Stoichiometric Studies on the Carbonylative Trifluoromethylation of Aryl Pd(II) Complexes using TMSCF3 as the Trifluoromethyl Source

Article abstract of DOI:10.1021/acs.organomet.9b00849

We have performed a series of stoichiometric studies in order to identify viable steps for a hypothetical catalytic cycle for the palladium-mediated carbonylative coupling of an aryl bromide with TMSCF₃. Our work revealed that benzoyl Pd(II) complexes bearing Xantphos or tBu₃P as the phosphine ligands, which are generated from the corresponding Pd^II(Ph)Br complexes exposed to stoichiometric ¹³CO from ¹³COgen, were unable to undergo transmetalation and reductive elimination to trifluoroacetophenone. Instead, in the presence of base and additional CO, these organometallic complexes readily underwent reductive elimination to the acid fluoride. Attempts to determine whether the acid fluoride could represent an intermediate for acetophenone production were unrewarding. Only in the presence of a boronic ester did we observe some formation of the desired product, although the efficiency of transformation was still low. Finally, we investigated the reactivity of four phosphine-ligated Pd^II(Ph)CF₃ complexes (Xantphos, DtBPF, tBu₃P, and triphenylphosphine) with carbon monoxide. With the exception of the tBu₃P-ligated complex, all other metal complexes led to the facile formation of trifluoroacetophenone. We also determined in the case of triphenylphosphine that CO insertion occurred into the Pd-Ar bond, as trapping of this complex with n-hexylamine led to the formation of n-hexylbenzamide.

Full text of DOI:10.1021/acs.organomet.9b00849

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Stoichiometric Studies on the Carbonylative Trifluoromethylation of
Aryl Pd(II) Complexes using TMSCF3 as the Trifluoromethyl Source
Katrine Domino, Martin B. Johansen, Kim Daasbjerg, and Troels Skrydstrup*
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ABSTRACT: We have performed a series of stoichiometric
studies in order to identify viable steps for a hypothetical catalytic
cycle for the palladium-mediated carbonylative coupling of an aryl
bromide with TMSCF3. Our work revealed that benzoyl Pd(II)
complexes bearing Xantphos or tBu3P as the phosphine ligands,
which are generated from the corresponding PdII(Ph)Br complexes
exposed to stoichiometric 13CO from 13COgen, were unable to
undergo transmetalation and reductive elimination to trifluor-
oacetophenone. Instead, in the presence of base and additional CO, these organometallic complexes readily underwent reductive
elimination to the acid fluoride. Attempts to determine whether the acid fluoride could represent an intermediate for acetophenone
production were unrewarding. Only in the presence of a boronic ester did we observe some formation of the desired product,
although the efficiency of transformation was still low. Finally, we investigated the reactivity of four phosphine-ligated PdII(Ph)CF3
complexes (Xantphos, DtBPF, tBu3P, and triphenylphosphine) with carbon monoxide. With the exception of the tBu3P-ligated
complex, all other metal complexes led to the facile formation of trifluoroacetophenone. We also determined in the case of
triphenylphosphine that CO insertion occurred into the Pd−Ar bond, as trapping of this complex with n-hexylamine led to the
formation of n-hexylbenzamide.
catalyzed carbonylative trifluoromethylation of aryl halides
INTRODUCTION
■
has been reported.9 As depicted in Scheme 1, Wu and co-
The development of mild and efficient methods for the
introduction of fluorine has received widespread attention, as
selective incorporation of fluorine-containing motifs into
pharmaceutically relevant molecules can be used for the fine-
tuning of chemical and biological activity.1 Trifluoromethyl
ketones represent one such motif exploited for preparing
potent enzyme inhibitors but are also used as building blocks
for the synthesis of fluorine-containing pharmaceuticals,
trifluoromethylated heterocycles, and trifluoromethylated
analogues of natural compounds.2 Despite their high interest,
the use of trifluoromethyl ketones is limited by the existing
methods for their synthesis, which suffer from substrate-
determined regioselectivity, low functional group tolerance,
and side-product formation resulting from the reactive
nucleophilic trifluoromethylating reagents applied.3
Over the past decade, palladium-catalyzed cross-coupling
reactions of aryl halides with trifluoromethyl nucleophiles have
been extensively studied as a viable approach for the
introduction of a trifluoromethyl group onto (hetero)aryl
ring systems. However, this transformation has proven to be
highly challenging owing to the unique properties of the CF3
group, which renders the reductive elimination step challeng-
ing.4 Only a few ligands have been successfully employed in
this transformation, including Xantphos,5 BrettPhos,4a tri-tert-
butylphosphine,6 dfmpe,7 and bis(di-tert-butylphosphino)-
ferrocene,8 promoting the reductive elimination by steric
and/or electronic effects. To date, only one palladium-
Scheme 1. Pd-Catalyzed Carbonylative
Trifluoromethylation by Wu and Co-workers
workers rely on the use of carbon monoxide at 20 bar pressure
in combination with (PPh3)3CuCF3 as the organometallic
trifluoromethylating agent, effectively allowing for the con-
struction of trifluoroacetophenones from aryl iodides.
Since the palladium-catalyzed cross-coupling of aryl
chlorides with TESCF3 has been demonstrated by the
Buchwald group,4a we questioned whether a related three-
component carbonylative cross-coupling of aryl halides with
carbon monoxide and a trifluoromethyl nucleophile could
represent a convenient approach for accessing aryl trifluor-
omethyl ketones. Particularly, the electron-withdrawing effects
Received: December 15, 2019
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of the carbonyl intermediate on the metal center should
facilitate the reductive elimination to the new C−C bond.10
Furthermore, the coordination of carbon monoxide to
palladium(II) acyl complexes has been previously reported to
lower the activation barrier for a subsequent reductive
elimination.11
Palladium-catalyzed carbonylative versions of Negishi,
Suzuki, Sonogashira, Stille, and Hiyama couplings have already
been established for the synthesis of nonsymmetrical
ketones.16 These transformations are generally considered to
proceed via a four-step catalytic cycle (Scheme 3): (1)
We also questioned whether the more easily accessible
Ruppert−Prakash reagent could be applied as the nucleophilic
trifluoromethyl source in combination with aryl bromides and
in the presence of only stoichiometric amounts of carbon
monoxide, which would not only increase the applicability of
this synthetic transformation but also allow for carbon isotope
labeling of the target compounds.12 Inspired by a copper-
catalyzed cross coupling of aryl iodides with TMSCF3, we used
potassium fluoride for its activation in combination with
trimethylborate. It is speculated that the reactive trifluor-
omethyl anions formed upon fluoride addition are stabilized by
the presence of trimethylborate,13 which also circumvents the
formation of inactive Pd−CF3 species.
Scheme 3. General Catalytic Cycle for Carbonylative Cross-
Coupling Reactions
After extensive attempts to optimize this transformation, the
most efficient conditions obtained with this catalytic system are
shown in Scheme 2. On application of Xantphos-Pd-G414
oxidative addition of the Pd0 center into the Ar−X bond to
generate LnPdII(Ar)X (B), (2) insertion of carbon monoxide
resulting in the palladium(II) acyl complex C, (3) trans-
metalation of LnPdII(COAr)X (C) with an organometallic
reagent to form LnPdII(COPh)R (D), and finally (4) reductive
elimination to the unsymmetrical ketone and regeneration of
LnPd0 (A).
Scheme 2. Previous Attempts toward a Pd-Catalyzed
Trifluoroacetophenone Synthesis
This mechanism was proposed for the carbonylative
trifluoromethylation reported by Wu (Scheme 1), involving
aryl iodides, CO, and the organometallic reagent
(PPh3)3CuCF3. They demonstrated that exposure of the
isolated complex (PPh3)2Pd(Ph)CF3 formed by transmetala-
tion of the oxidative addition complex (PPh3)2PdII(Ph)I does
not insert CO with subsequent trifluoroacetophenone
formation (Scheme 4, top). In contrast, when the PdII acyl
complex (PPh3)2Pd(COPh)I is treated with (PPh3)3CuCF3,
trifluoroacetophenone is successfully generated in a 60% yield
(Scheme 4, bottom).9
a
Reactions were performed in a two-chamber setup. CO was released
from a solid precursor (see the Supporting Information for details).
b
Yields were determined by 19F NMR spectroscopy using
(trifluoromethoxy)benzene as an internal standard.
Scheme 4. Stoichiometric Experiments by Wu and Co-
workers9
along with 1.5 equiv of carbon monoxide released from COgen
in the two-chamber system COware, the carbonylative cross
coupling delivered up to 36% of the desired trifluoroaceto-
phenone with 10% of the side product 2 present. Without
trimethylborate, no fluorine-containing products were ob-
served. When Cs2CO3 was employed for the activation of
TMSCF3 instead of KF, the selectivity changed completely,
leading only to the bis(trifluoromethyl)carbinol 3 and its O-
trimethylsilylated derivative. Generally, the yields and product
distribution fluctuated between 10 and 20% in attempts under
identical conditions. Using strong bases, the formation of the
carboxylic acid fluoride was observed in some cases. In other
attempts, optimizing reagent ratios, temperature, solvents,
activating reagents, phosphine ligands, and Pd sources did not
provide increased yields or a more consistent performance of
the catalytic system. Furthermore, the reaction often delivered
mixtures of trifluoroacetophenone 1 accompanied by the side
products 2 and 3, thought to arise from the addition of the
trifluoromethyl anion to the desired product 1 to form the
bis(trifluoromethyl)carbinol 3,15 followed by an ensuing
alkoxycarbonylation to generate ester 2.
To gain insight into the reactivity of carbonylative cross-
coupling reactions with fluorinated organometallic reagents or
nucleophiles, we set out to perform a stoichiometric study of
the plausible intermediates in a hypothetical catalytic cycle. In
this paper, we report on our investigations on the feasibility of
each of the possible elementary steps employing the Ruppert−
Prakash reagent in combination with a fluoride source for its
activation. Since the ligands Xantphos, DtBPF, and tri-tert-
butylphosphine have previously been reported to support the
reductive elimination to aryl-CF3 from the corresponding
LPdII(Ph)CF3 complexes, and Xantphos is the only ligand
viable in the catalytic system, these ligands were chosen for our
B
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stoichiometric studies. In this work, we demonstrate new
reactivity of bidentate phosphine-ligated complexes of the type
LPdII(Ph)CF3 with the ligands Xantphos and DtBPF on
treatment with stoichiometric amounts of carbon monoxide.
With these complexes, CO insertion and reductive elimination
are sufficiently efficient under mild reaction conditions to
generate trifluoroacetophenone in high yields. On the other
hand, we uncover that the use of the monodentate ligand
PtBu3 only results in trace amounts of the desired ketone.
Furthermore, our investigations reveal that transmetalation of a
Pd(II) acyl complex with TMSCF3 activated by a fluoride
source is not a viable process.
such systems relies on the coordination strength of the
phosphine ligands. Palladium complexes with strongly bound
ligands successfully undergo transmetalation with CF3 anions,
while more weakly bonded ligands result in nonselective
exchange with halides and/or phosphine ligands.4a,5,19,20 To
investigate this event with the palladium(II) acyl complexes 5
and 7, a series of experiments was set up, the results of which
are revealed in Table 1. As can be seen in entry 1, when the
a
Table 1. Transmetalation Studies with LPd(COPh)Br
RESULTS AND DISCUSSION
■
Steps 1 and 2: Oxidative Addition of Bromobenzene
and Carbon Monoxide Insertion. The oxidative addition
complex of bromobenzene with Xantphos as the ligand was
prepared by treatment of Pd(dba)2 with 1 equiv of the
diphosphine ligand and excess bromobenzene in benzene at 80
°C for 4 h.17 Upon workup, the desired complex (Xantphos)-
Pd(Ph)Br (4) was obtained in a 61% yield. When complex 4
was subjected to 1.5 equiv of 13C-carbon monoxide in
acetonitrile at 40 °C for 1 h in the two-chamber system
COware, carbon monoxide insertion successfully resulted in
the formation of the palladium(II) acyl complex 5 with an 86%
conversion, as determined by the shift in the phosphine signal
from a singlet at 8.7 ppm to a doublet at 4.1 ppm in the 31P
NMR spectrum of the crude reaction mixture (eq 1).
b
entry
complex
variation
result
1
2
3
4
5
6
5
5
5
5
7
7
8 not obsd
8 not obsd
8 not obsd
8 not obsd
9 not obsd
9 not obsd
13CO (1.5 equiv)
B(OMe)3 (1.2 equiv)
without KF
13CO (1.5 equiv)
a
A representative procedure is provided in the Supporting
b
Information. The results were analyzed by 13C NMR, 31P NMR,
and 19F NMR spectroscopy.
(Xantphos)PdII acyl complex 5 was heated to 70 °C in
acetonitrile in the presence of 1.2 equiv of TMSCF3 and
equimolar amounts of potassium fluoride, the Pd−CF3
complex 8 was not observed. The signals in both the 31P
NMR and 13C NMR spectra indicated partial decomposition of
the PdII acyl complex, while by 19F NMR spectroscopy, 13% of
the recovered TMSCF3 provided the major signal. Similar
treatment of the (tBu3P)PdII acyl complex 7 (entry 5) did not
result in the generation of the corresponding Pd−CF3
complex, which should have presented itself by a signal at
approximately −27 ppm in the 19F NMR spectrum.6 When the
reaction was carried out in the presence of 1.5 equiv of 13C-
labeled carbon monoxide using compound 5, the formation of
Furthermore, formation of the PdII acyl complex was
confirmed by the appearance of a distinct triplet at 221.2
ppm in the 13C NMR spectrum arising from the metal-bound
13C-carbonyl group.
The oxidative addition complex containing P(tBu)3 as a
ligand was synthesized by stirring Pd(P(tBu)3)2 in excess
bromobenzene for 2 h at 70 °C, affording (tBu3P)Pd(Ph)Br
(6) in 75% yield after workup.18 Complex 6 behaved similarly
to the Xantphos-ligated complex 4, and full conversion to the
PdII acyl complex 7 was observed according to 31P NMR
spectroscopy accompanied by a doublet at 200.4 ppm in the
13C NMR spectrum when 6 was subjected to 1.5 equiv of 13C-
carbon monoxide at 40 °C for 1 h (eq 2).
8 was not detected. The disappearance of the signals in the 31
P
NMR and 13C NMR spectra and the formation of a complex
mixture according to the 19F NMR spectrum suggest
decomposition of the starting material. Treatment of the
palladium(II) acyl complex 7 under similar conditions did not
provide any improvement according to the NMR spectra in
comparison to the reaction without 13CO (entry 6).
Addition of 1.2 equiv of trimethylborate to the reaction
mixture of complex 5 with TMSCF3 did not aid the
transmetalation either, as only decomposition of the starting
material was observed by 13C NMR and 31P NMR spectros-
copy. There is literature precedence for a transmetalation event
between TMSCF3 with (Xantphos)Pd(Ph)F without activa-
tion of the silicon.5 Whether a similar event could occur with
the acyl complex 5 was tested by treating this organometallic
complex with 1.2 equiv of TMSCF3 and omitting the addition
of KF (entry 4). However, according to the 19F NMR
spectrum of the crude reaction mixture, the Pd−CF3 complex
8 was not formed. Only partial decomposition of the starting
material was observed.
Step 3: Transmetalation Studies. Previous reports have
established the feasibility of oxidative addition of palladium(II)
complexes to undergo transmetalation with nucleophilic
TMSCF3/fluoride systems.4a,b,19 The success of this step in
C
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All in all, these results indicate that the Pd−CF3 complexes
were not easily formed from the corresponding palladium(II)
acyl complexes. The results indicate that the Xantphos-ligated
complex is unstable when it is treated with TMSCF3 and
potassium fluoride, which is consistent with the previously
reported results by Grushin and Marshall, who observed the
facile displacement of Xantphos in attempting to transmetalate
[(Xantphos)Pd(Ph)I] with a similar mixture of TMSCF3 and
CsF.5
Since the addition of trimethylborate was previously found
to be necessary for the catalytic system to function, it was
hypothesized that potassium (trifluoromethyl)-
trimethoxyborate, K(MeO)3BCF3, could initially be formed
in the reaction mixture and subsequently act as the
trifluoromethylating agent. Use of K(MeO)3BCF3 as a CF3
source has previously been exploited by Gooβen and co-
workers in a Cu-catalyzed trifluoromethylation of aryl
iodides.21 However, when (Xantphos)PdII acyl complex 5
was treated with this reagent at 70 °C for 2 h (eq 3), no
bromide either; however, complexes 5 and 7 were both stable
under the reaction conditions. This lack of decomposition
could be explained by an equilibrium between either
(Xantphos)Pd(Ph)Br or (tBu3P)Pd(Ph)Br with (13CO)Pd-
(13COPh)Br in the presence of 13CO. However, analysis by
13C and 31P NMR spectroscopy of the reaction mixture in the
presence of 13CO did not reveal any new signals corresponding
to the latter complex.
In contrast, when the palladium(II) acyl complexes 5 and 7
were subjected to potassium fluoride under similar conditions,
formation of carboxylic acid fluoride 11 was obtained, as
revealed by a doublet at 17.3 ppm in the 19F NMR spectrum
and a doublet at 158.4 ppm in 13C NMR spectrum. Treatment
of the Xantphos complex 5 with potassium fluoride at 70 °C in
acetonitrile for 2 h provided a 10% conversion to 11, whereas
the conversion was increased to 22% by leaving the reaction
mixture for 18 h (Table 2, entries 1 and 2). When the reactions
Table 2. Reductive Elimination Studies to Benzoyl
a
Fluorides
transmetalated species was detected. Similarly, when the
experiment was repeated in the presence of 13C-carbon
monoxide, the PdII−CF3 complex 8 was not observed either.
On the other hand, a small amount of the corresponding
methyl benzoate was noted by 13C NMR spectroscopy (167.6
ppm)22 and GC-MS analysis, indicating the transfer of
methoxide instead of a trifluoromethyl group followed by
reductive elimination of the ester.23
Because the formation of Pd−CF3 complexes could not be
observed from the corresponding palladium(II) complexes 5
and 7 under the applied reaction conditions, we questioned
whether the role of the Pd-catalyzed carbonylation was to form
an activated intermediate, which upon subsequent reaction
with the Ruppert−Prakash reagent could generate the desired
aryl trifluoromethylketone. Inspired by the work of Arndtsen
and co-workers on the Pd-catalyzed synthesis of aroyl
chlorides,11 we initially tested if reductive elimination to
benzoyl bromide 10 could be a viable process.
b
complex
13CO (equiv)
time (h)
conversn to 11 (%)
5
5
5
5
7
7
7
7
0
0
1.5
1.5
0
0
1.5
1.5
2
18
2
18
2
18
2
18
10
22
68
>95
0
<5
15
61
a
A representative procedure is provided in the Supporting
b
Information. The results were analyzed by 13C NMR, 31P NMR,
and 19F NMR spectroscopy. Conversion was determined by 13C NMR
spectroscopy.
were performed in the presence of 1.5 equiv of 13C-carbon
monoxide, the yields were significantly improved, providing a
68% conversion in 2 h and full conversion when the reaction
mixture was heated for 18 h. In contrast, when the PdII acyl
complex 7 was subjected to potassium fluoride, no conversion
was noted after 2 h, and after an overnight experiment, only
trace amounts of benzoyl fluoride 11 could be detected.
Nevertheless, when the reaction mixture was exposed to 1.5
equiv of carbon monoxide, complex 7 provided a 15%
conversion to 11, and a significant increase to a 61%
conversion was obtained by prolonged heating of the reaction
mixture.
As illustrated in eq 4, heating palladium(II) acyl complexes 5
and 7 to 70 °C for 2 h in acetonitrile did not result in the
Reductive elimination of benzoyl fluoride from the
corresponding (PPh3)2Pd(COPh)F complex has previously
been reported by Grushin to occur at a temperature below 10
°C,25 and Pd-catalyzed carbonylative protocols for the
synthesis of aryl carboxylic acid fluorides have been similarly
developed.26 Furthermore, the necessity for carbon monoxide
to promote the reductive elimination has previously been
demonstrated by the Arndtsen group in their experimental and
formation of benzoyl bromide, as determined by the absence of
a carbonyl signal at approximately 166 ppm in the 13C NMR
spectrum.24 Instead, considerable decomposition of complex 5
was observed. Performing the reaction in the presence of 13C-
carbon monoxide did not result in the formation of benzoyl
D
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computational investigations of the mechanism for the Pd-
catalyzed synthesis of aroyl chlorides. In this work, it was
discovered that the reductive elimination did not take place
without the presence of carbon monoxide. Computational
calculations using PtBu3 as the ligand revealed that CO binding
to a free coordination site on the palladium complex prior to
reductive elimination could lower the activation energy for
reductive elimination of benzoyl chloride from 23.6 to 14.2
kcal/mol.11 Thus, the carboxylic acid fluoride could be a
plausible intermediate in a catalytic system (Scheme 2).
In 2018, Schoenebeck reported a decarbonylative trifluor-
omethylation of aroyl fluorides utilizing a Pd/Xantphos
catalyst system and TESCF3 as the trifluoromethyl source.29
By computational methods, they proposed that the Pd(II) acyl
complex (Xantphos)Pd(COPh)F is an intermediate efficiently
undergoing transmetalation with TESCF3 without external
activation to form (Xantphos)Pd(COPh)CF3. Moreover, they
found the activation energy for the reductive elimination step
from this Pd(II)−CF3 acyl complex to be lower than that for a
decarbonylation. With this, we speculated whether a similar
complex could be formed from an intermediate aryl acid
fluoride in our system. This suggestion was examined by
treating [1,1′-biphenyl]-4-carbonyl fluoride with TMSCF3 in
dioxane at 70 °C (Table 3). Without additives trace amounts
source of the product mixtures observed for this catalytic
system.
Synthesis of Phosphine-Ligated PdII(Ph)CF3 Com-
plexes. As we were not able to observe an efficient
transformation to trifluoroacetophenone from the attempted
transmetalation of the Pd(II) acyl complex with TMSCF3, we
speculated whether the reductive elimination to the target
compound could be achieved by an initial transmetalation
event with the oxidative addition complexes followed by CO
insertion. The trifluoromethyl tri-tert-butylphosphine complex
12 was prepared by treating ((o-tol)3P)Pd(OCOCF3)CF3 with
P(tBu)3, followed by the addition of diphenylzinc as reported
by Sanford et al. (eq 5),6 providing a 24% yield of the desired
complex 12. To synthesize the corresponding Xantphos
complex 13, we initially attempted to use a literature procedure
starting from (Xantphos)Pd(Ph)F.5 Unfortunately, the syn-
thesis of this fluoride complex resulted in decomposition of the
starting material or low conversion to the desired complex in
our hands. The (Xantphos)Pd(Ph)CF3 complex 13 is known
to reductively eliminate to produce trifluorotoluene when it is
heated to 50−80 °C, and therefore, it was important to
develop an alternative pathway under mild conditions for its
synthesis. Inspiration for this was drawn from the synthesis of
(DtBPF)Pd(Ph)CF3 reported earlier this year, in which
PdII(Ph)CF3 bearing labile 3-fluoropyridine ligands (complex
14) was used as an intermediate to allow ligand exchange at
room temperature.8 The Xantphos complex 13 was thus
prepared in a 91% yield by adapting this approach (eq 6).
Similarly, the 1,1′-bis(di-tert-butylphosphino)ferrocene com-
plex 15 could be generated in a 58% yield.
Table 3. Reactivity Studies of a Carboxylic Acid Fluoride
a
with TMSCF3
b
entry time (h)
X
additive (equiv)
yield 1/2/3 (%)
1
2
3
4
5
6
7
2
16
2
16
16
2
1.2
1.2
1.2
1.2
3.0
3.0
3.0
<5/0/0
<5/0/0
<5/0/0
<5/0/0
7/0/0
Pd(dba)2 (1) + Xantphos (1)
Pd(dba)2 (1) + Xantphos (1)
Pd(dba)2 (1) + Xantphos (1)
KF (1) + B(OMe)3 (3)
c
c
13/9/< 5
<5/38/31
16
KF (1) + B(OMe)3 (3)
a
A representative procedure is provided in the Supporting
b
Information. Yields were determined by 19F NMR spectroscopy
using (trifluoromethoxy)benzene as an internal standard. Reaction
c
performed at 80 °C in CPME/dioxane (2/1).
of trifluoroacetophenone 1 were formed (entries 1 and 2).
Similarly, in the presence of Pd(dba)2 and Xantphos in
stoichiometric amounts, only trace amounts of the trifluor-
omethylated ketone could be generated (entries 3 and 4).
When the amount of TMSCF3 was increased, 7% of 1 was
observed (entry 5). These results indicate that the Pd-
mediated trifluoromethylation of the carboxylic acid fluoride
is not a viable pathway at these temperatures. To further
elucidate the reactivity of the carboxylic acid fluoride in
relation to the catalytic system, it was treated with TMSCF3,
KF, and B(OMe)3 at 80 °C in the CPME/dioxane solvent
mixture for 2 h, resulting in formation of a mixture of
compounds 1−3. Furthermore, with a prolonged reaction time
of 16 h, 79% of the carboxylic acid fluoride was converted into
the side products 2 and 3, and only traces of the desired
trifluoroacetophenone 1 was observed (entry 7). As such,
formation of an intermediate aroyl fluoride species can be the
Finally, (PPh3)2PdII(Ph)CF3 was prepared from (PPh3)2Pd-
(Ph)F by treatment with TMSCF3 at rt for 1 h, following a
procedure reported by Grushin et al.5 As such, the
triphenylphosphine complex 16 was synthesized in an 83%
yield (eq 7).
E
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Step 4: CO Insertion and Reductive Elimination from
Phosphine-Ligated PdII(Ph)CF3 Complexes. Complexes
12, 13, and 15 are all known to reductively eliminate Ph−CF3
upon heating to 80 °C in benzene in the presence of additional
ligand.5,6,8 Interestingly, heating the Xantphos complex 13 in
benzene at 70 °C for 2 h in the presence of 1.5 equiv of 13C-
carbon monoxide resulted in the formation of 1,1,1-
trifluoroacetophenone (1a) in an 84% yield (Table 4, entry
Table 5. Competition Experiments between Reductive
Elimination and CO Insertion/Reductive Elimination from
a
(DtBPF)PdII(Ph)CF3
b
entry
variation
yield 1a/17 (%)
Table 4. Competition Experiments between Reductive
Elimination and CO Insertion/Reductive Elimination from
1
2
3
4
5
6
7
28/11
54/11
0/38
31/13
68/15
28/<5
>95/<5
18 h
a
(Xantphos)PdII(Ph)CF3
without 13CO
KF (1.2 equiv) added
KF (1.2 equiv) added, 18 h
40 °C
40 °C, 18 h
a
A representative procedure is provided in the Supporting
b
Information. Yields were determined by 19F NMR spectroscopy
using (trimethoxy)benzene as an internal standard.
b
entry
variation
yield 1a/17 (%)
Lowering the temperature to only 40 °C delivered a outcome
similar to that of the reaction at 70 °C, however with increased
selectivity toward trifluoroacetophenone 1a, while only traces
of 17 could be observed.
1
2
3
4
5
6
7
84/<5
0/35
without 13CO
Xantphos (1.2 equiv) added
KF (1.2 equiv) added
TMSCF3 (1.2 equiv) and KF (1.2 equiv) added
40 °C
71/<5
77/<5
81/<5
67/<5
97/0
Finally, when the reaction mixture was heated to 40 °C
overnight, full conversion to trifluoacetophenone 1a was
observed, thereby indicating that complex 15 is less stable at
70 °C than at 40 °C (entry 7). Nonetheless, it performs the
carbon monoxide insertion and reductive elimination effec-
tively, but with a lower rate than for the corresponding
Xantphos complex 13 (Table 4, entry 7).
40 °C, 18 h
a
A representative procedure is provided in the Supporting
b
Information. Yields were determined by 19F NMR spectroscopy
using (trimethoxy)benzene as internal standard.
Finally, the complex (tBu3P)PdII(Ph)CF3 (12) was tested
under similar reaction conditions. As shown in eq 8, when
1). The structure was confirmed by a distinct doublet at −71.4
ppm in the 19F NMR spectrum and a quartet at 179.8 ppm in
the 13C NMR spectrum along with only trace amounts of
α,α,α-trifluorotoluene (17). As summarized in Table 4, and as
expected, omission of 13CO did not lead to the formation of
trifluoroacetophenone 1a but instead led to a 35% yield of
trifluorotoluene 17. Generally, in reactions investigating
reductive eliminations, extra ligand is added to the reaction
mixture to stabilize the Pd0 complex and thus limit unwanted
side reactions.8 However, when Xantphos was added, a
decrease in the yield of 1a was observed (entry 3). Addition
of reagents such as potassium fluoride alone or in combination
with TMSCF3 did not change the reaction outcome (entries 4
and 5). When the temperature was lowered to 40 °C, a yield of
67% of 1a could be obtained in 2 h, whereas heating the
mixture to 40 °C for 18 h resulted in full conversion to the
trifluoroacetophenone 1a. Hence, the obtained results
demonstrate that (Xantphos)PdII(Ph)CF3 (13) effectively
undergoes CO insertion and reductive elimination when it is
subjected to carbon monoxide.
Further investigations revealed that the complex (DtBPF)-
PdII(Ph)CF3 (15) operated in a similar manner. As depicted in
Table 5, heating 15 to 70 °C in the presence of 13CO for 2 h
produced trifluoroacetophenone 1a in a 28% yield along with
an 11% yield of 17 as a side product. Increasing the reaction
time to 18 h improved the yield of 1a to 54%, while the
amount of α,α,α-trifluorotoluene remained unchanged (entry
2). When carbon monoxide was omitted, trifluorotoluene was
the sole product in the reaction, forming in a 38% yield (entry
3). Addition of potassium fluoride to the reaction mixture did
not have any influence on the yields (entries 4 and 5).
complex 12 was subjected to 13C-carbon monoxide at 70 °C
for 2 h, only trace conversion to the desired trifluoroaceto-
phenone 1a was noted. Traces of α,α,α-trifluorotoluene 17
were similarly observed along with complex 18 and side
product 19. These side products have been reported to arise
from complex 12 in the original thermolysis experiments by
Sanford and co-workers.6 They suggest that the side products
arise due to the empty site on palladium, which enables α-
fluoride elimination. The side products are released upon
transmetalation with Pd−CF3 or Pd−Ph and reductive
elimination. Since only trace amounts of trifluoroacetophenone
1a were observed, we conclude that complex 12 is inefficient
for CO insertion and reductive elimination of trifluoroaceto-
phenone 1a in comparison to complexes 13 and 15. As such,
this complex was not investigated further.
To conclude this study, we attempted to determine into
which of the two bonds (Pd−Ph vs Pd−CF3) CO was
inserting. As Wu reported that treatment of (PPh3)2PdII(Ph)-
CF3 did not result in formation of trifluoroacetophenone
(Scheme 4, top),9 we hypothesized that a slow reductive
elimination could be the cause of the low reactivity, and as
F
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such, this complex could represent an opportunity to examine
the Pd(II) acyl complex upon treatment with carbon
monoxide.
EXPERIMENTAL SECTION
■
Preparation of (Xantphos)Pd(Ph)Br (4). In an argon-filled
glovebox, a flame-dried 50 mL round-bottom flask was charged with
Pd(dba)2 (517.5 mg, 0.90 mmol), Xantphos (520.8 mg, 0.90 mmol, 1
equiv), benzene (25 mL), and bromobenzene (1.8 mL, 17.0 mmol, 19
equiv). The flask was sealed and brought out of the glovebox, and the
mixture was stirred at 80 °C for 4 h. The reaction mixture was filtered
through a pad of Celite with toluene and concentrated before Et2O
(10 mL) was added to force precipitation. The yellow solid was
isolated by filtration, washed with Et2O (3 × 10 mL), and dried under
vacuum to provide (Xantphos)Pd(Ph)Br (4; 459.2 mg, 61%) as a
yellow solid. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.61 (dd, J = 6.7,
2.6 Hz, 2H), 7.37−7.32 (m, 8H), 7.26−7.22 (m, 4H), 7.15−7.10 (m,
12H), 6.54 (bs, 2H), 6.28 (t, J = 7.2 Hz, 1H), 6.11 (t, J = 7.5 Hz, 2H),
1.79 (s, 6H). 13C NMR (101 MHz, CDCl3): δ (ppm) 157.2, 155.7 (t,
J = 5.7 Hz), 134.9 (t, J = 6.2 Hz), 134.6−134.5 (m), 132.1, 131.7 (t, J
= 22.3 Hz), 129.6, 129.2, 128.0 (t, J = 5.0 Hz), 127.2, 126.6, 124.4 (t,
J = 3.4 Hz), 122.2 (t, J = 21.1 Hz), 121.0, 36.3, 28.7. 31P NMR (162
Initially, the PdII−CF3 complex 16 was treated with 1.5
equiv of 13CO in benzene for 1 h. 13C NMR analysis revealed
the presence of a new quartet at 250.7 ppm indicating CO
insertion, whereas 19F NMR and 31P NMR spectroscopic
analysis revealed that full conversion of complex 16 was not
achieved. Upon treatment of the reaction mixture with N-
hexylamine, the 13C-carbonyl signal at 250.7 ppm disappeared
and new a singlet at 166.2 ppm appeared in the 13C NMR
spectrum of the reaction mixture, which is consistent with the
formation of the corresponding N-hexylbenzamide (eq 9).27
This preliminary result indicates that CO insertion takes place
into the Pd−Ph bond. In an attempt to obtain full CO
insertion, complex 16 was treated with 13C-labeled CO at 40
°C for 18 h. Surprisingly, instead of formation of a Pd(II) acyl
complex, a 66% yield of the trifluoroacetophenone 1a was
observed (eq 9). The formation of trifluoroacetophenone 1a
by subjecting complex 16 to carbon monoxide is in contrast to
the results reported by Wu and co-workers, who did not
observe formation of trifluoroacetophenone by treatment of
this complex with 20 bar of CO at 70 °C for 3 h. This
discrepancy in observations from the same initial complex may
be related to the pressure of CO used in the two experiments,
whereby higher pressures of CO may effectively surpress the
reductive elimination step.
MHz, CDCl3): δ (ppm) 8.7 (s, 2P). H, 13C, and 31P NMR spectra
1
are consistent with literature data.17
General Procedure 1: Preparation of L−PdII(COPh)Br
Complexes (5 and 7). In an argon-filled glovebox, in one chamber
of a two-chamber system was placed oxidative addition complex 4 or
6 (0.05 mmol) and solvent (0.5 mL). The second chamber was
charged with 9-methyl-9H-fluorene-9-carbonyl-13C chloride (18.3 mg,
0.08 mmol), HBF4P(tBu)3 (1.3 mg, 0.005 mmol, 6 mol %),
Pd(cod)Cl2 (1.3 mg, 0.005 mmol, 6 mol %), DMF (0.5 mL), and
Cy2NMe (32 μL, 0.15 mmol, 2 equiv). The chambers were
immediately sealed tightly with screw caps fitted with Teflon seals.
The two-chamber system was brought out of the glovebox, and the
contents were stirred in a preheated heating block at 40 °C for 1 h.
The two-chamber system was brought back into the glovebox, where
the reaction mixture was transferred directly to an NMR tube. The
NMR tube was sealed and brought out of the glovebox. A 31P NMR
spectrum was obtained to determine the conversion of starting
material to LPdII(COPh)Br complexes 5 and 7. 13C and 31P NMR
spectra for complexes 5 and 7 are available in the Supporting
Information. This mixture was used directly in the subsequent
reactions.
Preparation of (tBu3P)Pd(Ph)Br (6). In an argon-filled glovebox,
a flame-dried 8 mL vial was charged with Pd(PtBu3)2 (102.2 mg, 0.2
mmol) and bromobenzene (942 μL, 9.0 mmol, 45 equiv). The
reaction mixture was stirred at 70 °C for 2 h before the mixture was
transferred to a flame-dried 50 mL flask containing Et2O (8 mL) and
concentrated to a yellow oil under vacuum. The oily residue was
treated with pentane (12 mL) and placed in the glovebox freezer
(−35 °C) to force precipitation of a yellow solid that was collected by
filtration, washed with pentane, and dried under vacuum to provide
(tBu3P)Pd(Ph)Br (6; 69.5 mg, 75%) as a yellow solid. 1H NMR (400
MHz, THF-d8): δ (ppm) 7.22 (d, J = 7.7 Hz, 2H), 6.81−6.72 (m,
3H), 1.47 (d, J = 12.5 Hz, 27H). 31P NMR (162 MHz, THF-d8): δ
CONCLUSION
■
In summary, we have performed a series of stoichiometric
studies in order to identify viable steps for a hypothetical
catalytic cycle for the palladium-mediated carbonylative
coupling of an aryl bromide with TMSCF3. Our work revealed
that benzoyl Pd(II) complexes bearing Xantphos or tBu3P as
the phosphine ligands, being generated with stoichiometric
13CO from 13COgen from the corresponding aryl Pd(II)
bromide complexes were incapable of undergoing trans-
metalation and reductive elimination to trifluoroacetophenone.
Instead, in the presence of base and additional CO, these
organometallic complexes could undergo facile reductive
elimination to the acid fluoride. Attempts to determine
whether the acid fluoride could represent an intermediate for
acetophenone production were unrewarding. Only in the
presence of a boronic ester did we observe some formation of
the desired product, although the efficiency was still low.
Finally, we investigated the reactivity of four LnPd(Ph)CF3
complexes bearing phosphine ligands with carbon monoxide.
In all but one case, we observed the facile formation of
trifluoroacetophenone and also determined for one of the
complexes that CO insertion occurred into the Pd−Ar bond.
1
(ppm) 62.2 (s, 1P). H and 31P NMR spectra are consistent with
literature data.18
Preparation of Potassium (Trifluoromethyl)-
trimethoxyborate. In a flame-dried 50 mL flask were placed KF
(383.5 mg, 6.6 mmol), THF (15 mL), B(OMe)3 (0.90 mL, 8.1 mmol,
1.2 equiv), and TMSCF3 (1.04 mL, 7.1 mmol, 1.1 equiv) under an
argon atmosphere. The suspension was stirred at rt O/N. The
reaction mixture was concentrated to half volume under reduced
pressure, and dry pentane (11 mL) was added. The resulting colorless
precipitate was isolated by filtration, washed with pentane, and dried
under vacuum to provide potassium (trifluoromethyl)-
trimethoxyborate (903.9 mg, 60%) as a colorless solid. 1H NMR
(400 MHz, D2O): δ (ppm) 3.31 (s, 9H). 13C NMR (101 MHz,
D2O): δ (ppm) 48.9. CF3 was not resolved. 19F NMR (376 MHz,
1
D2O): δ (ppm) −74.7 to −74.9 (m, 3F). H, 19F, and 13C NMR
spectra are consistent with literature data.21,28
Preparation of (tBu3P)Pd(Ph)CF3 (12). In an argon-filled
glovebox, a flame-dried 20 mL vial was charged with [(o-tol)3P]2Pd-
(OC(O)CF3)(CF3) SI3 (150.0 mg, 0.15 mmol), PtBu3 (182.2 mg,
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0.90 mmol, 6 equiv), and THF (8.8 mL). The solution was cooled to
−35 °C in the glovebox freezer for 14 min. During this time, a flame-
dried 4 mL vial was charged with Ph2Zn (18.1 mg, 0.08 mmol, 0.55
equiv). After it was cooled, the reaction mixture was removed from
the freezer and Ph2Zn was immediately added. The vial containing
Ph2Zn was washed with the reaction solution and the wash was
recombined with the reaction mixture. The reaction was left at rt for 7
min and then moved to the glovebox freezer for 15 min. The solution
was concentrated under vacuum to give a yellow oil. The oily residue
was treated with pentane (2 mL), and the vial was shaken to force
precipitation of a yellow precipitate. The mixture was left in the
glovebox freezer for 5 min. The pentane layer was removed by
decantation, and the solid was washed with pentane (3 × 2 mL) and
Et2O (0.5 mL) and dried under vacuum to provide (tBu3P)Pd(Ph)-
147.6, 147.3 (d, J = 4.3 Hz), 140.3, 140.0, 139.8, 136.4, 127.1, 126.6
(d, J = 5.5 Hz), 125.9, 125.7, 123.5. The remaining quartet from Pd−
1
CF3 was not resolved by 13C NMR spectroscopy. H NMR and 19F
NMR spectra are consistent with literature data.8
Preparation of (DtBPF)Pd(Ph)(CF3) (15). In an argon-filled
glovebox, a flame-dried 25 mL round-bottom flask was charged with
(3-F-Py)2Pd(Ph)(CF3) (14; 150.0 mg, 0.34 mmol), 1.1′-bis(di-tert-
butylphosphino)ferrocene (162.5 mg, 0.34 mmol, 1.02 equiv), THF
(3.0 mL), and toluene (6.0 mL). The yellow solution was cooled to
−35 °C in the glovebox freezer for 15 min before the volatiles were
removed under vacuum. The remaining solids were redissolved in the
THF/toluene mixture and concentrated again under vacuum. This
process was repeated three times in total to remove 3-fluoropyridine.
The solids were collected by filtration, washed with pentane and Et2O,
and dried under vacuum to provide (DtBPF)Pd(Ph)CF3 (15; 142.4
mg, 58%) as a yellow solid. 1H NMR (400 MHz C6D6): δ (ppm) 8.10
(d, J = 7.8 Hz, 2H), 7.22−7.19 (m, 2H), 7.05 t, J = 7.2 Hz, 1H), 4.13
1
(CF3) (12; 16.2 mg, 24%) as a yellow solid. H NMR (400 MHz,
C6D6): δ (ppm) 7.66 (d, J = 7.7 Hz, 2H), 6.98 (t, J = 7.5 Hz, 2H),
6.88 (t, J = 7.2 Hz, 1H), 0.95 (d, J = 12.1 Hz, 27H). 31P NMR (162
MHz, C6D6): δ (ppm) 54.5 (q, J = 39.6 Hz, 1P). 19F NMR (376
1
(s, 4H), 3.95 (s, 4H), 1.37−1.19 (m, 36H). H NMR (400 MHz,
1
MHz, C6D6): δ (ppm) −28.3 (d, J = 40.0 Hz, 3F). H, 31P, and 19F
CD2Cl2): δ (ppm) 7.67 (d, J = 7.5 Hz, 2H), 6.69 (t, J = 7.5 Hz, 2H),
6.87 (t, J = 7.3 Hz, 1H), 4.41 (s, 8H), 1.41−1.15 (m, 36H). 31P NMR
(162 MHz, C6D6): δ (ppm) 38.0 (broad resonance, 1P), 32.5 (broad
resonance, 1P). 19F NMR (376 MHz, C6D6): δ (ppm) −13.3 (t, J =
31.5 Hz, 3F). 1H, 31P, and 19F NMR spectra are consistent with
literature data.8
NMR spectra are consistent with literature data.6
Preparation of (Xantphos)Pd(Ph)CF3 (13). In an argon-filled
glovebox, a flame-dried 25 mL round-bottom flask was charged with
(3-F-Py)2Pd(Ph)CF3 (14; 77.0 mg, 0.17 mmol), Xantphos (101.8
mg, 0.18 mmol, 1.02 equiv), THF (1.5 mL), and toluene (3.0 mL).
The yellow solution was cooled to −35 °C in the glovebox freezer for
15 min before the volatiles were removed under vacuum. The solid
was dissolved in a THF/toluene mixture (1/2) and concentrated
under vacuum again. This process was repeated three times in total to
remove the 3-fluoropyridine. The remaining solids were collected by
filtration, washed with pentane and Et2O, and dried under vacuum to
provide (Xantphos)Pd(Ph)CF3 (13; 130.1 mg, 91%) as a pale yellow
Preparation of (Ph3P)2Pd(Ph)CF3 (16). In an argon-filled
glovebox, a flame-dried 8 mL vial was charged with (Ph3P)2Pd(Ph)
F (SI5; 190.0 mg, 0.26 mmol), benzene (3 mL), and TMSCF3 (384
μL, 2.6 mmol, 10 equiv). The reaction mixture was stirred at rt for 1
h. The solution was concentrated, and pentane (3 mL) was added to
force precipitation. The solids were collected by filtration, washed
with pentane, and dried under vacuum to provide (Ph3P)2Pd(Ph)CF3
(16; 167.3 mg, 83%) as a colorless solid containing trace amounts of
benzene, which could not be removed by prolonged drying under
vacuum. 1H NMR (400 MHz, CD2Cl2): δ (ppm) 7.45−7.41 (m,
12H), 7.38−7.34 (m, 6H), 7.29−7.25 (m, 12H), 6.52 (d, J = 7.4 Hz,
2H), 6.28 (t, J = 7.3 Hz, 1H), 6.15 (t, J = 7.3 Hz, 2H). 31P NMR (162
MHz, CD2Cl2): δ (ppm) 26.6 (q, J = 13.3 Hz, 2P). 19F NMR (376
MHz, CD2Cl2): δ (ppm) −17.0 (t, J = 13.6 Hz, 3F). 31P and 19F
NMR spectra are consistent with literature data.5,29
1
solid. H NMR (400 MHz, CD2Cl2): δ (ppm) 7.68 (bs, 2H), 7.31−
6.91 (m, 26H), 6.69−6.68 (m, 3H), 1.79 (s, 6H). 31P NMR (162
MHz, CD2Cl2): δ (ppm) 18.0 (app q, J = 14.0 Hz, 2P, trace trans-13),
12.6 (bs, 1P, cis-13), 9.1−7.7 (m, 1P, cis-13). 19F NMR (376 MHz,
CD2Cl2): δ (ppm) −14.3 (t, J = 15.8 Hz, trace trans-13), −15.8 to
−16.0 (m, 3F, cis-13). 31P NMR (162 MHz, C6D6): δ (ppm) 17.6
(app q, J = 15.6 Hz, 2P, trans-13), 9.6 (br m, 1P, cis-13), 5.1−3.9 (br
m, 1P, cis-13). 19F NMR (376 MHz, C6D6): δ (ppm) −13.1 (t, J =
16.2 Hz, 3F, trans-13), −14.6 to −14.8 (br m, 3F, cis-13). 31P and 19
F
NMR spectra in C6D6 are consistent with literature data.5
ASSOCIATED CONTENT
■
Preparation of (3-F-Py)2Pd(Ph)(CF3) (14). In an argon-filled
glovebox, a flame-dried 20 mL vial containing a stir bar was charged
with [(o-tol)3P]2Pd(OC(O)CF3)(CF3) (SI3; 620.0 mg, 0.62 mmol),
3-fluoropyridine (853 μL, 9.93 mmol, 16 equiv), and THF (8.2 mL).
The mixture was cooled to −35 °C in the glovebox freezer for 15 min.
During this time, Ph2Zn (74.9 mg, 0.34 mmol, 0.55 equiv) was
weighed out into a flame-dried 4 mL vial. The reaction mixture was
removed from the freezer, upon which the preweighed Ph2Zn was
immediately added, resulting in a homogeneous solution. The vial
containing Ph2Zn was washed with the reaction mixture, and the wash
was recombined with the reaction mixture. The vial was sealed,
brought out of the glovebox, and stirred at rt for 20 min. The vial was
opened in air, H2O (1 mL) was added, and the mixture was stirred for
an additional 20 min. The solution was filtered through a pad of
Celite with dichloromethane, concentrated under reduced pressure,
and transferred to a separatory funnel with dichloromethane (20 mL).
The organic layer was washed with H2O (2 × 20 mL), dried over
Na2SO4, filtered, and concentrated under reduced pressure. The
remaining oil was treated with hexane (30 mL), resulting in formation
of a yellow precipitate. The solids were isolated by filtration, washed
with hexane (30 mL) and Et2O (2 mL), and dried under vacuum to
provide (3-F-Py)2Pd(Ph)(CF3) (14; 155.6 mg, 56%) as a colorless
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00849.
Experimental procedures, compound characterization,
and NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Troels Skrydstrup − Carbon Dioxide Activation Center
(CADIAC), Department of Chemistry, and the Interdisciplinary
Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus
C, Denmark; orcid.org/0000-0001-8090-5050;
Email: ts@chem.au.dk
Authors
Katrine Domino − Carbon Dioxide Activation Center
(CADIAC), Department of Chemistry, and the Interdisciplinary
Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus
C, Denmark
Martin B. Johansen − Carbon Dioxide Activation Center
(CADIAC), Department of Chemistry, and the Interdisciplinary
Nanoscience Center (iNANO) and Department of Engineering,
Aarhus University, 8000 Aarhus C, Denmark; orcid.org/
0000-0003-1638-6692
1
solid. H NMR (400 MHz, CD2Cl2): δ (ppm) 8.63−8.62 (m, 1H),
8.56 (app d, J = 5.2 Hz, 1H), 8.25−8.24 (m, 1H), 8.21 (app d, J = 5.4
Hz, 1H), 7.60−7.56 (m, 1H), 7.46−7.41 (m, 4H), 7.26 (app dt, J =
8.5, 5.3 Hz, 1H), 6.94 (t, J = 7.3 Hz, 2H), 6.87 (d, J = 7.2 Hz, 1H).
19F NMR (376 MHz, CD2Cl2): δ (ppm) −21.2 (s, 3F), −122.2 (s,
1F), −122.72 (bs, 1F). 13C NMR (101 MHz, CD2Cl2): δ (ppm)
161.4 (d, J = 22.2 Hz), 158.9 (d, J = 22.3 Hz), 153.5 (q, J = 9.8 Hz),
H
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(4) (a) Cho, E. J.; Senecal, T. D.; Kinzel, T.; Zhang, Y.; Watson, D.
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Kim Daasbjerg − Carbon Dioxide Activation Center
(CADIAC), Department of Chemistry, and the Interdisciplinary
Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus
C, Denmark; orcid.org/0000-0003-0212-8190
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.9b00849
Notes
The authors declare the following competing financial
interest(s): T.S. is co-owner of SyTracks A/S, which
commercializes the two-chamber system (COware) and 13C-
COgen.
(6) Ferguson, D. M.; Bour, J. R.; Canty, A. J.; Kampf, J. W.; Sanford,
M. S. Stoichiometric and Catalytic Aryl−Perfluoroalkyl Coupling at
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M. S. Aryl−CF3 Coupling from Phosphinoferrocene-Ligated
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ACKNOWLEDGMENTS
■
We thank the Danish National Research Foundation (grant no.
DNRF118), NordForsk (grant no. 85378), and Aarhus
University for financial support.
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