C?H and C?F Bond Activation Reactions of Fluorinated Propenes at Rhodium: Distinctive Reactivity of the Refrigerant HFO-1234yf

Article abstract of DOI:10.1002/anie.201902872

The reaction of [Rh(H)(PEt₃)₃] (1) with the refrigerant HFO-1234yf (2,3,3,3-tetrafluoropropene) affords an efficient route to obtain [Rh(F)(PEt₃)₃] (3) by C?F bond activation. Catalytic hydrodefluorinations were achieved in the presence of the silane HSiPh₃. In the presence of a fluorosilane, 3 provides a C?H bond activation followed by a 1,2-fluorine shift to produce [Rh{(E)-C(CF₃)=CHF}(PEt₃)₃] (4). Similar rearrangements of HFO-1234yf were observed at [Rh(E)(PEt₃)₃] [E=Bpin (6), C₇D₇ (8), Me (9)]. The ability to favor C?H bond activation using 3 and fluorosilane is also demonstrated with 3,3,3-trifluoropropene. Studies are supported by DFT calculations.

Full text of DOI:10.1002/anie.201902872

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Accepted Article
Title: C–H and C–F Bond Activation Reactions of Fluorinated Propenes
at Rhodium: Distinctive Reactivity of the Refrigerant HFO-1234yf
Authors: Thomas Braun, Maria Talavera, Cortney von Hahmann,
Robert Müller, Mike Ahrens, and Martin Kaupp
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902872
Angew. Chem. 10.1002/ange.201902872
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http://dx.doi.org/10.1002/ange.201902872

10.1002/anie.201902872
Angewandte Chemie International Edition
COMMUNICATION
C–H and C–F Bond Activation Reactions of Fluorinated Propenes
at Rhodium: Distinctive Reactivity of the Refrigerant HFO-1234yf
Maria Talavera,†,[a] Cortney N. von Hahmann,†,[a] Robert Müller[b], Mike Ahrens[a], Martin Kaupp,*[b] and
Thomas Braun*[a]
Dedicated to Prof. Helmut Werner on the occasion of his 85th birthday
Abstract: The reaction of [Rh(H)(PEt3)3] (1) with the refrigerant
HFO-1234yf (2,3,3,3-tetrafluoropropene) affords an efficient route to
obtain [Rh(F)(PEt3)3] (3) by C–F bond activation. Catalytic
hydrodefluorinations were achieved in the presence of the silane
HSiPh3. In the presence of fluorosilane the fluorido complex 3
provides a C–H bond activation followed by a 1,2-fluorine shift to
produce [Rh{(E)-C(CF3)=CHF}(PEt3)3] (4). Similar rearrangements of
HFO-1234yf were observed at [Rh(E)(PEt3)3] (E = Bpin (6), C7D7 (8),
Me (9)). The ability to favor C–H bond activation using [Rh(F)(PEt3)3]
(3) and fluorosilane is also demonstrated with 3,3,3-trifluoropropene.
Studies are supported by DFT calculations.
The fluoroolefin 2,3,3,3-tetrafluoropropene, HFO-1234yf, has
been identified as a replacement for the refrigerant 1,1,1,2-
tetrafluoroethane, HFC-134a, which was used in automobile air
conditioning systems.[10] Even with the interesting properties of
this olefin, reactivity studies with HFO-1234yf have been little
described.[11]
Ogoshi
et
al.
reported
on
catalytic
[11e]
monodefluoroborylations using a copper catalyst and B2pin2
as well as on catalytic monodefluorosilylations with a copper
fluorido complex and PhMe2Si(Bpin).[11g] Recently, Crimmin et al.
presented oxidative addition reactions of HFO-1234yf via the
activation of the C(sp2)–F bond at an aluminium(I) complex.[11h]
In this paper, we present the development of a new strategy for
C–H bond activation reactions: Conversions of HFO-1234yf and
3,3,3-trifluoropropene are initiated by a rhodium fluorido complex
in the presence of fluorosilane. Activation of HFO-1234yf is
distinctive and involves the subsequent rearrangement of the
fluorinated moiety by a 1,2-fluorine shift.
Fluorinated building blocks are not only an important inclusion in
hydrofluorocarbons used in pharmaceuticals, agrochemicals and
materials, but are also employed in refrigeration and air
conditioning.[1] The chemical, physical and environmental
properties of hydrofluoroolefins have led to their ubiquity in
cooling agents.[2]
Treatment of [Rh(H)(PEt3)3] (1) with HFO-1234yf at room
temperature provided selectively the rhodium fluorido complex
[Rh(F)(PEt3)3] (3) and 3,3,3-trifluoropropene conveniently and
Extensive research has been performed on the reactivity of late
transition metal complexes towards perfluorinated or highly
efficiently, in contrast to previous methods to access 3, which
4]
fluorinated olefins.[3] Carbon-fluorine bond activation[3a-d, can
7, 12]
require sources of HF and longer reaction times.[6b,
The
occur through the formation of a thermodynamically favored F–B,
F–Si or F–H bond, among others, implying that metal complexes
bearing boryl, silyl or hydrido ligands are useful for C–F bond
activation reactions.[5] In recent years, we have shown the
capabilities of various rhodium(I) complexes [Rh(E)(PEt3)3] (E =
H (1), Bpin (6, pin = pinacolate, O2C2Me4), Si(OEt)3, GePh3) in
C–F bond activation reactions of olefins such as
hexafluoropropene[6], 3,3,3-trifluoropropene[7] and cis-1,2,3,3,3-
pentafluoropropene.[6a] In contrast, C–H activation steps at
fluoroolefins are very rare and usually restricted to olefins not
fluorinated at the double bond. One example by Cowie et al.
demonstrates simultaneous C–F and C–H bond activations of
various polyfluorinated ethylene derivatives using a diiridium
hydrido carbonyl complex.[3h] Though C–H bond activations at
intermediate fac-[Rh(H)(CH2=CFCF3)(PEt3)3] (2) of this reaction,
identified by NMR spectroscopy at 253 K (see SI), exhibits the
coordination of the olefin at the rhodium center of 1 (Scheme 1).
When warmed up by 20 K, the carbon-fluorine bond activation
occurred, resulting in the formation of complex 3 as well as the
release of 3,3,3-trifluoropropene. Presumably, the mechanism
from intermediate 2 to complex 3 follows an insertion into the
Rh–H bond and a β-fluoride elimination. A nucleophilic attack of
1 at HFO-1234yf to yield 3 and the trifluoropropene without 2 as
intermediate can be an alternative mechanism, although this
requires a dissociation of the olefin from 2. Previously, olefin
coordination was observable with hexafluoropropene[6f] or 3,3,3-
trifluoropropene[7, 13] coordinating rhodium or nickel complexes,
and β-fluoride elimination reactions have been also proposed in
other systems using transition metals.[3e, 14]
rhodium are well-known with fluoroaromatics and also occur at
8]
[Rh(E)(PEt3)3],[6e,
they are often competing with C–F bond
activation reactions.[3b, 9]
[a]
[b]
Dr. M. Talavera, C. N. von Hahmann, Dr. M. Ahrens, Prof. Dr. T. Braun
Department of Chemistry, Humboldt Universität zu Berlin
Brook-Taylor-Straße 2, D-12489 Berlin, Germany
E-mail: thomas.braun@chemie.hu-berlin.de
Dr. R. Müller, Prof. Dr. M. Kaupp
Institut für Chemie, Theoretische Chemie/Quantenchemie, Sekr. C7
Technische Universität Berlin
Straße des 17. Juni 135, 10623 Berlin, Germany.
E-mail: martin.kaupp@tu-berlin.de
† These authors contributed equally
Supporting information for this article is given via a link at the end of the
document.
Scheme 1. Reaction of rhodium hydrido complex 1 with HFO-1234yf.
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10.1002/anie.201902872
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In order to study the catalytic hydrodefluorination of HFO-1234yf
to obtain 3,3,3-trifluoropropene, HSiPh3 and the olefin were
modeled by the generation of benzene and F2SiPh2, the
conversion is exergonic by ΔG = -87 kJ/mol.
reacted in a ≈0.55:1 ratio with 5 mol% of complex 1 as a catalyst, At this stage two distinct reaction steps may be identified: (i) a
achieving 90% of conversion based on the consumption of
silane (Scheme 2). This catalytic reaction represents one of the
C–H activation of HFO-1234yf and (ii) a rearrangement of the
fluorinated moiety involving a 1,2-fluorine shift. These two steps
have been probed in independent reactions. Thus, the system
3/FSiPh3 was tested as a tool for C–H bond activation. Indeed,
in a reaction with 3,3,3-trifluoropropene after 3 h the formation of
[Rh{(E)-CH=CH(CF3)}(PEt3)3] (7) (see SI) as well as complex 5
in a 70:30 ratio was observed (Scheme 4). Note that this
reaction also took place when using FSi(OEt)3. Furthermore, the
reactivity of complex 3 with FSiPh3 was tested in the absence of
the olefin. C–D activation of the solvent toluene-d8 occurred,
forming within 1 day isomers of [Rh(C7D7)(PEt3)3] (8) (SI) and 5
in a 95:5 ratio, respectively, as well as fluorosilicates (Scheme 4).
To study the fluoride migration, the rhodium methyl complex
[Rh(Me)(PEt3)3] (9), which is known to perform C–H activation
reactions,[20] was treated with HFO-1234yf. The isolated mixture
of isomers of [Rh(C7D7)(PEt3)3] (8) were also tested
independently with HFO-1234yf, and both reactions gave 4
(Scheme 3). Note that the conversion of [Rh(Me)(PEt3)3] (9) and
HFO-1234yf into 4 and methane is also computed to be
exergonic by ΔG = -103 kJ/mol. Importantly, the production of
pure complex 4 demonstrates that the fluorine shift proceeds
without the need of fluorosilane. This observation is further
few examples of catalytic C(sp2)–F bond hydrodefluorination of
15]
an olefin by late transition metals.[4b,
In contrast, using an
excess of HSiPh3 in regard to HFO-1234yf (≈1.67:1 ratio) and
8.3 mol% of complex as catalyst, 1,1,1-trifluoro-3-
triphenylsilylpropane and 1,1,1-trifluoropropane can be observed
in addition to 3,3,3-trifluoropropene in 2.2:1:35 ratio,
1
a
a
respectively, together with FSiPh3 (Scheme 2). The
hydrosilylation and hydrogenation products stem from 3,3,3-
trifluoropropene.[16] Note that the rhodium(I) fluorido complex 3
reacts immediately with HSiPh3 to form rhodium(I) hydrido
complex 1 and FSiPh3.[6a]
Scheme 2. Catalytic hydrodefluorination of HFO-1234yf.
supported by
a
reaction of
3
with the silane Z-
(CF3)CF=CH(SiMe2Ph) (SI) which also yields 4 and FSiMe2Ph
by Si–C bond cleavage instead of C–H activation (Scheme 3).
Surprisingly, the reaction solution of the stoichiometric
conversion of 1 to 3 in the presence of HFO-1234yf and one
equivalent of HSiPh3, showed on a slower time scale after 19 h
the formation of [Rh{(E)-C(CF3)=CHF}(PEt3)3] (4).[6a] Additionally,
minor amounts of [Rh(PEt3)4]+ (5)[17] and a complex which is
presumably [Rh(η6-C7D8)(PEt3)2]+ (see SI), as well as 3,3,3-
trifluoropropene and FSiPh3 were observed. [18] We assume that
the initially formed counteranion of the cationic species is

F2SiPh3(see also below), which then transforms into SiF5 . The
same reaction pathway can also be seen using HSiEt3 instead of
HSiPh3. It was then intriguing to discover that in an independent
reaction the rhodium fluorido complex 3, FSiPh3 and HFO-
1234yf within 1 h yielded the rhodium complex 4 by C–H
activation (Scheme 3), as well as [Rh(PEt3)4]+ (5) in a 92:8 ratio
and again minor amounts of [Rh(η6-C7D8)(PEt3)2]+.[19] These
results together with the absence of a reaction between free
phosphine or pure complex 3 and HFO-1234yf when there is no
silicon species present in the reaction mixture, allow us to
assume that the fluorosilane plays an important role in the
reaction. Importantly, in a PFA inliner the reaction proceeded
more slowly, completing in 3 h, suggesting that the formally
generated HF is consumed by the glass of the NMR tube.
However, in PFA, the HF is not evident in the reaction mixtures
but instead reacts further with FSiPh3 to provide F2SiPh2 and
benzene, which were both observed by GC/MS. Furthermore,
using the proton sponge, 1,8-bis(dimethylamino)naphthalene in
Scheme 3. Activation of HFO-1234yf and (CF3)CF=CH(SiMe2Ph); [Rh(PEt3)4]+
(5) was observed in the formation of 4 from 3 and FSiPh3 (ratio 4:5 = 92:8).[18]
a
PFA inliner the reaction duration decreased to 1.5 h.
Consistent with that observation, DFT calculations reveal that a
reaction from 3 to give 4 and HF would be uphill by ΔG = +49
kJ/mol (all reported reaction and activation free energies are
calculated at 298.15 K and 0.1 MPa). If the trapping of HF is
Scheme 4. C–H activations at
3 and fluorosilane; as second product
[Rh(PEt3)4]+ (5) was observed for the generation of 7 (ratio 7:5 = 70:30).[18]
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10.1002/anie.201902872
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COMMUNICATION
Based on these results, the following mechanism is suggested
(Scheme 5). It is conceivable that a fluorosilicate could be
formed in situ by the reaction of FSiPh3 with the fluorido complex
3,[21] by abstracting the fluorido ligand. Prior or concomitant
olefin coordination to rhodium leads to intermediate A, which in
our DFT computations features an interaction of the
A comparable reactivity of HFO-1234yf was further observed in
a reaction of the boryl complex [Rh(Bpin)(PEt3)3] (6) with HFO-
1234yf affording, in 10 min, again complex 4 and the cationic
rhodium(I) complex 5 in a 93:7 ratio.[29] The reaction was
monitored by low temperature NMR spectroscopy revealing at
253 K the generation of the hydrido complex
2 as an
fluorosilicate with the C–H bond at the metal-bound olefin.[22]
A
intermediate, which can be explained by an initial C–H activation
of HFO-1234yf to afford the rhodium hydrido complex 1 and
presumably concomitant generation of a fluorinated vinylborane.
At 273 K complex 2 evolves to yield 3 by C–F bond activation of
HFO-1234yf as observed before (Scheme 1). Finally, the
rhodium fluorido complex 3 reacted further in the presence of
HFO-1234yf to give the final products 4 and cationic complex 5
at 283 K. The latter step is presumably mediated by Lewis-acidic
borane species instead of a fluorosilane.
C–H bond activation transpires next from A, giving intermediate
B and the regeneration of fluorosilane together with HF with a
computed free-energy barrier of ΔGǂ = +38 kJ/mol relative to A
for the CH bond in the trans position to the CF3 group. This
step is exergonic by ΔG = -108 kJ/mol, if the generation of
F2SiPh2 and benzene is considered (Scheme 5 and Scheme
S47 in SI). The conversion is zero-order in the decrease of 3
and the formation of 4. When using the deuterated HFO-
1234yf[23] a small kinetic isotope effect of 1.2 was observed,
which is consistent with the proposed intermediate A and a
transition state for the C–H activation step still featuring a C–H
interaction.[24] Note that it has been suggested recently that C6F6
can act as a fluoride shuttle to abstract a proton from the
backbone of an imidazolinium cation.[25]
Independently, efforts to generate the cation [Rh(η2-
CH2CFCF3)(PEt3)3]+ from 3 on using various Lewis acids such as
BF3 or borate salts like NaBArF (sodium tetrakis(3,5-
4
bis(trifluoromethyl)phenyl)borate) were not successful but
always led to complex 5 with the corresponding counteranion.
The reactions of 5·BF4 with HFO-1234yf and TBAT
(tetrabutylammonium difluorotriphenylsilicate) or the anionic
The subsequent rearrangement from B into 4 might proceed via
metallacyclopropene complexes generated from the vinyl
ligand[26] or the generation of a fluorido complex bearing
trifluoropropyne as a ligand. Both mechanistic possibilities were
evaluated by DFT methods (see Scheme S48-S50 in the SI),
and fluoride migration is seen to be the rate-determining step in
both cases. However, the barrier for insertion/deinsertion of Rh
into the C–F bond of the olefin (cf. S48) is lower than the one for
migration of fluoride across the C–C bond (cf. S49) in the
metallacyclopropene intermediate (ΔGǂ = +153 kJ/mol vs +174
kJ/mol), rendering the former mechanism the favored reaction
pathway. Overall the computed barriers appear somewhat high
for a room-temperature reaction. Note that the fluorine shift and
subsequent steps are fast, and no intermediates could be
observed by NMR spectroscopy, even at low temperature. Using
THF-d8 as solvent did not increase the reaction rate or provide
intermediates. Note that Hu and coworkers have proposed that a
1,2-fluorine shift at cyclopropyl-substituted fluoroepoxides might
proceed via a concerted mechanism or a tight ion-pair.[27] Lewis-
acid-induced transformations at halogenated alkanes were also
reported.[1d, 28]
fluoroboronate, [(E)-CF3CH=CH(BpinF)]NMe4 (see SI) as
a
model for a vinylfluoroborate, were then investigated. Indeed,
complex 5·BF4 evolves to complex 4 within days through
complex 3 as an intermediate (Scheme 6). For the reaction of
5·BF4 with TBAT the initial generation of complex 3 and
fluorosilane was observed after 5 min. Full conversion into
complex 4 was achieved after 3 days. Apparently, refluorination
of the cation 5 is favored, but on a much slower time scale the
activation of HFO-1234yf is competing and occurs. The latter is
slower presumably because of low concentration of a borane or
silane as a fluoride acceptor. Therefore, a reaction of 5·BF4,
HFO-1234yf, TBAT and FSiPh3 was performed and indeed the
reaction was finished after 2 d. This is consistent with the fact
that a reaction of 3, FSiPh3, TBAT and HFO-1234yf (1.5 d) was
faster than the conversion of 3 with TBAT and HFO-1234yf into
4 (2 w), but slower than the reaction of 3 with FSiPh3 and HFO-
1234yf (1 h).
Scheme 6. Formation of complex 4 from the cation complex 5·BF4.[18]
In conclusion, a unique system to activate C–H bonds has been
reported, which involves a fluorido complex and a mildly acidic
fluorosilane.[30] The C–H activation step is induced by a F···H
Scheme 5. Proposed mechanism to form complex 4.
interaction involving the C–H bond and
a fluorosilicate.
Remarkably, at HFO-1234yf, a C–H activation followed by a 1,2-
fluorine shift occur. While C–F bond activation and C–H bond
activation are two well-known reactions, the combination of both
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10.1002/anie.201902872
Angewandte Chemie International Edition
COMMUNICATION
is scarce.[3h, 31] The reported strategy might open new routes for
the synthesis of fluorinated building blocks.
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Acknowledgements
We would like to acknowledge the AvH Foundation, the research
training group GRK 1582/2 “Fluorine as a Key Element” and the
CRC 1349 “Fluorine-Specific Interactions” funded by the
Deutsche Forschungsgemeinschaft (DFG, German Research
Foundation, project 387284271) for financial support.
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Keywords: C–F activation • C–H activation • fluorido complexes
• fluorinated refrigerants • silanes
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C–H and C–F Bond Activation
Reactions of Fluorinated Propenes at
Rhodium: Distinctive Reactivity of the
Refrigerant HFO-1234yf
Using the refrigerant HFO-1234yf together with [RhH(PEt3)3], a C–F bond activation
occurs to produce a rhodium fluorido complex. This reaction is followed by a C–H
bond activation mediated by the fluorosilane and a subsequent 1,2-fluorine shift.
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