New Vistas in Transmetalation with Discrete “AgCF3” Species: Implications in Pd-Mediated Trifluoromethylation Reactions

Article abstract of DOI:10.1002/chem.201802586

This work describes the employment of discrete “AgCF₃” complexes as efficient transmetalating agents to Pd^II to surmount overlooked challenges related to the transmetalation step in Pd-catalyzed trifluoromethylation processes. We report the participation of a unique silver ate (Cs)[Ag(CF₃)₂] complex, under stoichiometric and catalytic conditions, in the unprecedented one-pot formation of PhCF₃ using PhI as starting material. Moreover, we show that the transmetalation step, which is often ignored in these transformations, can also determine the success or failure of the coupling process.

Full text of DOI:10.1002/chem.201802586

A Journal of
Accepted Article
Title: New vistas in transmetalation with discrete "AgCF₃" species:
Implications in Pd-mediated trifluoromethylation reactions
Authors: Monica Helvia Perez-Temprano, Sara Martínez de Salinas,
Angel Luis Mudarra, Jordi Benet-Buchholz, Teodor Parella,
and Feliu Maseras
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
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to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201802586
Link to VoR: http://dx.doi.org/10.1002/chem.201802586
Supported by

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COMMUNICATION
New vistas in transmetalation with discrete “AgCF3” species:
Implications in Pd-mediated trifluoromethylation reactions
Sara Martínez de Salinas,[a] Ángel L. Mudarra,[a],[b] Jordi Benet-Buchholz,[a] Teodor Parella,[c] Feliu
Maseras, [a],[d] Mónica H. Pérez-Temprano*[a]
Abstract: This work describes the employment of discrete “AgCF3”
complexes, including unique (Cat)[Ag(CF3)2] salts, as efficient
transmetalating agents to PdII in order to tackle some of the usually
overshadowed limitations related to this step within the
trifluoromethylation area.
benchmark complex and their comparison to commercially
available nucleophilic reagents; and 2) the high efficiency of
[Ag(CF3)2]– species, only detected by NMR spectroscopy to date
and whose reactivity has been unrecognized for decades,7b-d in
one of the few productive PdII systems, in which the
transmetalation has been pointed out as a challenging step.
Over the past few years, organosilver(I) intermediates have
demonstrated their potential as nucleophilic coupling partners in
Potential unproductive transmetalations in Pd0/II-catalyzed Ar−CF3 couplings
◼
1
Ligand Displacement
Pd-catalyzed transformations.
However, their ability as
Mismatched Transmetalation
−
“ CF3
”
LnPdAr(CF3)
Ar
X
CF3
Ar
Ar
transmetalating agents is far from being fully exploited, most
likely due to their instability (e.g. photosensitivity).2 Therefore,
important fundamental questions such as the scope of the
transferred group or the reactivity of silver(I) ate complexes
remains essentially unexplored. In this context, a particularly
interesting test case is the synergistic Ag/Pd cooperation for the
transfer of a CF3 moiety. This group is a prevalent structural
motif in high-value molecules and organometallic scaffolds due
to its unique capability to modify physicochemical and/or
biological properties.3 A priori, the design of new CF3 shuttles for
their use in Pd0/II-catalyzed aryl trifluoromethylation could seem
unnecessary since the reductive elimination step is considered
the central problem associated with these processes.4 However,
a close look at the literature reveals that the transmetalation step
can also dramatically hamper the C–CF3 bond-forming reaction
(Figure 1). A slow nucleophilic trifluoromethylation leads to
undesired reactions by “mismatched” group exchanges.4a-b,d-e,h,5
Furthermore, CF3– ions can displace the stabilizing ligands on Pd,
forming inactive PdII(CF3)n species.4a, d-e, h-i,6
[LmPd(CF3)x(Ar)]
+
LnPd
LnPd
LnPd
X
L
L
Ph
“AgCF3”
Pd
(i)
Ph−I
I
◼
This work (i, ii and iii)
Synthesis of well-defined “AgCF3” ,
including unique [Cat][Ag(CF3)2] salts
L L = dppp
[LnPd0]
(iii)
(ii)
Xantphos
Relative reactivity towards a benchmark
system and further translation to
stoichiometric/catalytic coupling
L
L
Ph
Ph−CF3
Pd
CF3
Figure 1. Exploration of trifluoromethylsilver(I) nucleophiles as efficient CF3
shuttle to PdII systems.
We started our study by exploring the relative reactivity of
“AgCF3” complexes towards a PdII model system (Scheme 1).
Several considerations were taken into account for this initial
investigation. Firstly, (dppp)Pd(Ph)I (1) was selected as
benchmark complex since it contains a strong coordinating
Intrigued by these overlooked challenges, we envisioned to
surmount these limitations by exploring “AgCF3” complexes as
selective and rapid CF3 shuttles to PdII. We support our
hypothesis on the well-known lability of the Ag–CF3 bond of in
situ generated “AgCF3” species which readily form a silver(I) ate
[Ag(CF3)2]– complex through a CF3 exchange reaction in polar
solvents.7 Herein, we reveal the exceptional transmetalating
activity of well-defined isolated “AgCF3” compounds to PdII metal
centers (Figure 1), including: 1) their relative reactivity to a
ligand
that
prevents
the
formation
of
inactive
poly(trifluoromethyl)palladium compounds and the resulting
product (2) does not undergo Ph–CF3 coupling.8 Secondly, only
well-defined isolable trifluoromethylsilver(I) complexes were
targeted to avoid potential reproducibility issues associated with
in situ generated species. Finally, we pursued fast I-for-CF3
exchanges (< 30 minutes), to minimize possible undesired by-
products related to long reaction times.
Ph Ph
P
Ph Ph
P
[a]
Dr. S. Martínez de Salinas, A. L. Mudarra, Dr. J. Benet-Buchholz,
Prof. F. Maseras, Dr. M. H. Pérez-Temprano
Institute of Chemical Research of Catalonia (ICIQ)
The Barcelona Institute of Science and Technology (BIST)
Avgda. Països Catalans 16, 43007 Tarragona (Spain)
E-mail: mperez@iciq.es
Ph
I
Ph
THF, rt, Ar
< 30 min
+
“AgCF3”
+ “AgI”
Pd
Pd
P
P
CF3
Ph Ph
1
Ph Ph
2
benchmark PdII system
rapid transmetalation
well-defined isolable “AgCF3”
[b]
A. L. Mudarra
Scheme 1. Requirements for the initial study
Departament de Química Analítica i Química Orgànica, Universitat
Rovira i Virgili,
C/ Marcel·li Domingo s/n, 43007 Tarragona (Spain)
Dr. T. Parella
Servei de RMN, Facultat de Ciències
Universitat Autònoma de Barcelona, 08193 Bellaterra (Spain)
Prof. F. Maseras
Departament de Química, Facultat de Ciències
Universitat Autònoma de Barcelona, 08193 Bellaterra (Spain)
[c]
[d]
Before exploring the CF3 group transfer from Ag to
(dppp)Pd(Ph)I, we established as touchstone the
trifluoromethylation of 1 with the widely used nucleophilic
trifluoromethyl sources R3SiCF3 (R = Me,9 Et4b,h-i) in combination
with CsF. After 30 minutes, we only observed 2 in 19% and
traces using Me3SiCF3 and Et3SiCF3, respectively (Figure 2a).10
With these results as a reference, we then focused on the
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201802586
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activity of the scarce examples of isolated trifluoromethylsilver(I)
compounds reported in the literature to date: SIPrAgCF311 and
(bathophenanthroline)Ag(CF3).12 The treatment of (dppp)Pd(Ph)I
with 1.5 equiv of SIPrAgCF3 (3)13 resulted in <5% yield of
(dppp)Pd(Ph)(CF3) after 30 minutes (Figure 2a). We discarded
the exploration of the other known LAgCF3, bearing the
bathophenanthroline ligand,12 due to stability issues under our
reaction conditions.14 Notably, after some experimentation, using
bathocuproine, we were able to prepare 4 in THF, as a mixture
of (Bc)Ag(CF3) (4a) in equilibrium with an ionic [Ag(CF3)2]–
species (4b) (Figure 2b). Initially, we hypothesized that the
structure of 4b was [(Bc)2Ag][Ag(CF3)2], by analogy to related
copper compounds.15 However, a detailed NMR spectroscopic
analysis, including DOSY experiments, confirmed the absence
of [(Bc)2Ag]+ as the cation of 4b, on the basis of the higher
hydrodynamic radius measured for [(Bc)2Ag](SbF6) (5) (6.86 Å),
Inspired by the extraordinary transmetalating ability of 4, and
prompted by its different behavior from 3, we wondered whether
[Ag(CF3)2]– could be a non-innocent spectator, and participate as
CF3 shuttle.7 To unravel this key question, we targeted the
synthesis of two well-defined (Cat)[Ag(CF3)2] salts, Cat = NBu4
(8NBu4) or Cs (8Cs) to evaluate their relative stability and reactivity.
As shown in Figure 3, the reaction of AgOAc with 4 equiv of
Me3SiCF3, in THF at room temperature, in the presence of 4
equiv of KF and 1 equiv of NBu4OAc afforded a white crystalline
solid, 8NBu4, in 83% isolated yield. Following a similar synthetic
route, but using 2 equiv of CsF instead of the combination
KF/NBu4OAc, we synthesized (Cs)[Ag(CF3)2] in 85% yield as a
yellow solid. The structures of both salts, that can be stored for
months at -30 ºC under inert atmosphere in the dark,18 were
unambiguously confirmed by NMR spectroscopy, ESI-MS and
single crystal X-ray diffraction. It is worth mentioning the different
compared to 4 (4.33 Å). Challenged by this unexpected outcome, bonding situation between these ionic species. The X-ray
we performed computational studies which suggested,16 as the
most stable cation for 4b, a structure that contains the silver
center coordinated to a bathocuproine ligand along with two
molecules of THF, and a second Bc bound to the system
through stabilizing p-p interactions. 17 In line with the DFT
calculations, we observed the formation of [(Bc)Ag(THF)](SbF6)
(6), characterized by X-ray diffraction, upon exposure of 5 to
THF. With the structural information of 4a/4b in hand, we
assessed the reactivity of this equilibrium mixture. Gratifyingly,
the trifluoromethylation of (dppp)Pd(Ph)I with 1.5 equiv of 4
proceeded cleanly and quantitatively in 10 minutes to afford 2
and an iodo-bridged dimeric compound with Ag–Ag interactions
(7) (Figure 2a).
structure of 8NBu4 shows a linear bis(trifluoromethyl)argentate
paired together with the NBu4 cation. In sharp contrast, 8Cs
presents a rather unique structure, with the silver atoms forming
linear chains, and the cesium cations interacting with twelve
19
different fluorine atoms.
Having synthesized and fully-
characterized these singular ionic species, we next investigated
their efficiency as nucleophilic trifluoromethyl sources. To our
delight, the reaction of (dppp)Pd(Ph)I with 0.75 equiv of either of
the two silver salts resulted in the quantitative formation of 2 in
20
10 minutes, when using 8Cs, and in 93% yield for 8NBu4
.
Me3SiCF3 (4 equiv)
◼ Synthesis
KF (4 equiv)
Me3SiCF3 (4 equiv)
NBu4OAc (1 equiv)
CsF (2 equiv)
(NBu4)[Ag(CF3)2]
8NBu4 (83%)
AgOAc
(Cs)[Ag(CF3)2]
8Cs (85%)
THF, rt, 22 h, Ar
THF, rt, 19 h, Ar
(a)
touchstone
Cs
Cs
Cs
Ph Ph
P
Ph Ph
P
R3SiCF3/CsF
(2 /4 equiv)
30 min
2 (quantitative)
Cs
Cs
F
C
Ph
Ph
I
4 (1.5 equiv)
F
+
F
Pd
Pd
F
F
F
F
10 min
ꢀ
C
C
F
P
CF3
P
F
Me
Ph
Ph
Ph Ph
2
Ph Ph
1
I
N
N
8NBu4
Ag
Ag
Ag
Ag
19% for R = Me
traces for R = Et
2
8Cs
SIPrAgCF3
Me
7
(3, 1.5 equiv)
◼ Reactivity
Ph Ph
30 min
Ph Ph
traces of 2
(Bc)AgCF3
+ (Cat)[CF3AgI]
P
Ph
I
P
Ph
(Cat)[Ag(CF3)2] (8, 0.75 equiv)
Pd
Pd
(b)
THF, rt, 10 min, Ar
+ (Cat)[AgI2]
P
P
CF3
Me3SiCF3 (2 equiv)
4a
1
2
Ph Ph
Ph Ph
bathocuproine (1 equiv)
AgF
93% for 8NBu4
100% for 8Cs
THF, rt, 72 h, Ar
80 %
(3 equiv)
[Cat][Ag(CF3)2]
4b
4a
▪ 4a and 4b are observed by 19F NMR
Elucidation ofthe nature of the cation of 4b
Figure 3. Synthesis, Characterization and Reactivity of (Cat)[Ag(CF3)2] (Cat =
NBu4, Cs)
Ph
Ph
Ph
Ph
N
N
Ph
Next, we aimed at evaluating the real potential of our silver
nucleophiles, focusing our attention on one of the few PdII
systems which affords relatively facile PhCF3 coupling. In 2006,
Grushin et al. reported the first example of C–CF3 bond-forming
reductive elimination from an isolated Xantphos-based PdII
derivative, synthetized by treatment of (Xantphos)Pd(Ph)F with
Me3SiCF3.4a In this work and subsequent elegant mechanistic
studies,4d the authors explained in detail, not only the challenges
associated to the reductive elimination from this system, but also
the appealing difficulties related to unfruitful attempts to achieve
N
N
N
N
Ag
Ag
N
N
Ph
Solv
Cation of 6
5
Solv
Ph
Initial proposal
(discarded by 1H DOSY NMR)
Our current proposal
(supported by DFT and experimental evidences)
Figure 2. (a) Reactivity of Me3SiCF3, Et3SiCF3, 3 and 4 towards 1. (b)
Synthesis and characterization of 4.
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Stoichiometric conditions
the nucleophilic trifluoromethylation of (Xantphos)Pd(Ph)I (9)
using Me3SiCF3/CsF, such as: i) ligand displacement by CF3 ;
Pd(dba)2/Xantphos (1/1 equiv)
–
(Cs)[Ag(CF3)2] (0.75 equiv)
and/or ii) the formation of unproductive Ph–Ph homocoupling.
Encouraged by our previous results, we envisioned that our
silver nucleophiles could overcome these shortcomings and
provide the unprecedented formation of PhCF3 using 9 as
starting material (Scheme 2). Following the same strategy used
for 1, we first defined as touchstone the reactivity of 9 with
R3SiCF3/F– (R = Me, Et), under comparable reaction conditions
to those reported previously for the high-yielding formation of
PhCF3 from (Xantphos)Pd(Ph)(CF3) (10).4a,d As expected, we
observed the formation of the coupling product in low yields in
the presence of CsF (14% and 20%, for Me3SiCF3 and Et3SiCF3
respectively). Then, we examined the reactivity of 9 with our
most efficient Ag–CF3 sources, 4a,21 8NBu4 and 8Cs. We were
pleased to observe that all these transformations led to the
Toluene, 95 ºC, 3 h, Ar
91%a
I
CF3
11
Pd(dba)2/Xantphos (0.2/0.2 equiv)
(Cs)[Ag(CF3)2] (1 equiv, slow addition)
(excess)
Toluene/THF (3:1), 95 ºC, 5 h, Ar
56%a
Catalytic conditions
Scheme 3. Stoichiometric and catalytic reactions. 19F NMR analysis using
fluorobenzene as internal standard.
a
In summary, this work presents the potential of discrete “AgCF3”,
including a unique (Cs)[Ag(CF3)2] salt, as CF3 shuttle to PdII
systems. Our results, not only provide the first reported example
of stoichiometric and catalytic one-pot formation of Ph–CF3
starting from PhI, but also confirm the crucial role of the
nucleophile in the transmetalation step, which can be decisive,
enabling or preventing the product formation. Further work
towards unravelling the potential of silver nucleophiles as
transmetalating agents is currently ongoing in our laboratory.
targeted product in moderate to excellent yields (4: 70%,22 8NBu4
:
42%, and 8Cs: 84%). The lower reactivity of (Bc)Ag(CF3) and
(NBu4)[Ag(CF3)2] can be ascribed to unproductive pathways:23
ligand scrambling between both metals which affords
(Bc)Pd(Ph)(CF3)
for
4,
and
the
formation
of
poly(trifluoromethyl)complexes and decomposition to AgIII for
18,23
8NBu4
.
For 8Cs, we corroborated the rapid and selective CF3
transfer by observing full conversion of 9 into 10 in less than 10
minutes in C6H6 at room temperature.
Acknowledgements
Entry CF3 source (equiv) 11 (%)b
We thank the CERCA Programme/Generalitat de Catalunya and
the Spanish Ministry of Economy, Industry and Competitiveness
(MINECO: CTQ2017-87792-R, CTQ2016-79942-P, CTQ2015-
64436-P, AIE/FEDER, EU, and Severo Ochoa Excellence
Accreditation 2014-2018, SEV-2013-0319) for the financial
support. S. M. S. thanks Severo Ochoa Excellence Accreditation
for post-doctoral contract. A. L. M. thanks La Caixa-Severo
Ochoa programme for a predoctoral grant. We thank to the
Research Support Area of ICIQ. The authors also thank Prof.
Rubén Martín and Dr. Alex Shafir for useful discussions.
“CF3” (XX equiv)
CF3
11
1
2
3
4
5
Me3SiCF3/CsF (2/4)
Et3SiCF3/CsF (2/4)
4a (1.5)
14
20
70
42
84
Xantphos (1 equiv)
O
I
C6H6, 90 ºC
3 h, Ar
Ph2P
PPh2
Pd
8NBu4 (0.75)
9
Ph
8Cs (0.75)
Scheme 2. Thermolysis of
9
with different “CF3–” sources. aReaction
conditions: 9 (0.006 mmol), Xantphos (0.006 mmol), C6H6 (0.01 M) under Ar. b
19F NMR analysis using fluorobenzene or 4,4’-difluorobiphenyl as internal
standards.
On the basis of these promising data, we next examined the
compatibility of 8Cs with all elementary steps involved in the
catalytic cycle, under stoichiometric conditions in the presence
of excess of PhI. As previously described, the accumulation of
the oxidative addition product, (Xantphos)Pd(Ph)I, could favor
the mismatched transmetalation shown in Figure 1. Delightfully,
using Pd(dba)2 as Pd0 source and 30 equiv of PhI, we observed
the desired product (11) in slightly higher yield (91%) when
compared to the entry 5 of Scheme 2 (Scheme 3), along with the
regeneration of the oxidative addition product.24 This result
points out the capability of 8Cs for precluding the side-product
formation that could potentially lead to dead-end routes under
catalytic conditions using Xantphos as ligand. Indeed, as a
proof-of-concept, preliminary results show the formation of 11 in
56% yield by slow addition of 8Cs under catalytic conditions in
the presence of 60 equiv of PhI (Scheme 3).25
Abbreviations
Dppp = 1,3-Bis(diphenylphosphino)propane
Bc = Bathocuproine
SIPr = Bis(1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)
Keywords: silver • transmetalation • trifluoromethylation •
organometallic synthesis • structural elucidation
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Chemistry - A European Journal
COMMUNICATION
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to HCF3 and other by-products in polar coordinating solvents such as
THF or DMF. See SI for further details.
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solvent. Full computational details provided in the Supporting
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in the ioChem-BD repository M. Alvarez-Moreno, C. de Graaf, N. Lopez,
F. Maseras, J. M. Poblet, C. Bo, J. Chem. Inf. Model. 2015, 55, 95.
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than any of the computed cationic alternatives. Indeed, the comparison
between the calculated/experimental ratio for 4a:4b, 92:8/98:2 in THF
and 58:42/66:34 in DMF, also supports the assignment for 4b-cat. See
SI for further details.
[5]
Other
potential
“mismatched”
group
exchanges
involve
disproportionation of (Ln)Pd(Ar)X and/or (Ln)Pd(Ar)(CF3), for
representative examples, see: a) V. V. Grushin, J. William, W. J.
Marshall, J. Am. Chem. Soc. 2009, 131, 918; b) M. S. Remy, T. R.
Cundari, M. S. Sanford, Organometallics, 2010, 29, 1522; c) S.-L.
Zhang, Z.-Q. Deng, Phys. Chem. Chem. Phys. 2016, 18, 32664 and ref.
3d and 3g.
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S11-16.
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9444; b) L. Carreras, L. Rovira, M. Vaquero, I. Mon, E. Martin, J. Benet-
Buchholz, A. Vidal-Ferran, RSC Advances 2017, 7, 32833.
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Tyrra, Heteroat. Chem. 2002, 13, 561.
[20] Since we only added 0.75 equiv of 8, the quantitative formation of 2
suggests that (Cat)[(CF3)AgI] is also capable of acting as CF3 shuttle.
[21] In benzene, the equilibrium is totally shifted towards 4a, see p. S17.
[22] This experimental result, along with those obtained in THF for the
benchmark system, suggest the participation of 4a and 4b in the CF3
group exchange.
[8]
[9]
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[23] See supporting information for further details on the thermolysis of 9
with the different CF3.
[10] We also carried out the reaction of complex 1 with (Phen)CuCF3, other
commercially available nucleophilic trifluoromethylating reagent. This
copper compound only afforded 2 in 12% yield. See SI for further
details.
[11] SIPrAgCF3 (3) was prepared using a modified procedure from B. K.
Tate, A. J. Jordan, J. Bacsa, J. P. Sadighi, Organometallics 2017, 36,
964. See SI for further details.
[12] Z. Weng, R. Lee, W. Jia, Y. Yuan, W. Wang, X. Feng, K.-W. Huang,
Organometallics 2011, 30, 3229.
[13] CCDC 1588501 (3), CCDC 1566841 (4a), CCDC 1566842 (5), CCDC
1566843 (6), CCDC 1566844 (S1), CCDC 1566845 (7), CCDC
1566839 (8Cs) and CCDC 1588502 (8NBu4) contain the supplementary
[24] Grushin observed the formation of the mismatched product
(Xantphos)Pd(CF3)I during the thermolysis of (XantPhos)Pd(Ph)(CF3)
and PhI during 8 h at 70 ºC. See SI p 5-7 from reference 4a.
[25] The formation of 11 in moderate yield (56% yield or catalyst turnover
number of 5.6) is not due to the well-precedented unproductive
reactions discussed along the text, but for practical challenges related
to the use of 8Cs as transmetalating agent under catalytic conditions.
The product yield was calculated taking as limiting reagent 8Cs and
considering the transmetalation of both CF3 groups. See SI pS84 for
further details.
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10.1002/chem.201802586
Chemistry - A European Journal
COMMUNICATION
Layout 1:
COMMUNICATION
Disclosed herein it is the
potential of [LAgCF3] and
S. Martínez de Salinas, A. L.
Mudarra, J. Benet-Buchholz,
T. Parella, F. Maseras, M. H.
Pérez-Temprano*
[Cs][Ag(CF3)2]
(Cat)[Ag(CF3)2]
as
CF3 shuttles to PdII, in the
context of one of the hidden
Benchmark
system
Translation
to a productive
system
Page No. – Page No.
challenges
in
trifluoromethylation
processes:
transmetalation step.
New
transmetalation
discrete “AgCF3” species:
Implications in Pd-mediated
trifluoromethylation
reactions
vistas
in
with
the
This article is protected by copyright. All rights reserved.