Catalytic C-F bond activation of hexafluoropropene by rhodium: Formation of (3,3,3-trifluoropropyl)silanes

Article abstract of DOI:10.1002/anie.200700711

A clean break: The novel catalytic conversion of hexafluoropropene into (3,3,3-trifluoropropyl)silanes by C-F activation has been developed (see scheme). The reactions are catalyzed by the rhodium complex [Rh{(Z)-CF= CF(CF₃)}(PEt₃)₃], proceed at room temperature, and are highly selective. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200700711

Angewandte
Chemie
DOI: 10.1002/anie.200700711
Carbon–Fluorine Bond Activation
À
Catalytic C F Bond Activation of Hexafluoropropene by Rhodium:
Formation of (3,3,3-Trifluoropropyl)silanes**
Thomas Braun,* Falk Wehmeier, and Kai Altenhöner
Dedicated to Professor Helge Willner on the occasion of his 60th birthday
The activation of carbon–fluorine bonds by transition-metal
centers is an established process in organometallic chemis-
try.[1] Current interests include the development of new routes
to higher-value fluorinated compounds from easily accessible
precursors. The strategy involves the selective cleavage of a
We have already shown that hexafluoropropene can be
converted into 1,1,1-trifluoropropene by using dihydrogen as
hydrogen source, but the transformation was not catalytic.[9,10]
The hydrodefluorination reaction is based on the activation of
hexafluoropropene by [RhH(PEt3)3] (1a) to yield the rho-
À
=
C F bond in a highly fluorinated substrate to obtain
dium derivative [Rh{(Z)-CF CF(CF3)}(PEt3)3] (2). The latter
fluorinated building blocks that then can be functionalized
further in the coordination sphere of the metal.[1,2] There have
been striking advances in the synthesis of fluorinated
complex reacts with dihydrogen to give 1,1,1-trifluoropro-
pene and the fluoro complex [RhF(PEt3)3] (3).
Herein we present our results on the reactivity of 2 with
tertiary silanes. The studies led to the development of a
catalytic process for the conversion of hexafluoropropene
into (3,3,3-trifluoropropyl)silanes by C F bond activation.
The reactions proceed at room temperature with good TONs
and are very selective. They are unique in that 1) they involve
a rare catalytic C F bond activation of a fluorinated alkene,
and 2) the catalytic cycle, along with the hydrodefluorination
steps, involves the formation of a C Si bond.
Treatment of a solution of 2 with an excess Ph3SiH led to
the selective formation of the (3,3,3-trifluoropropyl)silane 4a
and the dihydridosilyl complex cis-fac-[Rh(H)2(SiPh3)-
(PEt3)3] (5)[10] (Scheme 1). An experiment with substoichio-
metric amounts of Ph3SiH also gave selectively the silane 4a
and complex 5, but the starting compound 2 was still present.
À
molecules using C F activation reactions, but catalytic
reactions are still rare. The transformations include a very
limited number of cross-coupling reactions.[3,4] In another
exceptional example, carbon–silicon bonds are formed by
catalytic conversions of functionalized fluorobenzenes, such
as fluoroacetophenones or (fluorophenyl)oxazolines, with
hexamethyldisilane using [Rh(cod)2]BF4 as catalyst (cod =
cyclooctadiene).[5] Most of the other examples that have
been reported involve simple hydrodefluorination steps.[1,2]
Very little has been reported on stoichiometric or even
À
À
À
À
catalytic transformations that involve the cleavage of a C F
bond in a fluorinated olefin.[1,4,6–8] Again, almost all of the
reactions involve hydrodefluorinations. The only exception
involves a palladium-catalyzed cross-coupling reaction of 1,1-
difluoro-2-naphtylpropene with [(tolyl)ZnCl] to give mono-
and ditolyl derivatives.[4] Holland and co-workers showed that
hexafluoropropene can be converted catalytically into a
mixture of pentafluoropropenes with a turnover number
(TON) of 9.8.[7] Here, a diketiminate iron(II) fluoro complex
served as the catalyst. In a very recent example, Peterson and
McNeill reported on the rhodium-catalyzed hydrodehaloge-
nation of vinylfluoride or chlorofluoroethylenes in the
presence of HSiEt3 to give ethane with comparable TONs.[8]
[*] Prof. Dr. T. Braun
Institut für Chemie, Humboldt Universität zu Berlin
Brook-Taylor-Str. 2, 12489 Berlin (Germany)
Fax: (+49)30-2093-6939
Scheme 1. Reactivity of 2 towards silanes.
E-mail: thomas.braun@chemie.hu-berlin.de
F. Wehmeier, K. Altenhöner
Fakultät für Chemie, Universität Bielefeld
Postfach 100131, 33501 Bielefeld (Germany)
As 5 is in equilibrium with 1a, it can be used as the starting
À
compound for the C F bond activation of hexafluoropropene
(Scheme 1).[10] Based on these results, we could develop
catalytic cycles for the formation of trifluoropropylsilanes
from hexafluoropropene. In an initial experiment we inves-
tigated the reaction of hexafluoropropene with an excess of
Ph3SiH in the presence of 2 as catalytic precursor (0.7% based
on the amount of silane). We observed a selective trans-
formation of hexafluoropropene into 4a at room temperature
[**] We would like to acknowledge the Deutsche Forschungsgemein-
schaft and the Fonds der Chemischen Industrie for financial
support. We are also grateful to Prof. P. Jutzi (Bielefeld) for his
support, to M. Ahijado and S. Hinze (Berlin) for experimental
assistance, and to Dr. D. Noveski (Bielefeld) for performing
preliminary experiments.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
with a TON of 90 based on the amounts of (3,3,3-trifluoro-
Cs2CO3 led to a TON of 15 for the conversion of hexafluoro-
propyl)silane 4a produced (Scheme 2, Table 1). The silane 4a
propene into 4e (Scheme 2, Table 1).
À
was characterized by NMR spectroscopy, mass spectrometry
Mechanistically, the C F bond activation steps might be
1
and elemental analysis.[11] The H NMR spectrum shows two
induced by the RhI hydrido species [RhH(PEt3)3] (1a) or by a
RhI silyl species [Rh(SiR3)(PEt3)3] (A; Scheme 3). Both
À
complexes should be capable to form products from C F
bond activation such as 2 along with HF or fluorosilane,
respectively.[9,15] The mechanism of the C F bond activation
À
steps is, as yet, not clear. It might involve an oxidative
À
addition of a C F bond or an olefin-insertion/b-fluorine-
elimination pathway.[1,6f,10,15,16] However, oxidative addition
of tertiary silanes to the RhI propenyl intermediates leads to
RhIII hydrido complexes. In the case of 2 this complex would
be an isomer of B (Scheme 3). After reductive elimination of
the less-fluorinated alkene, both reaction cycles are com-
pleted by generation of either 1a or A. The former can be in
equilibrium with dihydridosilyl species C (for example, 5 or 6)
which can be considered as resting states of the catalytic
system. In a final step, after three defluorination reactions, a
hydrogenation of 3,3,3-trifluoropropene D or hydrosilylation
of the silylalkene E occurs (Scheme 3). The fact that the
catalytic reactions are very selective and that only the
trifluoropropane with one silyl group was formed might
Scheme 2. Catalytic derivatization of fluorinated alkenes; [a] Et3N/
Cs2CO3 was added.
Table 1: Catalytic formation of (3,3,3-trifluorpropyl)silanes from hexa-
fluoropropene.
À
indicate that at least two C F bond activation steps are
À
followed by a reductive elimination of a C H bond to give the
Entry
Substrate
Product
TON
silyl species [Rh(SiR3)(PEt3)3] (A). Finally we note that
=
1
HSiPh3
4a
4a
4b
4c
4d
4e
4e
90
54
35
35
12
2.5
15
complexes such as cis-mer-[Rh(H)2{(Z)-CF CF(CF3)}-
2[a]
3
HSiPh3/H2
HSiPh2Me
HSiPhMe2
HSiEt3
HSi(OMe)3
HSi(OMe)3
(PEt3)3][9] can be produced from 2 and H2, and these
complexes may play a minor role in the catalytic conversions.
Dihydrogen can be formed within the catalytic cycle
either by reaction of HF with free tertiary silane to give
fluorosilane or by reaction of HF with the hydride 1a to give
the fluoro complex [RhF(PEt3)3] (3). In principle 1a can be
regenerated from 3 by reaction of 3 with a tertiary silane.[10,17]
We have no indication that cis-fac-[Rh(H)2(SiPh3)(PEt3)3] (5)
serves as dihydrogen source because we did not observe any
loss of H2 from 5 in stoichiometric experiments. Instead 5 was
found to liberate only Ph3SiH.
To determine the possible influence of H2 on the catalytic
transformations, we studied the catalytic activity of 2 towards
hexafluoropropene in the presence of H2 and Ph3SiH. The
conversions were performed in benzene at room temperature
in the presence of the bases Et3N/Cs2CO3 to trap the HF that
may have been generated. We obtained selectively the silane
4a together with Ph3SiF as the only fluorinated products
(Scheme 2) with a TON of 54 based on the formation of 4a.
We did not observe the generation of propane or 1,1,1-
trifluoropropane, which suggests that no hydrogenation of
3,3,3-trifluoropropene occurred. The use of D2 instead of H2
resulted in the formation of considerable amounts of 4a, but
also of several deuterated isotopologues of 4a, which were
4
5
6
7[a]
[a] Base added: NEt3/Cs2CO3.
signals of higher order for the methylene groups at d = 1.57
and 2.13 ppm. The signals can be properly simulated as AA’
and BB’ parts of an AA’BB’X3 spin system (see the Support-
ing Information).[12] Ph2MeSiH and PhMe2SiH can also be
used in comparable transformations to give the (3,3,3-
trifluoropropyl)silanes 4b and 4c with a TON of 35 for
both catalytic cycles. Et3SiH converts into 4d, whereas no
reaction was observed for iPr3SiH (Table 1).
To further test the limits of this catalytic route, we checked
the reactivity of 2 towards the electronically very different
silane (MeO)3SiH.[13] In a stoichiometric experiment we
found that on treatment of 2 with (MeO)3SiH, the trifluoro-
propylsilane 4e and the hydrido complex cis-fac-[Rh(H)2{Si-
(OMe)3}(PEt3)3] (6) were generated (Scheme 1). Complex 6
can be synthesized through an independent reaction pathway
by treatment of the hydride 1a with (MeO)3SiH. The
rhodium–phosphorus coupling constants in the 31P NMR
spectra of 6, as well as a T1 relaxation time of 1.2 s at 300 K
(600 MHz) for the hydride nuclei (1H NMR: d =
À11.34 ppm), suggest the presence of classical hydrido ligands
at a RhIII center.[14] A catalytic experiment showed that 2 can
also be used as a catalytic precursor to convert hexafluoro-
propene into 4e in the presence of (MeO)3SiH, but the TONs
were very low (2.5). However, addition of the bases Et3N/
2
not characterized. The H spectrum also reveals deuterium
exchange between the free silane and D2, as indicated by the
presence of Ph3SiD. Nevertheless, D2 might play a particular
role in the catalytic cycle either in the hydrodefluorination
steps or in a hydrogenation reaction of a silyl derivative of
3,3,3-trifluoropropene. The generation of the nondeuterated
compound 4a is remarkable and indicates that the hydro-
silylation of 3,3,3-trifluoropropene is an important step.
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Scheme 3. Possible reaction pathways for the formation of (3,3,3-trifluoroproyl)silanes from hexafluoropropene.
Indeed, in an independent experiment we showed that
À
[1] Reviews of C F bond activation by transition-metal complexes:
[RhH(PEt3)4] (1b) can serve as a catalyst for the hydro-
silylation of 3,3,3-trifluoropropene, and silane 4a was
obtained with a TON of 138 (unoptimized, Scheme 2).
In conclusion, we have reported the catalytic conversion
of hexafluoropropene and tertiary silanes to give (3,3,3-
trifluoropropyl)silanes. The reactions proceed at room tem-
perature with reasonable TONs and are highly selective.
According to the NMR spectroscopic data, hexafluoropro-
pene is converted quantitatively into the (3,3,3-trifluoropro-
pyl)silanes. The conversions also represent the first catalytic
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À
C F bond activation of a highly fluorinated alkene that does
not result in simple hydrodefluorination products. Instead
higher-value fluorinated compounds are formed. We propose
that the defluorination of hexafluoropropene initially results
in either 3,3,3-trifluoropropene or a silyl derivative of 3,3,3-
trifluoropropene by replacement of the fluorine with a
hydrogen or silicon atom. These defluorination reactions
are coupled consecutively with a catalytic hydrosilylation or
hydrogenation reaction. Notably the transformation can be
applied for electronically very different silanes such as
Ph3SiH, Et3SiH, and (MeO)3SiH. The product 4e is used as
starting compound for the formation of aerogels and poly-
silsesquioxanes.[18]
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c) H. Guo, F. Kong, K. Kanno, J. He, K. Nakajima, T. Takahashi,
Organometallics 2006, 25, 2045 – 2048; d) T. Schaub, M. Backes,
Received: February 15, 2007
Published online: June 4, 2007
À
Keywords: alkenes · C F activation · fluorine · rhodium · silanes
.
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Communications
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[14] Spectroscopic data for 6: IR (n-hexane): n˜ = 2063, 1955 cmÀ1
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1J(RhH) = 1J(RhH’) = 18.1, 2J(PaH) = 2J(Pa’H’) = À130.5, 2J-
(PaH’) = 2J(Pa’H) = 22.5, 2J(PbH) = 2J(PbH’) = 18.2, 2J(HH’) =
b
À
32.4 Hz, 2H, Rh H), 0.98 (m, 9H, P CH2CH3), 1.05 (m, 18H,
PaCH2CH3), 1.20 (m, 6H, PbCH2CH3), 1.44 (m, 12H,
PaCH2CH3,), 3.80 ppm (s, 9H, Si(OMe)3). 31P NMR (202 MHz):
d = 5.9 (dt, 1J(RhP) = 85.1, 2J(PbPa) = 23.8 Hz, Pb), 15.1 ppm (dd,
1J(RhP) = 101.6, 2J(PbPa) = 23.8 Hz, Pa).
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