Trifluoromethylthiolation of aryl iodides and bromides enabled by a bench-stable and easy-to-recover dinuclear palladium(I) catalyst

Article abstract of DOI:10.1002/anie.201501617

Abstract While palladium catalysis is ubiquitous in modern chemical research, the recovery of the active transition-metal complex under routine laboratory applications is frequently challenging. Described herein is the concept of alternative cross-coupling cycles with a more robust (air-, moisture-, and thermally-stable) dinuclear Pd^I complex, thus avoiding the handling of sensitive Pd⁰ species or ligands. Highly efficient C-SCF₃ coupling of a range of aryl iodides and bromides was achieved, and the recovery of the Pd^I complex was accomplished via simple open-atmosphere column chromatography. Kinetic and computational data support the feasibility of dinuclear Pd^I catalysis. A novel SCF₃-bridged Pd^I dimer was isolated, characterized by X-ray crystallography, and verified to be a competent catalytic intermediate. Pd double team: The cross-coupling enabled by an air-, moisture-, and thermally stable dinuclear Pd^I complex was explored. Highly efficient C-SCF₃ coupling of a range of aryl iodides and bromides was achieved and the catalyst was recovered by simple column chromatography, thus highlighting its robustness and the possibility for catalyst recycling. Kinetic and computational data support the feasibility of dinuclear Pd^I catalysis.

Full text of DOI:10.1002/anie.201501617

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201501617
German Edition: DOI: 10.1002/ange.201501617
Cross-Coupling
Trifluoromethylthiolation of Aryl Iodides and Bromides Enabled by
a Bench-Stable and Easy-To-Recover Dinuclear Palladium(I)
Catalyst**
Guoyin Yin, Indrek Kalvet, and Franziska Schoenebeck*
Abstract: While palladium catalysis is ubiquitous in modern
chemical research, the recovery of the active transition-metal
complex under routine laboratory applications is frequently
challenging. Described herein is the concept of alternative
cross-coupling cycles with a more robust (air-, moisture-, and
the reactivities at such dinuclear palladium(I) sites, and the
role of such dimers in catalysis has generally been ascribed to
being off-cycle precursors to the actual catalytically active
palladium(0) species.[7] Indeed, the PdI dimer 1[8] has found
use as efficient pre-catalyst in cross-coupling reactions
(Figure 1).[9] By contrast, the iodinated analogue 2 does not
thermally-stable) dinuclear PdI complex, thus avoiding the
0
ꢀ
handling of sensitive Pd species or ligands. Highly efficient C
SCF3 coupling of a range of aryl iodides and bromides was
achieved, and the recovery of the PdI complex was accom-
plished via simple open-atmosphere column chromatography.
Kinetic and computational data support the feasibility of
dinuclear PdI catalysis. A novel SCF3-bridged PdI dimer was
isolated, characterized by X-ray crystallography, and verified
to be a competent catalytic intermediate.
P
alladium-catalyzed coupling reactions belong to the most
utilized synthetic tools in modern academic and industrial
research.[1] These transformations generally proceed via the
oxidation states (0) and (II) of mononuclear palladium
complexes. However, many palladium(0) catalysts and pop-
ular ligands are not air-stable,[2,3] thus requiring their handling
and storage under inert conditions. A common approach to
increasing the operational simplicity is the employment of air-
stable precursors [e.g., palladium(II) pre-catalysts[4]] which
release the catalytically active components in situ. While
these developments have found widespread applications, the
recovery of the sensitive in situ generated palladium species
after reaction completion is less straightforward.[5] Conse-
quently, under routine laboratory applications, most
employed catalysts are simply disposed of.
Figure 1. PdI-dimer catalysis concept (bottom).
act as an efficient reservoir for Pd0.[10] Its dinuclear Pd–Pd
core remains intact in the presence of a variety of nucleo-
philes, bases, and reaction conditions.[11] Notably, we found 2
to be completely stable to oxygen as a solid; it has been stored
in our labs on the bench for over three months without any
observed decay to date. We envisioned that these stability
features make 2 an attractive candidate for exploring
dinuclear PdI catalysis.
We recently reported our initial success in the area, that is,
a PdI-dimer-catalyzed I!Br halogen exchange of 9-iodo-
anthracene with NBu4Br.[12] Our detailed mechanistic study of
this halogen exchange process supported the mechanism
presented in Figure 1. Notably, typical Pd0 catalysis did not
trigger this halogen exchange under analogous reaction
conditions, thus highlighting the potential for distinct reac-
tivities at such dinuclear PdI–PdI sites. While this study was
a proof-of-concept, the halogen exchange had not shown
a wide substrate scope. We hypothesized that this was because
of inefficient I–Br exchange at PdI (i.e., conversion of 2 into 3
in Figure 1), and hence the limited formation of the reactive
We envisioned that utilizing the more robust + 1 oxidation
state of palladium directly as a catalyst through alternative
dinuclear coupling cycles would be advantageous in terms of
operational simplicity, convenience, and sustainability, since
the stable catalytic entity should easily be recoverable without
the need for special precautions or technology. In the
d9 configuration, palladium favors a dimeric appearance,
[6]
ꢀ
featuring distinct Pd Pd bonds. Little is understood about
[*] Dr. G. Yin, I. Kalvet, Prof. Dr. F. Schoenebeck
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
E-mail: franziska.schoenebeck@rwth-aachen.de
Homepage: http://www.schoenebeck.oc.rwth-aachen.de/
[**] We thank RWTH Aachen University and MIWF NRW for funding. We
gratefully acknowledge the computing time granted on the RWTH
Bull Cluster in Aachen (grant number JARA0091).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201501617.
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species 3. Thus, we subsequently shifted our attention towards
nucleophiles that would present a greater driving force, and
focused on the PdI–PdI-catalyzed conversion of aryl halides
into ArSCF3 with [SCF3]ꢀ as nucleophile.
species had taken place at room temperature in THF within
one hour, as judged by the observation of a single resonance
at d = 93.82 ppm [relative to (EtO)3PO as internal standard].
X-ray crystallographic analysis confirmed that the SCF3-
bridged PdI dimer 4 had formed (Figure 2).[22] This novel
complex features an interesting cis arrangement of the SCF3
Trifluoromethylthiolation has received considerable
attention[13,14] owing to the importance of ArSCF3 compounds
in pharmaceutical and agrochemical research as a result of
their remarkable lipophilicity properties.[15] The direct metal-
catalyzed functionalization of aryl halides to ArSCF3 con-
stitutes an attractive strategy in this context. However, only
three catalytic protocols to functionalize aryl halides have
ꢀ
units and a Pd Pd bond length of 2.57 ꢀ, which is in line with
[6,23]
ꢀ
distances previously reported for Pd Pd single bonds.
In
analogy to its iodine-bridged counterpart, 4 is completely
stable in air.
To test the potential of 4 in functionalizing aryl iodides, we
subsequently studied the stoichiometric reactivity of 4 with
the aryl iodide 5 (Figure 2). This reaction resulted in clean
conversion of 5 into the ArSCF3 6 with concomitant
formation of 2 (d = 101.5 ppm) and the mixed PdI dimer,
featuring an iodine and SCF3 bridge (d = 98.8 ppm). No
signals other than those corresponding to the PdI dimers were
observed by 31P NMR spectroscopy. These results suggest that
direct reactivity of the SCF3-derived 4 with ArI seemed
indeed possible.
To gain additional support we undertook kinetic inves-
tigations of the 4-mediated trifluoromethylthiolation of 9-
iodoanthracene (7). Under pseudo-first-order conditions (7
was employed in considerable excess), we determined a first-
order kinetic dependence in 4 and an overall activation
barrier of DG° = 28.0 ꢁ 3.9 kcalmolꢀ1 for the ArI!ArSCF3
exchange process.
We subsequently examined whether these kinetic data
would also be in the range of computationally predicted
barriers for a mechanism proceeding by direct oxidative
addition of 4 to 9-iodoanthracene. We applied the computa-
tional method M06L along with the implicit solvation model
CPCM to account for toluene and two different basis sets
[def2TZVP and 6-311 ++ G(d,p)/LANL2DZ] for our stud-
ies.[24,12b] Figure 3 presents the full free-energy profile of the
stoichiometric I!SCF3 exchange. The direct oxidative addi-
tion by 4 to 7 was found to be energetically feasible and
endergonic, and is in line with the spectroscopic data in
Figure 2 which only showed the PdI dimers as stable
phosphine-containing intermediates. The reductive elimina-
tion via TS-2 was calculated to be rate-limiting. Within error
limits, the calculated barriers are in reasonable agreement
with the experimentally determined barrier. Overall, the I!
SCF3 exchange reaction is thermodynamically driven, and
exergonic overall by DGrxn ꢂ ꢀ21 kcalmolꢀ1 (Figure 3).
Encouraged by these mechanistic data, and having
established that 4 serves as an efficient trifluoromethylthio-
lation agent, we subsequently set out to explore the corre-
sponding catalytic transformation. Pleasingly, by using
2 mol% of 2 along with the easily accessible SCF3-source
(Me4N)SCF3,[25] a range of aryl iodides were successfully
trifluoromethylthiolated in toluene at 808C. Table 1 summa-
rizes the results. Electron-rich and electron-poor aryl iodides
were converted into ArSCF3 in excellent yields. The trans-
formation was found to be compatible with aldehyde, ketone,
ester, ether, nitro, cyano, and amine functional groups
(Table 1). Pleasingly, two heterocyclic examples were also
trifluoromethylthiolated in good yields. As such, our PdI-
catalyzed protocol offers a substantially wider substrate scope
been realized to date.[16] Buchwald and co-workers made
0
ꢀ
a seminal contribution in developing a Pd -catalyzed Ar
SCF3 bond-formation of aryl bromides.[17] While there have
been no other reports of successful Pd-catalyzed SCF3
couplings of alternative aryl halides, Vicic and Zhang
developed a [Ni(cod)2]/bipyridine-catalyzed protocol to con-
vert certain aryl iodides and bromides into ArSCF3.[18,19]
While electron-rich aryl iodides showed good conversions,
interestingly, the more electron-deficient ArI analogues gave
less than 50% yield of ArSCF3. Alternative methods to
functionalize aryl iodides require either stoichiometric
amounts of a copper salt[20] or, under copper catalysis,
ortho-directing groups to be present.[21]
Our previous fundamental mechanistic studies in the area
of PdI dimer reactivity suggested that the key requirements
for successful catalysis at PdI–PdI sites are that firstly the
nucleophile of interest must be capable of replacing the
bridging iodines at PdI–PdI, and secondly the same nucleo-
phile also needs to be able to stabilize the resulting dinuclear
PdI framework.[12] To date, there is no SCF3-derived PdI dimer
known. Thus, we initially set out to synthesize the SCF3-
bridged PdI dimer 4 by comproportionation of [Pd0(PtBu3)2]
and [PdII(SCF3)2] (Figure 2). 31P NMR spectroscopic analysis
indicated that conversion into a single phosphine-containing
Figure 2. Preparation of the SCF3-bridged PdI dimer 4 and stoichio-
metric reactivity with 5 (yield relative to 4, 2 equiv of 6 can form).
(EtO)3PO was used as an internal standard for the 31P NMR analysis.
Thermal ellipsoids are shown at 50% probability.
2
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Figure 3. Free-energy path of I!SCF3 exchange between 4 and 7, calculated at CPCM (toluene) M06L/6-311+ +G(d,p) [SDD for Pd,I] or CPCM (toluene)
M06L/def2TZVP (in parentheses). Geometries were optimized at B3LYP/6-31G(d) [with LANL2DZ for Pd,I]. Values are in kcalmolꢀ1 at 298 K.
Table 1: PdI-catalyzed SCF3 coupling of aryl iodides to ArSCF3.[a]
Table 2: PdI-catalyzed SCF3 coupling of aryl bromides to ArSCF3.[a]
[a] 2 (3.6 mg, 0.004 mmol), ArBr (0.2 mmol), (Me4N)SCF3 (52 mg,
0.3 mmol), toluene (1.0 mL). Yield of isolated product given. [b] 4 mol%
catalyst 2 was used.
nished in very good yields, by using only 2 mol% of the PdI
dimer (Table 2) and tolerating carbonyl, cyano, ether, and CF
functional groups.
[a] 2 (3.6 mg, 0.004 mmol), ArI (0.2 mmol), (Me4N)SCF3 (52 mg,
0.3 mmol), toluene (1.0 mL). Yield of isolated product given. [b] Yield
determined by 19F NMR analysis versus PhCF3 as an internal standard.
If our calculated mechanism in Figure 3 was indeed
operative, the more labile 1 would be expected to be
generated as a catalytic intermediate in the SCF3-coupling
of aryl bromides. Although 1 is likely rapidly converted into
the brominated analogue of 3 (or to 4) in the presence of
(Me4N)SCF3, mechanistically we cannot rule out that [Pd0] is
being released in the catalytic coupling process. However, we
succeeded in the recovery of the PdI–PdI catalyst in the form
of bridged complex 4 after SCF3 coupling of a number of aryl
than the established metal-catalyzed SCF3-coupling protocols
of aryl iodides.
To further test the generality of the coupling enabled by
PdI–PdI, we embarked on extending our coupling protocol to
aryl bromides. Our computational studies predicted roughly
3 kcalmolꢀ1 greater activation energy for the individual
coupling steps for the reaction with ArBr (see the Supporting
Information for the computed full profile), and catalysis
therefore appeared viable. Thus, we subjected a number of
aryl bromides to the trifluoromethylthiolation with 2. Pleas-
ingly, the corresponding SCF3-coupled products were fur-
bromides. By using
a slightly higher catalyst loading
(10 mol%), we recovered 4 from the reaction with 4-
bromo-1,1’-biphenyl in 76%, with 5-bromo-2-methoxybenz-
aldehyde in 64%, and 1-bromonaphthalene in 69%.[26] This
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catalytic transformation, this process would either be rever-
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ꢀ
reuse of the recovered 4 as the catalyst in another C SCF3
coupling of 4-bromo-1,1’-biphenyl gave high conversion into
the desired product (88%) after 7 hours at 608C. The catalyst
4 was subsequently recovered in 68% yield (Scheme 1).
Scheme 1. Demonstration of recyclability of the PdI dimer.
In conclusion, we herein demonstrated a highly efficient
and operationally simple SCF3-coupling protocol of a range of
aryl iodides and bromides enabled by a bench-stable dinu-
clear PdI catalyst and the easily accessible (Me4N)SCF3
reagent. The catalyst was shown to be very persistent and
was recovered by ordinary column chromatography in an
open laboratory atmosphere. The air-stability and straightfor-
ward recoverability of the PdI catalyst poses a considerable
practical advantage over more sensitive Pd0- or Ni0-catalyzed
processes. While a catalyst per definition is not consumed
during a reaction, practical obstacles in recoverability fre-
quently still lead to its loss. We envisage that the herein
explored alternative coupling concept at dinuclear PdI sites
presents a promising advance in this context. Our future
research is directed at exploring the full potential of this
catalysis concept.
Keywords: cross-coupling ·
density functional theory calculations · palladium ·
reaction mechanisms · synthetic methods
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Pazenok, J.-P. Vors, F. R. Leroux, Beilstein J. Org. Chem. 2013,
9, 2476; b) P. Chen, G. Liu, Synthesis 2013, 2919; c) T. Liang,
C. N. Neumann, T. Ritter, Angew. Chem. Int. Ed. 2013, 52, 8214;
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Billard, Eur. J. Org. Chem. 2014, 2415.
[21] J. Xu, X. Mu, P. Chen, J. Ye, G. Liu, Org. Lett. 2014, 16, 3942.
[22] Selected X-ray data for 4: C26H54F6P2Pd2S2, M = 813.50, brown
rod; space group Pnma; a = 15.016(5), b = 14.811(5), c =
15.214(5) ꢀ; V= 3384(2) ꢀ3; Z = 4; 1calcd = 1.609 gcmꢀ3; T=
100(2) K; reflections collected: 33035, independent reflections:
2996
[R(int) = 0.1112],
R1 = 0.0884,
wR2 = 0.2358.
[14] For noncatalytic or indirect synthetic methods to generate
ArSCF3 from aryl sulfides or disulfides with trifluoromethyla-
tion reagents, see: a) V. N. Boiko, G. M. Shchupak, L. M.
Yagupolskii, J. Org. Chem. USSR 1977, 13, 972; b) C. Waksel-
man, M. Tordeux, J. Chem. Soc. Chem. Commun. 1984, 793; c) T.
Umemoto, S. Ishihara, Tetrahedron Lett. 1990, 31, 3579; d) C.
Wakselman, M. Tordeux, J. L. Clavel, B. R. Langlois, J. Chem.
Soc. Chem. Commun. 1991, 993; e) N. Roques, J. Fluorine Chem.
2001, 107, 311; f) G. Blond, T. Billard, B. R. Langlois, Tetrahe-
dron Lett. 2001, 42, 2473; g) C. Pooput, M. Medebielle, W. R.
Dolbier, Org. Lett. 2004, 6, 301; h) F. Leroux, P. Jeschke, M.
Schlosser, Chem. Rev. 2005, 105, 827. For syntheses in which
“SCF3” functions as nucleophile or electrophile, see: i) T. Billard,
B. R. Langlois, Tetrahedron Lett. 1996, 37, 6865; j) D. J. Adams,
A. Goddard, J. H. Clark, D. J. Macquarrie, Chem. Commun.
2000, 987; k) D. J. Adams, J. H. Clark, J. Org. Chem. 2000, 65,
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T. Billard, Angew. Chem. Int. Ed. 2012, 51, 10382; Angew. Chem.
2012, 124, 10528.
CCDC 995456 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[23] The novel dimer complex 4 has a slightly shorter Pd–Pd distance
than the commercial PdI dimer 1 (2.62 ꢀ). Also see Ref. [8].
[24] Gaussian 09, Revision D.01, M. J. Frisch et al. (see the Support-
ing Information for full reference).
[25] (Me4N)SCF3 was prepared according to the procedure devel-
oped by Tyrra and co-workers. See: D. Naumann, B. Hogea,
Y. L. Yagupolskii, W. E. Tyrra, J. Fluorine Chem. 2003, 119, 101.
See the Supporting Information for details.
[26] The reaction was performed at 608C for 7 h. There was no visible
sign of catalyst degradation (as judged by 31P NMR spectro-
scopic analysis). All phosphine-containing signals observed
belonged to the PdI dimers. The quantification of PdI dimer
was performed by 19F NMR spectroscopic analysis versus an
internal standard. See the Supporting Information for more
details.
Received: February 18, 2015
Revised: March 12, 2015
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Cross-Coupling
G. Yin, I. Kalvet,
F. Schoenebeck*
&&&& — &&&&
Trifluoromethylthiolation of Aryl Iodides
and Bromides Enabled by a Bench-Stable
and Easy-To-Recover Dinuclear
Palladium(I) Catalyst
Pd double team: The cross-coupling
enabled by an air-, moisture-, and ther-
mally-stable dinuclear PdI complex was
was recovered by simple column chro-
matography, thus highlighting its robust-
ness and the possibility for catalyst
recycling. Kinetic and computational data
support the feasibility of dinuclear PdI
catalysis.
ꢀ
explored. Highly efficient C SCF3 cou-
pling of a range of aryl iodides and
bromides was achieved and the catalyst
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!