Cobalt-catalyzed borylation of aryl halides and pseudohalides

Article abstract of DOI:10.1021/acs.organomet.6b00161

We report the first Co-catalyzed borylation of aryl halides and pseudohalides with bis(pinacolato)diboron (B₂pin₂). The synthesis of two new Co(II) complexes of oxazolinylferrocenylphosphine ligands is described. Upon activation with LiMe, the Co complex catalyzes the borylation reactions of aryl bromides, iodides, sulfonates, arenediazonium salts, and even aryl chlorides under mild conditions, providing the borylated products in excellent to moderate yields and with high functional group tolerance.

Full text of DOI:10.1021/acs.organomet.6b00161

Article
pubs.acs.org/Organometallics
Cobalt-Catalyzed Borylation of Aryl Halides and Pseudohalides
Wubing Yao,†,‡ Huaquan Fang,‡ Sihan Peng,‡ Huanan Wen,‡ Lei Zhang,‡ Aiguo Hu,*,†
and Zheng Huang*,‡
†Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of
Science and Technology, Shanghai 200237, People’s Republic of China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, 345 Lingling Road, Shanghai 200032,
People’s Republic of China
S
* Supporting Information
ABSTRACT: We report the first Co-catalyzed borylation of
aryl halides and pseudohalides with bis(pinacolato)diboron
(B2pin2). The synthesis of two new Co(II) complexes of
oxazolinylferrocenylphosphine ligands is described. Upon
activation with LiMe, the Co complex catalyzes the borylation
reactions of aryl bromides, iodides, sulfonates, arenediazonium
salts, and even aryl chlorides under mild conditions, providing
the borylated products in excellent to moderate yields and with
high functional group tolerance.
oxazolinylferrocenylphosphine ligands and their applications in
the first Co-catalyzed borylations of aryl (pseudo)halides with
bis(pinacolato)diboron (B2pin2). Importantly, the Co catalyst is
effective for borylations of the challenging aryl chlorides under
mild conditions.
INTRODUCTION
■
Due to their stability, low toxicity, and ease of handling,
arylboronic acids and arylboronate esters have become
indispensable synthetic intermediates for C−C bond-forming
reactions.1 The traditional procedures for the preparation of
these boron compounds involve the reaction of aryllithium or
Grignard reagents with suitable organoboron nucleophiles.2
However, these methods suffer from poor functional group
tolerance and multistep synthesis. Thus, recently there has been
increasing interest in the development of transition-metal-
catalyzed methods for the synthesis of arylboronic acid
derivatives. Two important classes of metal-catalyzed trans-
formations are catalytic borylations of aryl halides3 and C−H
bond activation/borylations.4,5
RESULTS AND DISCUSSION
■
The synthesis of the ferrocenyloxazoline-derived N,P ligands
and the corresponding Co complexes is outlined in Scheme 1.
Deprotonation of ferrocenyloxazoline, followed by the addition
of diphenylchlorophosphine or diisopropylchlorophosphine,
formed ligands 1a,b containing Ph and iPr phosphino
substituents, respectively.14 Treatment of the ligands with
CoCl2 in THF afforded complexes 2a (87%) and 2b (90%) in
The majority of the catalyst systems for borylations of aryl
halides use the noble metal Pd6 or Ni.7 During the past decade,
environmentally more friendly and low-cost base-metal
catalysts, including Cu,8 Zn,9 and Fe10 complexes, have been
developed.11 These systems in general show lower catalytic
efficiency than the Pd catalysts. Moreover, the borylations of
inexpensive, widely available aryl chlorides using base metals
other than Ni7g−k are rare. Thus, the development of cost-
effective and environmentally benign base-metal catalysts for
efficient borylation of aryl halides, in particular for trans-
formations of aryl chlorides, is of great interest.
Scheme 1. Synthesis of Oxazolinylferrocenylphosphine
Ligands and the Corresponding Co Complexes
Cobalt is an attractive alternative to the widely used Pd metal
because it is abundant and inexpensive. To the best of our
knowledge, the Co-catalyzed borylations of aryl halides have
remained unknown, although stoichiometric borylation of aryl
bromides using Co complexes has previously been reported.12
Driven by our interest in employing base-metal catalysts for the
preparation of synthetically useful organoboronic acid deriva-
tives,13 we report here the preparation of Co complexes of
Special Issue: Organometallics in Asia
Received: February 26, 2016
© XXXX American Chemical Society
A
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high yields. The solid-state structures of 2a,b adopt distorted-
tetrahedral coordination geometries at the Co center (Figure
1).
and 63% yields, respectively (Table 1, entries 1−3). Varying
amounts of benzene (6−16%) were also detected as the
protodehalogenation product in these reactions (see Table S7
in the Supporting Information for the product distributions).
Encouraged by these results, we studied Co complexes of
various bidentate ligands, such as bis(phosphine), bipyridine,
pyridinyl oxazoline, phosphino pyridine, and phosphino
oxazoline, for the borylation of 6a. While most complexes of
bidentate ligands afforded the desired product 7a in low to
moderate yields (see Table S7), the reaction using the new Co
complex 2a ligated by the oxazolinylferrocenylphosphine ligand
gave 7a in 80% yield (entry 4), along with 7% of benzene.
The effects of solvent, base, and activator on the conversion
of 6a to 7a were next examined by using 2a as the catalyst
precursor. The solvents have marked impact on the borylation
reaction. MTBE proved to be the best medium among the
solvents studied; dichloromethane (DCM) and toluene are not
suitable for the borylation (Table 1, entries 4−9). The catalytic
efficiency was improved by substitution of the base from
NaOMe to KOMe (89%, entry 10), while the reactions using
NaOH and K2CO3 gave very low yields of 7a (entries 11 and
12). Using NaBHEt3 as the catalyst activator resulted in the
formation of 60% of 7a and 18% of benzene (entry 13).
Interestingly, the reaction occurred in the absence of the
activator, albeit with a lower yield of 7a (entry 14, 54%) in
comparison to the reaction with the activator. Under the
optimized conditions, the Co complex 2b bearing the iPr
substituent on the P atom is even more effective than the Ph-
substituted analogue 2a; the reaction using 2b gave 95% of 7a
and 4% of benzene (90% isolated yield, entry 15). Finally, a
control experiment with KOMe and LiMe, but with no Co
precatalyst, was conducted. In this case no borylation product
was observed (entry 16).
Figure 1. Crystal structures of complexes 2a,b.
We commenced the studies of the Co-catalyzed borylation of
aryl halide by assessing the catalytic activities of previously
reported PNN15 and PNP16 pincer Co complexes (3−5, see
Table 1), including 3 developed by our group for efficient
alkene hydroborations.13c Using NaOMe (1.5 equiv) as the
base and LiMe (10 mol %) as the catalyst activator,13b the
coupling between bromobenzene (6a) and B2pin2 (1.5 equiv)
in the presence of the pincer Co precatalyst (5 mol %) occurred
at 50 °C in MTBE. After 24 h, the reactions with 3−5 gave the
desired borylation product, phenylboronate ester 7a, in 34, 51,
Table 1. Borylation of Bromobenzene (6a) with B2pin2:
a
Catalyst Screening and Condition Optimization
Using the catalyst generated from 2b and LiMe, and with
KOMe as the base, we explored the substrate scope with
respect to aryl halides for the borylation reactions (Table 2).
Aryl bromides containing both electron-donating and -with-
drawing groups in the para positions (6c−j) underwent
borylations to form the arylboronate products in good to
excellent yields. The ortho-substituted substrate 6b gave the
desired product 7b (54%) in moderate yield. Importantly, a
wide range of functional groups including ether (6d), amine
(6e,f), cyanide (6h), ketone (6i), and amide (6j) can be
tolerated. Furthermore, heteroaromatic bromides, such as
bromo-substituted 1,3-benzodioxole (6k), benzofuran (6l),
thiophene (6m), and quinoline (6n), were efficiently borylated.
The Co-catalyzed borylations of aryl iodides (6a-I, 6o-I, and
6b-I) produced the corresponding arylboronate esters in useful
yields. More importantly, borylations of aryl sulfonates
including aryl mesylates (6a-OMs, 6c-OMs, and 6p-OMs)
and aryl tosylate (6a-OTf) proceeded smoothly under the
standard conditions. It is worth noting that the sterically
demanding aryl mesylate (6p-OMs) bearing a bulky
trimethylsilyl group in the ortho position gave the borylated
product 7p in 87% isolated yield. In addition, aryldiazonium
salts are suitable substrates, as demonstrated by the direct
coupling between 6d-N2BF4 and B2pin2 to form 7d in 72%
yield after 24 h at 50 °C (Table 2). Thus, the Co-catalyzed
borylation reaction provides a mild method for transforming
various aryl pseudohalides into arylboronate esters.
b
entry
[Co]
solvent
base
activator
yield (%)
1
3
MTBE
MTBE
MTBE
MTBE
THF
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
KOMe
LiMe
LiMe
LiMe
LiMe
LiMe
LiMe
LiMe
LiMe
LiMe
LiMe
LiMe
LiMe
NaBHEt3
34
2
4
51
3
5
63
4
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
80
5
60
6
Et2O
70
7
DME
38
8
DCM
0
9
PhMe
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
14
10
11
12
13
14
15
16
89
NaOH
16
K2CO3
KOMe
4
60
54
KOMe
KOMe
LiMe
LiMe
95 (90)
0
KOMe
a
Reaction conditions: 6a (0.26 mmol), B2pin2 (0.39 mmol), base
(0.39 mmol), [Co] (0.013 mmol), activator (0.026 mmol), and
solvent (1.5 mL) under Ar at 50 °C for 24 h.
Despite the cost advantages of using aryl chlorides as the
coupling reagents, there have been only a few reports of
borylations of aryl chlorides using base-metal (Ni) catalysts.
b
Yields were determined by GC. The value in parentheses is the
isolated yield.
B
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electron-rich and -neutral substrates for borylations.6d The
reactions of aryl chlorides with ester and CF3 groups in the para
positions gave the desired products (7t−v) in moderate yields.
The reaction is also sensitive to the steric effects of the
substrates: an aryl chloride possessing an o-methyl group gave
the product 7b in only 43% yield.
Table 2. Co-Catalyzed Borylation of Aryl Bromides and
Iodides and Aryl Pseudohalides
a
Mechanistic studies of this Co-catalyzed borylation of aryl
halides are underway in our laboratory. To probe the possibility
of a radical-mediated reaction pathway, we conducted the
reactions of 6a with B2pin2 in the presence of commonly used
radical scavengers (Table 4). The addition of 9,10-dihydroan-
Table 4. Effects of Radical Scavengers on the Borylation of
a
6a
entry
additive
amt of additive (equiv)
yield (%)
1
2
95
79
9,10-dihydroanthracene
BHT
0.5
2.0
0.5
2.0
0.5
2.0
60
3
4
74
81
a
Reaction conditions unless specified otherwise: 6 (0.26 mmol),
TEMPO
53
B2pin2 (0.39 mmol), KOMe (0.39 mmol), 2b (0.013 mmol), LiMe
(0.026 mmol), and MTBE (1.5 mL) under Ar at 50 °C for 24 h. Yields
were determined by 1H NMR. Isolated yields are given in parentheses.
< 5
a
Reaction conditions: 6a (0.26 mmol), B2Pin2 (0.39 mmol), 2 (0.013
b
mmol), KOMe (0.39 mmol), LiMe (0.026 mmol), additive (0.5 or 2.0
equiv), and MTBE (1.5 mL) under Ar at 50 °C for 24 h. Yields were
determined by GC.
Using CH3CN (1.5 mL) as the solvent.
The previously reported Ni-catalyzed borylation reactions
typically require high temperatures (80−100 °C) for good
conversion.7g−k To our delight, this Co catalyst system is
applicable to the borylations of aryl chlorides under mild
conditions (Table 3). The reactions of most substrates
occurred at 50 °C with 5 mol % of the catalyst, furnishing
the borylated products in good yields. However, electron-
deficient aryl chlorides appeared to be more difficult than
thracene (entry 2) or BHT (entry 3) resulted in a slight
decrease of the yield of 7a in comparison with that without the
radical scavenger, whereas the reactions in the presence of 0.5
and 2.0 equiv of TEMPO gave 7a in only 53% and <5% yields,
respectively (entry 4). However, it should be noted that
TEMPO may promote decomposition of B2pin2.11c,17
In addition, a radical-clock experiment using 2-(3-butenyl)-
bromobenzene (8) as the substrate was carried out (eq 1). The
a
Table 3. Co-Catalyzed Borylation of Aryl Chlorides
aryl radical of 8 is known to undergo rapid cyclization.9b,c,18
Under the standard catalytic conditions, the reaction of 8 with
B2pin2 gave the borylated internal alkenes 9 (45%) and 10
(26%) as the major products, as well as other unidentified
species (see the Supporting Information for details). No
detected amount of the radical cyclization product 12 was
observed by NMR analysis. The expected borylated terminal
alkene 11 was also not detected. It is therefore not clear if the
lack of the cyclization product 12 in this reaction is due to a
nonradical reaction pathway or is due to facile olefin
isomerization.
a
Reaction conditions unless specified otherwise: 6-Cl (0.26 mmol),
B2pin2 (0.39 mmol), KOMe (0.39 mmol), 2b (0.013 mmol), LiMe
(0.026 mmol), and MTBE (1.5 mL) under Ar at 50 °C for 24 h. Yields
were determined by 1H NMR. Isolated yields are given in parentheses.
b
2b (0.026 mmol), LiMe (0.052 mmol).
C
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Article
acetate (4/1) as eluent to give the pure ligand (0.97 g, 68%). 1H NMR
(400 MHz, CDCl3): δ 4.93 (s, 1H), 4.58 (t, 2JHH = 8.0 Hz, 1H), 4.46
(s, 1H), 4.27 (s, 1H), 4.18 (s, 5H), 3.83 (t, 2JHH = 8.0 Hz, 1H), 2.08−
CONCLUSIONS
■
In summary, we have developed the first Co-catalyzed
borylation of aryl halides and pseudohalides, including aryl
chlorides, using an oxazolinylferrocenylphosphine-supported
Co complex. Featuring the use of nonprecious base-metal
catalyst, mild reaction conditions, wide substrate scope, and
tolerance toward many functional groups, this method could be
an attractive route for the synthesis of arylboronate esters.
Further studies regarding the mechanistic implications of these
observations and Co-catalyzed borylations of alkyl halides are
ongoing.
2
2.03 (m, 1H), 1.76−1.72 (m, 1H), 1.50 (q, JHH = 8.0 Hz, 3H), 1.29
(d, 2JHH = 4.0 Hz, 4H), 1.15 (q, 2JHH = 4.0 Hz, 3H), 0.89 (q, 2JHH = 8.0
Hz, 4H), 0.73 (t, 2JHH = 8.0 Hz, 3H). 31P NMR (162 MHz, CDCl3): δ
−2.19. 13C NMR (101 MHz, CDCl3): δ 74.1, 71.9, 71.2, 70.9, 70.6,
61.3, 26.1, 22.2, 21.9, 21.3, 19.8, 18.1. HRMS (MALDI-FT): m/z [M]+
calculated for C20H29NO54FeP, 384.1377; found, 384.1381.
Synthesis of Cobalt Complex 2a.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out using
standard Schlenk, high vacuum, and glovebox techniques. Glassware
was dried in a 140 °C oven over 4 h prior to use. Ferrocenecarboxylic
acid (97+%) and (R)-(−)-2-1-propanol (97%) were purchased from
Accela ChemBioCo., Ltd., People’s Republic of China, and used as
received. Bis(pinacolato)diboron (B2pin2; 99%), BHT (99%), 9,10-
dihydroanthracene (98%), chlorodiisopropylphosphine (96%), and
chlorodiphenylphosphine (97%) were purchased from TCI and used
as received. Dibromomethane (99%), CoCl2 (97%), and KOMe (90+
%) were purchased from Alfa and used as received. LiMe (3.1 M in
diethoxymethane) was purchased from Aldrich and used as received.
NaBHEt3 (1.0 M in THF) was purchased from Acros and used as
received. All solvents, aryl halides, and aryl pseudohalides were
obtained from commercial sources and dried and degassed according
to standard procedures. The cobalt complexes 3,13d 4,15 and 5,16 2-(3-
butenyl)bromobenzene (8),19 N-(1-hydroxy-2-propyl)-
ferrocenamide,20 2-ferrocenyl-4,5-dihydro-4-(1-methyl)oxazole,20 and
the ligand 2-[2-(diphenylphosphino)ferrocenyl]-4,5-dihydro-4-(1-
methyl)oxazole (1a)14,20,21 were synthesized according to literature
procedures.
In an argon-filled glovebox, CoCl2 (0.30 g, 2.30 mmol, 0.9 equiv) and
THF (30 mL) were placed in a 50 mL Schlenk tube. After the mixture
was stirred for 2 h, the ligand (1.15 g, 2.50 mmol, 1.0 equiv) was
added. The reaction mixture was stirred at room temperature for 24 h.
After the removal of THF by evaporation, the solids were washed with
n-hexane and dried under vacuum to give complex 1 as a brown solid
(1.17 g, 87%). Anal. Calcd for C26H24Cl2CoFeNOP: C, 53.55; H, 4.15;
N, 2.40. Found: C, 53.75; H, 4.51; N, 2.18.
Synthesis of Cobalt Complex 2b.
In an argon-filled glovebox, CoCl2 (0.30 g, 2.30 mmol, 0.9 equiv) and
THF (30 mL) were placed in a 50 mL Schlenk tube. After the mixture
was stirred for 2 h, the ligand (0.97 g, 2.50 mmol, 1.0 equiv) was
added. The reaction mixture was stirred at room temperature for 24 h.
After the removal of THF by evaporation, the solids were washed with
n-hexane and dried under vacuum to give complex 2 as pale green
solids (1.07 g, 90%). Anal. Calcd for C20H28Cl2CoFeNOP: C, 46.63;
H, 5.48; N, 2.72. Found: C, 46.84; H, 5.74; N, 2.36.
Typical Procedure for the Borylation of Aryl Halides. In an
argon-filled glovebox, a 5 mL dried Schlenk tube equipped with a
magnetic stir bar was charged with cobalt complex 2b (6.7 mg, 13.0
μmol), MTBE (1.5 mL), and LiMe (9.0 μL, 26.0 μmol, 3.1 M in
diethoxymethane). The solution was stirred at 25 °C for about 1 min
until the solution turned gray-black. B2pin2 (0.1 g, 0.39 mmol, 1.5
equiv), KOMe (30 mg, 0.39 mmol, 1.5 equiv), and bromobenzene (6a,
28 μL, 0.26 mmol, 1.0 equiv) were then added to the solution in
sequence. The tube was then sealed with a Teflon plug under an argon
atmosphere and removed from the glovebox. The solution was stirred
at 50 °C for 24 h. After that, MTBE and other volatiles were removed
under high vacuum. The residue was dissolved in ethyl acetate and
filtrated though Celite. The filtrate was collected, the volatiles were
removed under reduced pressure, and the crude product was analyzed
by 1H NMR spectroscopy or GC analysis, with dibromomethane or n-
undecane as an internal standard to obtain the GC or 1H NMR yields.
After that, the crude product was purified by silica gel column
chromatography.
NMR spectral data were obtained on Aglient 400, Varian 400 MHz,
1
and Varian 300 MHz instruments. H NMR spectra were referenced
to residual protio solvent peaks or the TMS signal (0 ppm), and 13C
NMR spectra were referenced to the solvent resonance. 31P NMR
chemical shifts were referenced to an external H3PO4 standard. Data
for 1H NMR are recorded as follows: chemical shift (δ, ppm),
multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet or
unresolved, br = broad singlet, coupling constant(s) in Hz,
integration). Data for 13C NMR are reported in terms of chemical
shift (δ, ppm). Elemental analyses and high-resolution mass
spectrometry (HRMS) were carried out by the Analytical Laboratory
of the Shanghai Institute of Organic Chemistry (CAS). GC-MS
analysis was performed on Agilent 7890A gas chromatograph coupled
to an Agilent 5975C inert mass selective detector.
Synthesis of 2-[2-(Diisopropylphosphino)ferrocenyl]-4,5-di-
hydro-4-(1-methyl)oxazole (1b).
To a solution of 2-ferrocenyl-4,5-dihydro-4-(1-methyl)oxazole (1.0 g,
3.7 mmol, 1.0 equiv) in n-hexane (120 mL) was added TMEDA (0.7
mL, 4.9 mmol, 1.3 equiv). The solution was cooled to −78 °C. nBuLi
(2.5 M, 2.0 mL, 4.9 mmol, 1.3 equiv) was then added to the above
solution. The solution was stirred at −78 °C for 2 h and then warmed
to 0 °C, followed by the addition of iPr2PCl (0.74 g, 4.9 mmol, 1.3
equiv). The reaction mixture was stirred at 0 °C for 15 min and then
quenched with a saturated aqueous solution of NaHCO3 (50 mL,
degassed). The product was extracted with CH2Cl2 (2 × 50 mL). The
combined organic solution was washed with a saturated aqueous
solution of NaHCO3 (50 mL), H2O (50 mL), and brine (50 mL). The
organic layer was separated, dried with anhydrous MgSO4, filtered, and
concentrated to give a red-brown oil. The crude product was purified
by silica gel column chromatography using petroleum ether/ethyl
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00161.
Experimental procedures, characterization and spectral
data (PDF)
Crystallographic data for 2a (CIF)
Crystallographic data for 2b (CIF)
D
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Article
(6) Selected examples of Pd-catalyzed borylation of aryl halides and
pesudohalides: (a) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun.
1981, 11, 513−519. (b) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org.
Chem. 1995, 60, 7508−7510. (c) Murata, M.; Watanabe, S.; Masuda,
Y. J. Org. Chem. 1997, 62, 6458−6459. (d) Chow, W. K.; So, C. M.;
Lau, C. P.; Kwong, F. Y. Chem. - Eur. J. 2011, 17, 6913−6917.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for A.H.: hagmhsn@ecust.edu.cn.
*E-mail for Z.H.: huangzh@sioc.ac.cn.
Notes
■
(e) Furstner, A.; Seidel, G. Org. Lett. 2002, 4, 541−543. (f) Billingsley,
̈
The authors declare no competing financial interest.
K. L.; Barder, T. E.; Buchwald, S. L. Angew. Chem., Int. Ed. 2007, 46,
5359−5363. (g) Billingsley, K. L.; Buchwald, S. L. J. Org. Chem. 2008,
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Am. Chem. Soc. 2010, 132, 17701−17703. (i) Tang, W.; Keshipeddy,
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C. H. Org. Lett. 2011, 13, 1366−1369. (j) Kawamorita, S.; Ohmiya, H.;
Iwai, T.; Sawamura, M. Angew. Chem., Int. Ed. 2011, 50, 8363−8366.
(k) Molander, G. A.; Trice, S. L. J.; Kennedy, S. M. Org. Lett. 2012, 14,
4814−4817. (l) Molander, G. A.; Trice, S. L. J.; Kennedy, S. M. J. Org.
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(n) Wang, L.; Cui, X.; Li, J.; Wu, Y.; Zhu, Z.; Wu, Y. Eur. J. Org. Chem.
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ACKNOWLEDGMENTS
■
We acknowledge financial support from the National Basic
Research Program of China (2015CB856600), the National
Natural Science Foundation of China (No. 21422209,
21432011, 21272255, and 21421091), and the CAS/SAFEA
International Partnership Program for Creative Research
Teams.
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