N-Arylpyridiniophosphines: Synthesis, Structure, and Applications in Au(I) Catalysis

Article abstract of DOI:10.1021/acscatal.8b03271

The synthesis and characterization through NMR and X-ray crystallography of a series of N-arylpyridiniophosphines and their corresponding Au(I)-derivatives are reported. Because of their acceptor properties, pyridiniophosphines efficiently enhance the ele

Full text of DOI:10.1021/acscatal.8b03271

Research Article
pubs.acs.org/acscatalysis
Cite This: ACS Catal. 2018, 8, 10457−10463
N‑Arylpyridiniophosphines: Synthesis, Structure, and Applications
in Au(I) Catalysis
Hendrik Tinnermann,† Leo D. M. Nicholls,† Tim Johannsen,† Christian Wille,‡ Christopher Golz,†
Richard Goddard,‡ and Manuel Alcarazo*,†
†Institut fu
Germany
‡Max-Planck-Institut fu
̈
̈ ̈ ̈
r Organische und Biomolekulare Chemie, Georg-August-Universitat Gottingen, Tammannstraße 2, 37077 Gottingen,
̈
r Kohlenforschung, Kaiser Wilhelm Platz 1, 45470 Mulheim an der Ruhr, Germany
̈
S
* Supporting Information
ABSTRACT: The synthesis and characterization through NMR and X-
ray crystallography of a series of N-arylpyridiniophosphines and their
corresponding Au(I)-derivatives are reported. Because of their acceptor
properties, pyridiniophosphines efficiently enhance the electrophilicity of
the Au atom in the complexes they form. In our study, this is translated
into higher reactivity of the corresponding Au catalysts, which is
demonstrated in two mechanistically differentiated cycloisomerizations.
Moreover, the steric protection and probably also the electronic
stabilization provided by the N-aryl substituents make the active Au-
species more robust and slow down its rate of decay. This allows for an
appreciable reduction of the catalyst loadings.
KEYWORDS: ligand design, α-cationic phosphines, Buchwald phosphines, Au(I)-catalysis, cycloisomerizations
reduced σ-donor ability and enhanced π-acceptor character if
INTRODUCTION
■
compared with their neutral counterparts. These distinguishing
attributes have been employed in our group to develop a series
of mainly Au-6 and Pt-,7 but also Rh-catalysts8 of enhanced
catalytic activities.9 However, the use of α-cationic ligands is
not free of shortcomings: as a general rule, the reduced
donation from the phosphorus center to the metal makes the
bond between these two atoms significantly weaker. Moreover,
the Coulomb repulsion between the ligand and the usually also
positively charged metal center is expected to reduce the
stability of the catalytically relevant species even more. As a
result, although catalysts derived from α-cationic ligands
exhibit unmatched reactivity, they are more prone to
decomposition than those based on typical phosphines. This
is particularly true in the case of monodentate ancillary ligands
where stabilizing chelate effects are not present.5
Stimulated by the beneficial effects of Buchwald ligands in
Au-promoted transformations2 and expecting that a pendent
aryl substituent directed toward the metal should also provide
additional robustness to the complexes derived from cationic
phosphines, we envisaged the design of a new generation of α-
cationic phosphines that incorporates the privileged biaryl
architecture. Herein, we bring this idea into practice with the
synthesis of N-arylpyridiniophosphines. As can be observed in
Figure 1, these salts can be considered as formal derivatives of
The success harvested by Buchwald biarylphosphines in
forming highly active Pd-catalysts is generally attributed to
the unique combination of electronic and steric properties
derived from their structural design. Specifically, their strong σ-
donor ability together with the effective steric shielding of the
metal by the biaryl rest facilitate the formation of low
coordinated species [L1Pd], which display enhanced reactivity
toward both oxidative addition and reductive elimination. As a
consequence, the employment of Buchwald ligands is
associated with short reaction times, low catalyst loadings,
mild reaction conditions, and a broad substrate scope in a wide
range of C−C and C−N coupling reactions.1
The privileged architecture of dialkylbiarylphosphanes has
also found widespread use in the area of Au-catalysis.2 Both the
steric protection of the active metal center and the weak but
still stabilizing electronic interaction between the Au atom and
the π-system of the lateral arene probably account for the
robustness of the catalysts derived from these architectures.3 It
is also worth mentioning that because of the bulkiness of
Buchwald ligands the formation of diaurated species, which
have been recognized as resting states or even dead ends of the
catalytic cycles leading to enyne cycloisomerizations, is less
favored.4
Our research group has been involved during the past few
years in the design and synthesis of α-cationic phosphines of
various structure.5 These ancillary ligands are unique in that
the positive charge that they bear is directly attached to the
donor phosphorus center, and for this reason, they display
Received: August 16, 2018
Revised: September 26, 2018
© XXXX American Chemical Society
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a
dialkylbiarylphosphanes in which the bridging carbon atom has
been substituted by a nitrogen center.10
Scheme 1. Synthesis of N-Arylpyridiniophosphines
Figure 1. Design of N-arylpyridiniophosphines.
EXPERIMENTAL SECTION
■
Synthesis and Characterization of N-Arylpyridinio-
phosphines. We previously reported a three-step route for
the synthesis of N-phenylpyridiniophosphines starting from
pyridone 1 consisting of an initial N-phenylation with
iodobenzene to afford 2,11 which was transformed into the
corresponding chloropyridinium salt 3 by treatment with oxalyl
chloride. Finally, nucleophilic attack of diphenylphosphine to
the 2-chloropyridinium ring of 3 efficiently afforded the desired
α-pyridinium phosphine 4 (Scheme 1a).10b This protocol
however encounters serious difficulties delivering N-arylpyr-
idones in acceptable yields when the aryl rests to be
incorporated at the N-position are bulkier than just a phenyl
group. For this reason, the development of an alternative
synthetic strategy was crucial. At that juncture, we decided to
prepare N-arylpyridinium salts 6a−x following the classical but
very reliable Zincke synthesis, by reaction of appropriate
anilines with dinitro substituted pyridinium salt 5.12 The
necessary chloride functionality at the ortho-position to
nitrogen was further installed through a two-step sequence
consisting of the deprotonation of salts 6a−c with LiHMDS at
−100 °C followed by trapping of the in situ formed pyridin-2-
ylidene with elemental sulfur. This leads to thiopyridones 7a−
c, which were subsequently transformed into the desired
chloropyridinum salts 8a−c by reaction with oxalyl chloride.13
The scaling up of the synthetic sequence just described to
gram scale was possible for all substitution patterns. Finally,
condensation of 8a−c with a range of secondary phosphines
under microwave irradiation rendered the target N-arylpyr-
idiniophosphines 9 as white to light brown solids in moderate
to good yields. Due to the low basicity of bis(3,5-
bis(trifluoromethyl)phenyl)-phosphine, the nucleophilic sub-
stitution at the pyridinium ring does not proceed to any
appreciable extent; however, 9bc is obtained in moderate yield
when the polyfluorinated phosphine is previously deproto-
nated with KH to enhance its nucleophilicity (see Scheme 1b
and the Supporting Information).
a
Reagents and conditions: (a) iodobenzene (1 equiv), CuBr (10 mol
%), Cs2CO3 (2.1 equiv), DMSO, 60 °C, 95%; (b) oxalyl chloride (3
equiv), Cl(CH2)2Cl, and then NaBF4 (4 equiv), 71%; (c) Ph2PH (2
equiv), THF, 65 °C, 71%; (d) ArNH2, n-Butanol, 72 h, reflux, and
then NaBF4 (excess), 6a (62%); 6b (43%); 6c (34%); (e) LiHMDS
(1 equiv), S8 (0.25 equiv), THF, 7a (64%); 7b (70%); 7c (74%); (f)
oxalyl chloride (3 equiv), DMF (0.1 mL), Cl(CH2)2Cl, 8a (57%); 8b
(97%); 8c (99%); (g) Method A: R2PH (3 equiv), THF, 120 °C,
μwave, 12 h, and then NaSbF6, 9aa (50%); 9ab (55%); 9ba (69%);
9bb (69%); 9ca (53%); 9cb (32%); Method B: KH (8 equiv), R2PH
(1 equiv), THF, −78 °C → r.t., and then NaSbF6,12 h, 9ac (70%);
9bc (44%).
Evaluation of Stereoelectronic Properties. The X-ray
analyses of single crystals of compounds 9aa, 9ac, 9ba, and 9cb
are displayed in Figure 2. In the solid state, the sum of angles
around P1 (304.9°, 9aa; 308.2°, 9ac; 305.7°, 9ba; and 304.0°,
9cb) are comparable to that observed in PPh3 (309°). These
similar degrees of pyramidalization indicate retention of the
free electron pair at phosphorus despite the added positive
charge. Also informative is the comparison of the metrical
parameters of pyridiniophosphines with their parent Buchwald
Figure 2. Crystal structures of 9aa, 9ac, 9ba, and 9cb. Hydrogen
atoms and SbF6 anions were omitted for clarity; ellipsoids are set at
50% probability.15
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structures. If the expected slight shortening of the three C−N
bonds around N1 compared with Csp2−Csp2 bonds in 9 is
ignored, no other significant variation is observed. This means
that the formal exchange of the biaryl rest by a N-
arylpyridinium moiety has minimal influence on the ligand
structure. Thus, the steric parameters in 9 remain identical to
the ones of the parent (uncharged) Buchwald ligands. This is
confirmed by comparison of the percent buried volume (%
parameter to rank the electronic properties of the new ligands.
Cyclic voltammetry measurements afford values for the
oxidation of ligands 4, 9aa, 9ab, 9ca, and 9cb at the interval
between 1.225 and 1.302 V (calibrated versus Fc+/Fc), which
are very similar potentials to the one measured for (MeO)3P
(1.287 V). Although consideration of these values should be
done with care because of the irreversible nature of the
oxidation monitored, we can prudently assume that ligands of
general structure 9 are defined by a basicity comparable to that
characteristic of phosphites (Table 1). Hence, we conclude
V
Bur) and the topographic steric maps for both families of
ligands (see Figure 3 and the Supporting Information).14
a
Table 1. Electrochemical Oxidation Potentials
entry
ligand
Ep(ox)
1
2
3
4
5
6
7
SPhos
(PhO3)P
4
9aa
9ab
0.570
1.287
1.285
1.302
1.271
1.287
1.225
9ca
9cb
Figure 3. Steric maps for 9ab (left) and SPhos (right). Both ligands
are characterized by an identical %VBur = 49.
a
Oxidation peak potentials reported in V. Calibrated vs ferrocene/
ferrocenium, Bu4NPF6 (0.1 M) in CH2Cl2.
In a first attempt to determine the Tolman electronic
parameter of the new cationic phosphines, ligand 9bb was
chosen as a proxy and reacted with [IrCl(cod)]2 (Scheme
2).16,17 The expected complex 10 could be detected by NMR
that the formal exchange of the phenylene bridge by a
pyridinium moiety in Buchwald-type phosphines significantly
reduces the basicity of the parent ligands without altering, not
even minimally, their unique steric properties.
Synthesis of Au(I)-Complexes. Encouraged by this
analysis and willing to exploit the stereoelectronic properties
of the newly prepared pyridiniophosphines in catalysis, we first
prepared a series of Au(I)-complexes 12−17 by phosphine
displacement of Me2S in (Me2S)AuCl (Scheme 3).18 The
a
Scheme 2. Synthesis and X-ray Structure of Iridacycle 11
a
Scheme 3. Synthesis of Au-Complexes 12−17
a
Reagents and conditions: (a) (Me2S)AuCl (1 equiv), CH2Cl2, rt, 1h,
12 (98%); 13 (96%); 14 (98%); 15 (98%); 16 (96%); 17 (89%).
decision to employ Au-catalysis for the initial screening of N-
arylpyridiniophosphines comes from our long-standing interest
in Au-catalyzed processes that benefit from strong acceptor
ligands.6,19
a
Reagents and conditions: (a) [IrCl(cod)]2, CH2Cl2, 76%. Hydrogen
atoms, except the hydride one, and SbF6 anions omitted for clarity;
ellipsoids are set at 50% probability.15
Indicative of the formation of the desired Au(I)-complexes is
the displacement of the 31P {1H}NMR chemical shifts upon
coordination from the range of δ = −7.3 to −1.7 ppm in the
free cationic ligands to δ = 29.7−48.8 ppm in the
corresponding Au-complexes (see the Supporting Informa-
tion). The combination of massive steric overcrowding with a
significantly reduced σ-donor ability is presumably the reason
why the reaction of 9ca and 9cb with (Me2S)AuCl is not clean,
thus precluding the isolation of the corresponding Au-
complexes in analytically pure form.
analysis; however, the reaction did not stop at that point, and
instead, the Ir center oxidatively added to the meta C−H bond
of the pyridinium moiety to afford the iridacyclobutane
hydride complex 11. The formation of this iridacycle was
originally suggested by the appearance of a characteristic
hydride signal at δ = −14.81 ppm (2JH−P = 11.4 Hz) in the 1H
NMR spectrum and subsequently confirmed by X-ray
crystallography (Scheme 2).
Because the oxidative addition process just described seems
to be general for the pyridiniophosphine series of ligands, the
oxidation potential Ep(ox) was chosen as an alternative
The solid-state structures of complexes 12, 13, 14, and 17,
determined by X-ray diffraction analysis, are depicted in Figure
4. The measured Au1−P1 bond lengths for these complexes
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Scheme 4. Ligand Effect on the Au-Catalyzed
Cycloisomerization of 18 to Cyclobutene 19
a
Reaction time, 90 min.
structure and further activated by an adjacent Lewis acidic
borane.23 Cyclobutene 19 was isolated in 73% yield employing
2 mol % of 21; however, the reaction time needed to be
prolonged for 12 h. All together, these experiments suggest
that optimal results should be obtained by the use of a strong
π-acceptor ancillary ligand, which additionally should be able
to efficiently protect the active catalytic species from
decomposition.
Figure 4. Crystal structures of 12, 13, 14, and 17. Hydrogen atoms
and SbF6 anions and solvent acetonitrile in the case of 14 were
omitted for clarity; ellipsoids are set at 50% probability.15
vary from 2.12 to 2.23 Å, a slightly shorter range than that
measured for the corresponding neutral analogues (P1−Au1,
2.22−2.30 Å).3a This is probably a consequence of the more
internal electron pair at phosphorus in pyridiniophosphines,
which imposes a shorter P1−Au1 distance to maximize orbital
overlap. This agrees with the lower donor power of cationic
phosphanes. Moreover, in all the structures measured a contact
shorter than the sum of van der Waals radii (3.36 Å) is
observed between the Au atom and the flanking aryl ring.20 For
12 and 14, it is the Cortho closest to Au with distances of 3.136
and 3.177 Å respectively. In 13 the Au−Cipso distance is the
shortest, 3.149 Å, while in 17 the Au atom interacts
preferentially with the oxygen atom of one of the methoxy-
substituents (Au1−O2; 3.092 Å). In all cases the geometry of
the P−Au−Cl fragment is slightly distorted from the expected
linearity (P1−Au1−Cl1 angles: 172.8° in 12; 176.8° in 13;
176.0° in 14 and 177.2° in 17). Most likely, this deviation
originates from the steric requirements of the biaryl rest.
Catalysis. As a preliminary study to determine the relative
electrophilicity of complexes 12−17 and test their potential as
catalysts, we initially compared their performance in the [2 +
2] cycloaddition of 1,8-enyne 18 into cyclobutene 19.21,22 This
benchmark reaction, originally reported by Gagosz, was chosen
because the archetypical Au-catalyst, (Ph3PAu(SbF6), 2 mol
%) is only able to afford low conversion (16% yield); probably
due to fast catalyst decomposition (Scheme 4). The use of
XPhos as ancillary ligand under similar conditions improves
the yield of 19 to 48% (24 h); very likely, the steric protection
of the 2,4,6-tri(isopropylphenyl) flanking group and the
stronger electron-donation of this ancillary ligand better
safeguard the catalytic species from deactivation. On the
other hand strong π-acceptor ligands such as phosphites also
seem to be beneficial for this transformation. Compound 19
was obtained in 51% yield after only 90 min using 2 mol % of
20, yet catalyst decomposition again prevented the reaction
from proceeding to conclusion (Scheme 4). In terms of
isolated yield of the desired product, the best catalytic system
reported to date for this transformation is 21, consisting of a
monocationic Au center embedded in a robust chelating
The conversion versus time plot for precatalysts 13, 14, 16,
SPhosAuCl, and 20 under otherwise identical conditions (2
mol % Au, r.t., CH2Cl2) is shown in Figure 5. All catalysts
Figure 5. Ligand effect on the Au-catalyzed [2 + 2] cycloaddition of
1,8-enyne 18 into cyclobutene 19. Conditions: 18 (0.1 M in CH2Cl2,
2 mol % [Au], 2 mol % AgSbF6, r.t. Conversions determined by GC.
screened overtake the performance of SPhosAuCl, surely due
to the augmented electrophilicity that they impart on the Au
center. However, for precatalyst 14 the high initial activities
decay after approximately 1 h, and only partial conversion into
19 is finally achieved. We speculate that for this system the
protecting steric effect of the mesityl residue is not sufficient to
compensate for the weakness of the P−Au bond. Note that the
basicity of the P atom in 14 is significantly reduced because it
bears three strongly electron-withdrawing groups: the
pyridinium substituent and two 3,5-bis(trifluoromethyl)phenyl
groups.
In contrast, catalysts 13 and 16, both decorated with
cyclohexyl substituents, exhibit excellent performance; their
activities only seem to decay when no more substrate is
available. In fact, after optimization of the reaction conditions,
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18 could be transformed into cyclobutene 19 (95% yield, 16 h)
employing a catalyst loading of only 0.2 mol % of 16.
Encouraged by this result, pyridiniophosphines were addi-
tionally screened in a more complex scenario. Our group has
recently reported the enantioselective synthesis of [6]helicenes
via double 6-endo-dig hydroarylation of appropriate diynes
22.6a,b In order to obtain the desired reactivity, strong Lewis
acidity at the metal is again required;24 additionally, catalytic
loadings of up to 10 mol % are necessary to ensure complete
conversion of the starting material.6a,b Hence, we considered it
interesting to test the performance of precatalysts 13−17 on
this particular cycloisomerization.
observed with a good 83:17 ratio favoring helicene 24a (Table
2, entry 4). Moreover, the catalytic activity of the active gold
species decays slowly, and accordingly, the catalyst loading
could be reduced to 2 mol % with identical results (Table 2,
entry 8). Further reduction of the catalyst loading to 1.5 mol %
is still possible for more reactive substrates. These conditions
suffice to transform 22b and 22c into [6]helicenes 24b and
24c, respectively, with excellent yields and complete endo
selectivity. The X-ray structure of 24b is depicted in Figure 6.
The first set of results collected is shown in Table 2 (entries
1−7); a fixed catalyst loading of 5 mol % was employed. At
a
Table 2. Ligand Effect in the Synthesis of [6]Helicenes
Figure 6. Crystal structure of 24b. Hydrogen atoms were omitted for
clarity; ellipsoids are set at 50% probability.15
These results additionally demonstrate the beneficial effects
provided in Au-catalysis by the use of ancillary ligands able to
combine a Buchwald-type structure with Lewis acidity at the P
atom.
b
c
entry
subst.
catalyst (mol %)
yields (%)
ratio 23:24:25
1
2
3
4
5
6
7
22a
22a
22a
22a
22a
22a
22a
22a
22b
22c
Ph3PAuCl (5)
SPhosAuCl (5)
(PhO)3PAuCl (5)
13 (5)
49
10
59:22:19
100:0:0
1:65:34
0:83:17
0:31:69
0:67:33
0:45:55
0:83:17
0:100:0
0:100:0
CONCLUSIONS
■
In summary, we report an efficient method for the preparation
of N-arylpyridiniophosphines of different structures and the
corresponding Au-derived complexes. Our experiments suggest
exceptional electrophilicity at the Au center and importantly,
much longer survival of the actual catalytic species if compared
with those derived from N-alkylpyridinio phosphines.10 This
has been used to improve yields and significantly reduce the
catalyst loadings in the chosen model transformations. We
suspect that a potentially large number of Au-catalyzed
transformations might benefit from the use of the ligands
herein described. Ongoing research in our group targets this
challenging field, as well as the application of N-arylpyridinio
phosphines in processes catalyzed by other metals.25
>95
>95
>95
>95
>95
>95
14 (5)
16 (5)
17 (5)
13 (2)
13 (1.5)
13 (1.5)
d
8
9
e
f
91
e
f
10
93
a
Reactions were carried out in CH2Cl2 at r.t. and quenched after 45
min. Combined yields of the mixture of 23a, 24a and 25a.
Determined by H NMR and/or HPLC. 1 h. 2 h for 24b and 6 h
for 24c. Isolated yield.
b
c
d
e
1
f
first sight, it can be appreciated that the SPhos or even Ph3P
ligands are too basic for this transformation; the reactions
proceed slowly and tetrahelicene 23a, the intermediate where
only one of the two alkynes has been hydroarylated,
accumulates. This is not surprising because the second
cyclization is believed to be the rate-determining step of the
complete process. Catalysts 14 and 17, both bearing a
pyridinium group and two bis(trifluoromethyl)phenyl sub-
stituents attached to the phosphorus center were subsequently
tested. Both displayed exceedingly higher reactivities; after only
45 min the initial substrate has disappeared. It is of note,
however, that low selectivity toward the formation of the
desired [6]helicene is observed, as in this case fulvene 25a, the
product of one 5-exo-dig and one 6-endo-dig cyclization is the
main compound obtained (Table 2, entries 5 and 7).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.8b03271.
■
S
Experimental, synthesis of new compounds, and
analytical data (PDF)
X-ray data for 24b (CIF)
X-ray data for 9aa (CIF)
X-ray data for 9ab (CIF)
X-ray data for 9ac (CIF)
X-ray data for 9ba (CIF)
X-ray data for 9cb (CIF)
X-ray data for 11 (CIF)
X-ray data for 12 (CIF)
X-ray data for 13 (CIF)
X-ray data for 14 (CIF)
A much better balance between reactivity and selectivity is
obtained with catalyst 13. Complete conversion of 22a was
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X-ray data for 17 (CIF)
Research Article
Gu, L.; Goddard, R.; Alcarazo, M. Synthesis and reactivity of α-
cationic phosphines: the effect of imidazolinium and amidinium
substituents. Dalton Trans 2016, 45, 1872−1876. (d) Carreras, J.;
AUTHOR INFORMATION
Corresponding Author
*E-mail: manuel.alcarazo@chemie.uni-goettingen.de.
̌
Gopakumar, G.; Gu, L.; Gimeno, A. M.; Linowski, P.; Petuskova, J.;
■
Thiel, W.; Alcarazo, M. Polycationic Ligands in Gold Catalysis:
Synthesis and Applications of Extremely π-Acidic Catalysts. J. Am.
Chem. Soc. 2013, 135, 18815−18823. (e) Petuskova, J.; Bruns, H.;
Alcarazo, M. Cyclopropenylylidene-Stabilized Diaryl and Dialkyl
Phosphenium Cations: Applications in Homogeneous Gold Catalysis.
Angew. Chem., Int. Ed. 2011, 50, 3799−3802.
̌
ORCID
Richard Goddard: 0000-0003-0357-3173
Manuel Alcarazo: 0000-0002-5491-5682
(7) (a) Dube, J. W.; Zheng, Y.; Thiel, W.; Alcarazo, M. α-Cationic
Arsines: Synthesis, Structure, Reactivity, and Applications. J. Am.
Chem. Soc. 2016, 138, 6869−6877. (b) Carreras, J.; Patil, M.; Thiel,
W.; Alcarazo, M. Exploiting the π-Acceptor Properties of Carbene-
Stabilized Phosphorus Centered Trications [L3P]3+: Applications in
Pt(II) Catalysis. J. Am. Chem. Soc. 2012, 134, 16753−16758.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Generous financial support from the Deutsche Forschungsge-
meinschaft (AL 1348/4-2 and INST 186/1237-1) is gratefully
acknowledged.
́
(8) Gu, L.; Wolf, L. M.; Zielinski, A.; Thiel, W.; Alcarazo, M. α-
Dicationic Chelating Phosphines: Synthesis and Application to the
Hydroarylation of Dienes. J. Am. Chem. Soc. 2017, 139, 4948−4953.
(9) For an additional theoretical analysis, see: García-Rodeja, Y.;
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