A Highly Active Ylide-Functionalized Phosphine for Palladium-Catalyzed Aminations of Aryl Chlorides

Article abstract of DOI:10.1002/anie.201810696

Ylide-functionalized phosphine ligands (YPhos) were rationally designed to fit the requirements of Buchwald–Hartwig aminations at room temperature. This ligand class combines a strong electron-donating ability comparable to NHC ligands with high steric demand similar to biaryl phosphines. The active Pd species are stabilized by agostic C?H???Pd rather than by Pd–arene interactions. The practical advantage of YPhos ligands arises from their easy and scalable synthesis from widely available, inexpensive starting materials. Benchmark studies showed that YPhos-Pd complexes are superior to the best-known phosphine ligands in room-temperature aminations of aryl chlorides. The utility of the catalysts was demonstrated by the synthesis of various arylamines in high yields within short reaction times.

Full text of DOI:10.1002/anie.201810696

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Accepted Article
Title: A Highly Active Ylide-Functionalized Phosphine for Palladium-
Catalyzed Aminations of Aryl Chlorides
Authors: Philip Weber, Thorsten Scherpf, Ilja Rodstein, Dominik Lichte,
Lennart T. Scharf, Lukas Gooßen, and Viktoria H. Gessner
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10.1002/anie.201810696
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A Highly Active Ylide-Functionalized Phosphine for Palladium-
Catalyzed Aminations of Aryl Chlorides
Philip Weber,[b][+] Thorsten Scherpf,[a][+] Ilja Rodstein,[a] Dominik Lichte,[b] Lennart T. Scharf,[a] Lukas J.
Gooßen*[b] and Viktoria H. Gessner*[a]
Abstract: Ylide-functionalized phosphine ligands (YPhos) were
rationally designed to fit the requirements of Buchwald-Hartwig
aminations at room temperature. This ligand class combines a
strong electron-donating ability comparable to NHC ligands with high
steric demand similar to biaryl phosphines. The active Pd species
are stabilized by agostic C–H∙∙∙Pd rather than by Pd-arene
interactions. The practical advantage of YPhos ligands arises from
their easy and scalable synthesis from widely available, inexpensive
starting materials. Benchmark studies showed that YPhos-Pd
complexes are superior to the best-known phosphine ligands in
room-temperature aminations of aryl chlorides. The utility of the
catalysts was demonstrated by the synthesis of various arylamines
in high yields within short reaction times.
of the ylide, the electron-releasing properties of these so-called
YPhos ligands were found to exceed even those of commonly
used N-heterocyclic carbenes (NHCs). First applications in gold-
catalyzed hydroamination reactions revealed high activities of
the YPhos-complexed cationic gold species. Thus, we became
interested in further exploiting the utility of our ligands in other
catalytic transformations and turned our attention towards
Buchwald-Hartwig-type C–N coupling reactions, which are of
substantial industrial importance due to the widespread
presence of arylamines in natural products, agrochemicals, and
pharmaceuticals.[7] The rapid progress in this field is the result of
the rational design of customized ligands, as showcased by
Buchwald’s biaryl phosphine ligands,[ 8 ] Hartwig’s ferrocenyl
phosphines,[9] Beller’s N-substituted heteroaryl phosphines,[10]
11
]
]
bulky trialkylphosphines[
or N-heterocyclic carbenes.[ 12
Homogenous catalysis is an integral part of modern organic
synthesis, finding widespread applications ranging from the
large-scale manufacturing of fine chemicals to the stereo-
controlled synthesis of complex molecules. The development of
catalysts with new activity levels that drive future advances in
selectivity, sustainability, and efficiency in chemical synthesis is
intimately linked to the discovery of new ligand systems. To date,
phosphines are the dominating class of ligands, and are used in
key processes such as coupling reactions, hydrogenations,
hydroformylations, or other hydrofunctionalizations.[1] In these
reactions, electron-rich phosphine ligands are often needed to
stabilize active metal species, to facilitate bond activation
processes, or to displace other ligands from the metal center.
Thus, typical commercially available and industrially relevant
Despite substantial progress made over the past two decades,
most catalysts require elaborate catalyst preformation and/or
operate at elevated temperatures.
Figure 1. Examples for important phosphines used in homogenous catalysis.
]
3 ]
phosphines, such as the cataCXium®,[ 2 DalPhos,[
or
The YPhos ligand YMePCy2, which bears a methyl group in
the ylide backbone, was envisaged as a suitable lead structure
for applications in palladium catalysis (Scheme 1). The methyl
group renders the phosphorus center highly electron-rich, with a
donor strength exceeding those of simple phosphines such as
PtBu3 or PAd3 and comparable to those of NHCs. In the first test
reactions, however, the amination of aryl bromides and chlorides
using YMePCy2 with various palladium sources and KOtBu as the
base gave variable yields at room temperature that we found
difficult to reproduce. To gain insights into the origin of these
inconsistent and unsatisfactory results, a solution of the ligand
with Pd(PPh3)4 was analyzed by NMR spectroscopy. The
spectra showed consumption of the ligand and selective
formation of a new product, which was unambiguously identified
as the diphenylphosphine 1, rather than a Pd complex of
YMePCy2. These results suggested that under the reaction
conditions, the P–CPh bond of the phosphonium moiety is
activated by Pd0 and transferred to the more basic PCy2 group.
Since the donor strength of 1 is only moderate, amination
became inefficient at room temperature as soon as 1 was
formed.
Buchwald-type biaryl ligands, contain electron-donating
cyclohexyl (Cy), tert-butyl (tBu) or adamantyl (Ad) groups (Figure
1).[4] The development of yet more potent catalysts, which would
operate under milder reaction conditions, is limited by the finite
electron-donating capacity of these alkyl substituents.
Recently, we reported on a novel class of monodentate
electron-rich phosphine ligands bearing an ylide substituent at
the phosphorus atom.[5,6] Depending on the substitution pattern
[a]
Faculty of Chemistry and Biochemistry,
Chair of Inorganic Chemistry II, Ruhr University Bochum,
Universitätsstr. 150, 44801 Bochum, Germany
E-mail: viktoria.gessner@rub.de
[b]
[+]
Evonik Chair of Organic Chemistry, Ruhr University Bochum,
ZEMOS, Universitätsstr. 150, 44801 Bochum, Germany.
E-mail: lukas.goossen@rub.de
These authors contributed equally.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/ange.201xxxxxx.
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10.1002/anie.201810696
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COMMUNICATION
– and in the range of the Buchwald-type biaryl ligands (e.g.
JohnPhos: %Vbur = 50.9%),[14b] which is also in line with the
Cy
rather high stability of
Y PCy2 towards air. The calculated
Me
Tolman electronic parameter of L1 amounts to 2050.1 cm–1.[15,16]
This value is considerably smaller than that of simple
phosphines (e.g. TEP(PCy3) = 2058.1 cm–1[16]), suggesting that
Cy
Y
Me
PCy2 is a stronger electron donor with a donor capacity
comparable to N-heterocyclic carbenes (c.f. 1,3-dimesityl-
imidazol-2-ylidene, IMes: TEP = 2050.7 cm–1[17]).
To evaluate the stability of L1 in the presence of Pd0, we
prepared
Scheme 1. (Left) Decomposition of YMePCy2 to diphenylphosphine 1; (Right)
molecular structure of 1.
a
Pd0–L1 adduct from L1 and Pd2dba3. NMR
experiments confirmed the integrity of the P–C bond in L1 over
24 hours. Diffusion of pentane into a toluene solution of Pd2dba3
and one equivalent of L1 led to the formation of dark red crystals
suitable for XRD analysis. The resulting molecular structure
corresponds to the monophosphine-dba complex L1∙Pd(dba)
(Figure 2).[18] The structure confirms the bulkiness of the ligand
and the protection of the metal centre, which was already
suggested by the calculated buried volume (vide supra). The
Pd–P (2.3318(9) Å) and Pd–C bonds (2.118(4) and 2.144(4) Å)
to the dba ligand are short, but comparable to an analogous
biaryl phosphine complex.[18]
In order to preserve the structural integrity of the backbone
and, thus, the donor strength of our ligand, we replaced the
PPh3 group by PCy3 with the rationale that Pd-mediated P–C
bond activation is slower for P-alkyl bonds. The synthesis of
Cy
Y
Me
PCy2 turned out to be remarkably facile, even on multi-
gram scale (Scheme 2).[13] L1 is accessible in only two steps
from the simple, commercially available starting materials ethyl
iodide, PCy3, and Cy2PCl. The phosphonium salt 2 formed in the
first step is converted to L1 by deprotonation with one equivalent
of n-butyllithium and subsequent addition of 0.5 equivalents of
Cy2PCl. The ylide intermediately formed from 2 acts as substrate
as well as base and deprotonates the α-phosphino-substituted
phosphonium salt formed in situ to the final ligand. This protocol
was scaled up to yield L1 in more than 10 grams. The
phosphonium salt 2 was recycled (in 87% yield) and used for
Cy
further ligand synthesis.
Y PCy2 proved to be reasonably
Me
stable, showing no more than traces of oxidation or
decomposition products upon storage in air for at least 24 hours.
Thus, the solid can be handled in air when setting up reactions.
After 7 days, however, visible decomposition reactions occur.
Figure 2. (Top) Molecular structure of L1∙Pd(dba) and contour map of the
Laplacian of the bonding region between the L1 and palladium; (bottom) metal
ligand interaction in PdII complexes with L1 and biaryl phosphine ligands.
Interestingly, a short Pd–H contact of only 2.09(2) Å is
observed in the solid state, suggesting the presence of a further
stabilizing interaction between the ligand and the Pd0 center.
QT/AIM studies showed a bond critical point between Pd and H
with an electron density of 0.044 e, which supports an agostic
interaction (Figure 3).[19] NBO analysis, the calculated Wiberg
bond indices, and natural charges are also well in line with this
interpretation (see the SI). In the 1H NMR spectrum, no
indication for a strong interaction in solution was observed, thus
indicating an only weak C–H∙∙∙Pd interaction in the L1∙Pd(dba).
Similar stabilizing agostic interactions have been reported in
other catalytically active, low-coordinate Pd complexes with
bulky ligands.[20] Hence, de-coordination to open a free binding
site should be facile and require less energy compared to the
breaking of the arene–Pd interaction in the Buchwald
systems.[21] Here, rotation around the P–C bond was shown to
become critical for fast conversion at room temperature.[ 22 ]
Based on the special structural features of L1 revealed above,
one would expect Pd complexes with this YPhos ligand to be
ideally suited for Buchwald-Hartwig-type reactions.
Scheme 2. (Top) Preparation of L1; (Bottom) Molecular structures of L1 and
its gold complex (ellipsoids at the 50% level, hydrogen atoms omitted for
clarity).
The ligand is characterized by two doublets in the 31P{1H}
NMR spectrum at 1.0 and 30.6 ppm with a large coupling
constant of 128.9 Hz. This suggest a syn-arrangement of the
methyl and PCy2 groups in solution, which is in agreement with
the solid-state structure of YMePCy2 (Scheme 2, bottom).[5] The
gold chloride complex of L1 was prepared to calculate the buried
[14]
volume (%Vbur
)
and to quantify the steric properties of the
ligand. With %Vbur = 48.5%, the steric protection by L1 is higher
than by simple phosphines – including PAd3 (%Vbur = 40.5%)[6d]
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10.1002/anie.201810696
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Cy
To test the performance of
Y
Me
PCy2 in C–N cross-coupling
heterocycles such as pyridine, can also be coupled at room
temperature. The latter are particularly interesting because of
their presence in many pharmaceutically relevant compounds.
Although the catalyst is highly efficient at room temperature, it
can also be employed at slightly elevated temperatures to
achieve higher conversion rates.
reactions and to compare its activity to known catalysts with
other ligand systems, we chose the C–N coupling of 4-
chlorotoluene and 4-fluorochlorobenzene with N-methylaniline at
room temperature as test reactions. The screening of different
reaction conditions revealed an excellent performance of 0.5
mol% L1 with 0.25 mol% Pd2dba3 or 0.5 mol% Pd(OAc)2 using
1.5 equivalents of KOtBu in various solvents (see Table 1 and
SI). For example, full conversion was observed within 1 hour for
the amination of 4-chlorotoluene in dioxane. Almost equal
reactivity was observed in THF. Under these reaction conditions,
none of the established C–N coupling catalysts – including the
biaryl phosphine complexes P1 and P2 (with RuPhos (L2) and
BrettPhos (L3)), the PEPPSI-IPr complex P5 or the Ad2Pd(nBu)-
based precatalyst cataCXium® A Pd G3 – showed comparable
activity. Even when employing the biaryl ligand JohnPhos (L5)
under the optimal literature conditions,[ 23 ] higher conversions
were observed with L1.
Table 1. Evaluation of the ligand performance.
R
(Pre)catalyst
base
solvent
Yield [%][a]
KOtBu
NaOtBu
KOtBu
KOtBu
NaOtBu
KOtBu
KOtBu
KOtBu
KOtBu
NaOtBu
NaOtBu
THF
95
Me
Me
Me
F
L1∙Pd2dba3
L1∙Pd2dba3
L1∙Pd2dba3
L1∙Pd2dba3
L1∙Pd2dba3
L1∙Pd2dba3
L1∙Pd(OAc)2
P1 - P5
THF
52
dioxane
THF
99
83
F
THF
45
F
dioxane
THF
88
F
87
F
THF
<1
F
L2,L3 or L5∙Pd2dba3
THF
<1
[b]
F
L5∙Pd(OAc)2
toluene
toluene
10 (84)[c]
78 (88)[c]
[b]
F
L1∙Pd(OAc)2
[a] Reactions conditions: Aryl chloride (1 mmol), N-methylaniline (1.1 mmol),
THF (2 mL), catalyst solution in THF, base (1.5 eq.), r.t., 1 h; Yields were
determined either by 19F-NMR analysis using α,α,α-trifluorotoluene as internal
standard or by GC with n-tetradecane as internal standard. [b] 1 mol% ligand.
[c] after 19h.
Scheme 3. Scope of C–N cross-coupling reactions using ligand L1. Reaction
conditions: aryl halide (1.0 mmol), amine (1.1 mmol), L1 (0.5 mol%), Pd2dba3
(0.25 mol%), KOtBu (1.5 mmol), THF (2.0 mL). All yields are isolated yields.
Next, we explored the scope of the optimized protocol
(Scheme 3). L1 performs outstandingly for a variety of aryl
chlorides. Even electron-rich chlorides such as 5-chloro-1,3-
dimethoxybenzene are coupled with N-methylaniline within 1
hour. More demanding arenes with ortho substituents, as well as
A wide variety of amines with different steric and electronic
properties can be coupled under the same conditions. Piperidine
and morpholine showed full conversion within less than one hour.
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Even challenging amines such as tert-butylamine or 2,6-
diisopropylaniline, which usually need high catalyst loadings,
elevated temperatures, and long reaction times,[24] were coupled
under standard conditions. To the best of our knowledge, such a
high performance in the coupling of a wide range of aryl
chlorides and amines including challenging electron-rich and
bulky substrates has never been observed with any other ligand
at room temperature.[25]
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In conclusion, an ylide-substituted phosphine (YPhos) was
designed that imparts outstanding activity to Pd complexes in C–
N cross-coupling reactions. Even challenging substrates are
converted at room temperature without the necessity to fine-tune
the ligand specifically for a given amine or to prepare defined
pre-catalysts. The YPhos ligand is easily accessible even on
multi-gram scale. The unusual activity of the catalyst can be
explained by the strong donor properties of the ligand and its
unique architecture, which stabilizes the low-coordinated Pd
species by a weak intramolecular C–H∙∙∙Pd interaction. This
interaction is readily cleaved to open up a free coordination site
for the substrates. The results impressively demonstrate the
beneficial properties of YPhos ligands in homogenous catalysis,
which are further explored at the moment.
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Acknowledgements
We thank the European Research Council (StartingGrant:
YlideLigands 677749), the German Research Foundation
(Cluster of Excellence EXC1069 “RESOLV”) for financial support,
and Umicore AG & Co. KG for the donation of catalysts.
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Keywords: Ylides • Phosphine ligands • Catalysis • Agostic
interactions • Cross-coupling reactions
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[7]
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Entry for the Table of Contents
COMMUNICATION
The ylide-functionalized phosphine,
Philip Weber, Thorsten Scherpf, Ilja
Cy
Y
Me
PCy2, which is easy to
Rodstein, Dominik Lichte, Lennart T.
Scharf, Lukas J. Gooßen* and Viktoria
H. Gessner*
synthesize in few steps, shows
excellent performance in a series of
C-N cross coupling reactions at room
temperature. This efficiency can be
explained by the strong electron-
donating properties of the ligand and
its unique architecture, which allows
for the stabilization of the active Pd
species by a weak C-H∙∙∙Pd
Page No. – Page No.
A Highly Active Ylide-Functionalized
Phosphine for Palladium-Catalyzed
Aminations of Aryl Chlorides
interaction.
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