A cobalt-catalyzed alkene hydroboration with pinacolborane

Article abstract of DOI:10.1002/anie.201310096

An extremely efficient cobalt catalyst for the hydroboration of both vinylarenes and aliphatic α-olefins with pinacolborane is described, providing the anti-Markovnikov products with excellent regio- and chemoselectivity, broad functional-group tolerance,

Full text of DOI:10.1002/anie.201310096

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DOI: 10.1002/anie.201310096
Synthetic Methods
A Cobalt-Catalyzed Alkene Hydroboration with Pinacolborane**
Lei Zhang, Ziqing Zuo, Xuebing Leng, and Zheng Huang*
Abstract: An extremely efficient cobalt catalyst for the hydro-
boration of both vinylarenes and aliphatic a-olefins with
pinacolborane is described, providing the anti-Markovnikov
products with excellent regio- and chemoselectivity, broad
functional-group tolerance, and high turnover numbers (up to
19800). The alkene hydroboration route is further extended to
a two-step, one-pot hydroboration and cross-coupling of
alkylboronates with aryl chlorides.
catalysts that show high regio- and stereoselectivity in the
hydroboration of 1,3-dienes with pinacolborane (HBpin).[9] In
2013, we reported that a [(tBuPNN)FeCl2] pincer complex
1 (Figure 1) is even more efficient than the well-documented
Rh and Ir catalysts in the hydroboration of a-olefins with
A
lkylboronic acid derivatives are versatile intermediates for
organic synthesis, as they are utilized in the construction of
[1]
À
À
À
various C C, C O, and C N bonds. In the context of the
3
2
3
3
À
À
transition-metal-catalyzed C(sp ) C(sp ) and C(sp ) C(sp )
bond-forming reactions, the Suzuki–Miyaura coupling using
C(sp3) organoboron compounds has emerged as a powerful
protocol because of the thermal stability of the nucleophiles,
nontoxic nature of the inorganic by-products, operational
simplicity, and wide functional-group compatibility.[2] Tradi-
tionally, alkylboronic acid derivatives are synthesized by
addition of alkyllithium or alkylmagnesium reagents to
suitable boron compounds.[3] However, the use of reactive
organometallic reagents is restricted to substrates without
sensitive functional groups. Recently, transition-metal-cata-
lyzed borylations have been developed for the preparation of
alkylboronic acid derivatives. For example, Hartwig and co-
workers reported a direct alkylboronate synthesis involving
Rh-catalyzed alkane borylation with B2Pin2.[4] In 2012, Liu,
Marder, Steel, and co-workers reported a Cu-catalyzed
borylation of alkyl halides with B2Pin2 to form alkylboronates
with diverse functionalities.[5]
Figure 1. Synthesis of [(tBuPNN)CoCl2] (2), [(iPrPNN)CoCl2] (3), and
[(iPrPNN)FeCl2] (4), and the solid-state structures of the cobalt com-
plexes 2 and 3.
HBpin.[10] However, the Fe catalyst is unreactive toward
internal olefins. In a subsequent report, Obligacion and
Chirik demonstrated that bis(imino)pyridine iron dinitrogen
complexes are effective for hydroboration of a-olefins and
cycloalkenes.[11] More recently, Greenhalgh and Thomas
showed that a combination of FeCl2 and a bis(imino)pyridine
ligand, upon activation with EtMgBr, is also active for alkene
hydroborations.[12] There are limitations to the scope of iron
catalysts: preliminary studies in our group showed that a-
olefins containing allyl ethers and ketones cannot be effi-
ciently hydroborated.[13]
Very recently, there has been noteworthy progress in the
development of cobalt catalysts for alkene hydrogenations[14]
and hydrosilylations.[15] For example, Hanson and co-workers
reported a novel aliphatic PNP pincer CoII alkyl catalyst for
the hydrogenation of olefins, ketones, aldehydes, and imin-
es.[14a,b] Chirik and co-workers developed an enantiopure Co
complex of a bis(imino)pyridine ligand for the asymmetric
hydrogenation of gem-disubstituted alkenes.[14c] They also
showed that a Co complex of an biscarbene pyridine ligand is
an effective precatalyst for the hydrogenation of sterically
hindered alkenes.[14d] However, examples of cobalt-catalyzed
alkene hydroborations are very rare.[16]
With respect to atom economy, alkene hydroboration is
a more attractive approach to alkylboronate esters.[1b,6]
Although dialkylboranes react readily with alkenes, the
addition of dialkoxyboranes to alkenes occurs sluggishly in
the absence of catalyst. Most catalytic alkene hydroborations
involve complexes of precious metals, such as Rh[7] and Ir.[7k,8]
However, the low abundance, economic constrains, and
environmental concerns over noble metals have inspired the
investigation of earth-abundant and inexpensive base–metal
alternatives. In 2009, Ritter described iminopyridine iron
[*] L. Zhang, Z. Zuo, Dr. X. Leng, Prof. Dr. Z. Huang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
345 Lingling Road, Shanghai 200032 (China)
E-mail: huangzh@sioc.ac.cn
Herein, we report the preparation of PNN pincer Co
complexes and their application in Co-catalyzed alkene
hydroborations. This system is remarkably efficient for the
anti-Markovnikov hydroboration of both vinylarenes and
aliphatic a-olefins with HBpin. Most reactions proceeded to
completion with 0.005–0.05 mol% of catalyst with or without
solvent. Beyond high activity, the new base–metal system
offers broad functional-group tolerance and excellent chemo-
[**] We gratefully acknowledge the financial support from the National
Natural Sciences Foundation of China (No. 21272255, 21121062),
the “1000 Youth Talents Plan”, and the Chinese Academy of
Sciences.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310096.
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Chemie
selectivity. In addition, we describe a two-step, one-pot
protocol comprising Co-catalyzed hydroboration and
a Pd-catalyzed cross-coupling of alkylboronates with aryl
chlorides.
the hydroboration of both vinylarenes and aliphatic a-olefins.
a
The reaction of 5a with only 0.05 mol% of 3 in THF at 258C
afforded isolated product 6a in 91% yield after 15 minutes
(Table 1, entry 8). Under the “solvent-free” conditions, the
reaction proceeded to completion within three minutes,
The synthesis of the [(RPNN)CoCl2] (2: R = tBu; 3: R =
iPr) complexes is outlined in Figure 1. Treatment of Milsteinꢀs
PNN ligands, which contain phosphino tert-butyl and phos-
phino isopropyl groups, with CoCl2 resulted in the Co pincer
complexes 2 and 3, respectively, in good yields. The iron
analogue 4, which is supported by the iPrPNN ligand, was also
prepared in a similar procedure.[17] Figure 1 includes the solid-
state structures of 2 and 3, which show a distorted square-
pyramidal geometry at the mononuclear Co site.[18]
corresponding to
a
turnover frequency of 40000 hÀ1
(Table 1, entry 9). In the reaction with 7a, the catalyst loading
could be reduced to 0.01 mol% and isolated product 8b was
obtained in nearly quantitative yield (95%) after one hour
(Table 1, entry 11). The desired hydroboration products were
detected as the sole products in each instance (Table 1,
entries 7–11).
To investigate the scope and limitation of the cobalt
system, we used complex 3 as the precatalyst for the
hydroboration of a diverse array of vinylarenes. The results
are summarized in Table 2. All reactions were selective for
the formation of the linear hydroboration products. Most
reactions proceeded to completion within 15 minutes at 258C
Our initial experiments focused on assessing the activity
of the Co and Fe precatalysts for the hydroboration of
vinylarenes and aliphatic a-olefins with HBpin. In our
previous report, we showed that the Fe precatalyst 1, upon
activation with NaBHEt3, is efficient in the hydroboration of
aliphatic a-olefins (Table 1, entry 2). For vinylarenes, how-
ever, the hydroboration process was accompanied by a dehy-
Table 2: Hydroboration of vinylarenes (5b–n) with HBpin catalyzed by 3.
Table 1: Co- and Fe-catalyzed alkene hydroboration.[a]
Substrate
3
t
Yield
[%][b]
[mol%]
[min]
1
2
5b
5c
0.05
0.05
15
15
97
93
Olefin Precat. ([mol%]) t [min] 6a or 8a, yield [%] Yield [%]
of 6a’
3
5d
0.05
15
93
1[b] 5a
2[b] 7a
1 (2)
1 (0.25)
4 (1)
4 (1)
2 (1)
2 (1)
3 (1)
3 (0.05)
3 (0.05)
3 (0.05)
3 (0.01)
30
10
60
60
60
60
60
15
3
6a, 34[c]
8a, 95
6a, 48[c]
8a, 95
6a, 93
8a, 75
6a, 92
6a, 91
6a, >99
8a, 95
8a, 95
65[c]
–
4
5
6
7
8
9
10
5e
5 f
5g
5h
5i
0.05
0.1
0.05
0.05
0.1
15
15
15
15
30
15
15
96
93
97
89
90
92
80
3
4
5
6
7
8
5a
7a
5a
7a
5a
5a
52[c]
–
–
–
–
–
–
–
–
5j
5k
0.05
0.1
9[d] 5a
10 7a
11 7a
11
12
13
5l
0.1
0.1
5
15
15
30
93
92
n.r.
15
60
5m
5n
[a] Reaction conditions: HBpin (0.5 mmol) and 5a or 7a (0.5 mmol) in
THF (1 mL) at 258C. Unless otherwise specified, yields shown are of
isolated products. [b] Results from Ref. [10a]. Reaction conditions:
HBpin (0.5 mmol) and 5a or 7a (1 mmol) in THF (1 mL) at 258C.
[c] Yield was determined by 1H NMR analysis using an internal standard.
[d] Under solvent-free conditions, HBpin (5.0 mmol) and 5a (5.0 mmol).
Reaction conditions: HBpin (0.5 mmol) and alkenes (0.5 mmol) in THF
(1 mL) at 258C. Yields shown are of isolated products. n.r. =no reaction.
drogenative borylation to form vinylboronate (Table 1,
entry 1).[10a] The catalysis with the new Fe complex
[(iPrPNN)FeCl2] (4) led to similar results. The reaction with
1-nonene (7a) occurred smoothly (Table 1, entry 4), but the
reaction with styrene (5a) gave the hydroboration product 6a
in 48% and vinylboronate ester 6a’ in 52% yield (entry 3). In
sharp contrast, the use of the Co complex 2 engendered 6a in
a high yield (93%) in the hydroboration of 5a. No dehydro-
genative borylation product was observed in this reaction
(Table 1, entry 5). However, this precatalyst is less efficient
for the hydroboration of 7a, giving 8a in 75% yield after one
hour and 78% yield after five hours. To our delight, the less
sterically hindered Co complex 3 proved highly effective in
using only 0.05 mol% of 3, and provided isolated products in
yields higher than 90%. The method worked efficiently for
vinylarenes that bear electron-donating and electron-with-
drawing groups. A methyl substituent is tolerated at all the
positions on the aromatic ring (6b–d). Halogen-substituted
vinylarenes are compatible with the reaction conditions (6g–
j). Ester functionalities are also tolerated, as shown by the
isolation of 6k in 80% yield. The hydroboration of N-
vinylcarbazole gave 6m in 92% yield. However, a-methyl-
styrene (5n) was unreactive in the hydroboration, even
though a higher catalyst loading was used.
Complex 3 was next investigated as a precatalyst for the
hydroboration of aliphatic olefins that contain various func-
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Table 3: Hydroboration of various aliphatic alkenes (7a–o) with HBpin
catalyzed by 3.
With a remarkably active catalyst for the alkene hydro-
boration in hand, we sought to develop procedures to conduct
this transformation on a large scale. Using only 50 ppm
(0.005 mol%) of 3, the hydroboration of 5a (50 mmol) with
HBPin (50 mmol) occurred at 258C under neat conditions to
afford isolated product 6a in 99% yield after one hour
[Eq. (1)]. The result corresponds to a turnover number
(TON) of 19800, which represents the highest turnover
number observed for any metal-catalyzed alkene hydrobora-
tion with HBpin.
Substrate
Product
3
t
Yield
[mol%] [min] [%][b]
1
2
3
4
5
6
7
7b
7c
7d
7e
7 f
8b 0.05
8c 0.05
8d 0.05
8e 0.05
8 f 0.05
8g 0.05
8h 0.05
15
15
15
15
15
15
30
91
86
97
87
96
96
87
7g
7h
8
7i
7j
8i 0.1
15
15
94
72
9[a]
8j
1
1
Given the quantitative conversion of alkenes to alkylbor-
onates and the extremely low loading of the Co catalyst in
most reactions, we anticipated that the crude products could
be used for sequential transformations without purification.
Organotrifluoroborates are valuable synthetic intermediates
in Suzuki cross-couplings.[2c,19] Treatment of alkylboronate 6g,
generated in situ from 5g, with aqueous KHF2, afforded the
corresponding trifluoroborate in a high yield [Eq. (2)].
10
7k
8k
15
92
11[a]
12
7l
8l
1
15
15
76
96
7m
8m 0.05
13
14
7n
7o
8n
5
30
15
n.r.
94
8o 0.05
Reaction conditions: HBpin (0.5 mmol) and alkenes (0.5 mmol) in THF
(1 mL) at 258C. Yields shown are of isolated products. [a] HBpin
(0.5 mmol) and 7 (0.75 mmol).
tional groups. The results are summarized in Table 3. Simple
alkenes, such as 4-methyl-1-pentene, g-phenylpropene, and
cyclohexylethene were hydroborated in high yield. Functional
groups, including silyl (8e, 87%), silyl ether (8 f, 96%), halide
(8g, 96%), amine (8h, 87%), and tertiary amide (8i, 94%)
were tolerated under the reaction conditions. Allyl ethers,
which are challenging substrates, were also compatible, as
shown by the isolation of 8j in 72% yield. It should be noted
the Co system is highly chemoselective for the hydroboration
of ketone-functionalized alkenes. The reaction of 2-allyl-
cyclohexanone 7k with HBpin exclusively formed the alkene
hydroboration product 8k in 92% yield. No ketone hydro-
boration product was detected in this reaction. Similarly, the
hydroboration of 6-hepten-3-one (7l) gave the desired
product selectively in 76% yield.
More interestingly, the hydroboration products could
react in situ with aryl chlorides to afford the Suzuki–Miyaura
cross-coupling products in good yields by using a Pd/RuPhos
catalyst system (Scheme 1; RuPhos = 2-dicyclohexylphos-
phino-2’,6’-diisopropoxybiphenyl).[20] This method represents
the first one-pot process for the direct synthesis of alkyl–aryl
coupling compounds from alkenes and aryl chlorides with
HBpin as the hydroboration reagent. These reactions high-
light several advantages compared to the conventional
method utilizing B-alkyl-9-BBN (9-BBN = 9-borabicyclo-
[3.3.1]nonane) reagents as the coupling partners. First, the
handling of the robust alkylboronates is very simple, whereas
The catalytic activity of the Co system is very sensitive to
the olefin substitution pattern. In 4-vinyl-cyclohexene (7m),
the hydroboration of the terminal olefin occurred selectively
without the hydroboration of the internal olefin (8m, 96%).
Even more noteworthy, gem-disubstituted alkenes, such as 2-
methyl-1-pentene (7n) and 5n (see Table 2) are unreactive
under the Co-catalyzed hydroboration conditions. The high
sensitivity of the Co catalyst to the steric effects of alkene
substrates enables for chemoselective hydroboration of
monosubstituted alkenes in the presence of gem-disubstituted
alkenes. For example, the reaction of 2-methylhexa-1,5-diene
(7o) afforded exclusively the product resulting from the
hydroboration of the less sterically hindered double bond (8o,
94%).
Scheme 1. One-pot Co-catalyzed hydroboration and Pd-catalyzed cross-
coupling of alkylboronates with aryl chlorides. Yields shown are of
isolated products.
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8863; f) K. Burgess, W. A. Van der Donk, S. A. Westcott, T. B.
the air-sensitive B-alkyl-9-BBN compounds are difficult to
handle. Second, the alkylboronate approach can be used to
prepare compounds containing gem-disubstituted alkenes
(e.g., 10d, 93%), whereas this functional group is readily
hydroborated when alkylborane cross-coupling reactions are
carried out.[21]
In conclusion, we have disclosed an unprecedented
cobalt-catalyzed alkene hydroboration with pinacolborane.
Featuring the use of sustainable, cost-efficient, and environ-
mentally benign cobalt catalysts with loadings in the ppm
range, 100% atom economy, mild reaction conditions, high
conversion, excellent regio- and chemoselectivity, broad
functional-group compatibility, and operational simplicity,
the cobalt-catalyzed alkene hydroboration is a practical route
to alkylboronates. We have also reported a convenient one-
pot protocol for the Suzuki–Miyaura coupling of alkylboro-
nates generated in situ with aryl chlorides. Mechanistic
investigations of the cobalt-catalyzed hydroboration are in
progress and will be reported in due course.
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Received: November 20, 2013
Published online: February 3, 2014
Keywords: alkenes · cobalt · cross-coupling · hydroboration ·
.
synthetic methods
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[19] G. A. Molander, B. Canturk, Angew. Chem. 2009, 121, 9404;
Angew. Chem. Int. Ed. 2009, 48, 9240.
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[20] The Pd/RuPhos catalyst has proved to be effective for cross-
couplings between alkylboronic acid derivatives and aryl halides.
See Ref. [2c,5,19] and S. D. Dreher, S.-E. Lim, D. L. Sandrock,
G. A. Molander, J. Org. Chem. 2009, 74, 3626.
[21] For examples of the hydroboration of gem-disubstituted alkenes
with 9-BBN and subsequent Pd-catalyzed Suzuki–Miyaura
cross-couplings, see: a) N. Miyaura, T. Ishiyama, H. Sasaki, M.
Ishikawa, M. Sato, A. Suzuki, J. Am. Chem. Soc. 1989, 111, 314;
b) N. Miyaura, M. Ishikawa, M. Sato, A. Suzuki, Tetrahedron
Lett. 1992, 33, 2571. For comparison, we also carried out an
experiment with 7o as the substrate and 9-BBN as the hydro-
boration reagent. The hydroboration was conducted in the
absence of a catalyst. Using the same coupling conditions, the
reaction gave 10d in 26% yield (determined by 1H NMR
analysis) and its isomer ( ꢀ 7%) arising from the hydroboration
of the gem-disubstituted alkene in 7o and subsequent coupling.
Notably, a product ( ꢀ 5%) resulting from the hydroboration of
both monosubstituted alkene and gem-disubstituted alkene and
subsequent coupling with two aryl chlorides was also detected by
GC/MS and GC analysis.
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