2-Phosphinoimidazole Ligands: N-H NHC or P-N Coordination Complexes in Palladium-Catalyzed Suzuki-Miyaura Reactions of Aryl Chlorides

Article abstract of DOI:10.1021/acs.organomet.1c00165

We report the synthesis of two palladium 2-(dialkylphosphino)imidazole complexes and demonstrate their activity as catalysts for Suzuki-Miyaura reactions with (hetero)aryl chlorides at room temperature. Our mechanistic studies demonstrate that these palladium complexes exist as an equilibrium mixture between the P-N coordinated and N-H NHC forms of ligand. Our studies suggest that the N-H NHC form may be important for high catalytic activity in Suzuki-Miyaura reactions with aryl chlorides. These reactions proceed at or near room temperature in good to excellent yields. Heteroaryl chlorides are also reactive at lower catalyst loadings.

Full text of DOI:10.1021/acs.organomet.1c00165

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Communication
2‑Phosphinoimidazole Ligands: N−H NHC or P−N Coordination
Complexes in Palladium-Catalyzed Suzuki−Miyaura Reactions of
Aryl Chlorides
Erin E. Martinez, Alexandra J. S. Larson, Sydney K. Fuller, Kathryn M. Petersen, Stacey J. Smith,
and David J. Michaelis*
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ABSTRACT: We report the synthesis of two palladium 2-(dialkylphosphino)-
imidazole complexes and demonstrate their activity as catalysts for Suzuki−Miyaura
reactions with (hetero)aryl chlorides at room temperature. Our mechanistic studies
demonstrate that these palladium complexes exist as an equilibrium mixture
between the P−N coordinated and N−H NHC forms of ligand. Our studies suggest
that the N−H NHC form may be important for high catalytic activity in Suzuki−
Miyaura reactions with aryl chlorides. These reactions proceed at or near room
temperature in good to excellent yields. Heteroaryl chlorides are also reactive at
lower catalyst loadings.
he Suzuki−Miyaura cross-coupling reaction is arguably
one of the most versatile reactions for creating Csp2−
nonoxidative P−aryl insertion by palladium into the C−P
bond of a 2-(diarylphosphino)imidazole ligand such as 1a
(Figure 1).7 Under our standard reaction conditions, the N−H
of complex 2a is deprotonated to form an X-type imidazolyl
ligand.6a Tolman electronic parameter comparisons between
T
Csp2 bonds. Since their discovery 40 years ago,1 palladium-
catalyzed Suzuki−Miyaura reactions have been used in the
synthesis of herbicides, pharmaceuticals, fine chemicals, and
natural products.2 One reason this reaction is favored over
other C−C coupling methods is due to its wide utility; mild
reaction conditions, combined with a vast number of
commercially available boronic acids, make the Suzuki−
Miyaura reaction more practical than other couplings with
stoichiometric organometallic reagents. Aryl bromides and
iodides are typically employed as cross-coupling partners and
can give high yields in short reaction times. Aryl chlorides,
however, are much more challenging substrates and the
development of new catalysts to address this issue is an
ongoing focus in catalysis research. Aryl chlorides are more
shelf-stable, more readily available than the corresponding
bromides and iodides, and much less expensive.3 However, this
increased stability also makes them harder to activate in the
Suzuki−Miyaura reaction, leading to higher temperatures and
catalyst loadings.4,2b
Figure 1. 2-Phosphinoimidazole-based palladium catalysis.
Recently, we reported the synthesis of monosubstituted N−
H N-heterocyclic carbene (NHC) complexes of palladium and
demonstrated their high activity in Suzuki−Miyaura reactions
with aryl bromides.5 N−H NHC complexes and the
corresponding imidazolyl-type ligands are known to enable
efficient catalysis in cross-coupling and other classes of
reactions by serving as bifunctional catalysts.6 We demon-
strated that the N−H NHC catalyst (2a) could be accessed via
Received: March 18, 2021
Published: May 14, 2021
https://doi.org/10.1021/acs.organomet.1c00165
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this ligand and other common NHC ligands showed that the
anionic imidazolyl ligand of 2a was significantly more donating
in Suzuki−Miyaura cross-couplings. The vCO value was shifted
by >40 cm−1 for DFT-calculated Ni(CO)3(NHC) structures.5
Unfortunately, diarylphosphine complexes like 2a had very low
reactivity toward cross-couplings with aryl chloride substrates.
Beller has previously reported that 2-(dialkylphosphino)-
imidazole ligands (3 or 4) enable highly efficient Pd-catalyzed
Suzuki−Miyaura reactions and Buchwald−Hartwig aminations
with aryl chloride substrates.8 On the basis of this precedence
and our recent findings, we wondered whether P-alkyl-
substituted phosphinoimidazole ligands also undergo P−
imidazole bond insertion to form N−H NHC complexes
with palladium and if these N−H NHC species were
responsible for the high activity of the catalysts as observed
by Beller and other researchers.9 In this report, we synthesize
two new P-alkyl 2-phosphinoimidazole palladium complexes
and demonstrate that an equilibrium exists between the P−N
phosphinoimidazole complex and the corresponding N−H
NHC complex.10 These studies suggest that the N−H NHC
palladium complex may play an important role in catalysis
when 2-phosphinoimidazole ligands are employed in palla-
dium-catalyzed reactions.
potential of the P-alkyl phosphinoimidazole ligands to form
N−H NHC complexes with palladium and suggests that these
types of N−H NHC ligands may help facilitate Suzuki
reactions in catalysis.7b
We next sought to investigate the catalytic efficiency of these
new palladium complexes in the Suzuki−Miyaura reaction with
less reactive aryl chloride substrates. We chose 4-chloroben-
zaldehyde for our optimization studies because it is known to
be a difficult substrate; the aldehyde is unstable at high
temperature, which leads to poor yields and conversions under
standard Suzuki conditions.11 We found that at ambient
temperature diphenylphosphino catalyst 2a provided only 5%
conversion to product (Table 1, entry 1). This result is
Table 1. Optimization of Suzuki−Miyaura Reaction with
Aryl Chlorides
To begin, we synthesized the corresponding ditert-butyl
(1b) and dicyclohexyl (1c) phosphinoimidazole ligand
derivatives of 1a, which have previously been reported by
Beller (Figure 2a).9e As observed previously, when ligand 1a is
a
b
entry
catalyst
% conv.
5
1
2
3
4
2a
5b
2c + 5c
2c
5b
2c + 5c
6a
6b
100 (79)
100 (76)
94 (70)
0
39
100
100
8
c
5
c
6
7
8
9
7
10
11
PdCl2
1b
5b
5b
5b
6
0
28
34
100
100
13
d
12
e
13
f
14
g
15
5b
5b
h
16
a
Reaction run with catalyst (0.02 mmol, 2%), 4-chlorobenzaldehyde
(1.0 mmol, 1 equiv), phenylboronic acid (1.5 mmol, 1.5 equiv), and
sodium hydroxide (2.0 mmol, 2 equiv) at room temperature for 4 h in
Figure 2. (a) Synthesis of Pd-phosphinoimidazole complexes. (b) X-
ray crystal structures of 5b and 2c.
b
3.3 mL of toluene with 5% methanol. Percent conversion determined
by 1H NMR by comparing product formed to remaining starting
material aryl chloride. Numbers in parentheses indicate isolated yield.
c
d
e
Run in 100% toluene. With Cs2CO3 (2 equiv). With NaOtBu (2
mixed with PdCl2 in methanol, complex 2a is isolated as the
only product, and complex 5a is not observed. In contrast,
when ligand 1b containing P-tBu substituents was used,
complex 5b was isolated in 96% yield as the only product of
the reaction. X-ray quality single crystals of 5b were grown, and
the X-ray structure was obtained to confirm its structure as a
P−N palladium complex (Figure 2b). Complex 5b was
previously reported.9f When dicyclohexyl derivative 1c was
reacted, a mixture (∼4:1) of products 2c and 5c was observed
by NMR (69% yield). When the mixture was subjected to
crystallization conditions, an X-ray quality crystal of 2c was
obtained (Figure 2b). The X-ray structure of 2c confirms the
f
g
equiv). Run in 100% EtOH with K2CO3 as base. Run with 1 mol %
5b for 14 h. Run with 0.1 mol % 5b for 24 h.
h
consistent with what we had observed previously with aryl
chloride substrates. However, when complex 5b or the mixture
of 2c and 5c were employed (entries 2−3), both reactions
proceeded to 100% conversion in less than 4 h (79% isolated
yield for catalyst 5b, R = t-butyl). Importantly, we saw only a
small difference in reactivity between the t-butyl (5b) and
cyclohexyl (2c + 5c) substituted catalysts. This was intriguing
to us because the ratio of the N−H NHC and P−N complexes
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Communication
(2 and 5) are dramatically different for the two ligands. When
pure N−H NHC complex 2c was employed (after purification
by crystallization), the reaction conversion remained high
(entry 4). Additionally, we observed that methanol was
essential as a cosolvent to achieve high rates of catalysis with
both t-butyl and cyclohexyl complexes. The absence of
methanol in the reactions leads to no product formation
with 5b (entry 5) and 39% conversion with the mixture of 2c +
5c (entry 6). Previously we have found that methanol is
essential for formation of the N−H NHC complex to occur,5
which suggests that the N−H NHC form of the t-butyl catalyst
(2b) may actually be present in the reaction. The fact that the
mixture of 2c + 5c does give some product in the absence of
methanol further supports our hypothesis that the N−H NHC
form of the catalyst is important for efficient catalysis. When
we performed the Suzuki reaction with 20 mol % catalyst 5b,
we observed in the 31P NMR clear signs of formation of the
N−H NHC complex 2b (characteristic Pd-PR2OMe peaks in
the 31P NMR) and mass spectrometry confirmed the presence
of complex 2b in the reaction (see the Supporting
Information). These results suggest that the N−H NHC
form of these palladium complexes may in fact be the active
species in catalysis that enables high reactivity in the Suzuki−
Miyaura reaction with aryl chlorides.
Figure 3. Substrate scope for Suzuki−Miyaura reaction.
Our optimization studies showed that the resulting catalytic
activity of our new phosphinoimidazole palladium complexes
in Suzuki reactions with aryl chlorides was comparable to those
reported previously.8 Although the catalyst loading is higher
with our catalyst, the reaction can be run at lower
temperatures, which can provide important advantages with
sensitive substrates. Thus, we performed additional studies to
further optimize the reactivity of our catalyst. We found that
when compared to state-of-the-art PEPPSI (6a),11 cinnamyl−
NHC (6b),12 and diamine (7)13 palladium catalysts, our
phosphinoimidazole catalysts performed as well or better
under our standard conditions (Table 1, entries 7−9). We
found that the phosphinoimidazole ligand is essential for
catalysis to occur (entry 10) and that palladium is necessary for
product formation (entry 11). We also screened other bases
(entries 12−13) and found that sodium hydroxide was the best
base for the reaction under these conditions. The reaction can
also proceed in ethanol with K2CO3 as the base with excellent
reactivity (entry 14).12b,14 Finally, we tested how much we can
lower the catalyst loading and still maintain a high yield of
product when the reaction is run at room temperature. At 1
mol % 5b, the reaction reached completion in 14 h (entry 15).
Unfortunately, lower concentrations resulted in minimal
reactivity (entry 16).
With our optimized conditions in hand, we next wanted to
confirm that catalyst 5b maintained high reactivity across a
variety of (hetero)aryl chloride substrates (Figure 3). Many of
the substrates tested required elevated temperatures (40−80
°C) in order to provide high yields (see the Supporting
Information for details). Good functional group tolerance was
seen with aryl chlorides containing both electron-donating and
-withdrawing functional groups (9a−9n). Notably, 4-chlor-
ophenol (9b) is a particularly challenging substrate for Suzuki
cross-couplings because it is deprotonated under the reaction
conditions, and we only achieved a small conversion to
product. More sterically hindered substrates (9k−9n) also
underwent the Suzuki−Miyaura reaction readily. Heteroaryl
chlorides, which are notoriously difficult substrates for Suzuki
reactions, also proceeded in good yields under increased
reaction times (9o−9v). Due to the difficult nature of
activating heteroaryl chlorides, these reactions required higher
temperatures, but they could be run with 1% of catalyst. We
were particularly happy to find that various N-heterocycles also
gave moderate to good yields with our catalyst (9q−9v).
In conclusion, we have synthesized and characterized two
new palladium catalysts with 2-(dialkylphosphino)imidazole
ligands. Our studies demonstrate the presence of an
equilibrium between the palladium P−N phosphinoimidazole
species and the corresponding N−H NHC complex in the
presence of methanol and during catalysis. Our studies suggest
that the N−H NHC form of the phosphinoimidazole catalyst
may be important for high catalytic activity in catalysis with
these ligands. These catalysts were optimized for the Suzuki−
Miyaura reaction and were shown to perform with higher
reactivity that P-aryl-substituted 2-phosphinoimidazole ligands
in cross-coupling reactions with aryl chloride substrates at or
near room temperature. Heteroaryl chlorides are also reactive
at lower catalyst loadings to give moderate to good yields.
Future work in our lab will include further mechanistic and
computational studies to investigate the role of the N−H NHC
(2b) versus P−N (5b) palladium complexes during catalysis,
as well as applying 2-(dialkylphosphino)imidazole ligands to
other transition metal catalyzed reactions.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00165.
■
sı
Full procedures for the synthesis of ligands, Pd−NHC
and Pd−P−N complexes, and conducting catalytic
reactions (PDF)
Accession Codes
CCDC 2063985 and 2071170 contain the supplementary
crystallographic 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
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Communication
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AUTHOR INFORMATION
■
Corresponding Author
David J. Michaelis − Department of Chemistry and
Biochemistry, Brigham Young University, Provo, Utah 84602,
United States; orcid.org/0000-0003-3914-5752;
Email: dmichaelis@chem.byu.edu
Authors
Erin E. Martinez − Department of Chemistry and
Biochemistry, Brigham Young University, Provo, Utah
84602, United States
Alexandra J. S. Larson − Department of Chemistry and
Biochemistry, Brigham Young University, Provo, Utah
84602, United States
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Complexes as Highly Active and General Bifunctional Catalysts in
Sustainable Cross-Coupling Reactions. ACS Catal. 2019, 9 (8),
6993−6998. (b) Kuwata, S.; Hahn, F. E. Complexes Bearing Protic N-
Heterocyclic Carbene Ligands. Chem. Rev. 2018, 118, 9642−9677.
(c) Jahnke, M. C.; Hahn, F. E. Complexes with Protic (NH,NH and
NH,NR) N-heterocyclic Carbene Ligands. Coord. Chem. Rev. 2015,
293−294, 95−115. (d) Kuwata, S.; Ikariya, T. Metal−Ligand
Bifunctional Reactivity and Catalysis of Protic N-Heterocyclic
Carbene and Pyrazole Complexes Featuring β-NH Units. Chem.
Commun. 2014, 50, 14290−1430. (e) Hahn, F. E. Substrate
Recognition and Regioselective Catalysis with Complexes Bearing
NR,NH−NHC Ligands. ChemCatChem 2013, 5, 419−430. (f) Kuwa-
ta, S.; Ikariya, T. β-Protic Pyrazole and N-Heterocyclic Carbene
Complexes: Synthesis, Properties, and Metal−Ligand Cooperative
Bifunctional Catalysis. Chem. - Eur. J. 2011, 17, 3542−3556.
(7) (a) Chang, Y.-C.; Chang, C.-H.; Wang, L.-W.; Liang, Y.-H.; Hu,
D.-F.; Weng, C.-M.; Mao, K.-C.; Hong, F.-E. Imidazolio-Substituted
Secondary Phosphine Oxides as Potential Carbene Reagents.
Polyhedron 2015, 100, 382−391. (b) Abdellah, I.; Lepetit, C.;
Canac, Y.; Duhayon, C.; Chauvin, R. Imidazoliophosphines are True
N-Heterocyclic Carbene (NHC)−Phosphenium Adducts. Chem. -
Eur. J. 2010, 16, 13095−13108.
Sydney K. Fuller − Department of Chemistry and
Biochemistry, Brigham Young University, Provo, Utah
84602, United States
Kathryn M. Petersen − Department of Chemistry and
Biochemistry, Brigham Young University, Provo, Utah
84602, United States
Stacey J. Smith − Department of Chemistry and Biochemistry,
Brigham Young University, Provo, Utah 84602, United
States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00165
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
We acknowledge the National Science Foundation Chemical
Synthesis Program for support of this work (CHE-1665015).
Notes
(8) (a) Zapf, A.; Beller, M. The Development of Efficient Catalysts
for Palladium-Catalyzed Coupling Reactions of Aryl Halides. Chem.
Commun. 2005, 431−440. (b) Harkal, S.; Rataboul, F.; Zapf, A.;
Fuhrmann, C.; Riermeier, T.; Monsees, A.; Beller, M. Dialkylphos-
phinoimidazoles as New Ligands for Palladium-Catalyzed Coupling
Reactions of Aryl Chlorides. Adv. Synth. Catal. 2004, 346, 1742−
1748.
The authors declare no competing financial interest.
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