Cobalt-Catalyzed Regioselective Hydroboration of Terminal Alkenes

Article abstract of DOI:10.1002/ejoc.201701047

Cobalt(II) catalysts based on flexible PNP or NNN ligands were explored for the regioselective hydroboration of alkenes. A known Co^II–PNP pincer complex was found to efficiently catalyze alkene hydroboration with excellent anti-Markovnikov selectivity, whereas a newly synthesized dinuclear Co^II–NNN complex was found to catalyze the hydroboration of a range of aromatic terminal alkenes with good Markovnikov selectivities (up to 98:2, b/l). This represents a rare example of Markovnikov selectivity in the hydroboration of alkenes using an inexpensive, flexible-ligand-supported dinuclear cobalt catalyst.

Full text of DOI:10.1002/ejoc.201701047

A Journal of
Accepted Article
Title: Cobalt-Catalyzed Regioselective Hydroboration of Terminal
Alkenes
Authors: Guoqi Zhang, Jing Wu, Haisu Zeng, Jessica Cheng, Michelle
C Neary, Shengping Zheng, and Man Wang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201701047
Link to VoR: http://dx.doi.org/10.1002/ejoc.201701047
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10.1002/ejoc.201701047
European Journal of Organic Chemistry
COMMUNICATION
Cobalt-Catalyzed Regioselective Hydroboration of Terminal
Alkenes
Guoqi Zhang,*[a] Jing Wu, [a][b] Man Wang, [a][c] Haisu Zeng,[a][b] Jessica Cheng,[a] Michelle C. Neary,[b]
and Shengping Zheng[b]
catalysts,[7] our group reported the first manganese-catalyzed
Abstract: Cobalt(II) catalysts built on flexible PNP or NNN ligands
were explored for the regioselective hydroboration of alkenes. While
the known CoII-PNP pincer complex catalyzes efficient alkene
hydroboration with excellent anti-Markovnikov selectivities, the newly
synthesized dinuclear CoII-NNN complex was found to catalyze the
hydroboration of a range of aromatic terminal alkenes with good
Markovnikov selectivities (up to 98 : 2, b/l). This represents a rare
example of inexpensive, flexible ligand-supported dinuclear cobalt
complex for Markovnikov hydroboration of alkenes.
hydroboration of terminal alkenes with Markovnikov selectivity in
2016.[8] In the same year, nickel and iron complexes with N-
heterocyclic carbene (NHC) ligands were reported to catalyze
the Markovnikov hydroboration of a range of aromatic terminal
alkenes.[9,10a] Very recently, Lu’s group also reported an iron-
catalyzed Markovnikov hydroboration of alkenes using amide-
based tridentate ligands.[10b]
Cobalt is one of most common earth abundant metals, and
cobalt-catalyzed hydroboration has received considerable
attention in recent years, along with the remarkable progress in
cobalt hydrogenation catalysis.[4,5,11] The Chirik group
investigated a range of cobalt alkyl complexes built on imine
ligands for alkene isomerization-hydroboration.[4c,4d] In addition,
enantioselective hydroboration of alkenes have also been
carried out using chiral cobalt catalysts.[12] Whereas the majority
of cobalt-catalyzed hydroboration of alkenes was achieved with
anti-Markovnikov selectivity, recent work indicated that selective
Markovnikov hydroboration of alkenes could be achieved
The hydroboration of alkenes provides a convenient route to
alkylboronic acid derivatives that have widespread applications
as versatile intermediates in
a number of bond-forming
reactions.[1] In recent years, transition metal-catalyzed alkene
hydroboration using boronate esters has been an established
method for the synthesis of valuable alkylboronic acids.[2]
However, precious metal catalysts containing Ru, Rh, or Ir were
required in most examples.[3] There is a current urgency in
replacing precious metals with earth abundant ones in
sustainable chemistry, as the low abundance, high cost and
heavy metal toxicity of noble metals have largely limited their
utilization in chemical and pharmaceutical industries. To this end,
catalysts based upon first-row transition metals, including copper,
iron, cobalt and nickel, have been reported for the efficient and
selective hydroboration of alkenes by several groups.[4-6] While
the vast majority of known nonprecious metal catalysts were
found to promote hydroboration of terminal alkenes with anti-
Markovnikov selectivity, a few catalysts with earth abundant
metals were recently revealed to carry out the unusual
Markovnikov alkene hydroboration.[7-10] After reports of Cu(I)
through
Co-NNN
complexes.[4d,13]
Nevertheless,
the
development of new cobalt catalysts for Markovnikov
hydroboration is still highly desired.
Scheme 1. Cobalt(II) pincer complexes 1 and 2.
Following the recent observation by Hanson and Zhang[14] on
the CoII pincer complex (PNP)Co (1, Scheme 1) and the ionic
(H-PNP)Co·BArF4 (2) that enables mild and highly efficient
hydrogenation and transfer hydrogenation of alkenes,
aldehydes, ketones and imines, as well as a variety of C-N and
[a]
Prof. Guoqi Zhang, Jing Wu, Man Wang, Haisu Zeng, Jessica
Cheng
Department of Sciences
John Jay College and PhD in Chemistry Program, The Graduate
Center of City University of New York
New York, 10019, New York, United States
E-mail: guzhang@jjay.cuny.edu
Homepage: http://www.jjay.cuny.edu/faculty/guoqi-zhang
Jing Wu, Haisu Zeng, Dr. Michelle C. Neary, Prof. Shengping Zheng
Department of Chemistry
Hunter College, City University of New York
New York, 10065, New York, United States
Current address: New York Military Academy, 78 Academy Avenue,
Cornwall on Hudson, NY 12520
C-C bond-forming reactions via
a “hydrogen-borrowing”
strategy,[15] we were also interested to explore whether this
catalyst is effective for the alkene hydroboration. Further efforts
were dedicated to the development of new cobalt catalysts
built on pincer-type NNN ligands instead of conventional PNP
ligands, which are expensive and not readily available
(Scheme 2). Although rigid tridentate ligands containing
pyridine or imine have shown unique applications in catalytic
hydrofunctionalizations,[4-13] the utilization of flexible tridentate
NNN ligands in hydroboration remains scarce. Herein, we
[b]
[c]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201701047
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COMMUNICATION
report the synthesis of new dinuclear CoII-alkyl complexes based
on flexible NNN ligands and the regiodivergent hydroboration of
aromatic terminal alkenes by using the Co-PNP or Co-NNN
complex.
The demonstrated ability of
2 in hydrogenation and
dehydrogenation processes prompted us to explore its reactivity
in hydroboration of alkenes, as well as other related cobalt
complexes. We began our initial reactivity tests for the
hydroboration of styrene with pinacolborane (HBPin) by using
the known cobalt complexes 1 and 2. The neutral cobalt pincer
complex 1 (1 mol %) was first examined for the reaction of
styrene (1.0 mmol) and pinacolborane (1.2 mmol) in Et2O at
room temperature. 1 was found to be a very poor catalyst for the
hydroboration of styrene, and the hydroborated product 5a and
6a were detected by GC analysis in only 15% yield, along with
several unidentified byproducts (entry 1, Table 1). However,
complex 2 proved to be quite efficient for the same reaction,
quantitatively affording the linear and branched products 5a and
6a with a ratio of 98 : 2 (l/b) after one hour (entry 2). The anti-
Markovnikov selectivity is unexceptional to the known cobalt-
catalyzed alkene hydroboration.
4
Scheme 2. The synthesis and ORTEP structures of dinuclear cobalt(II)
complexes 3 and 4 (thermal ellipsoid plotted at 50% probability level, H atoms
are omitted for clarity).
Next, we sought to design and synthesize new cobalt(II)
alkyl catalysts based on a readily available nitrogen ligand,
aiming at replacing the expensive phosphorus ligand in 2. Thus,
a tridentate NNN pincer analogue of PNP was selected to
prepare a cobalt alkyl complex similar to 1 (Scheme 2).
However, either the one-step metalation of the NNN ligand with
Co(py)2(CH2SiMe3)2 or two-step reactions using CoCl2 and
LiCH2SiMe3 sequentially (for details, see SI) furnished a novel
dinuclear cobalt complex (3) different from the structure of 1, as
confirmed by the single crystal X-ray structural analysis. The
air-sensitive compound 3 was characterized by IR and elemental
analysis under a N2 atmosphere. The X-ray structure reveals
that
3
is
a
dimeric complex containing two cobalt(II)
centers(Scheme 2). The structure of 3 is centrosymmetric with
two cobalt centers being closely contacted, and the Co…Co
distance is 2.6800(3) Å, similar to the N, P-supported Co-Co
bond.[16] Each cobalt center is tetra-coordinate in a tetrahedral
geometry with three N atoms (two from one ligand and one from
another) and one C atom of the CH2SiMe3 group. Each ligand
uses only two of its N atoms to coordinate to cobalt, with one
side N atom being uncoordinated. The central N is deprotonated
to adopt a μ2-N coordination mode, so that the cobalt remains in
its divalent oxidation state. Interestingly, two crystal polymorphs
of 3 were serendipitously obtained from different batches of
products by crystallization from a diethyl ether solution of the
compound in repeated reactions (see SI for details).
Table 1. Catalyst screening for cobalt-catalyzed hydroboration of styrene with
pinacolborane.[a]
3
Entry
Catalyst
Solvent
Yield (%)[b]
Ratio[c]
(5a + 6a)
(5a/6a)
1
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
1
2
3
4
5
6
7
15
>99
99
trace
54
0
-
2
98 : 2
3
5 : 95
4
-
3+HBArF
CoCl2
None
4
11 : 89
-
-
0
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10.1002/ejoc.201701047
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COMMUNICATION
8d
75
62
40
14 : 86
26 : 74
15 : 85
3
3
3
THF
selectivity. The ratio of products 5a : 6a was determined by GC
analysis as 5 : 95 (l/b) with the branched one being the major
product, as identified by NMR spectra (entry 3, Table 1). This is
in sharp contrast with the results obtained by using catalysts 1
and 2. In addition, the dinuclear cobalt(II) 4 was found to be
completely inactive for the hydroboration of styrene (entry 4),
probably due to the absence of a highly reactive Co-C bond in
its structure. Several control experiments were further performed.
The combination of 3 with two equiv. of HBArF4 led to lower yield
and poorer regioselectivity (entry 5). Reactions did not proceed
at all while using CoCl2 as a catalyst or without a catalyst
(entries 6 and 7). Other solvents such as THF, pentane, and
toluene were examined for the 3-catalyzed reactions; however,
inferior results were obtained (entries 8-10). Thus, the results
show that 2 and 3 are most efficient catalysts in Et2O and at
room temperature for excellent anti-Markovnikov and
Markovnikov selectivities, respectively.
9d
pentane
toluene
10d
[a] Conditions: styrene (1.0 mmol), pinacolborane (1.2 mmol), catalyst (1 mol
% for 1 and 2, 0.5 mol % for 3 and 4) and solvent (1 mL), 25°C, 4 h, N2. [b]
Determined by GC analysis with hexamethylbenzene as an internal standard.
[c] Determined by GC analysis. [d] Reaction run for 24 h.
In contrast, when a tetradentate N4 ligand was employed to
synthesize a similar cobalt complex through the two-step
metalation procedure (see SI), a new dinuclear complex 4 was
isolated and structurally characterized by X-ray crystallography.
In 4, the N4 ligand binds to the cobalt(II) centers in a tridentate
fashion, with a pendant NH group remaining uncoordinated.
Each cobalt(II) ion is surrounded by four N atoms from two
ligands, residing in a distorted tetrahedral environment. Two of
the side amino groups are deprotonated so that the cobalt(II)
ions are divalent without the involvement of CH2SiMe3 groups.
Similar to 3, the two cobalt(II) centers in 4 are also supported by
two μ2-N bridges, showing an even stronger Co…Co contact of
2.5591(4) Å.
Table 3. The substrate scope of 3-catalyzed regioselective hydroboration of
alkenes.a
Table 2. The substrate scope of 2-catalyzed anti-Markovnikov hydroboration
of alkenes.a
[a] Conditions: alkene (1.0 mmol), HBpin (1.2 mmol), cobalt 3 (0.5 mol %), and
Et2O (1 mL), 4 h. Yields are of isolated products. Ratios are of products 5 : 6
(l/b), as determined by GC analysis. [b] GC yield.
Using active catalysts 2 and 3 under the optimized reaction
conditions, we next extended the substrate scope for the
synthesis of linear (5) and branched (6) alkylboronate products,
respectively. First, a variety of styrene derivatives were chosen
to react with pinacolborane in the presence of 2 (1 mol %) and
the products were analyzed by GC-MS and isolated by column
chromatography. The results are summarized in Table 2. Both 2-
methyl and halo-substituted in the ortho- or para-position of
styrene gave high yields of the linear products 5b-f with
excellent anti-Markovnikov selectivities. The strong electron-
withdrawing trifluoromethyl-substituted styrene furnished the
[a] Conditions: alkene (1.0 mmol), HBpin (1.2 mmol), cobalt 2 (1 mol %), and
Et2O (1 mL), 4 h. Yields are of isolated products. Ratios are of products 5 : 6
(l/b), as determined by GC analysis. [b] ratio of linear to two branched
products. [c] GC yield.
With the new cobalt complexes 3 and 4 in hand, catalytic
tests for hydroboration of styrene under the same conditions as
for 1 and 2 were conducted (Table 1). Pleasingly, neutral
complex 3 (0.5 mol %, equivalent to 1 mol % Co) showed very
high activity toward the reaction with unusual Markovnikov
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10.1002/ejoc.201701047
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COMMUNICATION
reaction with equally high yield and selectivity. Whereas
electron-donating 4-methoxy-substituted styrene afforded
product after 4 h in a little lower yield (70%) with high selectivity,
the reaction proceeded perfectly with 4-acetoxystyrene. 2-
Vinylnaphthalene also proved to be a suitable substrate for the
anti-Markovnikov hydroboration to yield 85% 5j in 97 : 3 l/b ratio.
In a glovebox under N2, bis(diethylaminoethyl)amine (107.5 mg, 0.50
mmol) was dissolved in Et2O (10 mL) in a 20 mL scintillation vial at room
temperature. Co(py)2(CH2SiMe3)2 (196 mg, 0.50 mmol, prepared in situ)
was added in one portion upon stirring. The resultant dark-green solution
was stirred for 2 hours and filtered through a celite pad. The deep green
filtrate was allowed to evaporate to ca. 1 mL under N2 and then cooled to
-30ºC and stored for 3 days. Black (dark green) parallelepiped crystals
were collected and filtered, and then washed with cold pentane and dried
under vacuum. Yield: 85 mg (47%). FT-IR (solid, cm-1): 2968s, 2940s,
2809s, 1446m, 1373s, 1347m, 1248s, 1231s, 1200m, 1115m, 1066w,
1042s, 892s, 849s, 810s, 737s, 716s, 668m, 628w. Anal. Calcd. (%) for
C32H78Co2N6Si2 (3): C, 53.30; H, 10.90; N, 11.66. Found: C, 53.71; H,
10.85; N, 11.29.
The reaction with trans-β-methylstyrene as
a
substrate
proceeded well to give the linear product 5k selectively with the
observation of branched products, despite the fact that the
conversion was moderate, indicating an alkene isomerization
has occurred during the reaction. In addition, aliphatic terminal
alkenes, including 1-hexene, 1-octene, vinylcyclohexene and 4-
phenyl-1-butene, are all converted to the corresponding linear
products in good yield and with extremely high selectivities (5l-
o). One exception is the internal alkene, cyclohexene, which
reacted sluggishly in the presence of catalyst 2.
General Procedure for CoII-Catalyzed Hydroboration
In a glovebox under N2 atmosphere, catalyst 2 (16 mg, 0.01 mmol) or 3
(3.6 mg, 0.005 mmol) was dissolved in diethyl ether (1.0 mL) in a 5 mL
glass vial equipped with a stir bar. Alkenes (1.0 mmol) and pinacolborane
(154 mg, 1.2 mmol) were then added. The reaction mixture was allowed
to stir at room temperature for 4 hours. After completion of the reaction,
the solvent was evaporated and the crude reaction mixture was first
analyzed by GC-MS using a dilute CH2Cl2 solution to determine the ratio
of isomeric products or any hydroborated products, and then the product
was isolated by flash column chromatography with SiO2 using ethyl
acetate/hexane as eluents. The pure products were characterized by 1H
and 13C NMR spectroscopies.
When dinuclear 3 (0.5 mol %) was used as the catalyst, a
series of alkene substrates were examined, and the results are
shown in Table 3. It was found that the reactivity and
regioselectivity are substrate-dependent. 4-Halo-substituted
styrene furnished the reaction with only moderate yields for the
branched products 6b-d, and the regioselectivity was poor for 4-
fluorostyrene, whereas better results were obtained for styrenes
with Cl or Br substituents. However, good yields and high
Markovnikov selectivities were observed for styrenes with strong
electron-donating or electron–withdrawing groups (6e-6g). 2-
Vinylnaphthalene furnished the reaction with high yield, yet
slightly lower selectivity (80 : 20, b/l). Unfortunately, the reaction
with 2-chlorostyrene did not proceed smoothly, and branched 6i
was detected in a low yield, despite the fact that the selectivity
was good. It was noted that styrenes containing functional
groups such as nitro, cyano, or amino and 4-vinylpyridine were
not suitable substrates for the cobalt-catalyzed hydroboration
using either 2 or 3, probably due to the competitive coordination
to metal centers. It was further noticed that using 3 as a catalyst,
aliphatic alkenes were also converted to the corresponding
products with anti-Markovnikov selectivities, which is not
surprising as Markovnikov hydroboration has been rarely
achieved for aliphatic alkenes. Interestingly, unlike 2, 3 showed
high activity for the hydroboration of internal alkene, affording
the desired product 5p in 90% yield.
Acknowledgements
We are grateful to donors of the American Chemical Society Petroleum
Research Fund for partial support of this work (54247-UNI3). We also
acknowledge the support from the CUNY Collaborative Research
Incentive Program, the PSC-CUNY awards (69069-0047, 60328-0048)
and the Program for Research Initiatives for Science Majors (PRISM) at
CUNY John Jay College.
Keywords: Hydroboration • Alkenes • Markovnikov selectivity •
Cobalt catalysts • N ligands
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Entry for the Table of Contents
COMMUNICATION
Guoqi Zhang,*, Jing Wu, ManWang,
Haisu Zeng, Jessica Cheng, Michelle C.
Neary, and Shengping Zheng
Page No. – Page No.
Title: Cobalt-Catalyzed Regioselective
Hydroboration of Terminal Alkenes
Cobalt(II) catalysts built on flexible PNP or NNN ligands were found to enable
regiodivergent hydroboration of alkenes. While a CoII-PNP complex catalyzes
efficient alkene hydroboration with excellent anti-Markovnikov selectivities, the new
dinuclear CoII-NNN complex catalyzes the hydroboration of a range of aromatic
terminal alkenes with good Markovnikov selectivities. This represents a rare
example of an inexpensive dinuclear cobalt catalyst for Markovnikov hydroboration.
This article is protected by copyright. All rights reserved.