Highly reactive, single-component nickel catalyst precursor for Suzuki-Miyuara cross-coupling of heteroaryl boronic acids with heteroaryl halides

Article abstract of DOI:10.1002/anie.201207428

One for all: The coupling of a range of nitrogen- and sulfur-containing heteroaryl halides with five-membered nitrogen-, oxygen-, and sulfur-containing heteroaryl boronic acids were achieved in high yields with only 0.5 mol % of the single-component nickel precatalyst [(dppf)NiCl(cinnamyl)] (dppf=1,1′- bis(diphenylphosphanyl)ferrocene). The reaction demonstrates good functional group compatibility, and is easily conducted on a large scale without a dry box. Copyright

Full text of DOI:10.1002/anie.201207428

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DOI: 10.1002/anie.201207428
Heterobiaryl Cross-Coupling
Highly Reactive, Single-Component Nickel Catalyst Precursor for
Suzuki–Miyuara Cross-Coupling of Heteroaryl Boronic Acids with
Heteroaryl Halides**
Shaozhong Ge and John F. Hartwig*
The Suzuki–Miyaura (S-M) cross-coupling reaction is the
heteroaryl halides with five-membered heteroaryl boronic
acids. These coupling reactions occur in high yields with only
0.5 mol% of nickel and no added ligand. Moreover, the low
reactivity of this nickel precatalyst to air and moisture makes
these reactions practical to conduct on large or small scale.
Our studies on nickel-catalyzed S-M cross-couplings were
based on recent findings that a single-component, nitrile-
ligated nickel catalyst improved the enantioselectivity of the
a arylation of ketones.[10] Most of the prior nickel-catalyzed S-
M reactions were conducted with a precatalyst and excess of
an added ligand, such as [Ni(cod)2] (cod = cyclo-1,5-octa-
diene) with PCy3 or PPh3,[3g,5a,d,6] or the nickel(II) precatalysts
[(dppf)NiCl2], [(dppp)NiCl2], and [(PCy3)2NiCl2] (dppf = 1,1’-
bis(diphenylphosphanyl)ferrocene, dppp = l,3-bis(diphenyl-
phosphanyl)propane) with 0–2 equivalents of added
PCy3.[3a–c,5c,7,8] Even the aryl/nickel(II) chloride complex
[trans-(PPh3)2NiCl(1-naphthyl)], which is a potential reaction
intermediate, required high catalyst loading (5–10 mol% Ni)
and 1–2 equivalents of added ligand per catalyst for the
coupling of aryl tosylates and mesylates with aryl boronic
acids or esters.[5b,e]
À
most frequently conducted catalytic process to construct C C
bonds in medicinal chemistry,[1] and these coupling reactions
are conducted most frequently with palladium catalysts.[2] The
replacement of the precious metal palladium with the first-
row, abundant metal nickel for S-M couplings could signifi-
cantly reduce the cost of the catalyst. S-M couplings catalyzed
by nickel complexes, because of the high reactivity of
nickel(0) toward chloroarenes could allow some of the more
challenging coupling reactions to occur with catalysts con-
taining simple ligands. Several nickel catalysts have been
reported for this class of cross-coupling reaction,[3] and these
nickel catalysts are unusually active for the coupling of phenol
derivatives[4] such as sulfonates,[5] ethers,[6] esters,[7] carba-
mates,[8] carbonates, and sulfamates.[8b] However, there are
two general drawbacks to the existing nickel catalysts for S-M
cross-couplings: high catalyst loading (3–10 mol% Ni) is
required to achieve the coupling in high yield, and the scope
of these cross-couplings does not encompass reactions that
form hetero-biaryl products, which are important for medic-
inal and agrochemical applications.
Catalytic syntheses of the hetero-biaryl compounds are
challenging because the ligation of the heteroaryl coupling
partner can poison the catalyst. The majority of such catalytic
hetero-biaryl syntheses have been achieved by palladium-
catalyzed Suzuki–Miyaura or Stille–Migita cross-couplings
with a 2–4 mol% palladium loading.[9] Nickel catalysts have
also been employed to conduct such coupling reactions.
However, the scope of the couplings between two heteroaryl
reagents catalyzed by a nickel complex is limited to the
specific reactions of 2- or 3-thienyl neopentylglycolboronate
with pyrid-3-yl mesylate or sulfamate.[5d,e] No nickel catalyst
that reacts with high turnover numbers and couples two
heteroaryl substrates has been reported.
We initiated our studies of S-M cross-coupling reactions of
heteroaryl boronic acids with heteroaryl halides by exploring
reactions catalyzed by [(binap)Ni(h2-NCPh)] (binap = 2,2’-
bis(diphenylphosphanyl)-1,1’-binaphthyl). The use of a cata-
lyst that couples these reagents under mild reaction con-
ditions is important because of the instability of many
heteroaryl boronic acids. The coupling reactions catalyzed
by this nitrile complex did occur, but with limited scope (for
details, see the Supporting Information). We investigated
reactions catalyzed by the analogous dppf-ligated nickel(0)
nitrile complex, but this complex was too unstable to isolate in
pure form.
Thus, we sought alternative discrete nickel precatalysts
containing bis(phosphine)s besides binap. We considered that
dppf-ligated allylnickel complexes could readily generate the
reactive nickel(0) intermediate. Well-defined palladium allyl
complexes ligated by phosphine ligands are more active for
cross-coupling reactions that form C-C and C-N bonds than
those generated in situ by combining [Pd2(dba)3] (dba =
dibenzylideneacetone) or Pd(OAc)2 with phosphine
ligands.[11] Phosphine-ligated allylnickel complexes are well
known,[12] but their potential as catalysts for cross-coupling
reactions has not been tapped.[13]
We report
a single-component nickel catalyst that
addresses these current limitations. The dppf-ligated cinna-
mylnickel(II) chloride (dppf = 1,1’-bis(diphenylphosphanyl)-
ferrocene) couples a range of nitrogen- and sulfur-containing
[*] Dr. S. Ge, Prof. J. F. Hartwig
Department of Chemistry, University of California, Berkeley
CA, 94720-1460 (USA)
E-mail: jfhartwig@berkeley.edu
Homepage: http://www.cchem.berkeley.edu/jfhgrp/index.html
The new nickel complex [(dppf)Ni(cinnamyl)Cl] (1) was
prepared by a one-pot reaction of cinnamyl chloride with
[(dppf)Ni(cod)], which was generated in situ from [Ni(cod)2]
and dppf in THF [Eq. (1)]. This reaction afforded complex
[**] The authors acknowledge the financial support of this work from the
NIH (GM-58108).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207428.
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Table 1: Coupling of sulfur- and oxygen-containing 2-heteroaryl boronic
acids with heteroaryl halides catalyzed by 1.[a]
1 as a brown solid in 86% yield upon isolation. Compound
1 was stable under air in a closed vial at 08C for at least two
weeks without noticeable decomposition, as determined by
1H and 31P NMR spectroscopy.
The rate of generation of nickel(0) from 1 was studied
with 2-thienyl boronic acid as a nucleophile and K3PO4 as
a base in THF in the presence of 1 equivalent of dppf to trap
the resulting nickel(0) [Eq. (2)]. This reaction afforded
[(dppf)2Ni0] quantitatively in 10 minutes at room temper-
ature. On the basis of this rapid and quantitative generation of
nickel(0), we evaluated 1 as the catalyst precursor (0.5 mol%)
for the coupling of 2-thienyl boronic acid with 3-chloropyr-
idine in the presence of K2CO3(H2O)1.5 in acetonitrile at 508C.
This reaction afforded the coupling product in 81% yield
upon isolation. For comparison, the reaction catalyzed by the
nickel(II) precursors (e.g. [(PPh3)2NiCl(1-naphthyl)]/2PPh3
or 2PCy3 and [(dppf)NiCl2]/Zn) used previously for S-M
couplings of aryl boronic acids with aryl halides was
conducted under the same reaction conditions. However,
the reactions of these catalysts with the heteroaromatic
substrates studied here gave the coupling product in less than
30% yield upon isolation (for details, see the Supporting
Information).
To explore the scope of this catalyst for the preparation of
hetero-biaryl compounds, we evaluated the coupling of
a series of heteroaryl halides with several 2-heteroaryl
boronic acids. These boronic acids undergo deboronation at
elevated temperatures, and coupling of these boronic acids
with (hetero)aryl halides has been achieved only recently with
2 mol% of the discrete [chloro(Xphos){2-(2-aminoethyl)phe-
nyl}palladium(II)] precatalyst.[9d]
The results of the couplings catalyzed by 1 are summar-
ized in Table 1. These reactions were conducted either with
K2CO3(H2O)1.5 as a base in acetonitrile at 508C or with K3PO4
as base in 1,4-dioxane at 708C. Cross-couplings of 2-hetero-
aryl boronic acids with heteroaryl halides occurred with just
0.5 mol% of 1 and no added ligand. The 2-heteroaryl boronic
acids that undergo this process include 2-furanyl (entries 1a–
10a, 11, and 12), 2-thienyl (entries 1b–10b), benzo[b]furan-2-
yl (entries 13a–22a and 23), and benzo[b]thien-2-yl (entries
13b–22b and 24–28) boronic acids. In general, these 2-
heteroaryl boronic acids coupled with a range of unsubsti-
tuted and substituted pyridyl halides containing electron-
donating groups (Me and OMe) or electron-withdrawing
groups (CN, CF3, CO2Me) in good to excellent yields, despite
the coordinating properties of pyridines. We presume that the
catalyst is stable to the high concentrations of pyridines
[a] Reaction conditions: heteroaryl halide (0.400 mmol), 2-heteroaryl
boronic acid (0.800 mmol), compound 1 (2.0 mmol, 0.5 mol%), K2CO3-
(H2O)1.5 (1.60 mmol), CH3CN (1.0 mL), 508C, 12 h. Yields of isolated
products are listed. [b] K3PO4 (1.60 mmol), 1,4-dioxane (1.0 mL), 708C.
[c] 1 mol% compound 1.
because of the chelating property of the dppf ligand. The
coupling of these 2-heteroaryl boronic acids with 2-bromo-
pyrimidine did not occur, even though these boronic acids
reacted with 5-bromopyrimidine to form the corresponding
hetero-biaryl in high yields (entries 3a, 3b, 14a, and 14b).
This nickel-catalyzed S-M coupling tolerated unprotected
NH2 functionality (entry 25) and electrophilic functionalities
such as an aldehydes (entries 8a, 8b, and 26), ketones with
enolizable hydrogen atoms (entries 11 and 27), esters (entries
19a and 19b), and nitriles (entries 20a and 20b). In contrast
to the coupling of aryl mesylates and aryl neopentylglycol-
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boronate catalyzed by 6 mol% [Ni(cod)2] and 12 mol% PCy3,
which were reported to be intolerant of hydroxy functiona-
lity,[5d] the coupling of benzo[b]thien-2-yl boronic acid with 2-
chloro-5-hydroxymethylpyridine, bearing a free hydroxy
group, catalyzed by 0.5 mol% of 1 occurred in high yield
(entry 28).
The coupling of heteroaryl halides with 3-heteroaryl
boronic acids also occurred with broad scope. However, the
appropriate combination of base and solvent is critical for
these couplings to occur in high yields. For example, 3-furanyl
or 3-thienyl boronic acid coupled with 3-chloropyridine in
high yields when the coupling reactions were conducted with
K3PO4 as a base in 1,4-dioxane at 808C. However, either no
coupling (for 3-thienyl boronic acid) or very low yield (37%
for 3-furanyl boronic acid) were obtained when these
reactions were conducted in acetonitrile with K2CO3(H2O)1.5
as a base.
containing heterocycles has not been achieved with nickel
catalysts.
We studied initially the coupling of 1-Boc-pyrrole-2-
boronic acid with heteroaryl halides catalyzed by 1, and the
results are summarized in Table 3. The scope of heteroaryl
halides that underwent this process encompassed a range of 3-
pyridyl (entries 5–8) and 2-pyridyl (entries 9 and 10) halide
derivatives bearing electron-donating (OMe and Me) and
Table 3: Coupling of 1-Boc-pyrrole-2-boronic acid with heteroaryl halides
catalyzed by 1.[a]
The reactions of 3-heteroaryl boronic acids with hetero-
aryl halides occurred with a scope similar to that of the
reactions of 2-heteroaryl boronic acids in Table 1, and
selected examples are summarized in Table 2. Unsubstituted
3-chloropyridine reacted with these boronic acids in high
yields (entries 1 and 5). Heteroaryl halides containing two
nitrogen atoms, for example, 5-bromopyrimidine, also reacted
with these boronic acids in high yields (entries 2 and 6). The
reactions of these boronic acids with substituted chloropyr-
idines bearing electron-withdrawing groups (entries 3 and 7)
or electron-donating groups (entries 4 and 8) also occurred in
high yields.
[a] Reaction conditions: heteroaryl halide (0.400 mmol), 1-Boc-pyrrole-2-
boronic acid (0.800 mmol), 1 (2.0 mmol, 0.5 mol%), 12 h. Method A:
K2CO3(H2O)1.5 (1.60 mmol), CH3CN (1.0 mL), 508C, 12h. Method B:
K3PO4 (1.60 mmol), 1,4-dioxane (1.0 mL), 708C, 12h. Yields of isolated
products are listed.
Table 2: Coupling of five-membered 3-heteroaryl boronic acids with
heteroaryl halides catalyzed by 1.[a]
electron-withdrawing (CF3, CN, CO2Me) groups. In general,
these coupling reactions afforded the corresponding hetero-
biaryl products in good to excellent yields (82–96%) upon
isolation. This coupling also occurred in high yields with 2-
chloropyrazine (entry 2) and quinolinyl halides (entries 3 and
4). The ortho-substituted 3-bromo-4-methylpyridine reacted
with this boronic acid in 92% yield (entry 5). A sulfur-
containing heteroaryl bromide also coupled with this boronic
acid in high yield (entry 11).
Although the cross-couplings of five-membered hetero-
aryl boronic acids with heteroaryl halides occurred in high
yields in almost all cases we examined, the few limitations on
this reaction we observed should be noted. Among all of the
five-membered ring heteroaryl halides tested, only 2-chlor-
obenzoxazole and 2-chlorobenzothiazole did not form the
coupled product from reactions with any of the five-mem-
bered ring heteroaryl boronic acids employed in this study.
Among six-membered ring heteroaryl halides, 2-halopyrimi-
dine did not react, as noted earlier in this paper. Although
each five-membered ring heteroaryl boronic acid we inves-
tigated coupled in high yield with good scope, pyridyl boronic
acids did not react under either conditions A (K2CO3(H2O)1.5
as base in acetonitrile) or conditions B (K3PO4 as base in 1,4-
dioxane). Nickel-catalyzed coupling of pyridylboronates will
be the subject of future studies.
[a] Reaction conditions: heteroaryl halide (0.400 mmol), 3-heteroaryl
boronic acid (0.800 mmol), 1 (2.0 mmol, 0.5 mol%), K3PO4 (1.60 mmol),
1,4-dioxane (1.0 mL), at 808C for 8 h. Yields of isolated products are
listed.
The S-M coupling between two nitrogen-containing
heterocyclic coupling partners is challenging, presumably
because of the instability of azole boronic acids, the electron-
rich nature and instability of many haloazoles, and the
coordinating property of pyridines. Such coupling has been
realized only recently with palladium catalysts.[9a,b] Nickel-
catalyzed S-M couplings of azine and azole electrophiles have
been reported previously, but the coupling of two nitrogen-
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Compared with the air-sensitive and thermally unstable
nickel(0) precursor [Ni(cod)2], the single-component nickel
precursor 1 is convenient to use. It is stable indefinitely when
stored under nitrogen at room temperature, and it is
sufficiently stable to air and moisture to be weighed on the
benchtop. Reactions catalyzed by 1, which was stored for
more than six months under a nitrogen atmosphere, were
indistinguishable from those conducted with the freshly
prepared 1. For example, the reaction of 2-thienyl boronic
acid with 3-chloropyridine catalyzed by 0.5 mol% of 1 (stored
in a nitrogen-filled dry box for more than six months)
afforded 3-(thien-2-yl)pyridine in 84% yield, and this result is
comparable to that catalyzed by the freshly prepared 1.
Moreover, these coupling reactions can be conducted with
catalyst weighed in air. The coupling of 2-thienyl boronic acid
with 5-bromopyrimidine conducted on a 4.0 mmol scale in
degassed acetonitrile catalyzed by 0.5 mol% of 1 (stored
under air in a closed vial at 08C for 15 days and weighed in
air) afforded 5-(thien-2-yl)pyrimidine in 89% yield (this
reaction was set up outside a dry box; see the Supporting
corresponding reactions assembled inside a dry box. Thus,
these coupling reactions can be conducted in a manner
suitable for practical applications.
In summary, we have prepared the new, discrete nickel(II)
complex [(dppf)Ni(cinnamyl)Cl] (1) by the oxidative addition
of cinnamyl chloride to [(dppf)Ni(cod)], and this nickel
precursor catalyzes the couplings of five-membered hetero-
aryl boronic acids with a range of nitrogen- and sulfur-
containing heteroaryl halides. The desired hetero-biaryl
products were obtained in good to excellent yields upon
isolation, with only a 0.5 mol% loading of 1. This coupling
reaction was easily conducted on large scale without a dry
box. Such efficient coupling reactions result from the rapid
generation of the catalytically active nickel(0) species from
the nickel precursor [(dppf)Ni(cinnamyl)Cl] under mild S-M
coupling conditions. Further studies of other classes of cross-
coupling reactions catalyzed by related single-component
nickel precatalysts are ongoing in our laboratory.
Information). The result of this reaction was indistinguishable Experimental Section
Procedure for coupling of heteroaryl boronic acids with heteroaryl
from that conducted with 0.5 mol% of the freshly prepared
1 (0.400 mmol scale, carried out inside a dry box), and it
delivered the product in 92% yield (entry 3b; Table 1).
To assess the generality of the procedure for conducting
reactions outside a dry box, we conducted 11 examples of
couplings to evaluate the scope of the reactions under these
reaction conditions with different boronic acids and function-
alized aryl halides. We compared these results to those of
reactions conducted under fully anaerobic conditions. The
results of the reactions with catalysts weighed in air are shown
in Table 4. These reactions include the coupling of heteroaryl
halides containing aldehyde, enolizable ketone, nitrile, amino,
ester, and hydroxy functionality with oxygen-, sulfur-, and
nitrogen-containing heteroaryl boronic acids. The rates and
yields of these reactions were comparable to those of the
halides catalyzed by 1 (For Table 4): In air, a 4 mL screw-capped vial
was charged with the heteroaryl halide (0.400 mmol, if solid), the
heteroaryl boronic acid (0.800 mmol), K2CO3(H2O)1.5 or K3PO4
(1.60 mmol), 1 (2.0 mmol), and a magnetic stirring bar. The vial was
then sealed with a screw-cap containing a PTFE septum and the air in
the vial was replaced with nitrogen using a vacuum/nitrogen sequence
(three times). Degassed CH3CN or 1,4-dioxane (1 mL) and heteroaryl
halides (0.400 mmol, if liquid) were added using an air-tight syringe,
and the mixture was heated at 508C or 708C for 12 h and then
quenched with H2O (0.5 mL). The mixture was extracted with diethyl
ether (3 mL ꢀ 2), and the combined ether fraction was dried over
Na2SO4. All the volatile materials were evaporated under reduced
pressure, and the residue was purified by chromatography on silica
with a CombiFlash system. Detailed procedures on the purification of
the hetero-biaryl products and data on the characterization of them
are contained in the Supporting Information.
Received: September 14, 2012
Published online: November 7, 2012
Table 4: Coupling of five-membered heteroaryl boronic acids with
heteroaryl halides catalyzed by 1 that was weighed in air.[a]
Keywords: biaryls · cross-coupling · homogeneous catalysis ·
.
nickel · synthetic methods
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