Reversible Insertion of Carbon Dioxide at Phosphine Sulfonamido PdII-Aryl Complexes

Article abstract of DOI:10.1021/acs.organomet.0c00560

Phosphine sulfonamido Pd(II) complexes bearing p-anisyl ([(P,N)Pd(4-OMe-C6H4)L]; L = C5H5N, 5-py; L = OS(CH3)2, 5-dmso) or carboxylato ([(P,N)Pd(OOCR)], 1-X: R = 4-OMe-C6H4, 1-pAn; R = tBu, 1-Piv; R = C6H5, 1-Ph) ligands were synthesized and their structures and dynamic behaviors were characterized. The compound 5-dmso can effectively be converted to 1-pAn by migratory insertion of carbon dioxide into the Pd-aryl bond with precoordination of the CO2 molecule. Additionally, carboxylato-substituted compounds 1-X can serve as catalysts for protodecarboxylation reactions of bis-o-OMe-substituted carboxylic acids at room temperature without the need for an external proton source. Stoichiometric decarboxylative coupling of these acids with styrene selectively yields 1,1-diarylated olefins.

Full text of DOI:10.1021/acs.organomet.0c00560

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Article
Reversible Insertion of Carbon Dioxide at Phosphine Sulfonamido
PdII−Aryl Complexes
̈
̈
Gregor Voit, Sangeth Jenthra, Markus Holscher,* Thomas Weyhermuller, and Walter Leitner*
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ABSTRACT: Phosphine sulfonamido Pd(II) complexes bearing
p-anisyl ([(P,N)Pd(4-OMe-C6H4)L]; L = C5H5N, 5-py; L =
OS(CH3)2, 5-dmso) or carboxylato ([(P,N)Pd(OOCR)], 1-X: R
t
= 4-OMe-C6H4, 1-pAn; R = Bu, 1-Piv; R = C6H5, 1-Ph) ligands
were synthesized and their structures and dynamic behaviors were
characterized. The compound 5-dmso can effectively be converted
to 1-pAn by migratory insertion of carbon dioxide into the Pd−aryl
bond with precoordination of the CO2 molecule. Additionally,
carboxylato-substituted compounds 1-X can serve as catalysts for
protodecarboxylation reactions of bis-o-OMe-substituted carbox-
ylic acids at room temperature without the need for an external
proton source. Stoichiometric decarboxylative coupling of these
acids with styrene selectively yields 1,1-diarylated olefins.
the focus was on Ru(PNP)-pincer complexes, and in selected
cases the computed energy span (i.e. the effective activation
barrier in a catalytic cycle29,30) yielded values low enough to be
experimentally feasible.31 Due to drawbacks, which were
induced by the rather diverse coordination spheres of the
structurally flexible octahedrally coordinated ruthenium
complexes, no catalytic activity could be observed exper-
imentally, which in turn could be satisfactorily explained by
DFT computations.32 Therefore, we turned toward alternative
and potentially active d8 Pd(II) complexes, for which it can be
expected that in chemical reactions the square-planar
coordination geometry is not altered during the catalytic
events. Furthermore, various palladium carboxylato complexes
are well known and accessible experimentally as well as
spectroscopically. Additionally, all postulated elementary steps
of the catalytic carboxylation cycle have already been shown to
be possible using palladium complexes.13,33−35 The key step of
these reactions is the insertion of CO2 into a palladium−aryl
bond, which we therefore investigated in more detail. In this
process, we also focused on its reversibility, due to possible
applications in decarboxylation processes.
INTRODUCTION
■
Insertion of carbon dioxide (CO2) into transition-metal−aryl
bonds is an elementary step for the utilization of CO2 as a
building block in chemical synthesis.1−4 In addition to that,
this reversible reaction is also involved in the decarboxylation
of aromatic carboxylic acids. This type of reaction can be
employed for utilizing low-value and nontoxic carboxylic acids
as sustainable resources in C−C bond formation processes
such as arylations or olefinations.5,6 Regarding the utilization of
CO2, various aromatic carboxylic acids are desirable target
compounds, due to their wide application in pharmaceuticals
and other high-value compounds.7,8 There are plenty of
examples using CO2 as a carbon source in homogeneously
catalyzed carboxylation reactions on preactivated arenes, such
as organo-zinc9 or -boron10−12 compounds or aryl (pseudo)-
halides.13−17 Yet, to achieve high atomic efficiency, the direct
catalytic carboxylation of non-preactivated arenes is very
desirable to stay in line with the principles of green chemistry.
In the last decade, among others, the groups of Hou, Nolan,
and Iwasawa were able to realize catalytic direct carboxylation
of arenes which still require strongly electron withdrawing
substituents,18−20 directing groups at the aromatic com-
pound,21−25 or addition of stoichiometric amounts of organo-
metallic reagents.26−28 Insertion of CO2 into metal−aryl bonds
is always a key step of these transformations.
Herein we report on the direct stoichiometric insertion of
carbon dioxide into a Pd(II)−anisyl bond at phosphine
Received: August 24, 2020
We have been searching for catalysts that enable the direct
carboxylation of simple arenes without the need for
stoichiometric additives, by developing plausible reaction
scenarios for various catalysts. These scenarios were then
evaluated by DFT calculations, contributing to a computa-
tionally guided design of late-transition-metal catalysts. Initially
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sulfonamido complexes as well as the ability of such substituted
complexes for additive-free, catalytic protodecarboxylation of
carboxylic acids at room temperature.
the turnover-determining intermediate (TDI), whereas the
insertion of carbon dioxide into the Pd−aryl bond via
transition state TS5-1 is the highest in energy (TDTS). This
results in a total energy span of 32.3 kcal mol−1, which is at the
border of experimental feasibility. The Gibbs free reaction
energy ΔG = 11.2 kcal mol−1 shows the reaction to be
endergonic in accordance with expectation, indicating the need
for product stabilization strong enough to generate an overall
exergonic reaction. The addition of an amine base, for example,
could render the reaction thermodynamically favorable. As the
transformation of 5-solv to 1 and vice versa is the crucial point
of this reaction, these parts were investigated in more detail.
Complex Synthesis. The phosphine sulfonamido palla-
dium carboxylato complexes 1-X and phosphine sulfonamido
palladium p-anisyl compound 5-dmso were synthesized. The
latter bears a relatively weak coordinating dimethyl sulfoxide
ligand, which is expected to stabilize the 14-VE species to
enable synthetic accessibility but not to inhibit reactivity upon
reaction with CO2.
RESULTS AND DISCUSSION
■
In analogy to our previous work,31,32 at first a catalytic cycle
was computed, involving all necessary elementary reactions, as
shown in Scheme 1. We chose a bidentate phosphine
Scheme 1. DFT Computed (B97-D3(BJ)/def2-
TZVP(ECP), SMD, CH2Cl2) Catalytic Cycle for the Direct
Carboxylation of Arenes with CO2 Using 1-pAn (a) and
Computed Gibbs Free Energy Profile for R = 4-OMe (b)
To synthesize carboxylato complexes 1-X the known
compound 1′-Me36 was treated with 1 equiv of the
corresponding carboxylic acid in dichloromethane at room
temperature to achieve compounds 1-pAn, 1-Piv, and 1-Ph in
high yields as orange solids (Scheme 2).
Scheme 2. Synthesis of Pd(II)-Carboxylato Complexes 1-X
via 1′-Me
For the synthesis of 5-dmso the p-anisyl moiety was first
introduced in analogy to a procedure by Grushin et al.37 via
oxidative addition of 1-iodo-4-methoxybenzene to [Pd(dba)2]
in pyridine to give [PdI(4-OMe-C6H4)(py)2] in high yield
(Scheme 3). Addition of the sodium salt of the phosphine
sulfonamido ligand (L-Na) to this compound in dichloro-
methane at room temperature resulted in the formation of
pyridine-stabilized 5-py in high yields. To exchange the
stronger σ-donor ligand pyridine by DMSO, the relatively
low boiling point of pyridine was utilized and the desired 5-
dmso could be obtained as a colorless solid in excellent yield
by dissolving 5-py in dimethyl sulfoxide and removing all
volatiles under reduced pressure at 50 °C several times38
(Scheme 3).
sulfonamido ligand (L) that had been reported by Mecking
and co-workers,36 leaving two cis standing coordination sites
for catalysis. Anisole was selected as a substrate, due to its
highly nucleophilic carbon atom likely facilitating CO2
insertion.31 The cycle mainly consists of a C−H activation
step of an arene, promoted by the carboxylato ligand of
compound 1 via a CMD type mechanism, and insertion of
carbon dioxide into the newly formed Pd−aryl bond of
compound 5, regenerating carboxylato complex 1 (Scheme 1
a).
Solid-State Structures and Dynamic Behavior. Single
crystals suitable for X-ray diffraction analysis of 1-pAn, 1-Ph, 5-
py, and 5-dmso were obtained by slow diffusion of n-pentane
in solutions of these compounds in THF or CD2Cl2 (Figure 1
and the Supporting Information).
According to the computed Gibbs energies of all involved
intermediates and transition states (Scheme 1b) the carbox-
ylato complex 1 is the most stable compound and therefore is
B
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Scheme 3. Synthesis of Pd(II)-p-Anisyl Complexes 5-py and 5-dmso
In all solid-state structures the chelating six-membered ring
adopts a typical39 boatlike conformation and the N-aryl moiety
lies roughly perpendicular to the square-planar metal center,
being slightly distorted toward the spatially more remote
pseudoequatorial P-aryl group. The Pd−O47 distance (217.5
pm) of the κ-O-coordinating dmso ligand in 5-dmso is just
slightly larger than Pd−N41 (212.2 pm) in 5-py, indicating
weaker coordination of dmso, considering the larger van der
Waals radius of nitrogen in comparison to oxygen. Both p-
anisyl-substituted compounds 5-solv comprise exo2 config-
uration of the P-aryl substituents: i.e. the pseudoaxial arene’s o-
methoxy group points in the direction opposite to the metal
center.40 This is in accordance with previous reports on P-o-
anisyl-substituted phosphine sulfonato compounds39 as well as
1′-Me.36 In contrast, carboxylato complexes 1-X show exo3
configuration in the solid state with both methoxy substituents
occupying the space in the z direction of the square-planar
coordination sphere. This rather unusual configuration might
be enabled by distortion of the square-planar geometry
through the κ2-O,O′-coordinating carboxylato ligand.
substituted phosphine sulfonato complexes, where this ring-
flip process was determined to occur quickly in comparison to
P−aryl rotation.39
1
p-Anisyl-substituted compounds 5-solv show in H as well
as in 31P NMR spectra (see the Supporting Information)
i
broadening for all signals and two separated Pr methine and
four methyl signals (being partially isochronous for 5-dmso),
indicating hindrance of both processesaryl rotation and
chelate ring inversionat room temperature. The determi-
nation of coalescence temperatures for methine as well as
methyl protons at Tc,methine = 40 °C and Tc,methyl = 31 °C,
respectively, results in an energy barrier for the process lowest
in energy of ΔG⧧ = 15.5 kcal mol−1 (Figure 3 and the
Supporting Information). On consideration of the observations
for 1-pAn, this process is assigned as chelate ring inversion.
The observed decelerated dynamic behavior in comparison to
P-o-anisyl-substituted phosphine sulfonato complexes as well
as 1-X likely reflects the higher steric hindrance caused by the
demanding N-aryl moiety as well as the smaller P−Pd−C51
angle (vide infra).
Stoichiometric Insertion of CO2. Insertion of CO2 into
the Pd−aryl bond to interconvert 5-dmso to 1-pAn (Scheme
4) is expected to be the crucial step in the postulated catalytic
carboxylation of anisole. DFT calculations predict experimental
feasibility for stoichiometric CO2 insertion into 5-dmso with
an energy barrier for the insertion step of ΔG⧧ = 19.1 kcal
mol−1 in DCM and a Gibbs free reaction energy of ΔG = −1.9
kcal mol−1. For experimental validation, 10 μmol of 5-dmso
was dissolved in 1 mL of dichloromethane and pressurized
with 35 bar of CO2. After it was stirred for 16 h at 40 °C ,the
mixture was depressurized, the volatiles were removed under
reduced pressure, and the obtained orange solid was analyzed
by NMR spectroscopic methods.
In analogy to P-o-anisyl-substituted phosphine sulfonato
complexes, carboxylato compounds 1-X show sharp signals in
1H NMR spectra at room temperature, except for those
corresponding to ortho and meta substituents at the P-aryl
moieties, indicating hindered P−aryl rotation.39 For 1-pAn,
i
signals for Pr-methyl groups appear as two distinct, sharp
doublets at 0.97 and 1.22 ppm, respectively, which split into
four signals at lower temperatures (see the Supporting
Information). This indicates that either N−aryl rotation or
inversion of the chelating six-membered ring is slow on this
1
NMR time scale at room temperature. The H−1H-NOESY
spectrum (Figure 2) shows the spatial proximity of a methyl
group belonging to the signal at 0.97 ppm to the ligand’s
backbone protons (5-H, 6-H) and NOE interactions between
the signal at 1.22 ppm and the ortho protons of the carboxylato
ligand. This shows that methyl group 18a is not interconverted
to 18d at room temperature, as expected for an aryl rotation
process, indicating a lower energy barrier for the chelate ring
inversion process. This stands in line with P-o-anisyl-
Except for minor impurities which could partially be
identified as anisole and the Pd(I) species LPd-PdL (see the
Supporting Information), full conversion of 5-dmso with CO2
to 1-pAn and DMSO was observed (Figure 4). Special care
had to be taken that the utilized DCM or CD2Cl2 featured a
very high purity and a nearly complete absence of proton
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1
Figure 3. Methine proton region of variable-temperature H NMR
spectra (CDCl3, 300 MHz) of 5-dmso for coalescence temperature
determination.
Scheme 4. Stoichiometric Insertion of CO2 (20 bar) into 5-
dmso to Yield 1-pAn in DCM at 40 °C
Figure 1. Solid-state structures of 5-dmso (top) and 1-pAn (bottom)
with 50% probability ellipsoids. Hydrogen atoms and cocrystallized
solvent molecules and 1-py are omitted for clarity. Due to a partial
change in the crystal quality of 1-pAn during X-ray data collection the
refinement quality allows only for assigning atom connectivities.
Figure 4. 1H NMR spectra (CD2Cl2, r.t., 400 MHz) of 5-dmso before
(a) and after reaction with CO2 and workup (b) and of independently
synthesized 1-pAn (c).
Information) as well as further purification of the reaction
product by recrystallization from DCM with n-pentane.
The origin of the carboxylato functionality from CO2 could
be verified by conducting the reaction with 13CO2. 13C NMR
spectra of this reaction reveal that the peak for the quarternary
carbon atom of the carboxylato unit at 185.1 ppm largely
intensified. It shows cross coupling with ortho protons of the
1
anisolato ligand in H−13C-HMBC spectra, which can also be
1
Figure 2. Relevant region of the H−1H-NOESY spectrum (CD2Cl2,
observed in 1H NMR spectra with 3JCH = 3.8 Hz. Again, for the
unpurified reaction mixture these signals are slightly shifted
and strongly broadened due to partial DMSO coordination.
The insertion reaction is strongly altered by coordinating
solvents, resulting in a conversion of only 30% after 16 h when
4 equiv of DMSO was added and complete suppression by
addition of 20 equiv of DMSO or conducting the reaction in
THF (entries 1−4 in Table 1). The rate constant is also
dependent on the applied CO2 pressure, resulting in a
conversion of 70% at 5 bar of CO2 (entries 5 and 6). These
observations suggest a migratory insertion mechanism with
rt, 400 MHz, mixing time 0.8 s) of 1-pAn and simplified solid-state
structure marked with relevant NOE interactions.
sources, oxygen, and inhibitors, such as amylene, which
irreversibly deactivate the complex. The minor deviation in
chemical shifts and broadening of certain signals, especially of
o-carboxylato protons and those altered by increasing steric
hindrance (vide infra) can be explained by partial coordination
of formed DMSO to 1-pAn. This was confirmed by addition of
minor amounts of DMSO to 1-pAn (see the Supporting
D
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tri- and dimethoxybenzoic acids than for p-anisic acid. This is a
result of the fact that the trimethoxy- and dimethoxy-
substituted acids are richer in energy while the transition
states for the decarboxylations are lower in energy for these
compounds in comparison with p-anisic acid. In contrast to
previous reports on palladium-catalyzed protodecarboxylations
no external proton source is necessary to achieve a high
reaction rate,6,41,43 indicating that protonation occurs
effectively by the aromatic carboxylic acid via a six-membered
transition state, as postulated for its back-reaction (TS 4-3;
Scheme 1). In contrast to the insertion reaction, proto-
decarboxylation is not hampered by coordinating solvents such
as THF and acetone, reflecting the stronger coordination
ability of the carboxylic acid to the 14-VE aryl intermediates in
comparison to CO2.
Table 1. Stoichiometric Insertion of CO2 into 5-dmso at 40
°C after 16 h
pCO2
(bar)
additive (amt
(equiv))
conversion
a
entry
solvent (V (mL))
(%)
1
2
3
4
5
6
7
35
35
35
35
20
5
CD2Cl2 (1)
CD2Cl2 (1)
CD2Cl2 (1)
THF (3)
CD2Cl2 (1)
CD2Cl2 (1)
100
30
0
DMSO (4)
DMSO (20)
0
100
70
100
35
DIPEA (15)
DCM (2.5),
anisole (0.5)
a
Determined via 1H NMR spectroscopy, neglecting minor side
product formation.
To get deeper insight into the reaction mechanism and
investigate if the computed catalytic cycle is in accordance with
the experiment, the decarboxylation reaction of TMBA was
conducted with the compound 1-Piv as a catalyst precursor, to
avoid signal overlap of the precursor’s carboxylato ligand and
the substrate in 1H NMR spectra. The reaction was conducted
precoordination of the CO2 molecule to the Pd center and the
substitution step of the dmso ligand by CO2 being highest in
energy. This is in contrast to phosphine sulfonato palladium
methyl complexes, for which Jordan and co-workers observed
insertion of CO2 into Pd−methyl bonds via a SE2 mechanism
and therefore only reactivity for anionic complexes.33 Addition
of the sterically demanding amine base DIPEA and anisole to
the reaction mixture does not alter the reaction (entry 7),
indicating that there is no strong coordination of these
molecules to the palladium center that might inhibit possible
future catalytic reactions, where bases are essential for product
stabilization and anisole is used as a substrate. DFT
calculations predict an exo3 configuration of the transition
state for CO2 insertion, indicating stabilization relative to 5-
solv by attractive interactions between the ligand’s methoxy
groups and the palladium center (dPd−Oax = 296.4 pm).
Therefore, P-o-anisyl substituents might be necessary pre-
requisites for this reaction to occur.
1
in an NMR tube in CD2Cl2 and monitored over time by H
NMR spectroscopy (Figure 5). Directly after addition of the
Decarboxylation Experiments. After successful trans-
formation of 5-dmso to 1-pAn, we investigated processes that
involve the transformation of carboxylato complexes to
palladium aryl species. The back-reaction of the postulated
carboxylation of arenes, catalytic protodecarboxylation of
aromatic carboxylic acids, fulfills this requirement, is exergonic,
and shows reasonable energy spans to overcome (Scheme 1).
Therefore, the catalyst precursor 1-pAn and various
aromatic carboxylic acids were dissolved in DCM and stirred
at room temperature with no other additives. While 2,4,6-
trimethoxybenzoic acid (TMBA) and 2,6-dimethoxybenzoic
acid underwent quantitative decarboxylation at room temper-
ature even at catalyst loadings as low as 0.5 mol % within 20 h
(Scheme 5), o-anisic acid revealed much lower activity and 4-
methoxybenzoic acid was not decarboxylated under this
conditions at all. In accordance with these findings, previous
reports have also indicated ortho substituents to be a
prerequisite for decarboxylation reactions at palladium
systems6,41,42 and our own DFT calculations (not shown)
support this as well, as they predict lower theoretical TOFs for
1
Figure 5. Time-dependent H-NMR spectra of the decarboxylation
reaction of 40 mmol of TMBA with 4 mmol of 1-Piv in 0.4 mL
CD2Cl2 at rt before addition of TMBA (a), after 10 min (b), after 90
min (c), and after full conversion of TMBA and re-formation of the
catalyst precursor 1-Piv after 110 min (d).
solvent, free pivalic acid (shown by a black heart) was
detected, indicating activation of the catalyst precursor by
protonation of the more basic pivalato ligand by trimethox-
ybenzoic acid (shown by a black club). A single palladium
species could be observed while the reaction takes place,
containing a trimethoxy aryl moiety (shown by a black
diamond). Due to the relative instability of this compound no
detailed analytics could be performed. A comparison of NMR
spectra revealed strong similarity to carboxylato compounds 1-
X, rather than to the palladium aryl species 5-solv, including
sharp signals in the aromatic region. This suggests a
carboxylato intermediate (1 in Figure 1) to be the most stable
in energy, standing in accordance with the proposed cycle.
After all of the carboxylic acid is converted to 1,3,5-
trimethoxybenzene (shown by a black spade), the initial
catalyst precursor 1-Piv is quantitatively re-formed.
Scheme 5. Catalytic Protodecarboxylation of Bis-ortho-
Substituted Aromatic Carboxylic Acids
E
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and prepared analogously. All other solvents were purified by
redestillation, degassdc by bubbling with argon for several hours,
and stored over 4 Å molecular sieves. Compounds L-Na and 1′-Me
were synthesized via literature procedures.36 All other commercially
available reagents and starting materials were supplied by Sigma-
Aldrich, Acros Organics, or ABCR and used as received. NMR spectra
were recorded with a Bruker Avance 300, Bruker Ascend 400, or
The presence of aryl intermediates in the catalytic cycle
could be determined by addition of styrene to the reaction
mixture in THF.44 Although the protonation step is effectively
performed by the aromatic carboxylic acids, stoichiometric
decarboxylative coupling of 2,6-dimethoxybenzoic acid with
styrene could be observed, selectively yielding 1,3-dimethoxy-
2-(1-phenylvinyl)benzene (Scheme 6). This rather exclusive45
1
Bruker Avance 600 spectrometer at the temperatures given. H and
13C NMR spectra were referenced to tetramethylsilane; 31P NMR
spectra were referenced to an external 85% H3PO4 solution. Coupling
constants are given in Hz. X-ray diffraction data were collected at 100
K on a Bruker AXS-Enraf-Nonius-Kappa-CCD diffractometer with
monochromated Mo Kα radiation (λ = 0.71073 Å). Electrospray mass
spectra (ESI-MS) were recorded with a Thermo Scientific Q Exactive
Plus or LTQ-FT-Ultra instrument. Elemental analysis was performed
with a PerkinElmer CHNS/O analyzer 2400 instrument. Computa-
tional details of the DFT calculations are given in the Supporting
Information.
Scheme 6. Stoichiometric Decarboxylative Coupling of 2,6-
Dimethoxybenzoic Acid with Styrene at 1-pAn ([Pd]) in
a
THF (Top) and Intermediates Leading to 2,1- or 1,2
Insertion Products (Bottom)
Synthesis of [PdI(4-OMe-C6H4)(py)2].
In a 30 mL Schlenk tube, 600 mg of [Pd(dba)2] (1.04 mmol, 1 equiv)
and 609 mg of 4-iodoanisole (2.64 mmol, 2.5 equiv) were suspended
in 6 mL of dry pyridine. After the gray suspension was stirred at room
temperature for 30 min, 20 mL of diethyl ether was added, causing a
gray solid to precipitate. After the mixture was stirred for a further 30
min the solid was filtered and washed three times with 4 mL of Et2O,
yielding 490 mg of [Pd(I)(4-OMe-C6H4)(py)2] (0.98 mmol, 94%).
Due to rapid decomposition of the target compound in all common
deuterated solvents, no NMR spectroscopic analysis could be carried
out. ESI-MS(pos) [m/z] for [M − I + CH3CN]+, [C19H20N3OPd]+:
found (calcd), 412.0636 (412.0636). Anal. Calcd for C17H17IN2OPd:
C, 40.95; H, 3.44; N, 5.62. Found: C, 42.33, H, 3.95, N, 6.25.
Synthesis of 5-py.
a
The exact structure of [Pd]′ was not determined.
regioselectivity to 1,1-disubstituted olefins is likely caused by
the steric bulk of the ligand’s N-aryl moiety, forcing styrene
coordination in a manner only allowing a 2,1-insertion
mechanism.
SUMMARY AND CONCLUSIONS
■
In this study we reported the syntheses and reactivity of
phosphine sulfonamido palladium complexes bearing carbox-
ylato or aryl ligands and investigated these in insertions of CO2
into Pd−aryl species and decarboxylations of aromatic acids,
respectively. The p-anisyl-substituted compound 5-dmso can
effectively be converted to the corresponding anisolato
complex 1-pAn by insertion of carbon dioxide into the
palladium−aryl bond. This reaction occurs via a migratory
insertion pathway with precoordination of the CO2 molecule
to the palladium center. Additionally, the ability of carboxylato-
substituted compounds to catalytically protodecarboxylate o-
methoxy-substituted aromatic carboxylic acids at room
temperature without the need of an external proton source
was demonstrated. These compounds can also stoichiometri-
cally couple aromatic carboxylic acids with styrene in a
decarboxylative manner to selectively yield 1,1-disubstituted
olefins.
In a 30 mL Schlenk tube, 338 mg of [Pd(I)(4-OMe-C6H4)(py)2]
(678 μmol, 1 equiv) and 435 mg of L-Na (746 μmol, 1.1 equiv) were
suspended in 15 mL of DCM. After it was stirred for 30 min at room
temperature, the grey suspension was filtered via a syringe filter
(PTFE, 0.20 μm). The solution was concentrated to a volume of 5
mL and layered with 20 mL of n-pentane. The colorless crystals were
collected via filtration, washed twice with 5 mL of n-pentane, and
dried in vacuo, yielding 470 mg of the compound 5-py (551 μmol,
81%). Crystals suitable for X-ray diffraction analysis could be obtained
EXPERIMENTAL SECTION
■
1
by layering a solution of 5-py in CD2Cl2 with n-pentane. H-NMR
Materials and General Considerations. All manipulations of
air- and moisture-sensitive materials were carried out under an
atmosphere of argon (Argon 4.8 Spectro, Westphalen AG) using
standard glovebox and Schlenk techniques. Acetone, THF, pyridine,
DMSO, and DCM were purchased at Acros Organics as extra dry,
stored over 4 Å molecular sieves, and degassed by flushing for several
hours with Ar. DCM for insertion experiments was purchased
inhibitor free at Sigma-Aldrich, degassed by three consecutive freeze−
pump−thaw cycles, and stored over 4 Å molecular sieves. It was only
used when the water content, as measured by Karl Fischer titration,
was below 5 ppm. Deuterated solvents were supplied by Eurisotop
(400 MHz, CD2Cl2, 25 °C): δ [ppm] 9.63 (br, 1H, 8a-H), 7.97 (m,
2H, 23-H), 7.93 (vddd, 3JHH = 7.8 Hz, 4JHH = 4.9 Hz, 5JHH = 1.1 Hz,
1H, 6-H), 7.64 (br, 1H, Ar-H), 7.48 (br, 2H, Ar-H), 7.40 (m, 2H, Ar-
H), 7.29 (t, 3JHH = 7.4 Hz, 1H, 25-H), 7.19 (br, 1H, Ar-H), 7.09 (br,
1H, Ar-H), 7.00 (t, 3JHH = 7.3 Hz, 1H, 16-H), 6.92-6.82 (m, 5H, 24-H
and Ar-H), 6.74 (br, 1H, Ar-H), 6.63-5.52 (br, 5H, Ar-H), 3.79 (br,
1H, 17-H), 3.70 (s, 3H, 27-H), 3.55 (br, 1H, 17-H), 3.41 (s, 6H, 26-
H), 1.52 (br, 3H, 18-H), 1.20 (br, 3H, 18-H), 0.82 (br, 3H, 18-H),
0.29 (br, 3H, 18-H). 13C{1H} NMR (101 MHz, CD2Cl2, 25 °C): δ
[ppm] 161.5, 160.0, 156.3, 150.8, 147.2, 145.1, 142.3, 141.8, 137.6,
F
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136.2, 134.3, 133.8, 130.1, 127.4, 126.7, 125.0, 124.8, 123.8, 123.6,
120.7, 113.4, 111.8, 56.9 (s, C27), 54.9 (s, C26), 28.4 (s, C17), 26.2
(s, C18), 25.7 (s, C18), 24.9 (s, C18), 23.9 (s, C18). 31P{1H} NMR =
(121 MHz, CD2Cl2, 25 °C): δ [ppm] 18.58 (br). ESI-MS(pos) [m/
z]: for [M + H]+, [C44H48N2O5PPdS]+: found (calcd), 853.205070
(853.20509). Anal. Calcd for C44H47N2O5PPdS: C, 61.93; H, 5.55; N,
3.28. Found: C, 62.17; H, 5.80, N, 3.48.
In a typical experiment 228 mg of 1′-Me (308 μmol, 1 equiv) and
32.5 mg of pivalic acid (318 μmol, 1 equiv) were dissolved in 12 mL
of DCM. After it was stirred for 2 h at room temperature, the orange
solution was filtered via a syringe filter (PTFE, 0.2 μm). The solution
was concentrated under reduced pressure to a volume below 5 mL
and layered with 12 mL of n-pentane, upon which orange crystals
precipitated. Filtration and drying under reduced pressure resulted in
194 mg of 1-Piv (253 μmol, 82%) as a yellow solid. Alternatively, the
reaction could be performed in THF and the mixture stirred overnight
with partial precipitation of the desired product.
Synthesis of 5-dmso.
1-pAn. 1H NMR (400 MHz, CD2Cl2, 25 °C): δ [ppm] 7.92 (ddd,
3JHH = 7.8 Hz, 4JHH = 4.7 Hz, 5JHH = 1.0 Hz, 1H, 6-H), 7.80 (br, 1H,
3
3
8a-H), 7.65 (vt, JHH = 7.9 Hz 2H, 10-H), 7.63 (vtt, JHH = 7.6 Hz,
4JHH = 1.3 Hz, 1H, 5-H), 7.52 (m, 2H 21-H), 7.45 (vtt, 1H, 3JHH = 7.6
Hz, 4JHH = 1.4 Hz, 4-H), 7.31 (vd, 3JHH = 7.7 Hz, 1H, 3-H), 7.26 (vt,
3
3JHH = 7.7 Hz, 1H, 16-H), 7.12 (d, JHH = 7.7 Hz, 2H, 15-H), 7.06
(br, 5H, 8b-, 9- and 11-H), 6.75 (m, 2H, 22-H), 3.77 (s, 3H, 25-H),
3
3.77 (br, 8H, 17- and 24-H), 1.32 (d, JHH = 6.8 Hz, 6H, 18b- and
18d-H), 1.07 (d, 3JHH = 6.8 Hz, 6H, 18a- and 18c-H). 13C{1H}-NMR
(101 MHz, CD2Cl2, 25 °C): δ [ppm] 185.7 (br, C19), 163.9 (s,
In a 30 mL Schlenk tube, 600 mg of compound 5-py (0.69 mmol)
was suspended in 20 mL of DMSO. With stirring all volatiles were
removed in vacuo at 45 °C. This procedure was repeated eight times,
C23), 161.9 (d, 2JPC = 3.1 Hz, C12), 150.7 (br, C14), 149.4 (d, 2JPC
=
12.6 Hz, C1), 138.2 (s, C13), 134.9 (br, C8), 134.6 (br, C10), 134.3
1
until the starting material could no longer be detected via H NMR
2
4
(d, JPC = 2.5 Hz, C3), 131.7 (d, JPC = 2.7 Hz, C5), 130.8 (s, C21),
129.5 (d, 3JPC = 8.8 Hz, C4), 127.4 (s, C16), 125.6 (d, 3JPC = 9.7 Hz,
C6), 125.3 (d, JPC = 59.6 Hz, C2), 124.8 (s, C20), 124.1 (s, C15),
spectroscopy. The remaining yellow, viscous oil was recrystallized
from 2 mL of dichloromethane by adding 25 mL of pentane, yielding
478 mg of the compound 5-dmso (0.55 mmol, 80%). Crystals
suitable for X-ray diffraction analysis could be obtained by layering a
solution of 5-dmso in THF with n-pentane. Due to the remaining
starting material in this sample, the crystals consisted of 67% of 5-py
and 33% of 5-dmso. 1H NMR (600 MHz, CD2Cl2, -30 °C): δ [ppm]
1
121.5 (br, C9), 113.7 (s, C22), 112.6(br, C11), 56.7 (br, C24), 55.9
(s, C25), 29.4 (s, C17), 25.0 (s, C18a and C18c), 24.5 (br, C18b and
C18d). 31P{1H}-NMR (162 MHz, CD2Cl2, 25 °C): δ [ppm] −0.30
(br). ESI-MS(pos) [m/z] for [M + Na]+, [C40H42NNaO7PPdS]+:
found (calcd), 840.1352 (840.1347). Anal. Calcd for
C40H42NO7PPdS: C, 58.72; H, 5.17; N, 1.71. Found: C, 57.94; H,
4.34; N, 1.66.
3
9.41 (ddd, JHH = 16.8 Hz, 4JHH = 7.5 Hz, 5JHH = 1.4 Hz, 1H, 8a-H),
3
4
5
7.82 (ddd, JHH = 7.8 Hz, JHH = 4.7 Hz, JHH = 1.0 Hz, 1H, 6-H),
7.58 (t, 3JHH = 7.8 Hz, 1H, 10a-H), 7.46 (ddd, 3JHH = 12.0 Hz, 4JHH
8.0 Hz, JHH = 0.7 Hz, 1H, 3-H), 7.35 (m, 2H 5-H and 9a-H), 7.24
=
1
1-Piv. H NMR (400 MHz, CD2Cl2, 25 °C): δ [ppm] 7.90 (ddd,
5
3JHH = 7.8 Hz, 4JHH = 4.6 Hz, 5JHH = 1.1 Hz, 1H, 6-H), 7.65-7.57 (m,
3H, 5-H and 10-H), 7.50 (br, 1H, 8a-H), 7.42 (tt, 3JHH = 7.6 Hz, 4JHH
3
(m, 2H 4-H and 10b-H), 7.16 (d, JHH = 8.3 Hz, 1H, 20a-H), 7.02
(m, 3H, 15-H and 16-H), 6.92 (dd, 3JHH = 8.2 Hz, 4JHH = 5.2 Hz, 1H,
11b-H), 6.80 (dd, 3JHH = 8.2 Hz, 4JHH = 3.6 Hz, 1H, 11a-H), 6.47 (d,
= 1.5 Hz, 1H, 4-H), 7.24-7.18 (m, 2H, 3-H and 16-H), 7.08 (d, 3JHH
=
7.6 Hz, 2H, 15-H), 7.04 (br, 5H, 8b-, 9- and 11-H), 3.78 (s br, 6H,
3JHH = 8.0 Hz, 1H, 20b-H), 6.41 (t, JHH = 7.6 Hz, 1H, 9b-H), 6.27
3
3
24-H), 3.60 (br, 2H, 17-H), 1.28 (d, JHH = 6.6 Hz, 6H, 18-H), 1.04
(dd, 3JHH = 8.4 Hz, 4JHH = 2.4 Hz, 1H, 21a-H), 6.10 (ddd, 3JHH = 13.1
Hz, 4JHH = 7.9 Hz, 5JHH = 1.0 Hz, 1H, 8b-H), 5.96 (dd, 3JHH = 8.4 Hz,
4JHH = 2.4 Hz, 1H, 21b-H), 3.85 (sept, 3JHH = 6.8 Hz, 1H, 17-H), 3.72
(vsept, 3JHH = 6.8 Hz, 1H, 17-H), 3.67 (s, 3H, 23-H), 3.47 (s, 3H, 24-
3
(d, JHH = 6.6 Hz, 6H, 18-H), 0.89 (s, 9H, 21-H). 13C{1H}-NMR
(101 MHz, CD2Cl2, 25 °C): δ [ppm] = 200.1 (br, C19), 161.9 (d,
2
2JPC = 3.1 Hz, C12), 150.7 (br, C14), 149.5 (d, JPC = 12.2 Hz, C1),
138.3 (s, C13), 136.9 and 135.9 (br, C8), 135.6 (br, C8), 134.6 (br,
3
2
4
H), 3.40 (s, 3H, 23-H), 1.42 (s, 3H, 25-H), 1.32 (d, JHH = 6.9 Hz,
3H, 18-H), 1.31 (s, 3H, 25-H), 1.28 (d, JHH = 6.8 Hz, 3H, 18-H),
C10), 134.3 (d, JPC = 2.7 Hz, C3), 131.7 (d, JPC = 2.3 Hz, C5),
3
129.7 (d, 3JPC = 8.8 Hz, C4), 127.2 (s, C16), 125.6 (d, 3JPC = 9.0 Hz,
C6), 125.4 (d, JPC = 58.4 Hz, C2), 124.0 (s, C15),, 121.5 (d, JPC
1.19 (d, 3JHH = 6.8 Hz, 3H, 18-H), 0.71 (d, 3JHH = 6.9 Hz, 3H, 18-H).
13C{1H} NMR (101 MHz, CD2Cl2, 25 °C): δ [ppm] 161.0 and 160.4
(br, C12), 157.0 (s, C22), 151.2 and 147.3 (br, C14), 145.1 (br, C8),
142.5 (s, C13), 136.0 (br, C1), 133.8 (br, C10), 132.0 (br), 130.1 (s,
C5), 127.7 (br) 127.1 (br, C4), 126.1 (br, C6), 124.3, 123.5, and
123.1 (br, C15 and C16), 120.7 (br), 119.2, 118.5, 113.2 (br, C21),
111.8 (br, C11), 111.3 (s, C20), 56.6 (br, C23), 55.5 (s, C23), 54.7
(s, C24), 37.5 (br, C25), 29.2 (s, C17), 28.5 (br, C17), 25.7, 25.6,
25.2, and 24.8 (s, br, C18). 31P{1H} NMR (162 MHz, CD2Cl2, 25
°C): δ [ppm] 22.53 (br). ESI-MS(pos) [m/z] for [M + Na]+,
[C41H48NNaO6PPdS2]+: found (calcd), 874.158290 (874.15877).
Anal. Calcd for C41H48NO6PPdS2: C, 57.78; H, 5.68; N, 1.64. Found:
C, 53.54; H, 5.52; N, 1.44.
1
4
=
4
11.5 Hz, C9), 112.5 (d, JPC = 5.6 Hz, C11), 56.6 (s, C24), 40.5 (s,
C20), 29.4 (s, C17), 26.7 (s, C21), 24.8 (s, C18), 24.5 (br, C18).
31P{1H}-NMR (162 MHz, CD2Cl2, 25 °C): δ [ppm] −2.52 (br). ESI-
MS(pos) [m/z] for [M + Na]+, [C37H44NNaO6PPdS]+: found
(calcd), 790.155270 (790.15540). Anal. calcd. for C37H44NO6PPdS:
C, 57.85; H, 5.77; N, 1.82. Found: C, 56.82; H, 5.71; N, 1.72.
1
1-Ph. H-NMR (300 MHz, CD2Cl2, 25 °C): δ [ppm] 7.93 (ddd,
3JHH = 7.8 Hz, 4JHH = 4.7 Hz, 5JHH = 1.0 Hz, 1H, 6-H), 7.70 (br, 1H,
3
8a-H), 7.65 (vt, JHH = 8.5 Hz, 2H. 10-H), 7.62 (m, 1H, 5-H), 7.57
(vd, 3JHH = 8.2 Hz, 2H, 21-H), 7.45 (m, 2H, 4-H and 23-H), 7.27 (m,
3
4H, 3-H, 16-H and 22-H), 7.12 (vd, JHH = 7.6 Hz, 2H, 15-H), 7.07
(br, 5H, 8b-H, 9-H and 11-H), 3.75 (br, 8H, 17-H and 24-H), 1.32
3
3
General Synthesis of Pd(II)-Carboxylato Complexes 1-X.
(d, JHH = 6.8 Hz, 2H, 18-H), 1.07 (d, JHH = 6.8 Hz, 6H, 18-H).
13C{1H} NMR (151 MHz, CD2Cl2, 25 °C): δ [ppm] 185.2 (br, C19),
161.4 (br, C12), 150.1 (br, C14), 149.4 (d, 2JPC = 12.0 Hz, C1), 137.5
5
(br, C13), 134.3 (d, JPC = 1.7 Hz C10), 133.7 (br, C3), 133.2 (s,
C23), 132.5 (s, C20), 131.8 (d, JPC = 2.6 Hz, C5), 129.6 (d, JPC
4
3
=
9.1 Hz, C4), 128.8 (s, C21), 128.5 (s, C22), 127.5 (s, C16), 125.6 (d,
3JPC = 8.9 Hz, C6), 124.7 (br, C2), 124.8 (C20), 124.1 (s, C15),121.7
(br, C9), 112.6 (br, C11), 56.7 (br, C24), 29.5 (s, C17), 25.0 (s,
C18). 31P{1H} NMR (162 MHz, CD2Cl2, 25 °C): δ [ppm] −0.45
(br).
General Procedure for Stoichiometric CO2 Insertion Experi-
ments. In a typical experiment, 8.5 mg (10 μmol) of 5-dmso was
G
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dissolved in 1−3 mL of inhibitor-free, dry dichloromethane or
CD2Cl2. The slightly yellowish solution was transferred to a 10 mL
stainless steel autoclave equipped with a magnetic stirring bar and a
glass inlet. The autoclave was pressurized with 5−35 bar of carbon
dioxide and stirred at 800 rpm in a preheated aluminum cone at 40 °C
for 16 h. After the reaction was completed, the vessel was
depressurized and the clear orange solution was transferred to a 7
mL Schlenk tube. Volatiles were removed under reduced pressure,
yielding the compound 1-pAn as an orange solid. Further purification
could be done by crystallization from DCM with n-pentane.
General Procedure for Protodecarboxylation Experiments.
In a typical experiment, 4−10 μmol of the catalyst precursors 1-X and
10−180 equiv of the corresponding carboxylic acid were dissolved in
0.5−5 mL of the given solvent in a 7 mL Schlenk tube. After the
mixture was stirred at room temperature overnight, the volatiles were
removed in vacuo and the solid was analyzed via NMR spectroscopic
methods. In the case of anisic acids as substrates, reactions were
conducted in deuterated solvents and analyzed without prior removal
of the solvent, due to the relatively low boiling point of anisole.
General Procedure for Stoichiometric Decarboxylative
Coupling Reactions.
Aachen, Germany; orcid.org/0000-0003-4088-8924;
Email: hoelscher@itmc.rwth-aachen.de
Walter Leitner − Institut für Technische und
Makromolekulare Chemie, RWTH Aachen University, 52074
Aachen, Germany; Max Planck Institute for Chemical Energy
Conversion, Mülheim an der Ruhr, Germany; orcid.org/
0000-0001-6100-9656; Email: walter.leitner@cec.mpg.de,
leitner@itmc.rwth-aachen.de
Authors
Gregor Voit − Institut für Technische und Makromolekulare
Chemie, RWTH Aachen University, 52074 Aachen, Germany
Sangeth Jenthra − Institut für Technische und
Makromolekulare Chemie, RWTH Aachen University, 52074
Aachen, Germany
Thomas Weyhermuller − Max Planck Institute for Chemical
̈
Energy Conversion, Mülheim an der Ruhr, Germany;
orcid.org/0000-0002-0399-7999
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00560
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
In a typical experiment, 3−10 μmol of catalyst precursors 1-X and
10−50 equiv of the corresponding carboxylic acid were dissolved in
0.6−10 mL of THF. A 15−75 equiv amount of styrene was added,
and the mixture was stirred overnight. The volatiles were removed in
vacuo, and the solid was analyzed via NMR spectroscopic methods.
The coupling product 1,3-dimethoxy-2-(1-phenylvinyl)benzene could
be further purified by column chromatography (SiO2, n-pentane/ethyl
acetate = 3/1).
This work has been carried out as part of the International
Research Training Group “Selectivity in chemo- and
biocatalysis” IRTG1628 (Seleca), which is funded by the
German research foundation.
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Markus Holscher − Institut für Technische und
Makromolekulare Chemie, RWTH Aachen University, 52074
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https://dx.doi.org/10.1021/acs.organomet.0c00560
Organometallics XXXX, XXX, XXX−XXX