Palladium-catalyzed cross-coupling reactions with fluorinated substrates: Mechanistic insights into the undesired hydrodehalogenation of aryl halides

Article abstract of DOI:10.1021/om200898t

We report here that the undesired hydrodehalogenation in cross-coupling reactions with fluorinated substrates involves water as a possible hydrogen source. Moreover, the product distribution (hydrodehalogenation vs carbon-carbon coupling) can be controlled by varying the phosphine substituents. Significant hydrodehalogenation occurs prior to the formation of Ar_F-Pd(II)-Br complexes. DFT calculations were used to evaluate a direct hydrodehalogenation route with a phosphine and water. These findings provide new mechanistic insight into aryl-Br bond activation with fluorinated substrates and selective arene functionalization.

Full text of DOI:10.1021/om200898t

Communication
pubs.acs.org/Organometallics
Palladium-Catalyzed Cross-Coupling Reactions with Fluorinated
Substrates: Mechanistic Insights into the Undesired
Hydrodehalogenation of Aryl Halides
Meital Orbach,† Joyanta Choudhury,†,§ Michal Lahav,† Olena V. Zenkina,† Yael Diskin-Posner,‡
Gregory Leitus,‡ Mark A. Iron,‡ and Milko E. van der Boom*,†
‡
†Departments of Organic Chemistry and Chemical Research Support, The Weizmann Institute of Science, 76100 Rehovot, Israel
S
* Supporting Information
ABSTRACT: We report here that the undesired hydro-
dehalogenation in cross-coupling reactions with fluorinated
substrates involves water as a possible hydrogen source.
Moreover, the product distribution (hydrodehalogenation vs
carbon−carbon coupling) can be controlled by varying the
phosphine substituents. Significant hydrodehalogenation oc-
curs prior to the formation of ArF−Pd(II)−Br complexes.
DFT calculations were used to evaluate a direct hydro-
dehalogenation route with a phosphine and water. These
findings provide new mechanistic insight into aryl−Br bond
activation with fluorinated substrates and selective arene
functionalization.
Chart 1. Molecular Structures of Compounds 1−7
luorinated compounds are widely used in the semi-
Fconductor and pharmaceutical industries. Their applica-
tions include electronic materials, solvents, lubricants, and
phase-transfer catalysts.1 Despite their many applications, their
formation is not trivial. The transition-metal-catalyzed direct
arylation, olefination, and alkynylation of polyfluoroarenes have
been reported recently.2 These and other cross-coupling
reactions (e.g., Sonogashira, Heck, Suzuki)3 to generate
carbon−carbon bonds with fluorinated substrates are often
hampered by the formation of strong metal−CF σ-bonds and
other competitive processes.4 In 2001, Espinet and Milstein
reported the first Heck catalysis with fluorinated aryl halides,5
three decades after its initial discovery.6 Likewise, it took about
two decades after the initial report on the Sonogashira reaction
to apply it to fluorinated substrates.7 One commonly observed
byproduct of cross-coupling reactions is hydrodehalogenation
(ArF−X → ArF−H; X = halide) of fluorinated aryl halides,
which affects the efficiency and selectivity of the overall
catalytic process.5,8 Here we report insight into some of the key
steps involved in the Sonogashira and related cross-coupling
reactions with the fluorinated aryl halide 1 (Chart 1). We have
previously used this type of compounds to study aspects of aryl
halide oxidative addition.9,10 Here we present a study that
examines (i) the importance of the phosphine ligand and
(adventitious) water in controlling the product distribution
(e.g., C−C bond formation versus hydrodehalogenation), (ii)
the mechanism of a direct hydrodehalogenation route of the
(polyfluoro)aryl bromides (without the need for a metal
catalyst), and (iii) evidence that a radical-based mechanism is
unlikely to play a role in the metal-catalyzed hydrodehaloge-
nation.
We reacted compound 1 with phenylacetylene under
Sonogashira cross-coupling conditions at 100 °C for 27 h
with Pd(PEt3)4 as the catalyst (6 mol %) in anhydrous DMF
(50 ppm of water; for details see the Supporting Information,
Table S1). This reaction results in hydrodehalogenation of 1 as
Special Issue: Fluorine in OrganometallicChemistry
Received: September 27, 2011
Published: November 2, 2011
© 2011 American Chemical Society
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Communication
the major product (2, 67%) and C−C coupling as the minor
product (3, 33%), as determined by 19F{1H} NMR, high-
resolution mass spectrometry (HRMS) and comparison with
authentic samples. The single-crystal structure of compound 2
is included in the Supporting Information (Figure S1). Using
Pd(PPh3)4 instead of Pd(PEt3)4 results in a substantial increase
in the formation of the desired C−C coupling product (3, 77%)
and only 23% of the hydrodehalogenation product (2).
Furthermore, this reaction is complete in only 4 h. Thus, the
product distribution can be significantly shifted by varying the
phosphine (PEt3 versus PPh3).
Hydrodehalogenation of fluorinated arenes has been
reported with Zn/NH3, H2/catalyst, or Grignard reagents.11,12
It is likely in our case that phosphine ligands dissociate from the
palladium catalyst and attack the substrate (1), generating a
transient arylphosphonium ion that undergoes hydrolysis,
which in turn generates the observed product of hydro-
dehalogenation (2). Indeed, reacting 1 with 3 equiv of PEt3 in
THF/H2O (6:1 v/v) at room temperature results (after 5 min)
in quantitative formation of 2. Hydrodehalogenation of 1 with
PPh3 requires elevated temperatures and prolonged reaction
times (e.g., 42% conversion after 43 h at 100 °C). Therefore,
some of the hydrodehalogenation products in palladium-
catalyzed cross-coupling reactions might result from a
combination of (adventitious) water and the use of nucleophilic
phosphine ligands. Hydrodehalogenation was also observed
when bromopentafluorobenzene was used under similar
conditions (Scheme S1, Supporting Information), but not
with bromobenzene.
in which the phosphine as well as the bromine are bound to the
ipso carbon, does not occur. Such a putative intermediate could
not be found, but rather a σ−π complex is formed with the
phosphine hovering over the aromatic ring. The product can be
described as a trigonal-bipyramidal phosphonium salt with a
weakly bound bromide (r(P−Br) = 2.878 Å). This allows the
halide to be exchanged for OH−. Proton transfer from the
hydroxyl group to the aryl moiety affords the experimentally
observed arene and phosphine oxide. The hydroxyl inter-
mediate and the proton-transfer transition state (TS2) are
shown in Figure 2. The reaction profiles for the reaction of
bromopentafluorobenzene and bromobenzene with PEt3 is
shown in Figure 3. The reaction will not occur, even at elevated
temperatures, in the nonfluorinated systems and varies from
fast to sluggish with the fluorinated compounds.
Figure 2. Calculated intermediate (left) and transition state (TS2,
right) for the proton transfer step in the reaction of bromopenta-
fluorobenzene to PEt3 (color scheme: C, gray; H, white; F, light blue;
O, red; P, orange).
The reaction between either compound 1 or bromobenzene
(in both cases hydrogen and fluorine substituted) and either
PEt3 or PPh3 to give the corresponding hydrodehalogenated
arene and phosphine oxide was examined using density
functional theory (DFT) at the SMD(THF)-DSD-BLYP/
maug-cc-pV(D+d)Z-PP//DF−PBE+dv2/pc1 level of theory
(Tables S2−S5; see the Supporting Information for full
computational details). Four reaction steps were considered
at both 298 and 373 K: (i) oxidative addition of the aryl
bromide by the phosphine (PEt3 or PPh3), (ii) hydrolysis of the
PV intermediate, (iii) the acid−base reaction between the HBr
liberated in the previous step with free phosphine, and (iv)
proton transfer from the hydroxyl group to the arene. Step iii
was found to always be unfavorable, and thus the reaction was
considered to result in free HBr. The first step is the organic
equivalent of an oxidative addition of an aryl halide to the PIII
center. The transition state (TS1) for the reaction of PEt3 and
bromopentafluorobenzene is shown in Figure 1 and is fairly
Figure 3. Reaction profiles for the reactions of bromopentafluor-
obenzene (X = F) and bromobenzene (X = H) with PEt3 at 298 and
373 K.
Nonetheless, this direct hydrodehalogenation route with a
phosphine and water is not the only possible pathway under
catalytic conditions. If only such a process were to be operating,
the amount of hydrodehalogenation would be limited by the
amount of the phosphine and adventitious water. Reacting
compound 1 under the aforementioned Sonogashira cross-
coupling conditions without Pd but in the presence of PEt3 (6
mol %) and all the other components (CuI, NEt3, alkyne)
results in only 13% hydrodehalogenation (2). Without
Pd(PEt3)4 and CuI no hydrodehalogenation was observed,
whereas with Pd(PEt3)4 but without CuI formation of
compound 2 was observed as the major product (∼70%).
Moreover, no reaction is observed with PPh3. These
observations show that the metal center also plays a critical
role in catalyzing the hydrodehalogenation.
Figure 1. Calculated transition state (TS1, left) and product (right)
for the oxidative addition of bromopentafluorobenzene to PEt3 (color
scheme: C, gray; H, white; F, light blue; Br, red; P, orange).
Further mechanistic insight is provided by several stoichio-
metric reactions between Pd(PEt3)4 or Pd(PPh3)4 with
compound 1. Reacting Pd(PEt3)4 and 1 in dry THF at room
similar for the other pairs of reactants. A nucleophilic aromatic
substitution (SNAr) pathway, involving a discrete intermediate
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Communication
temperature for 5 min results in the formation of complex 4 by
oxidative addition of the ArF−Br moiety to the d10 metal center.
The nature of this process could either be a concerted oxidative
addition or involve a one-electron transfer from the Pd0
complex to 1 followed by recombination of the PdI with the
resulting aryl radical (vide infra). Complex 4 was formed nearly
quantitatively (98%) with concurrent liberation of 2 equiv of
PEt3. Traces of PdBr2(PEt3)2 and compound 2 were observed
as well by 19F{1H} and 31P{1H} NMR spectroscopy. Perform-
ing this reaction at −70 °C and gradually raising the
temperature to 20 °C reduced the selectivity, as complex 4
was formed in 86% yield. This product (4) was isolated and was
fully characterized by 1H, 13C{1H}, 19F{1H}, and 31P{1H}
NMR spectroscopy as well as by HRMS, elemental analysis (C,
H, N), and single-crystal X-ray crystallography (Figure 4, left).
4 h results in the quantitative formation of the thermodynami-
cally favored complex 5. No hydrodehalogenation of compound
1 was observed. Performing this reaction in THF/H2O (6:1
v/v) does not change the reaction outcome.
Hydrodehalogenation is not observed when trans-4′-bromo-
4-stilbazole (8) (the nonfluorinated analogue of compound 1;
see Scheme S2 in the Supporting Information) is used. Only
selective η2-CC coordination (9) with Pd(PEt3)2 in THF/
H2O (6/1 v/v) is observed by 31P{1H} NMR spectroscopy at
room temperature. Thermolysis at 40 °C for 16 h results in the
quantitative formation of the isostructural nonfluorinated
analogue (10) of complex 4 (Scheme S2). Electrochemical
measurements (cyclic voltammetry and differential pulse
voltammetry) show that the redox potential of 1 (E1/2
=
−0.94 V vs Ag/Ag+) is less negative than of the nonfluorinated
compound 8 (E1/2 = −1.82 V), allowing for easier reduction of
1 (Figure S2, Supporting Information). The product
distribution between C−Br bond activation and hydro-
dehalogenation is clearly controlled by the nature of the
arene (ArF vs Ar).
Our observations indicate that the product distribution with
the fluorinated compound 1 can be directed by water or the
nature of the phosphine. The hydrodehalogenation reaction
proceeds via the formation of a phosphonium salt followed by
hydrolysis with water, even in the presence of the palladium
catalyst. The formation of phosphine oxides under exclusion of
oxygen is in agreement with water activation. Indeed, the use of
Figure 4. X-ray crystallography of complexes 4 (left) and 6 (right)
with thermal ellipsoids at the 50% probability level. Hydrogen atoms
are omitted for clarity, except for H1 of complex 6. Bond lengths and
angles are provided in the Supporting Information.
17
17O-enriched water results in Et3P O.
Other (parallel) pathways cannot be excluded for this model
system. For instance, one could propose that an electron-rich
d10 palladium center with three or four PEt3 ligands might
undergo electron transfer with the electron-poor substrate 1 to
afford an unobserved PdIBr(PEt3)x species and a radical
(1•).14−16 The observed PdBr2(PEt3)2 would then be formed
from a secondary reaction, whereas 2 is generated from
hydrogen (or deuterium) abstraction from water by 1•. This
mechanistic proposal is also in agreement with the different
reactivities of the nonfluorinated substrate (8) with Pd(PEt3)4
and of 1 with Pd(PPh3)4. However, these last two systems are
less likely to undergo a one-electron transfer process. DFT
calculations17 on an electron-transfer process show that the
reaction Pd(PR3)4 + 1 → Pd(PR3)3Br + 1• + PR3 has a reaction
energy of ΔG298(THF) = 11.4 kcal/mol for R = Et and 15.9
kcal/mol for R = Ph. Furthermore, it was shown (vide supra)
that water and not THF is the hydrogen source, which would
not be expected for a radical mechanism considering the large
difference in the hydrogen bond dissociation energies (BDEs)
of water (119 kcal/mol) and THF (92 kcal/mol).18 Thus,
single-electron transfer/hydrogen transfer is not expected to occur.
PdBr2(PEt3)2 could also result from a reaction of Pd(PEt3)4
with HBr formed by the reaction of 1 with free phosphine. This
possibility was confirmed by a stoichiometric reaction between
Pd(PEt3)4 and HBr, resulting in the formation of PdBr2(PEt3)2.
It is noteworthy that the reactivity of PdBr2(PEt3)2 and
complex 4 under Sonogashira cross-coupling conditions is
similar to that of Pd(PEt3)4, suggesting that the substrate-
consuming hydrodehalogenation process does not necessarily
reduce the amount of active catalyst (for details see the
Supporting Information, Table S1).
Performing the reaction in THF-d8 containing traces of water
isotopomers results also in the deuterated analogue of 2 (i.e.,
2′). Compound 2′ is not observed when rigorously dried THF-
d8 is used. Thus, THF is not the hydrogen source in this
reaction. Apparently, adventitious water plays a role in this
hydrodehalogenation process.
To further evaluate the possible role of water, we performed
the room temperature reaction between Pd(PEt3)4 and 1 in
THF/H2O (6:1 v/v). After 5 min, 19F{1H} NMR spectroscopy
showed the complete conversion of compound 1 into
compound 2 and the η2-CC coordination complex 6 (Figure
4, right) in a ratio of 1.4:1. 31P{1H} NMR spectroscopy shows
the formation of 6, PdBr2(PEt3)2, and Et3PO in a ratio of 3:1:1.
These compounds were also identified by field-desorption mass
1
spectrometry (FDMS), H and 31P{1H} NMR spectroscopy,
and comparison with authentic samples. The use of THF/D2O
(6:1 v/v) resulted in the formation of the ArF−D analogues 2′
and 6′, unambiguously demonstrating that water is the
hydrogen source. In the absence of compound 1, Pd(PEt3)4
is stable under these reaction conditions.13 In addition, complex
4 is stable in the presence of an excess of H2O at 100 °C for 2
days. Therefore, the observed hydrodehalogenation process (1
→ 2) occurs without intermediacy of complex 4 and this
suggests that PdBr2(PEt3)2 and Et3PO are products formed
from the reaction of water with transient species. Similar
observations were made with bromopentafluorobenzene;
selective oxidative addition occurs between this substrate and
Pd(PEt3)4, whereas exclusive hydrodehalogenation takes place
in THF/D2O (6:1 v/v) with either Pd(PEt3)4 or free PEt3
(Scheme S1, Supporting Information).
In contrast, the reaction of Pd(PPh3)4 and compound 1
initially results in the η2-CC coordination of 1 (7, 37% at
room temperature) (in addition to unreacted starting
materials). Thermolysis of this reaction mixture at 45 °C for
In summary, we provide mechanistic insight into hydro-
dehalogenation of fluorinated aryl halides. The stoichiometric
reactions indicate that this undesired process uses water as the
hydrogen source and is affected by the nature of the phosphine
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substituents. The electron-rich PEt3 ligand plays a double role.
It can dissociate and directly hydrodehalogenate fluorinated
aryl halides. In addition, it may generate reactive electron-rich
metal centers capable of promoting nonselective processes.
Electrochemical measurements confirm that the metal center of
Pd(PEt3)4 more readily “donates” electrons in comparison to
Pd(PPh3)4 (E1 = −0.28 and 0.10 V vs Ag/Ag+, respectively;
1/2
Figures S3 and S4 in the Supporting Information). Although
electron-rich metal complexes are generally used in cross-
coupling reactions with nonfluorinated substrates,3 they might
be less efficient with fluorinated aryl halides. Furthermore, our
findings show that hydrodehalogenation occurs prior to the
formation of ArF−PdII−Br complexes. The DFT calculations
suggest that single-electron transfer from Pd(PEt3)4 to 1 is
disfavored by at about 12 kcal/mol. Therefore, such a process
coupled with a hydrogen atom transfer pathway is not likely,
and some, as of yet unknown, pathway involving proton or
hydride transfer must be operating. Nonetheless, our study
based on stoichiometric reactions does not take into account all
components of the catalytic process (e.g., CuI, NEt3, alkyne).
However, we have shown that under catalytic conditions
Pd(PEt3)4 and not CuI induces hydrodehalogenation. Other
parallel hydrodehalogenation processes might also be operating.
Our observations are most likely not limited to the Sonogashira
cross-coupling reaction with a model substrate presented here
but are applicable to a wider range of processes with fluorinated
aryl halides and platinum-group metals.
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ASSOCIATED CONTENT
* Supporting Information
(10) (a) Zenkina, O. V.; Karton, A.; Shimon, L. J. W.; Martin, J. M.
L.; van der Boom, M. E. Chem. Eur. J. 2009, 15, 10025. (b) Zenkina,
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der Boom, M. E. Inorg. Chem. 2008, 47, 5114. (c) Zenkina, O.;
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E. Organometallics 2007, 26, 4528. (d) Strawser, D.; Karton, A.;
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(12) (a) For hydrodehalogenation of fluorinated arenes with
P(NEt2)3: Bardin, V. V.; Pressman, L. S. Russ. Chem. Bull. 1997, 46,
786. (b) For hydrodehalogenation of p-halogenoperfluoroanilines:
Kobayashi, H.; Sonoda, T.; Takuma, K.; Honda, N.; Nakata, T. J.
Fluorine Chem. 1985, 27, 1.
(13) Reacting Pd(PEt3)4 in THF/H2O (6:1 v/v) at room
temperature does not generate metal hydrides capable of hydro-
dehalogenation.
■
S
Text, figures, tables, and CIF files giving complete experimental
and computational details and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: milko.vanderboom@weizmann.ac.il.
■
Present Address
§Department of Chemical Sciences, Indian Institute of Science
Education and Research Bhopal, Bhopal 462023, India.
ACKNOWLEDGMENTS
■
This research was supported by the Helen and Martin Kimmel
Center for Molecular Design. J.C. thanks the EU (FP7
program) for an Incoming Marie Curie fellowship. Dr. A. C.
B. Lucassen is acknowledged for his assistance. M.E.v.d.B. is the
incumbent of the Bruce A. Pearlman Professorial Chair in
Synthetic Organic Chemistry.
(14) Pd(PEt3)4 and Pd(PPh3)4 undergo reversible phosphine
dissociation: Mann, B. E.; Musco, A. J. Chem. Soc., Dalton Trans.
1975, 1673.
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P. W. J. Chem. Soc., Dalton Trans. 1980, 1448. (c) Tsou, T. T.; Kochi, J.
K. J. Am. Chem. Soc. 1979, 101, 6319.
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