Markovnikov-Selective Hydroboration of Olefins Catalyzed by a Copper N-Heterocyclic Carbene Complex

Article abstract of DOI:10.1021/acs.organomet.9b00394

The efficient and atom-economical hydroboration of alkenes and alkynes using an N-heterocyclic carbene (NHC) copper hydroxide catalyst has been developed. An equimolar combination of substrate and HBpin allows for the selective Markovnikov hydroboration of alkenes and alkynes. A variety of functional groups were tolerated in good to excellent yield. This system features a facile reaction setup, atom economy, high selectivity, and an easily synthesized copper-NHC catalyst.

Full text of DOI:10.1021/acs.organomet.9b00394

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Markovnikov-Selective Hydroboration of Olefins Catalyzed by a
Copper N‑Heterocyclic Carbene Complex
Tarah A. DiBenedetto, Astrid M. Parsons, and William D. Jones*
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
*
S Supporting Information
ABSTRACT: The efficient and atom-economical hydroboration of
alkenes and alkynes using an N-heterocyclic carbene (NHC) copper
hydroxide catalyst has been developed. An equimolar combination of
substrate and HBpin allows for the selective Markovnikov hydro-
boration of alkenes and alkynes. A variety of functional groups were
tolerated in good to excellent yield. This system features a facile
reaction setup, atom economy, high selectivity, and an easily
synthesized copper−NHC catalyst.
INTRODUCTION
imidazol-2-ylidene). A variety of functional groups are
tolerated, giving products in good to excellent yield. The use
of equimolar amounts of borylating agent and substrate allow
for the facile transformation to the organoborane at room
temperature. Furthermore, low catalyst loading, neat con-
ditions, and atom economy provide for an efficient and low-
cost method for obtaining valuable organoboranes.
■
Organoboron compounds are highly valuable synthetic
intermediates used in a variety of organic transformations.
Mainly, the synthetic utility of the boronate group has been
1
illustrated in many Suzuki−Miyaura coupling reactions to form
2
C−C bonds. Classically, organoboron compounds are
3
synthesized via Grignard or organolithium reagents. Alter-
natively, the use of a transition-metal catalyst is an attractive
strategy to access the organoboron motif. Whereas many
catalytic advances have been made toward the synthesis of
organoboron compounds, selectivity and atom economy still
remain a challenge.
4
RESULTS AND DISCUSSION
■
We began our study by investigating the hydroboration of
styrene 2a catalyzed by copper hydroxide Cu(IPr)(OH) 1.
Initial screens exclusively yielded the Markovnikov product,
albeit in low yield. A drastic increase in the yield of 3a was
observed when the reaction was carried out under more
concentrated conditions (Table 1, entries 1 and 2). Running
the reaction neat allowed for the efficient conversion of 2a to
3a in high yield while conserving the Markovnikov selectivity
(entry 3). In an effort to develop an efficient, atom-economical,
and cost-effective transformation, lower catalyst loadings were
explored. Lower catalyst loadings were tolerated (entries 4 and
5), but upon decreasing the loading to 0.25 mol %, the reaction
did not go to completion after 24 h. Control reactions showed
that catalyst is necessary for this transformation, whereas the
addition of solvent is not (entries 6 and 7). For this system, 0.5
mol % catalyst loading and neat conditions proved to be
optimal for the hydroboration of 2a to 3a (entry 4).
With the optimized reaction conditions in hand, various
alkenes and alkynes were tested (Table 2). In general,
substrates bearing para-electron withdrawing groups are well
tolerated (Table 2, 3b−e). In the case of 3b and 3d, increasing
the catalyst loading to 1.0 mol % allowed for a higher
conversion to the borylated product. For substrates with
electron-donating substituents, borylation proved to be more
challenging. Compared with a substrate with a mild electron-
Over the years, many examples of first-row metal-containing
catalysts in conjunction with supporting ligands have been
5
−12
reported for hydroboration reactions.
Notably, N-hetero-
cyclic carbenes (NHCs) have attracted attention as tunable
13
ligands for catalysis. Recently, copper−NHC complexes have
been prevalent in the literature for transforming alkenes and
alkynes to their borylated counterparts. Using a combination of
a simple copper salt, an NHC, and a base, Hoveyda reported
14
the hydroboration of aryl-substituted alkenes and acyclic and
1
5
exocyclic 1,1-disubstituted aryl alkenes. More recently,
t
Whittlesey reported the use of (NHC)Cu(O Bu) complexes
for the hydroboration of alkynes; however, the scope was
16
limited to internal alkynes. Nickel and iron NHC complexes
have also been developed for the hydroboration of styrene and
vinyl arenes. In 2016, Schomaker developed a heteroleptic
nickel NHC complex for the Markovnikov-selective hydro-
17
boration of styrenes. That same year, Thomas developed a
switchable Markovnikov/anti-Markovnikov system catalyzed
18
by an alkoxy-tethered NHC iron complex. Despite the many
catalytic strategies that have been developed for selective
Markovnikov or anti-Markovnikov products, most systems still
form some amount of the undesired product.
Herein we report a Markovnikov-selective hydroboration of
alkenes and alkynes catalyzed by a transition-metal base
Cu(IPr)(OH) 1 (IPr = 1,3-bis(2,6-diisopropylphenyl)-
Received: June 11, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00394
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Organometallics
Article
a
a
Table 1. Optimization of Standard Reaction Conditions
Table 2. Hydroboration Substrate Scope
b
entry
cat. loading
THF (mL)
yield 3a (%)
1
2
3
4
5
6
7
1.0 mol %
1.0 mol %
1.0 mol %
0.50 mol %
0.25 mol %
1.0
0.5
18
39
96
98
80
0
0.5
0
a
Reaction conditions: alkene (0.5 mmol), HBpin (0.5 mmol),
catalyst, THF (mL), rt, 24 h (IPr = 1,3-bis(2,6-diisopropylphenyl)-
b
1
imidazol-2-ylidene). Yield determined via H NMR spectroscopy
versus 2,4,6-trimethylbenzaldehyde as an internal standard.
donating group (3f), a substrate with a strong electron-
donating group in the para position (3g) significantly
decreased the yield, even at a higher catalyst loading. Attempts
to further optimize the reaction conditions for 3g did not
drastically improve the conversion, indicating that substrates
with strong electron-donating groups are not well-tolerated
1
9
with this system. Next, substrates featuring ortho and meta
functionality were examined. meta-Methyl substitution (3i)
results in a slight decrease in yield (compared with 3f),
whereas ortho substitution (3k) gave poor conversion and
poor yield, attributable to steric hindrance. Increasing the
catalyst loading to 1.0 mol % did result in an increase in yield
for the meta-substituted derivative 3j; however, this was not
the case for ortho-substituted derivative 3l. Furthermore, the
scope of the reaction was extended to internal alkene 3m and
acrylate substrate 3n. Strikingly, no linear borylation product of
α-methylstyrene (3o) was observed, highlighting the selectivity
of this system for Markovnikov product.
Borylation of alkynes proceeded in a modest to high yield,
6−99% yield (Table 2, 3p−w). Unlike 3k, the borylation of
4
ortho-methyl-substituted alkyne 3q proceeded in high con-
version (93%), suggesting that steric hindrance is not
detrimental for the borylation of alkynes. However, ortho-
methyl substitution results in the loss of Markovnikov
selectivity, yielding a ∼1:1 ratio of the branched and linear
products. Unlike 3j, the borylation of meta-chloro-substituted
alkyne 3w proceeded in high conversion (80%) under standard
conditions. Methyl 4-vinylbenzoate was also tested, and the
corresponding borylated product 3t was obtained in high yield,
a
Yield determined via 1H NMR spectroscopy versus 2,4,6-
trimethylbenzaldehyde as an internal standard (IPr = 1,3-bis(2,6-
b
8
9%. Strongly donating para-methoxy-substituted alkyne 3s
diisopropylphenyl)imidazol-2-ylidene). 1.0 mol % catalyst loading.
c
19
was tolerated in modest yield. This was not the case for para-
methoxy-substituted alkene, 3g. Mildly donating para-methyl-
substituted alkyne converted to product 3v in high yield
Yield determined via F NMR spectroscopy versus 4-fluorobenzoic
d
acid as an internal standard. Trace amount of linear product
1
observed by GC−MS (<1%); not observed by H NMR spectroscopy.
e
f
Isolated yield. α,α,α-Trifluorotoluene used as the internal standard.
Branched product observed by GC−MS (<5%). 43% of the linear
(
87%), mirroring the reactivity of alkene 3f (91%). Strongly
g
h
withdrawing trifluoromethyl-substituted alkyne 3u proceeded
1
i
product observed by H NMR spectroscopy. 4% of linear product
in high yield (93%). The scope of the reaction was extended to
j
1
observed by H NMR spectroscopy. 8% of linear product observed
by H NMR spectroscopy.
3
-ethynylpyridine, and the corresponding product 3r was
1
obtained in high yield. Lastly, the practicality of this
transformation was illustrated by accessing 3p on a gram
scale (eq 1).
t
reported that the reaction of Cu(IPr)(O Bu) with triethox-
Moving forward, we sought to explore the mechanism of the
reaction. NHC ligands have been reported to stabilize low-
ysilane in C D generates a dimeric copper(I) hydride
6
6
20
complex. This complex is unstable at room temperature
20,21
0
coordinate, reactive copper(I) complexes.
In 2004, Sadighi
and will start to decompose within 1 h to give Cu precipitate.
B
DOI: 10.1021/acs.organomet.9b00394
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Organometallics
Article
CONCLUSIONS
■
We have successfully developed the highly Markovnikov-
selective copper(I)-catalyzed hydroboration of alkenes and
alkynes. It was found that substrates with electron-withdrawing
groups are well-tolerated, whereas substrates with strong
electron-donating groups hinder reactivity. Furthermore, steric
hindrance plays an important role, with ortho substitution of
the styrene being detrimental for catalysis. The electronic and
steric effects are less important for the hydroboration of
alkynes. The current strategy features high selectivity for the
Markovnikov product, excellent atom economy, low catalyst
loading, and neat reaction conditions making this an attractive
strategy for the hydroboration of alkenes and alkynes.
Given these results, we sought to examine the in situ
stoichiometric formation of Cu(IPr)(H) 4 in benzene-d6
(Scheme 1a). Indeed, the reaction of 1 with HBpin results
Scheme 1. Stoichiometric Experiments
EXPERIMENTAL SECTION
General. Unless otherwise noted, all organometallic compounds
were prepared and handled under a nitrogen atmosphere using
■
1
2
standard Schlenk and glovebox techniques. (1E,2E)-N ,N -Bis(2,6-
diisopropylphenyl)-ethane-1,2-diimine A was synthesized according
in a rapid reaction with the immediate appearance of an
22
26
intense yellow color. This observation is consistent with the
color change observed in the previously mentioned formation
to the literature procedure. 1,3-Bis(2,6-diisopropylphenyl)-1,3-
dihydro-2H-imidazol-2-ylidene B was synthesized according to the
27
20
literature procedure. Compound C (Cu(IPr)Cl) was synthesized
of a copper(I) hydride complex in C D . Furthermore, in
6
6
28
1
according to the literature procedure and purified according to the
tetrahydrofuran-d , the H NMR spectrum of 1 shows the
8
29
literature procedure. Compound 1 (Cu(IPr)(OH)) was synthesized
characteristic broad singlet of the OH group at δ −1.91. Upon
the addition of HBpin, the resonance at δ −1.91 disappears,
likely due to the formation of a copper hydride, which is well
30
according to the literature procedure.
1
2
Synthesis of (1E,2E)-N ,N -Bis(2,6-diisopropylphenyl)-ethane-
,2-diimine (A). A 500 mL round-bottomed flask containing a stir
1
23,24
precedented in the literature.
HBpin to a solution containing the proposed intermediate
Scheme 1b) indeed yields borylated product 3a, suggesting
The addition of 2a and
(
that the copper species formed is an on-cycle active catalytic
intermediate. (See the Supporting Information for more
details.)
With the insight gained from these mechanistic experiments
bar was charged with 2,6-diisopropylaniline (280 mmol, 1.00 equiv),
glyoxal (140 mmol, 0.500 equiv), 100% anhydrous ethanol (250 mL),
and two drops of formic acid. The reaction was stirred at room
temperature for 48 h. A yellow precipitate started to form after 4 h.
After 48 h, the reaction was gravity filtered. The yellow precipitate was
washed with cold methanol and dried on a vacuum line to afford A
20,23,24
along with literature precedent,
a plausible mechanism is
proposed (Scheme 2). The reaction of complex 1 with HBpin
Scheme 2. Proposed Catalytic Cycle
(
33.8 g, 64% yield). Compound A was used in the next step without
26
1
further purification. Spectroscopic data match the reported data.
H
NMR (400 MHz, CDCl ): δ 8.10 (s, 2 H), 7.22−7.14 (m, 6 H),
3
3
.00−2.88 (m, 4 H), 1.21 (d, J = 6.8 Hz, 24 H).
Synthesis of 1,3-Bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imi-
dazol-2-ylidene (B). A 50 mL round-bottomed flask containing a stir
bar was charged with paraformaldehyde (46.0 mmol, 1.10 equiv) and
4
M HCl/dioxane solution (63.0 mmol, 1.50 equiv). The reaction was
stirred until most of the paraformaldehyde dissolved. A 250 mL three-
necked round-bottomed flask equipped with an addition funnel and
stir bar was charged with A (42.0 mmol, 1.00 equiv) and ethyl acetate
(85.0 mL). The paraformaldehyde solution was then added to the
addition funnel and added dropwise over the course of 1 h. The
addition of the paraformaldehyde solution caused the yellow solution
containing A to turn orange. A white precipitate formed after 5 min.
After the slow addition was complete, the solution was orange/brown
with an additional white precipitate. The reaction was stirred at room
temperature for 16 h, at which point the precipitate was brown/
orange. The brown/orange solid was gravity filtered and washed with
ethyl acetate, affording a light-pink solid. The light-pink solid was
dissolved in chloroform and then concentrated in vacuo and dried on
generates the active catalyst 4, a Cu(I)−hydride complex.
Coordination of the alkene, followed by migratory insertion
gives species 5. Reaction of 5 with HBpin affords borylated
product 3, regenerating the active catalyst 4. The high
regiochemistry obtained can be attributed to the strong
preference for the branched isomer 5. This preference can
be attributed to the formation of the strong methyl β-C−H
bond in 5, as the metal−benzylic carbon bond is expected to
2
5
be weak.
C
DOI: 10.1021/acs.organomet.9b00394
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Organometallics
Article
a vacuum line to afford B (21.4 g, 86% yield). Spectroscopic data
removed from the glovebox and stirred at room temperature for 24 h.
The reaction was then filtered through a celite plug. The 1-dram vial
was washed with CH Cl (5 × 0.5 mL), and the washings were
31
1
match the reported data. H NMR (400 MHz, CDCl ): δ 10.04 (s,
3
1
H), 8.15 (s, 2 H), 7.58 (t, J = 7.8 Hz, 2 H), 7.35 (d, J = 7.8 Hz, 4
2
2
H), 2.51−2.40 (sept, J = 6.8, 4 H), 1.29 (d, J = 6.8 Hz, 12 H), 1.25 (d,
J = 6.9 Hz, 12 H).
filtered through the plug. The combined washings were concentrated
1
in vacuo and analyzed by H NMR spectroscopy utilizing 1,4,6-
Synthesis of Cu(IPr)Cl (C). An oven-dried 250 mL round-bottomed
flask equipped with a stir bar was charged with B (9.36 mmol, 1.54
trimethylbenzaldehyde as the internal standard. Note: In conjunction
with NMR analysis, the reaction was also analyzed using GC−MS.
The GC−MS trace showed no formation of the linear product, which
comes at a distinctly different retention time than the branched
product. See the Supporting Information. Note: Although reaction
conditions are neat, substrates that are solids are still able to be
borylated (2h, 2r, 2t). Note: The neat large-scale reaction is
extremely exothermic. The slow addition of HBpin is recommended.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00394.
equiv) and Cu O (6.08 mmol, 1 equiv). The flask was then fitted with
2
■
a reflux condenser capped with a septum, and the system was put
under a nitrogen atmosphere. The system was then charged with
toluene (40 mL) and refluxed for 4 days. The reaction was then
cooled, and CH Cl was added to dissolve any solid that had formed.
S
*
2
2
The reaction was transferred to a separatory funnel and washed with
water (2 × 100 mL) to remove any unreacted imidazolium salt. The
organic layer was dried over sodium sulfate and concentrated in
vacuo. The resulting dark-brown solid was transferred to a frit and
washed with pentane. The solid turned from dark-brown to light-
purple. The light-purple solid was recrystallized by dissolving the
product in CH Cl , stirring over K CO , and gravity filtering to
Details of the optimization of the borylation reaction,
NMR spectra of stoichiometric reactions, and product
characterization data (PDF)
AUTHOR INFORMATION
Corresponding Author
E-mail: jones@chem.rochester.edu.
ORCID
2
2
2
3
■
*
remove the K CO . The product was then precipitated by the slow
2
3
addition of pentane through an addition funnel. The solid was gravity
filtered and dried on a vacuum line to afford C (1.4 g, 31% yield).
32
1
Spectroscopic data match the reported data. H NMR (400 MHz,
CDCl ): δ 7.49 (t, J = 7.8 Hz, 2 H), 7.30 (d, J = 7.8 Hz, 4 H), 7.13 (s,
3
Tarah A. DiBenedetto: 0000-0001-8861-0740
Astrid M. Parsons: 0000-0002-3835-9110
William D. Jones: 0000-0003-1932-0963
2
1
H), 2.63−2.50 (sept, J = 6.8 Hz, 4 H), 1.30 (d, J = 6.9 Hz, 12 H),
.23 (d, J = 6.9 Hz, 12 H). Elemental analysis calculated for
C H N CuCl (MW 487.59 g/mol): C, 66.51; H, 7.44; N, 5.75.
27
36
2
Found: C, 66.59; H, 7.49; N, 5.53. Note: The final product varies in
color from pale-purple to off-white. Sometimes multiple recrystalliza-
tions are required to get a pure product. The purity of C greatly
affects the yield of 1.
Synthesis of Cu(IPr)(OH) (1). An oven-dried 150 mL round-
bottomed flask was charged with C (2.50 mmol, 1.00 equiv), CsOH
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge funding from the U.S. Department
of Energy (grant no. FG02-86ER-13569) for instrument usage
and chemicals. A.M.P. acknowledges funding from an NSF
GRFP award (DGE-1419118).
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(
D
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DOI: 10.1021/acs.organomet.9b00394
Organometallics XXXX, XXX, XXX−XXX