted in minor
improvements (Table 1). The use of HOAc (20 mol%) helped
3
Table 1: Screening of ligands for the C(sp )ÀH borylation of alanine-
[
a]
derived amide 1.
0
[14]
aggregation of Pd species. The use of tetraethylammonium
tetrafluoroborate (TEABF ) increased the yield to 73%. To
4
elucidate the role of the ligand and TEABF , we examined
4
the influence of L10 and additives on the rate profile (see the
Supporting Information). The ligand increased the initial rate
of the borylation reaction by a factor of five whereas TEABF4
did not influence the rate. The substrates containing less
electron-deficient amide auxiliaries gave much lower yields
(see the Supporting Information), which is consistent with our
previous observation that the acidity of aryl amides is
3
important for the C(sp )ÀH activation. We have previously
shown that the deprotonation of amides by inorganic bases
3
promotes the activation of C(sp )ÀH bonds, leading to the
[13]
formation of palladacycle intermediates. Control experi-
ments confirmed that O is needed to render this reaction
2
catalytic (see the Supporting Information). The requirement
II
IV
of O for catalytic turnover is not consistent with a Pd /Pd
2
II
0
catalytic cycle. Instead, the proposed Pd /Pd catalytic cycle is
most likely operative. Importantly, this reaction also pro-
ceeded under air to give the desired product in 56% yield.
With the optimized reaction conditions in hand, we then
subjected various carboxylic acid derived amides to the
[
a] Reaction conditions: substrate 1 (0.10 mmol), B pin (2.0 equiv),
2 2
Pd(OAc) (10 mol%), ligand (20 mol%), HOAc (20 mol%), KHCO3
(
by H NMR analysis of the crude product mixture using CH Br as the
internal standard. [b] TEABF (50 mol%) was added. [c] Yield of isolated
2
2.0 equiv), CH CN (1.5 mL), O , 808C, 15 h. The yield was determined
3 2
1
3
C(sp )ÀH borylation reaction conditions (Table 2). The
2
2
4
borylation of substrates containing a-quaternary centers is
efficient in general (4a–4l). Compound 3c, an amide
derivative of the drug gemfibrozil, was borylated to give 4c
in 70% yield. Aryl groups at the b- or g-positions are well
tolerated (4e and 4 f). It is worth mentioning that this
borylation method is highly monoselective in the presence of
two or three methyl groups, as the newly installed boron
moiety is quite bulky and may also coordinate to the amide
product.
[
13]
prevent substrate decomposition, and slightly improved the
yield (see the Supporting Information). As 2-picoline (L1) is
known to accelerate the CÀH arylation of alanine-derived
[
13]
amide 1, we began our ligand screening with L1. Disap-
pointingly, L1 gave a slightly lower yield, which indicates that
this ligand may not be compatible with the transmetalation or
reductive-elimination step. Among several other pyridine
ligands (see the Supporting Information), only 2,4,6-trime-
thoxypyridine improved the yield, namely to 45%. We then
turned our attention to the electron-rich tricyclic quinoline
ligands (L2 and L3) that were previously used to promote
[
9b,15]
3
auxiliary,
thus preventing a second b-C(sp )ÀH activation
of the products (4a–4 f). The monoselectivity observed with
isobutyric and pivalic acid substrates is a distinct advantage
over many b-CÀH functionalization reactions where mixtures
of the mono- and difunctionalization products are
[
16]
obtained.
Amides possessing a-protons are compatible
with the reaction conditions as well (4m–4r). We also
performed the borylation reactions using 5 mol% of the
palladium catalyst, which gave the borylated products (4d–
3
II
0
[13]
C(sp )ÀH olefination by Pd /Pd catalysis. Encouragingly,
the use of L3 improved the yield to 51%. Whereas simple
acridine (L4) gave a similar result to L3, installation of
a methoxy group at the 9-position of acridine increased the
yield to 55% (L5). Replacement of L5 with electron-deficient
3
4 f) in high yields. Importantly, the C(sp )ÀH borylation was
carried out on a gram scale without any additives to provide
II
4a in 67% yield (Scheme 2), which clearly indicates that Pd /
9
-chloroacridine (L6) drastically decreased the yield to 18%,
L is the real catalyst and that the additives only provide minor
suggesting that electron-rich substituents on the ligand are
beneficial. Guided by this observation, we systematically
introduced alkyl and alkoxy groups onto the quinoline ring
improvement.
Whereas the methylene CÀH borylation of an acyclic
amide derived from 2-ethylbutanoic acid (3t) gave the
borylated product in 25% yield (see the Supporting Informa-
tion), a variety of cyclic amides were borylated in syntheti-
cally useful yields (Table 3). For example, a phthalimido
group is tolerated under the reaction conditions (6c and 6g).
(
see the Supporting Information). In spite of the poor
reactivity resulting from the use of ligand L7, 2-methoxyqui-
noline (L8) provided the desired product in 58% yield.
Replacing the methoxy group by a more hindered isobutoxy
7
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3
[a]
3
[a]
Table 2: Substrate scope of the C(sp )ÀH borylation.
Table 3: b-C(sp )ÀH borylation of cyclic carboxylic acid derivatives.
[
a] Reaction conditions: 5a–5l (0.10 mmol), B pin (2.0 equiv), Pd-
2 2
(
(
OAc) (10 mol%), L10 (20 mol%), HOAc (20 mol%), TEABF4
2
50 mol%), KHCO (2.0 equiv), CH CN (1.5 mL), O , 808C, 15 h. Yields
3
3
2
of isolated products are given. [b] Pd(OAc) (5 mol%) and L10
2
(10 mol%) were used.
of Pd(OAc) [Eq. (1)]. This unprecedented low palladium
2
loading speaks to the importance of ligand effects in CÀH
activation reactions.
[
a] Reaction conditions: 3a–3r (0.10 mmol), B pin (2.0 equiv), Pd-
2 2
(
(
OAc) (10 mol%), L10 (20 mol%), HOAc (20 mol%), TEABF
2 4
50 mol%), KHCO (2.0 equiv), CH CN (1.5 mL), O , 808C, 15 h. Yields
3
3
2
of isolated products are given. [b] Pd(OAc) (5 mol%) and L10
2
(10 mol%) were used.
3
To illustrate the synthetic utility of this C(sp )ÀH bor-
ylation reaction (Scheme 3), we subjected substrate 7, which
is derived from dehydroabietic acid, to the borylation
conditions. Treating the crude borylation product with hydro-
gen peroxide in THF afforded the desired hydroxylated
product 8 in 70% yield over two steps. The borylated alanine
3
Scheme 2. Gram-scale C(sp )ÀH borylation.
2
reacted with N-ethylaniline in the presence of Cu(OAc) as
2
the catalyst and Ag CO as the oxidant to give the b-aminated
2
3
[17]
In contrast to the cis diastereomers obtained with cyclo-
propyl, cyclobutyl, and cyclopentyl substrates, the reactions of
the cyclohexyl and cycloheptyl substrates 5i and 5j afforded
mixtures of the cis and trans diastereomers in a 1:1 ratio (6i
and 6j). The equatorial directing group in cyclohexyl rings is
known to enable both cis and trans cyclopalladation owing to
the chair conformation. We also performed the borylation of
the bicyclo[2.2.2]octanes 5k and 5l to rapidly diversify this
class of three-dimensional drug scaffolds. Notably, Ir-cata-
product 9 in 90% yield. Cyclobutylboronate ester 6c was
easily converted into the trifluoroborate salt 10 in 95% yield
by using aqueous KHF in acetonitrile. Under the Suzuki–
2
Miyaura cross-coupling reaction conditions developed by
[18a]
Molander and co-workers,
10 was coupled with 1-chloro-4-
nitrobenzene to give the arylated cyclobutane 11 in moderate
yield. Whereas complete inversion of stereochemistry is
generally observed for reactions with seemingly similar
[15a,18]
alkyl boron reagents that proceed by transmetalation,
3
lyzed C(sp )ÀH borylation of carbocyclic systems has only
the transformation of 10 into 11 proceeded with retention,
which is consistent with a transmetalation step that is directed
by the amide auxiliary. The acidic amide is known to form an
effectively coordinating amidate after deprotonation under
[
1m]
been demonstrated with cyclopropylamine
and 2-cyclo-
[1k]
hexylpyridine substrates. To further demonstrate the effi-
ciency of this ligand-promoted borylation reaction, we con-
ducted the reaction of cyclobutyl substrate 5e with 1–2 mol%
[13]
basic conditions.
Finally, fluorination of the borylated
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Figure 1. Plausible mechanism for C(sp )ÀH borylation.
structure–reactivity relationship indicate that electron-rich
and sterically less hindered quinoline ligands are superior.
Scheme 3. Further transformations of the products obtained by C-
3
(
(
sp )ÀH borylation. a) B pin (2.0 equiv), Pd(OAc) (10 mol%), L10
2
2
2
20 mol%), HOAc (20 mol%), TEABF (50 mol%), KHCO (2.0 equiv),
4
3
CH CN, O , 808C, 15 h; b) H O , aqueous buffer (pH 7), THF, RT, 2 h;
3
2
2
2
Acknowledgements
c) N-ethylaniline (1.5 equiv), Cu(OAc) (10 mol%), Ag CO (2.0 equiv),
2
2
3
toluene, 1008C, 20 h; d) aq. KHF (4.5m, 5.1 equiv), CH CN, RT, 3 h;
2
3
e) 1-chloro-4-nitrobenzene (1.0 equiv), 10 (1.2 equiv), Pd(OAc)2
10 mol%), SPhos (20 mol%), Cs CO (3.0 equiv), cyclopentyl methyl
We gratefully acknowledge The Scripps Research Institute
and the NIH (NIGMS, 2R01GM084019) for financial sup-
port. We thank the NSF under the Science Across Virtual
Institutes program as part of the CCI Center for Selective CÀ
(
2
3
ether (CPME)/H O (6.7:1), N , 958C, 20 h; f) AgNO (20 mol%),
2
2
3
Selectfluor (3.0 equiv), TFA (4.0 equiv), DCM, H O, N , 508C, 6 h.
2
2
H Functionalization for funding a visiting student (R.T.,
CHE-1205646). We thank Prof. Donna G. Blackmond (The
Scripps Research Institute) for insightful discussions.
[19]
product 6c in the presence of AgNO3 and Selectfluor
proceeded smoothly to yield two diastereomers, trans-12
and cis-12, in 39% and 53% yield, respectively. The
Keywords: amino acids · borylation · CÀH activation ·
3
combination of C(sp )ÀH borylation with fluorination could
palladium · synthetic methods
find widespread application in drug discovery.
3
Our desire to improve this potentially powerful C(sp )ÀH
How to cite: Angew. Chem. Int. Ed. 2016, 55, 785–789
Angew. Chem. 2016, 128, 795–799
borylation method prompted us to perform further kinetic
studies on the dependence of the reaction on the concen-
tration of the palladium catalyst and substrate 1 (see the
Supporting Information). We found that the borylation
reaction is first order with respect to Pd and zero order in
the substrate. The observed intermolecular kinetic isotope
effect (KIE) of 3b and [D ]-3b (k /k = 3.3) suggests that CÀ
[
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6
H
D
3
H cleavage is the rate-limiting step in this C(sp )ÀH
borylation (see the Supporting Information). To further
investigate the reaction mechanism, we carried out a stoichio-
metric reaction of the palladacycle intermediate with B pin2
2
and obtained the desired borylated product in 59% yield (see
the Supporting Information). Based on these mechanistic
II
0
data, we propose the Pd /Pd catalytic cycle shown in
Figure 1.
In summary, we have developed a versatile palladium-
3
catalyzed b-C(sp )ÀH borylation of a wide range of carboxylic
acid derivatives using a quinoline-based ligand. The new
3
method is compatible with methyl C(sp )ÀH bonds in both a-
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3
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organic synthons through carbon–carbon and carbon–heter-
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Received: October 26, 2015
Published online: November 27, 2015
Angew. Chem. Int. Ed. 2016, 55, 785 –789
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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