Bis(dialkylphosphino)ferrocene-Ligated Nickel(II) Precatalysts for Suzuki-Miyaura Reactions of Aryl Carbonates

Article abstract of DOI:10.1021/acs.organomet.9b00543

Aryl carbonates, a common protecting group in synthetic organic chemistry, are potentially valuable electrophiles in cross-coupling reactions. Here, after performing a thorough evaluation of different precatalysts, we demonstrate that (dcypf)Ni(2-ethylphenyl)(Br) (dcypf = 1,1-bis(dicyclohexylphosphino)ferrocene) is an efficient precatalyst for Suzuki-Miyaura reactions using a variety of aryl carbonates as substrates. Mechanistic studies indicate that (dcypf)Ni(2-ethylphenyl)(Br), which contains a bidentate phosphine that binds in a trans geometry, is an effective precatalyst for these reactions for two reasons: (i) it rapidly forms the Ni(0) active species and (ii) it minimizes comproportionation reactions between the Ni(0) active species and both the unactivated Ni(II) precatalyst and on-cycle Ni(II) complexes to form catalytically inactive Ni(I) species. In contrast, the state of the art precatalyst (dppf)Ni(o-tolyl)(Cl) (dppf = 1,1-bis(diphenylphosphino)ferrocene), which contains a bidentate phosphine that binds in a cis geometry, forms Ni(I) species during activation and is essentially inactive for aryl carbonate couplings. Although the exact reasons on a molecular level why the dcypf system is more active than the dppf system are unclear, our results indicate that in general Ni catalysts supported by the dcypf ligand will give better performance for catalytic reactions involving substrates which undergo relatively slow oxidative addition, such as aryl carbonates.

Full text of DOI:10.1021/acs.organomet.9b00543

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Bis(dialkylphosphino)ferrocene-Ligated Nickel(II) Precatalysts for
Suzuki−Miyaura Reactions of Aryl Carbonates
Emily L. Barth,† Ryan M. Davis,† Megan Mohadjer Beromi,† Andrew G. Walden,† David Balcells,‡
Gary W. Brudvig,† Amira H. Dardir,† Nilay Hazari,*,† Hannah M. C. Lant,† Brandon Q. Mercado,†
and Ian L. Peczak†
†Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
‡Hylleraas Centre for Quantum Molecular Sciences, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315
Oslo, Norway
S
* Supporting Information
ABSTRACT: Aryl carbonates, a common protecting group in
synthetic organic chemistry, are potentially valuable electro-
philes in cross-coupling reactions. Here, after performing a
thorough evaluation of different precatalysts, we demonstrate
that (dcypf)Ni(2-ethylphenyl)(Br) (dcypf = 1,1-bis-
(dicyclohexylphosphino)ferrocene) is an efficient precatalyst
for Suzuki−Miyaura reactions using a variety of aryl
carbonates as substrates. Mechanistic studies indicate that
(dcypf)Ni(2-ethylphenyl)(Br), which contains a bidentate
phosphine that binds in a trans geometry, is an effective
precatalyst for these reactions for two reasons: (i) it rapidly
forms the Ni(0) active species and (ii) it minimizes comproportionation reactions between the Ni(0) active species and both
the unactivated Ni(II) precatalyst and on-cycle Ni(II) complexes to form catalytically inactive Ni(I) species. In contrast, the
state of the art precatalyst (dppf)Ni(o-tolyl)(Cl) (dppf = 1,1-bis(diphenylphosphino)ferrocene), which contains a bidentate
phosphine that binds in a cis geometry, forms Ni(I) species during activation and is essentially inactive for aryl carbonate
couplings. Although the exact reasons on a molecular level why the dcypf system is more active than the dppf system are
unclear, our results indicate that in general Ni catalysts supported by the dcypf ligand will give better performance for catalytic
reactions involving substrates which undergo relatively slow oxidative addition, such as aryl carbonates.
to the current widely used systems for cross-coupling with Pd
catalysts, while in the case of most Ni-catalyzed cross-coupling
reactions these mechanistic studies still need to be performed.
Aryl carbonates are commonly used as protecting groups in
organic chemistry and can be readily prepared from phenols.9
There are potentially significant benefits in complex molecule
synthesis, in terms of the step count and overall efficiency, if
practical methods could be developed to use a protecting
group such as an aryl carbonate as an electrophile in cross-
coupling. The first report of the use of aryl carbonates in cross-
coupling was by Garg et al.7 They performed several Suzuki−
Miyaura reactions involving naphthyl carbonates under
reaction conditions that utilized 10 mol % of (PCy3)2NiCl2,
4 equiv of boronic acid, and 7.2 equiv of base at 130 °C for 24
h. Subsequently, Kuwano and Shimizu showed that a catalytic
system generated in situ from Ni(COD)2 (COD = 1,5-
cyclooctadiene) and 1,1-bis(dicyclohexylphosphino)ferrocene
(dcypf) could couple a limited number of phenyl carbonates,
in addition to a small range of naphthyl carbonates, at 60 °C.10
INTRODUCTION
■
Pd-catalyzed cross-coupling reactions represent one of the
most powerful synthetic methods and, due to their versatility
and reliability, have been utilized in numerous commercial
processes.1 In recent years, the significant research effort
dedicated to developing more sustainable catalysts for cross-
coupling using base metals such as Ni has revealed that there
are many reactions which are more effectively facilitated by Ni
catalysts in comparison with traditional Pd systems.2 Perhaps
the most notable example is the use of Ni catalysts for a variety
of cross-coupling and related reactions involving sp3-hybridized
substrates.3 However, it has also been demonstrated that Ni
catalysts are superior to Pd systems for the coupling of a
variety of nontraditional sp2-hybridized substrates. For
example, aryl nitriles,4 aryl trimethylammonium salts,5 aryl
fluorides,6 aryl carbamates,7 aryl esters,8 and aryl carbonates,7
which are the focus of this work, are all more effectively
coupled using Ni catalysts. Nevertheless, despite the impressive
variety of cross-coupling reactions which Ni catalysts can now
facilitate, in general the harsh reaction conditions and high
catalyst loadings limit the practicality of these reactions.
Further, detailed mechanistic studies over several decades led
Received: August 8, 2019
© XXXX American Chemical Society
A
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Figure 1. Comparison between previous and current work exploring Ni-catalyzed Suzuki−Miyaura reactions involving aryl carbonates.
Table 1. Optimization of the Suzuki−Miyaura Reaction between 2-Naphthyl tert-Butylcarbonate and 4-Methoxyphenylboronic
a
L
Acid Using NiII
(L = dppf, dippf, dcypf) as Precatalysts
(o‑tol)(Cl)
entry
ligand
dppf
dippf
dcypf
temp (°C)
time (h)
boronic acid (equiv)
catalyst loading(mol %)
water (equiv)
yield (%)
1
2
3
80
80
80
24
24
24
2.5
2.5
2.5
2.5
2.5
2.5
0
0
0
5
61
>99
4
5
dcypf
dcypf
80
80
4
2
2.5
2.5
2.5
2.5
0
0
>99
90
6
7
dcypf
dcypf
60
40
4
4
2.5
2.5
2.5
2.5
0
0
92
31
8
9
10
dcypf
dcypf
dcypf
60
60
60
4
4
4
1
1.5
2
2.5
2.5
2.5
0
0
0
64
74
87
b
11
12
13
dcypf
dcypf
dcypf
60
60
60
4
4
4
2.5
2.5
2.5
0.5
1
2
0
0
0
31
57
74
b
b
14
15
dcypf
dcypf
60
60
4
4
2.5
2.5
2.5
1.0
50
50
91
59
a
Reaction conditions: 2-naphthyl tert-butylcarbonate (0.133 mmol), 4-methoxyphenylboronic acid (0.133 mmol, 1 equiv; 0.200 mmol, 1.5 equiv;
0.266 mmol, 2 equiv; 0.333 mmol, 2.5 equiv), K3PO4 (1.8 equiv relative to boronic acid), naphthalene internal standard (0.0665 mmol, 0.5 equiv),
b
toluene (1 mL). Yields were determined by GC and are reported as the average of at least two trials. The reaction went to completion after 12 h of
reaction time; however, the reactions at 0.5 and 1.0 mol % did not reach completion even after 24 h.
However, this system still required high catalyst loadings (10
mol %) and extended reaction times (24−96 h) in order for
high yields to be obtained. In related chemistry, Molander et al.
reported that a closely related Ni(COD)2/2PCy3·HBF4 system
could couple a naphthyl carbonate electrophile with a
potassium aryltrifluoroborate nucleophile at 110 °C with 10
mol % catalyst loading.11 A conceptually different approach
was adopted by Chakraborti and co-workers, who showed that
heterobimetallic Ni/Pd nanoclusters could perform Suzuki−
Miyaura reactions using aryl carbonates, but high temperatures
(100 °C) were required, and the substrate scope was not
explored.12 At this stage, in order to develop practical coupling
reactions involving aryl carbonates, improvements are required
B
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Figure 2. Synthesis of multiple dcypf-ligated precatalysts containing different aryl ligands examined in this study. Inset: ORTEP of
dcypfNiII(2‑ethylPh)(Br) with thermal ellipsoids shown at 50% probability. Hydrogen atoms and solvent of crystallization have been removed for clarity.
in the substrate scope, reaction conditions, and catalyst
loadings.
more slowly than aryl sulfamates,14 we hypothesized that
dppfNiII
could potentially be a precatalyst for aryl
(o‑tol)(Cl)
Both our group and others have been exploring the use of
precatalysts of the type (dppf)Ni(aryl)(X) (dppf = 1,1-
bis(diphenylphosphino)ferrocene; X = Cl, Br) for a variety
of Suzuki−Miyaura reactions including those involving aryl
sulfamate electrophiles.13 Our results show that these
precatalysts give markedly better activity than systems
generated in situ from Ni(COD)2 or other “ligandless” Ni
precursors, likely because precatalysts increase the amount of
Ni in a catalytically active form.2e,13e However, despite the
increased catalytic efficiency of these precatalysts, they often
generate off-cycle Ni(I) species during activation.13b,c,e Given
the close relationship between aryl sulfamates and aryl
carbonates, we hypothesized that the use of well-defined
precatalysts of the type (PP)Ni(aryl)(X) (PP = bidentate
phosphine ligand) could lead to more active catalysts for
reactions with aryl carbonates. Here, we demonstrate that the
p r e c a t a l y s t ( d c y p f ) N i ( 2 - e t h y l p h e n y l ) ( B r )
carbonate coupling. However, our initial efforts to use 2.5 mol
% of dppfNiII
as a precatalyst for the coupling of 2-
(o‑tol)(Cl)
naphthyl tert-butylcarbonate with 4-methoxyphenylboronic
acid at 80 °C for 24 h indicated that it was essentially inactive
for this transformation (Table 1). One of the major problems
in using dppfNiII
as a precatalyst for coupling reactions
(o‑tol)(Cl)
involving aryl sulfamates is that inactive Ni(I) species are
formed during catalysis, which is also a potential problem in
aryl carbonate coupling.13c The proposed pathway for the
formation of Ni(I) species involves comproportionation
between a Ni(0) and Ni(II) species,13b,c,15 which DFT
calculations suggest is aided by the formation of a dimeric
intermediate containing an η2 interaction between one of the
phenyl groups on the phosphine atoms of dppf and one Ni
center.13c To address this potential issue, the precatalysts
(dippf)Ni(o-tolyl)(Cl) (dippfNiII(o‑tol)(Cl); dippf = 1,1-bis-
(diispropylphosphino)ferrocene) and (dcypf)Ni(o-tolyl)(Cl)
(
dcypfNiII(2‑ethylPh)(Br)), which contains a bidentate phosphine
(
dcypfNiII(o‑tol)(Cl)) were synthesized using literature methods.16
ligand that binds in a trans geometry, generates the most active
catalyst reported to date for Suzuki−Miyaura reactions
involving aryl carbonates (Figure 1). We show that this
precatalyst activates rapidly under the reaction conditions and
only forms a small amount of inactive Ni(I) species during
catalysis, which is likely a reason for its high activity. Our
results indicate that judicious selection of the ancillary ligand
can limit the formation of Ni(I) species during catalysis and
provide fundamental information about ligand selection that
may be relevant to the design of improved catalysts for a
variety of Ni-catalyzed cross-coupling reactions.
We predicted that the use of these precatalysts containing
bis(phosphino)ferrocenyl ligands with aliphatic substituents on
the phosphine donors may prevent the formation of a dimeric
intermediate with the same type of η2 interaction between Ni
and the ligand that was observed for dppf and as a result could
stop the generation of inactive Ni(I) species. Another
difference is that the bidentate phosphines have been shown
dcypf-
to bind in a trans geometry in dippfNiII
and
(o‑tol)(Cl)
NiII
in comparison with a cis geometry for
(o‑tol)(Cl)
16,17
dppfNiII
.
Under the same reaction conditions both
(o‑tol)(Cl)
dippfNiII
and dcypfNiII
are significantly more
(o‑tol)(Cl)
(o‑tol)(Cl)
active for aryl carbonate coupling than dppfNiII(o‑tol)(Cl), with
dippfNiII giving a 61% yield and dcypfNiII
giving
RESULTS AND DISCUSSION
■
(o‑tol)(Cl)
(o‑tol)(Cl)
quantitative conversion (Table 1, entries 2 and 3). These initial
results represent some of the mildest conditions under which
aryl carbonates have been coupled.
Optimization of a Catalyst System for Aryl Carbonate
Coupling. In previous work, we established that (dppf)Ni(o-
tolyl)(Cl) (dppfNiII(o‑tol)(Cl)) is a highly active precatalyst for
Suzuki−Miyaura coupling reactions involving aryl sulfama-
tes.13c Given recent results from Nelson and co-workers
showing that aryl carbonates undergo oxidative addition to
(dppf)Ni0(COD) (COD = 1,5-cycloctadiene) only slightly
dcypf-
Given the promising preliminary results using
NiII(o‑tol)(Cl) as a precatalyst, a full optimization of the reaction
temperature, time, amount of nucleophile, and catalyst loading
for the coupling of 2-naphthyl tert-butylcarbonate with 4-
C
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Table 2. Comparison of Selected Bond Lengths and Angles of dcypfNiII
and dppfNiII
(2‑ethylPh)(Br)
(2‑ethylPh)(Br)
complex
dcypfNiII
Ni−Br (Å)
Ni−P1, Ni−P2 (Å)
Ni−Caryl (Å) P1−Ni−P2 (deg) Br−Ni−Caryl (deg) P1−Ni−Caryl, P2−Ni−Caryl (deg)
2.4042(11)
2.1843(19)
1.893(7)
145.00(8)
161.1(2)
92.4(2)
(2‑ethylPh)(Br)
2.1877(18)
2.151(4), 2.281(4)
2.133(4), 2.278(4)
92.1(2)
83.9(4), 169.4(4)
84.8(5), 169.2(4)
a
dppfNiII
2.320(3)
2.323(2)
1.898(14)
1.905(14)
101.63(16)
102.43(16)
85.6(4)
86.1(4)
(2‑ethylPh)(Br)
a
Data obtained from ref 13e. Two sets of geometric parameters are included because there are two independent molecules in the unit cell.
Table 3. GC Yields for the Coupling of 2-Naphthyl tert-Butylcarbonate and 4-Methoxyphenylboronic Acid Using Precatalysts
a
of the Type dcypfNiII
(aryl)(X)
entry
[Ni]
2.5 mol % [Ni] yield (%)
1.0 mol % [Ni] yield (%)
1
2
3
4
5
6
7
8
dcypfNiII
dcypfNiII
dcypfNiII
dcypfNiII
dcypfNiII
dcypfNiII
dcypfNiII
dcypfNiII
49
87
78
92
83
82
89
83
28
38
47
57
60
63
64
73
(2‑OMePh)(Br)
(2,4‑xylyl)(Br)
(1‑naphthyl)(Br)
(o‑tol)(Cl)
(2‑CF3Ph)(Br)
(2,6‑xylyl)(Br)
(o‑tol)(Br)
(2‑ethylPh)(Br)
a
Reaction conditions: 2-naphthyl tert-butylcarbonate (0.133 mmol), 4-methoxyphenylboronic acid (0.333 mmol, 2.5 equiv), K3PO4 (0.599 mmol,
4.5 equiv), naphthalene internal standard (0.0665 mmol, 0.5 equiv), toluene (1 mL). Yields were determined by GC and are reported as the
average of at least two trials.
methoxyphenylboronic acid was performed (Table 1).
Notably, the reaction time can be truncated from 24 h to 4
h without a decrease in yield of cross-coupled product (entry 3
vs entry 4). In fact, at 80 °C, the reaction is almost complete
after only 2 h (entry 5). Furthermore, the temperature can be
reduced from 80 to 60 °C with no significant loss in catalytic
activity at 4 h; however, further reduction of temperature
results in a decreased yield (entries 6 and 7). Allowing the
reaction to run for 4 h appears to provide an excellent
compromise of a short reaction time and a high yield. Varying
the amount of boronic acid relative to the aryl carbonate
revealed that 1 equiv is sufficient to form the cross-coupled
product in moderate yield (entries 8−10), but optimal yields
require 2.5 equiv. This represents a decrease from the
previously reported conditions requiring 4 equiv.7 An
evaluation of catalyst loading showed that the optimal
precatalyst loading is 2.5 mol % (entry 6 vs entries 11−13),
which is also reduced from other reports of aryl carbonate
coupling where a catalyst is generated in situ rather than using
a precatalyst.10 Interestingly, the reaction is water tolerant, and
the addition of 50 equiv of water to the reaction (entries 14
and 15) did not impede catalytic activity. On the basis of this
screen, our optimal reaction conditions utilized 2.5 mol % of
dcypfNiII(o‑tol)(Cl) at 60 °C for 4 h with 2.5 equiv of boronic acid.
Precatalyst performance is typically determined primarily by
the nature of the ancillary phosphine ligand and the identity of
the aryl ligand, with the halide ligand generally being less
important.13e Recently, we performed detailed studies
exploring how the choice of the aryl group affects catalytic
relevant to dcypf, which binds in a trans geometry.16a,17
Additionally, in general, there has been significantly less work
exploring Ni precatalysts with a trans geometry; therefore,
there is a need for further studies of these systems which have
demonstrated improved catalytic activity for some cross-
coupling reactions. We synthesized seven precatalysts of the
form dcypfNiII
with different aryl ligands, selected to
(aryl)(Br)
provide diversity in both electronic and steric properties,
through the reaction of dcypfNiII
with the appropriate
Br2
arylmagnesium bromide reagent (Figure 2).16a The choice of
bromide as the halide was primarily based on synthetic ease, as
there are many more commercially available bromide-
containing Grignard reagents in comparison with chloride-
containing Grignard reagents, which are also often challenging
to prepare. All of the complexes were characterized by 1H, 13C,
and 31P NMR spectroscopy and elemental analysis, and in all
cases the 31P NMR spectra indicated that the dcypf ligand was
bound in a trans geometry, as only a single resonance was
present in the spectra. The one exception to this is the
dcypfNiII
precatalyst, which has two resonances in
(2‑OMePh)(Br)
the 31P NMR spectrum due to the presence of two rotamers in
a 0.6:1 ratio (see the Supporting Information). Furthermore,
the solid-state structure of dcypfNiII
confirms that
(2‑ethylPh)(Br)
the dcypf ligand binds in a trans fashion with a P1−Ni−P2
angle of 145.00(8)° (Table 2). The Ni−Caryl distance in
dcypfNiII
is 1.893(7) Å, which is similar to that
(2‑ethylPh)(Br)
observed in dppfNiII
(1.898(14) and 1.905(14) Å;
(2‑ethylPh)(Br)
two molecules are present in the asymmetric unit).13e
A
is
consequence of the trans geometry of dcypfNiII
(2‑ethylPh)(Br)
activity for the dppfNiII
scaffold.13e However, as
that it places the aryl ligand trans to the bromide ligand. In
contrast, in dppfNiII(2‑ethylPh)(Br) the aryl ligand is trans to one of
the phosphine donors of dppf. Due to the stronger trans
(aryl)(X)
previously described, the dppf ligand binds in a cis geometry;
thus, it is not clear if the trends observed for dppf are also
D
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Figure 3. Isolated yields for Suzuki−Miyaura reactions involving aryl carbonates and a variety of boronic acids catalyzed by dcypfNiII
.
(2‑ethylPh)(Br)
Yields are the average of at least two runs. Legend to reaction conditions: (a) 3.5 equiv of boronic acid, 6 equiv of K3PO4, and 6 h of reaction time;
(b) 5 mol % loading, 3.5 equiv of boronic acid, 6 equiv of K3PO4, 80 °C, and 24 h of reaction time; (c) 5 mol % loading, 3.5 equiv of boronic acid, 6
equiv of K3PO4, 100 °C, and 24 h of reaction time; (d) 10 mol % loading, 5 equiv of boronic acid, 8 equiv of K3PO4, 130 °C, and 48 h of reaction
time.
influence of the aryl ligand, the Ni−Br bond length in
dcypfNiII
(2.4042(11) Å) is approximately 0.08 Å
NiII
is the most active precatalyst (see the
(2‑ethylPh)(Br)
Supporting Information). On the basis of the low catalyst
loading, the relatively low reaction temperature (60 °C), the
reduced number of equivalents of boronic acid (2.5 equiv), and
(2‑ethylPh)(Br)
longer than the Ni−Br bond length in dppfNiII
(2‑ethylPh)(Br)
(2.320(3) and 2.323(2) Å).13e Precatalyst activation in
Suzuki−Miyaura reactions begins with transmetalation of the
bromide ligand with the boronic acid,13b and it is possible that
the short reaction time (4 h), it is clear that dcypfNiII
(2‑ethylPh)(Br)
generates the most active catalyst system for aryl carbonate
coupling reported to date and is an improvement over
alternative systems.7,10−12
the weaker Ni−Br bond in dcypfNiII
in comparison
to its dppf analogue enhances the rate of this process.
(2‑ethylPh)(Br)
All seven of our precatalysts of the type dcypfNiII(aryl)(Br) were
evaluated for the coupling of 2-naphthyl tert-butylcarbonate
and 4-methoxyphenylboronic acid using the optimized
conditions determined in Table 1 (Table 3). To test if there
Substrate Scope. Given the exceptional activity of
dcypfNiII
as a precatalyst for the coupling of 2-
(2‑ethylPh)(Br)
naphthyl tert-butylcarbonate and 4-methoxyphenylboronic acid
under our optimized conditions, we explored its activity with
other substrates. Initially, the scope for the coupling of 2-
naphthyl tert-butylcarbonate with different boronic acids was
investigated, and most reactions proceeded with good to
excellent yields (Figure 3). A lower than expected yield was
repeatedly obtained with phenylboronic acid (1a), but
reactions involving arylboronic acids containing electron-
donating substituents, such as 4-methoxyphenylboronic acid
(1b) and p-tolylboronic acid (1c), proceeded in high yields.
Notably, mono- and diortho substitution on the boronic acid
could be tolerated without a change in the optimized reaction
conditions (1d−f, respectively), and 1- and 2-naphthylboronic
acids (1f,g) were also coupled in high yield. With electron-
deficient boronic acids such as 4-trifluoromethylphenylboronic
acid (1h) and 3-furanylboronic acid (1i), increased amounts of
boronic acid (3.5 equiv) and base (6 equiv) and extended
reaction times (24 h) were required to obtain good yields. At
this stage, the reaction is not compatible with heteroarylbor-
onic acids (see the Supporting Information for more
information). The scope of the carbonate electrophile was
also examined but proved to be more limited than that of the
boronic acid. A 2-naphthyl carbonate with an ester substituent
in the 6-position was coupled in good yield (1j), but in general
was a halide effect, dcypfNiII
was included in the screen
(o‑tol)(Cl)
18
so that it could be directly compared with dcypfNiII
.
(o‑tol)(Br)
When the precatalysts were tested using a catalyst loading of
2.5 mol %, they all performed similarly after 4 h, with the
exception of dcypfNiII
(entry 1), which gave a
(2‑OMePh)(Br)
significantly lower yield of the cross-coupled product.
However, at shorter reaction times using 2.5 mol % precatalyst
loading (see the Supporting Information) and when the
loading was reduced to 1 mol %, significant differences in
catalytic activity were observed (Table 3). Interestingly,
dcypfNiII
gave a yield which was only slightly higher
(o‑tol)(Br)
than that observed with dcypfNiII
(entry 2 vs entry 7),
(o‑tol)(Cl)
which is consistent with our recent studies using related
dppfNiII
(X = Cl, Br) precatalysts in Suzuki−Miyaura
(o‑tol)(X)
reactions using aryl chlorides and aryl sulfamates, where the
identity of the halide ligand does not largely affect precatalyst
performance.13e At 1 mol % loading, there were no major
trends in precatalyst performance on the basis of either the
dcypf-
steric or electronic properties of the aryl ligand, but
NiII
operates the most efficiently and is able to
(2‑ethylPh)(Br)
produce 73% of cross-coupled product (entry 8). Comparative
dcypf-
studies with other substrates also suggest that
E
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Figure 4. Yields of cross-coupled and activation products as a function of reaction time for the Suzuki−Miyaura reaction of 2-naphthyl tert-
butylcarbonate and 4-methoxyphenylboronic acid catalyzed by dcypfNiII(2‑ethylPh)(Br). The error in yields is approximately 5%.
we were unable to couple aryl carbonates with electron-
donating substituents. When 1-naphthyl tert-butylcarbonate
was used as a substrate, increased catalyst loading (5 mol %),
temperature (80 °C), and reaction time (24 h) were required
to give a reasonable yield (1k). Presumably, this substrate is
more challenging than 2-naphthyl tert-butylcarbonate for steric
reasons, which makes oxidative addition more difficult.
Interestingly, under our more forcing conditions we were
able to couple 4-methoxy-1-naphthyl carbonate (1l), our only
example of an aryl carbonate with an electron-donating group.
A phenyl carbonate containing a strongly electron-withdrawing
trifluoromethyl group was coupled with modest yield but
required even higher temperatures (1m) (100 °C). The
unsubstituted substrate tert-butyl phenyl carbonate was even
more difficult to couple and required increased catalyst loading
(10 mol %), temperature (130 °C), and reaction time (48 h)
and still gave a low yield of product (1n). The observed trend
that naphthyl carbonates are considerably easier to couple in
comparison to phenyl carbonates is consistent with results for a
variety of other Ni-catalyzed cross-coupling reactions1g and is
likely due to the relative ease of oxidative addition of naphthyl-
containing substrates in comparison with simple phenyl
substrates.
Precatalyst Activation. In order to understand the high
catalytic activity observed using dcypfNiII
as a
(2‑ethylPh)(Br)
precatalyst, we examined the activation of this system to
generate Ni(0) species. Previous studies have implicated Ni(0)
as the catalytically active species when analogous dppf-ligated
σ-aryl precatalysts are employed in Suzuki−Miyaura reactions
involving sp2-hybridized substrates.13c−e In this case, we
propose that dcypfNiII
activates by transmetalation
(2‑ethylPh)(Br)
with arylboronic acid and base, followed by reductive
elimination of an organic biaryl activation product to generate
a Ni(0) unit that enters catalysis (Figure 4). Quantification of
the amount of the biaryl activation product as a function of
time provides information about the rate of activation and the
amount of Ni(0) that can potentially enter the catalytic cycle.
To probe activation, a catalytic reaction was performed
between 2-naphthyl tert-butylcarbonate and 4-methoxyphenyl-
boronic acid catalyzed by dcypfNiII
using the
(2‑ethylPh)(Br)
conditions optimized in Table 1. The amount of the activation
product, 2-ethyl-4′-methoxybiphenyl, was quantified over the
course of the reaction. As indicated by the data shown in
Figure 4, the activation of dcypfNiII
is extremely
(2‑ethylPh)(Br)
rapid; after 5 min, 93% of the precatalyst is activated. However,
over the course of the reaction, the observed amount of the
F
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Figure 5. (a) Synthesis of dcypf-ligated Ni(I) halides via comproportionation. (b) Solid-state structures and X-band EPR spectra taken at 77 K of
dcypfNiICl and dcypfNiIBr. ORTEP are shown with thermal ellipsoids at 50% probability. Hydrogen atoms and solvent of crystallization have been
removed for clarity. (c) Suzuki−Miyaura reaction using dcypfNiICl and dcypfNiIBr as precatalysts.
biaryl activation product never reaches the expected amount
predicted for complete activation of the precatalyst, even
though the reaction proceeds in greater than 90% yield. The
which would lower the amount of biaryl activation product
formed. Comproportionation processes of this type are well
precedented in dppf-ligated systems,13b,c,e,14 and even though
the dcypf ligand was selected to prevent the stabilization of the
dimeric intermediate proposed in these reactions, this off-cycle
reaction may still be able to occur. As opposed to related
systems in which well-defined Ni(I) species ligated by
ferrocenyl ancillary ligands exist,13b,c,e,15a there are no
documented examples of Ni(I) species ligated by dcypf. In
order to identify if these complexes are formed in catalysis, we
synthesized dcypf-supported Ni(I) halide complexes.19 Com-
dcypf-
high efficiency of activation is not exclusive to
NiII(2‑ethylPh)(Br), as a similar experiment conducted with
dcypfNiII
also indicates approximately 90% activation
(o‑tol)(Br)
of the precatalyst. Interestingly, when the chloride congener
dcypfNiII
is used, only 80% of the activation byproduct
(o‑tol)(Cl)
forms, which is consistent with its slower performance in
catalysis (see the Supporting Information). The activation of
dppfNiII(o‑tol)(Cl) was also monitored under these conditions (see
the Supporting Information). Surprisingly, given its poor
activity in catalysis (Table 1), dppfNiII(o‑tol)(Cl) activated rapidly
and after 4 h an 85% yield of the activation byproduct was
observed. This likely suggests that the poor performance of
proportionation of dcypfNi0
with dcypfNiII or dcypfNiII
C2H4
Cl2 Br2
produces paramagnetic species with solution magnetic mo-
ments consistent with S = 1/2 species (Figure 5a). Crystallo-
graphic characterization of the resulting complexes confirms
their identity as dcypfNiICl or dcypfNiIBr (Figure 5b). Notably, the
dcypf bite angle is contracted by about 40° in the two Ni(I)
dppfNiII
as a precatalyst for aryl carbonate coupling in
(o‑tol)(Cl)
comparison to dcypfNiII(2‑ethylPh)(Br) cannot be explained only by
differences in activation and that potentially the trans-spanning
dcypf ligand is better at promoting the elementary steps, such
as oxidative addition, in catalysis than the dppf ligand.
We hypothesized that the slightly less than quantitative
species in comparison to the Ni(II) species dcypfNiII
(2‑ethylPh)(Br)
such that the primary coordination sphere around the nickel
center is Y-shaped. These Ni(I) species were also characterized
by EPR spectroscopy performed at 77 K to provide a handle
with which to compare catalytic mixtures. The EPR spectrum
of dcypfNiICl (Figure 5b) shows an axial spectrum with g values
of 2.10 and 2.33, while the spectrum of dcypfNiIBr (Figure 5b)
shows a slightly rhombic spectrum with g values of 2.01, 2.12,
and 2.33. Additionally, hyperfine splitting consistent with the
dcypf-
yields of activation product during catalysis with
NiII(2‑ethylPh)(Br) could be due to the formation of Ni(I) species
during activation. Specifically, comproportionation between
dcypfNiII
and the active dcypfNi0 species could
(2‑ethylPh)(Br)
produce a Ni(I) halide species as well as a Ni(I) aryl species,
G
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Figure 6. (a) The abbreviated catalytic cycle, showing comproportionation pathways during activation and oxidative addition. (b) X-band EPR
spectrum following the reaction of 2-naphthyl tert-butylcarbonate and 4-methoxyphenylboronic acid catalyzed by dcypfNiII(2‑ethylPh)(Br) (left), which
is overlaid with the spectrum of the same reaction catalyzed by dppfNiII
(right). (c) X-band EPR spectrum following the reaction of 2-
(o‑tol)(Cl)
bromonaphthalene and 4-methoxyphenylboronic acid catalyzed by dcypfNiII(o‑tolyl)(Cl) overlaid with the spectrum of authentic dcypfNiIBr (left) and the
spectrum of the catalytic mixture overlaid with a simulated spectrum (right).
presence of a bromine nucleus is present in the spectrum of
dcypfNiIBr. It is notable that the time required to obtain high
yields of dcypfNiICl and dcypfNiIBr via comproportionation are
significantly longer than those required to synthesize the
corresponding dppf analogues, suggesting that comproportio-
nation of these species is indeed more difficult.13c Analysis of
the catalytic activity of both Ni(I) species for the coupling of 2-
naphthyl tert-butylcarbonate with 4-methoxyphenylboronic
acid demonstrates that they operate poorly (Figure 5c). This
indicates that if they are formed in catalysis using Ni(II)
precatalysts, they are likely inactive species.
EPR spectroscopy was performed to determine if Ni(I)
species are formed in a reaction between 2-naphthyl tert-
dcypf-
butylcarbonate and 4-methoxyphenylboronic acid using
NiII
as the precatalyst. After 4 h at 60 °C and 2.5
(2‑ethylPh)(Br)
mol % catalyst loading, EPR spectroscopy indicated that
H
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Article
approximately 12% of the Ni was in the form of Ni(I) (Figure
6b). Approximately the same amount of paramagnetic material
was present after 2 h (see the Supporting Information). In
comparison to the authentic EPR spectrum of dcypfNiIBr, the
spectrum of the catalytic mixture does not show bromine
hyperfine coupling. Instead, the spectrum of the catalytic
mixture is axial with a broad feature at g∥ and a triplet at g⊥
corresponding to hyperfine coupling to two slightly inequiva-
precatalyst was monitored using EPR spectroscopy, approx-
imately 22% of the Ni is in an EPR-active form at the end of
dcypf-
the reaction, in comparison to only 12% when
NiII
is used (Figure 6b). This result suggests that
(2‑ethylPh)(Br)
dcypf-supported species are less prone to comproportionation
than dppf-supported species and is also consistent with the
observation that approximately 85% of dppfNiII(o‑tol)(Cl) activates
under these conditions. Comparing the EPR spectrum of the
catalytic mixture with a simulated EPR spectrum of authentic
dppfNiICl suggests that the catalytic mixture contains multiple
EPR-active species, including dppfNiICl, which is presumably
formed as a result of comproportionation during precatalyst
activation (see the Supporting Information). Additionally, the
lent phosphorus nuclei. This suggests that dcypfNiII
(2‑ethylPh)(Br)
does not form detectable quantities of dcypfNiIBr during
activation. The rapid activation of the precatalyst and the
lack of aryl groups on the phosphine donors may prevent this
off-cycle reaction from occurring and contribute to the overall
efficiency of the precatalyst. However, at this stage the nature
of the side process which leads to the slightly less than
quantitative yields (93%) of the biaryl activation product when
reaction with dppfNiII
does produce a small amount of
(o‑tol)(Cl)
product, and the complexity of the EPR spectrum also suggests
that comproportionation occurs between the Ni(II) product
from oxidative addition and the Ni(0) active species.13c
Overall, we propose that, because oxidative addition of aryl
carbonates is more difficult than other substrates such as aryl
chlorides or aryl sulfmates,14 the Ni(0) active species has more
time to comproportionate with unreacted precatalyst and
become inactive. Therefore, a ligand which is less likely to
promote comproportionation is expected to give better results
in catalysis. This suggests that dcypf-supported precatalysts
may generally be more active in catalysis than dppf-supported
systems for substrates that do not readily undergo oxidative
addition.
dcypfNiII
is used as a precatalyst remains unclear.
(2‑ethylPh)(Br)
We propose that the small amount of EPR-active material
which was detected both during and after catalysis with
dcypfNiII
is generated because the Ni(II) complex
(2‑ethylPh)(Br)
dcypf-
formed after oxidative addition of the substrate,
NiII(2‑nap)(carbonate), undergoes comproportionation with the
active Ni(0) species (Figure 6a). This is a well-documented
process with other substrates for catalysts with dppf
ligands.13c−e,14,15 The Ni(I) species that could form after
comproportionation are dcypfNiI
and
dcypf-
(2‑naphthalene)
20
NiI(carbonate)
,
which would decrease the yield of the catalytic
reaction by siphoning the electrophile to an unproductive, off-
cycle reaction. To further probe the possibility of compro-
portionation occurring between the Ni(II) product of oxidative
addition and the Ni(0) active species, a Suzuki−Miyaura
reaction between the aryl halide 2-bromonaphthalene and 4-
CONCLUSIONS
In this work, we have shown that dcypfNiII
■
is a
(2‑ethylPh)(Br)
rapidly activating and selective precatalyst for Suzuki−Miyaura
reactions involving aryl carbonate electrophiles. It can couple
naphthyl carbonates, a limited number of phenyl carbonates,
and boronic acids with a variety of steric and electronic
properties under milder conditions and lower catalyst loadings
in comparison to previously reported systems. Our results
methoxyphenylboronic acid catalyzed by dcypfNiII
was
(o‑tol)(Cl)
monitored using EPR spectroscopy. In this case, given that the
precatalyst does not contain a bromine ligand, the Ni(I)
bromide complex dcypfNiIBr can only form via comproportio-
nation from the Ni(II) product from oxidative addition and the
active Ni(0) species. The resulting spectrum (Figure 6c)
contains bromine hyperfine splitting and is consistent with the
formation of dcypfNiIBr in catalysis. However, the EPR spectrum
also contains additional features and line broadening that are
not present in the EPR spectrum of authentic dcypfNiIBr, which
suggests that the observed spectrum is a composite of multiple
EPR-active species. We propose that the other species is
dcypfNiI(2‑naphthalene) (which is likely relatively unstable13d), the
other expected product from a comproportionation reaction
involving the Ni(II) species from oxidative addition and a
Ni(0) species (Figure 6a). Consistent with this interpretation,
the EPR spectra from catalysis agree with a simulation that
models the data as a linear combination of dcypfNiIBr and an
unknown species with hyperfine coupling to two phosphorus
nuclei. Additionally, we note that our simulation strongly
suggests that the other species is not dcypfNiICl, which could be
formed if comproportionation was occurring during activation
(see the Supporting Information).
suggest that one reason dcypfNiII
is a significantly
(2‑ethylPh)(Br)
more active precatalyst for Suzuki−Miyaura reactions involving
aryl carbonates in comparison with the state of the art
dppfNiII
precatalyst is because it is less likely to form
(o‑tol)(Cl)
inactive Ni(I) species. At this stage the exact mechanistic
reasons dcypfNiII
is less likely to form Ni(I) species
(2‑ethylPh)(Br)
are unclear. Possible reasons include that it could be (i) an
electronic effect related to changing from aryl substituents on
the phosphine donors to more donating alkyl substituents, (ii)
a steric effect related to having more bulky cyclohexyl
substituents on the phosphine donors as opposed to phenyl
substituents, or (iii) a geometric effect related to the fact that
dcypf can bind to the metal in a trans geometry whereas dppf
dcypf-
binds in a cis geometry. Given the high activity of
NiII(2‑ethylPh)(Br), we suggest that more detailed studies looking
at the exact role of the ligand on the elementary reactions and
decomposition events in catalysis would be beneficial. In
particular, there is a lack of information about how the
coordination geometry of bidentate phosphine ligands, cis or
trans, affects elementary reactions and more information on
the topic would be useful for catalyst design. From a synthetic
To provide further information about why the state of the
art precatalyst dppfNiII
is not active for aryl carbonate
(o‑tol)(Cl)
coupling, spin-integrated X-band EPR spectroscopy was
perspective, we propose that dcypfNiII
is likely to be
performed to compare the amount of paramagnetic material
(2‑ethylPh)(Br)
a more active catalyst than dppfNiII(o‑tol)(Cl) for other substrates
that undergo slow oxidative addition and intend to test this
hypothesis in ongoing studies in our laboratory.
produced by dcypfNiII
to that of dppfNiII
.
(o‑tol)(Cl)
(2‑ethylPh)(Br)
When the reaction between 2-naphthyl tert-butylcarbonate and
4-methoxyphenylboronic acid using dppfNiII
as the
(o‑tol)(Cl)
I
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Article
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00543.
■
S
Full characterization data and experimental procedures
(PDF)
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Coupling of Csp2 Halides with Alkyl Electrophiles. Acc. Chem. Res.
2015, 48, 1767−1775. (b) Cornella, J.; Edwards, J. T.; Qin, T.;
Kawamura, S.; Wang, J.; Pan, C.-M.; Gianatassio, R.; Schmidt, M.;
Eastgate, M. D.; Baran, P. S. Practical Ni-Catalyzed Aryl−Alkyl Cross-
Coupling of Secondary Redox-Active Esters. J. Am. Chem. Soc. 2016,
138, 2174−2177. (c) Qin, T.; Cornella, J.; Li, C.; Malins, L. R.;
Edwards, J. T.; Kawamura, S.; Maxwell, B. D.; Eastgate, M. D.; Baran,
P. S. A General Alkyl-Alkyl Cross-Coupling Enabled by Redox-Active
Esters and Alkylzinc Reagents. Science 2016, 352, 801−805.
(d) Twilton, J.; Le, C.; Zhang, P.; Shaw, M. H.; Evans, R. W.;
MacMillan, D. W. C. The Merger of Transition Metal and
Photocatalysis. Nat. Rev. Chem. 2017, 1, 0052. (e) Fu, G. C.
Transition-Metal Catalysis of Nucleophilic Substitution Reactions: A
Radical Alternative to SN1 and SN2 Processes. ACS Cent. Sci. 2017, 3,
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Alkyl Bond Formation: Another Dimension in Cross-Coupling
Chemistry. Science 2017, 356, eaaf7230.
Accession Codes
CCDC 1946181−1946183 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for N.H.: nilay.hazari@yale.edu.
ORCID
David Balcells: 0000-0002-3389-0543
Gary W. Brudvig: 0000-0002-7040-1892
Nilay Hazari: 0000-0001-8337-198X
■
(4) Yu, D.-G.; Yu, M.; Guan, B.-T.; Li, B.-J.; Zheng, Y.; Wu, Z.-H.;
Shi, Z.-J. Carbon−Carbon Formation via Ni-Catalyzed Suzuki−
Miyaura Coupling through C−CN Bond Cleavage of Aryl Nitrile.
Org. Lett. 2009, 11, 3374−3377.
Notes
The authors declare no competing financial interest.
(5) Blakey, S. B.; MacMillan, D. W. C. The First Suzuki Cross-
Couplings of Aryltrimethylammonium Salts. J. Am. Chem. Soc. 2003,
125, 6046−6047.
ACKNOWLEDGMENTS
■
N.H. acknowledges support from the NIHGMS under Award
Number R01GM120162. R.M.D. and M.M.B. thank the NSF
for support as NSF Graduate Research Fellows. The EPR
spectroscopy work was supported by the Department of
Energy, Office of Basic Energy Sciences, Division of Chemical
Sciences, grant DE-FG02-05ER15646 (G.W.B. and H.M.C.L.).
N.H. is a Camille and Henry-Dreyfus Foundation Teacher
Scholar. D.B. acknowledges support from the Research
Council of Norway through the Hylleraas Centre for Quantum
Molecular Sciences (project number 262695). We thank Dr.
Fabian Menges for assistance with mass spectrometry.
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(18) As part of this work, we also prepared the iodo compound
dcypfNiII(o‑tol)(I). As described in the Supporting Information, this gave
inferior catalytic activity for the coupling of 2-naphthyl tert-
butylcarbonate and 4-methoxyphenylboronic acid under our standard
conditions compared with dcypfNiII(o‑tol)(Br). As a consequence, this
compound was not studied further.
(19) The other product from comproportionation is a Ni(I) aryl
complex. Species of this nature are known to be highly unstable at
room temperature (see ref 13d), and as a consequence we did not
attempt to prepare a dcypf-supported Ni(I) aryl complex as part of
this work.
(20) As part of this work, we attempted to prepare an authentic
sample of dcypfNiI(carbonate) through the oxidative addition of 0.5 equiv
of 2-naphthyl tert-butylcarbonate to dcypf-ligated Ni(0) sources.
Unfortunately, we were unable to detect any clear signals for
dcypfNiI(carbonate) using EPR spectroscopy. We propose that this is
probably due to the instability of dcypfNiI(carbonate)
.
K
DOI: 10.1021/acs.organomet.9b00543
Organometallics XXXX, XXX, XXX−XXX