Markovnikov-Selective Hydroboration of Vinylarenes Catalyzed by a Cobalt(II) Coordination Polymer

Article abstract of DOI:10.1021/acs.orglett.8b03431

Highly efficient and practical hydroboration of alkenes has been catalyzed by an inexpensive and air-stable cobalt(II) coordination polymer (CP) in the presence of KO^tBu. Complete conversion of alkenes to alkylboronates were performed within just 5 min with low catalyst loading (0.025 mol%), achieving the record high turnover frequencies of up to 47 520 h^-1. For a range of vinylarenes, unusual Markovnikov selectivity was observed.

Full text of DOI:10.1021/acs.orglett.8b03431

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Markovnikov-Selective Hydroboration of Vinylarenes Catalyzed by a
Cobalt(II) Coordination Polymer
Guoqi Zhang,*,† Jing Wu,†,‡ Sihan Li,†,‡ Sean Cass,† and Shengping Zheng‡
†Department of Sciences, John Jay College and Ph.D. Program in Chemistry, The Graduate Center of the City University of New
York, New York, New York 10019, United States
‡Department of Chemistry, Hunter College, the City University of New York, New York, New York 10065, United States
S
* Supporting Information
ABSTRACT: Highly efficient and practical hydroboration of alkenes
has been catalyzed by an inexpensive and air-stable cobalt(II)
coordination polymer (CP) in the presence of KOtBu. Complete
conversion of alkenes to alkylboronates were performed within just 5
min with low catalyst loading (0.025 mol %), achieving the record high
turnover frequencies of up to 47 520 h−1. For a range of vinylarenes,
unusual Markovnikov selectivity was observed.
Scheme 1. State-of-the-Art Catalytic Regioselective
Hydoboration of Alkenes
ydrofunctionalization of alkenes constitutes a class of key
Hreduction reactions that have prominent applications in
current synthetic and petroleum chemical industries.1 Metal-
catalyzed hydroboration of alkenes is one of the most attractive
processes in this class, because it offers alkylboronates as key
and versatile synthetic intermediates in a direct, one-step
manner.2 Earlier success on this reaction has been built on
precious metal complex catalysts involving Ru, Rh, and Ir.3
Although good efficiencies (turnover frequency (TOF) values
3g
of up to 325 h−1
)
and excellent chemoselectivity,
regioselectivity, and enantioselectivity have been achieved
with these metals, their applications in industrial manufactur-
ing and sustainable synthesis are severely diminished by their
low abundance, high cost, and inherent environmental toxicity.
The requirement for replacing the precious-metal catalysts
with Earth-abundant elements has thus emerged.4,5
In the past decade, several alkene hydroboration catalysts
have been developed based on Earth-abundant metals, with the
majority being anti-Markovnikov selective for terminal
alkenes.6−8 However, Markovnikov hydroboration were rarely
approached with base-metal catalysts.9−12 Along with sparse
reports on CuI,9 MnII,10 NiII,11 and FeII (ref 12) complexes
that induced Markovnikov selectivity (in all examples, TOFs
were up to 330 h−1), cobalt-based catalysts proved to be the
most promising for high Markovnikov selectivity (Scheme 1).
For example, in 2015, Chirik and co-workers reported a cobalt
complex of terpyridine-catalyzed Markovnikov hydroboration
of styrene using pinacolborane (HBpin) with a 25:1 branch/
linear (b/l) product ratio and a TOF of 300 h−1.6d Later, the
Hollis group also revealed a (CCC)CoII-catalyzed hydro-
boration of styrene in high Markovnikov selectivity with a TOF
of 330 h−1.13 Thomas and co-workers developed a more
general (NNN)CoII catalyst for Markovnikov hydroboration of
a range of vinylarenes by using NaOtBu as an activator, while
reported TOFs were only up to 100 h−1.14a Very recently, the
Findlater group reported a simple Co(acac)3/PPh3-catalyzed
Markovnikov hydroborotion of styrenes with TOFs of up to 5
h−1 for several active substrates.14b We also observed cobalt(II)
catalyst of a flexible N3 ligand for Markovnikov hydroboration
of vinylarenes with TOFs up to 50 h−1,15 with connection to
Received: October 29, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03431
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
our recent interest in hydrogenation and dehydrogenation
catalysis with cobalt.16
activity. The yield of alkylboronates 2 and 3 were determined
to be 82% in 3 min and 99% in 5 min, while improving the 2/3
ratio slightly to 92:8, corresponding to a TOF of 47 520 h−1
(Table 1, entries 2 and 3). This represents the most efficient
alkene hydroboration catalyst thus far.3 Decreasing the amount
of KOtBu led to a lower yield, although comparable
regioselectivity was observed (Table 1, entry 4). Several
control experiments revealed that the combinations of free
terpyridine ligand or cobalt(II) chloride with KOtBu were
inactive and the reaction did not proceed in the absence of
cobalt precatalyst 1 or without an additive (Table 1, entries 5−
8). These results indicate the importance of the combined
system 1/KOtBu for generating an active catalyst species. We
further tested other basic additives (Table 1, entries 9−13) and
determined that, while NaOtBu, LiOiPr, and NaBH4 were
identically effective activators, the regioselectivities have
decreased in all cases. Finally, solvent effects were studied
under the same conditions (Table 1, entries 14−20). The
reactions conducted in nonpolar solvents and CH2Cl2 were
less efficient, yet those conducted in diethyl ether or under
neat conditions showed the same efficiency as in THF, despite
the regioselectivities being lower than that observed in THF. It
was also determined that, when the reaction was performed in
CH2Cl2 and in air, only trace alkene hydroboration was
detected (Table 1, entry 18).
Next, we explored the applicability of the optimized
conditions for a range of alkene substrates. Typically, the
reactions were performed in a 2 mmol scale using 1 (0.025
mol %) and KOtBu (1 mol %) as a catalyst in THF at ambient
temperature. The reaction mixture was examined and analyzed
via gas chromatography−mass spectroscopy (GC-MS) after 5
min and then the major regioisomer was isolated by column
chromatography on silica. The results are summarized in Table
2. The branched alkylboronate (2a) resulting from styrene
hydroboration could be obtained in a good isolated yield
(82%), and a 92:8 branched/linear selectivity. The hydro-
boration of sterically hindered 2-methylstyrene proceeded with
equal efficiency (TOF = 47 520 h−1) with decreased isolated
yield and slightly lower regioselectivity (80:20). Halogenated
styrenes are also suitable substrates for Markovnikov hydro-
boration, providing good isolated yields of branched products
(2c−2f) and comparable selectivity, although the TOFs were
relatively lower (in the range of 7840−23 760 h−1). Styrenes
bearing electron-donating groups in the 4-position, such as 4-
methoxystyrene and 4-acetoxystyrene, were hydroborated in
high TOFs (47 040 and 47 520 h−1) and good yields of
products 2g and 2h were isolated. In contrast, 4-trifluor-
omethylstyrene containing strongly electron-withdrawing
group was found to be a more challenging substrate, and, in
this case, only 66% conversion was detected in 1 h, while the
Markovnikov selectivity remained good (88:12). 2-Vinyl-
naphthalene was fully converted to the boronic esters in high
branched/linear regioselectivity (93:7) within 30 min,
corresponding to a TOF of 7920 h−1. In addition, several
internal alkenes were examined for regioselective hydro-
boration. Trans-β-methylstyrene was hydroborated selectively
with 2k being isolated in 92% yield and the TOF was as high as
47 520 h−1. In this case, only trace amounts of two other
regioisomers were observed. Equally high efficiency was
obtained when 2,3-benzofuran was used, along with a complete
control of regioselectivity for the C−B bond formation at the
benzylic position (Table 2, entry 12). Thus, product 2l has
been isolated in a high yield of 94%. Note that selective
Despite such progress, the catalysts developed thus far often
suffer from tedious ligand synthesis and purification, the
involvement of air-sensitive metal complexes or reagents, and
high catalyst loading.4−12 A higher efficiency and practical
method for catalytic alkene hydroboration is still unavailable.
In this contribution, we wish to report a practical, extremely
efficient, non-precious-metal, cobalt-catalyzed Markovnikov
hydroboration of vinylarenes with outstanding TOFs of up
to 47 520 h−1 at ambient temperature. Furthermore, this
catalyst also showed equally high efficiency toward internal
alkenes and good functional group tolerance.
We have previously reported that a cobalt(II) CP (1)
assembled from 4′-(4-pyridyl)-4,2′;6′,4″-terpyridine and
CoCl2 catalyzed the hydroboration of carbonyl compounds
in the presence of KOtBu as an activator.17,18 The structure of
1 features a one-dimensional linear polymer with CoII being in
a N4Cl2 environment that is similar to otherwell-defined
cobalt-based hydroboration catalysts (Scheme 1). An initial
catalytic test was performed using styrene as a model substrate
and the combination of 1 (0.1 mol %) and KOtBu (1 mol %)
as a catalyst at ambient temperature; it was found that effective
hydroboration (>99%) was accomplished within only 5 min,
affording good Markovnikov regioselectivity with a b/l product
ratio (2/3) of 90:10 (Table 1, entry 1). Further reducing the
loading amount of 1 to 0.025 mol % did not affect the catalytic
Table 1. Reactivity Test for 1-Catalyzed Hydroboration of
a
Styrene with HBpin
b
c
entry precatalyst
additive
solvent
yield (%) ratio, 2/3
d
1
2
1
1
1
1
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
none
KOtBu
NaOtBu
KOH
LiOiPr
LiNTf2
NaBH4
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
>99
82
99
90:10
92:8
92:8
e
3
f
4
64
90:10
g
h
5
6
7
8
L
−
−
−
−
−
−
−
−
g
h
CoCl2
g
h
1
none
1
1
1
1
1
1
1
1
1
1
1
g
h
9
99
95
99
34
98
95
80
92
99
44
<4
99
78:22
79:21
80:20
87:13
86:14
78:22
83:17
78:22
87:13
86:14
10
11
12
13
14
15
16
17
18
THF
toluene
pentane
benzene
Et2O
CH2Cl2
CH2Cl2
none
i
h
19
20
−
1
74:26
a
Conditions: styrene (1.0 mmol), pinacolborane (1.1 mmol),
precatalyst (0.025 mol %), additive (1 mol %), and solvent (0.5
b
mL), 25 °C, 5 min, N2. Yield of 2 + 3, determined by GC analysis
c
with hexamethylbenzene as an internal standard. Determined by GC
d
e
analysis. Reaction run using 0.1 mol % of 1. Reaction run for 3 min.
f
g
h
0.5 mol % KOtBu was used. No reaction. No ratio to measure or
ratio not measured. Reaction run in air.
i
B
DOI: 10.1021/acs.orglett.8b03431
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 2. CoII−CP-Catalyzed Regioselective Hydroboration of Alkenes
a
Conditions: alkene (2.0 mmol), pinacolborane (2.2 mmol), 1 (0.025 mol %) and KOtBu(1 mol %) in THF (1 mL), room temperature, N2. bRatio
c
of three possible regioisomers. Ratio of two regioisomers of the major alkene hydroboration products. The minor doubly hydroborated product
was isolated in 11% yield; see the SI.
hydroboration of 2,3-benzofuran was only sparsely explored in
the literature.6c The same conversion and selectivity were
achieved by using the Chirik’s catalyst (PPh3)3CoH(N2) (5
mol %) after 2 h (TOF < 10 h−1). Both cis- and trans-stilbenes
were also effective substrates, furnishing the reactions with 95%
and 96% isolated yields, respectively. The disubstituted
terminal alkene, α-methylstyrene, was found to be a
challenging substrate, affording the alkylboronates with 26%
conversion after 5 h, although a reasonable Markovnikov
selectivity was obtained (75:25; see Table 2, entry 15). For
alkyl alkenes, rapid hydroboration was also observed. For
instance, 1-hexene furnished the reaction with a TOF of
47 520 h−1; however, the selectivity was poor (b/l = 58:42). In
contrast, in the case of 1-octene as a substrate, the reaction
favors anti-Markovnikov selectivity with a b/l ratio of 36:64.
Trans-4-octene was also tested for the hydroboration;
however, the products include a complicated mixture of both
alkylboronate and octene isomers resulting from isomer-
ization−hydroboration,6b which will deserve further studies
on the optimization of selectivity. Finally, 5-hexen-2-one
containing both alkene and ketone functional groups was
selectively hydroborated on the alkene, with good anti-
Markovnikov selectivity (b/l = 10:90). The ketone-containing
alkylboronate 2n was isolated in 72% yield, even though a
C
DOI: 10.1021/acs.orglett.8b03431
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Letter
minor product (2n′) resulting from double hydroboration of
both ketone and alkene moieties was obtained in 11% yield
(see the Supporting Information (SI)). The regioselectivity
observed here is markedly different from that observed in the
work of Thomas, using discrete (NNN)CoCl2 as a precatalyst,
where the linear ketone-containing alkylboronate product was
obtained with a 70:30 b/l selectivity.14a
To further investigate the functional group tolerance of the
present 1-catalyzed Markovnikov hydroboration, we conducted
a fast robustness screen of reducible functional groups by using
the method introduced by Glorius.19 Thus, styrene hydro-
boration was examined under the optimized conditions with
added chemicals (1.0 equiv) containing different functional
groups and the results are summarized in Table 3. It is shown
examples (Table 3, entries 1−6), the additives were
determined not to be hydroborated, based on GC-MS analysis.
However, this catalyst showed different selectivity for styrene
hydroboration in the presence of ketones and aldehydes
(Table 3, entries 7−9).17
In conclusion, we developed a highly efficient and practical
cobalt-catalyzed method for selective hydroboration of alkenes.
The simple combination of air-stable cobalt(II) coordination
polymer and KOtBu constitutes, thus far, the most efficient
catalyst for alkene hydroboration, achieving outstanding TOFs
of up to 47 520 h−1. Remarkably, the Markovnikov selectivity
observed for vinylarenes also represents one of the rare
examples for base-metal-catalyzed hydroboration. This work
implicates a new approach to effective catalysts based on Earth-
abundant metals that outcompete non-precious-metal catalysts.
Table 3. Robustness Screen of 1-Catalyzed Hydroboration
of Styrene
a
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03431.
Additional experimental details and copies of NMR
spectra and crystallographic data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: guzhang@jjay.cuny.edu.
ORCID
Guoqi Zhang: 0000-0001-6071-8469
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to donors of the American Chemical Society
Petroleum Research Fund for partial support of this work (No.
54247-UNI3). We also acknowledge the support from the
PSC−CUNY awards (Nos. 69069-0047 and 60328-0048), the
Seed Grant from the Office for Advancement of Research and
the PRISM Program at John Jay College, CUNY.
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Conditions: styrene (1.0 mmol), pinacolborane (1.1 mmol), 1
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DOI: 10.1021/acs.orglett.8b03431
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