Consecutive Transformations of Tetrafluoropropenes: Hydrogermylation and Catalytic C?F Activation Steps at a Lewis Acidic Aluminum Fluoride

Article abstract of DOI:10.1002/anie.201707759

Functionalization reactions of the refrigerants HFO-1234yf (2,3,3,3-tetrafluoropropene) and HFO-1234ze (1,3,3,3-tetrafluoropropene) were developed. The selectivity and reactivity towards CF₃ groups of C?F activation reactions can be controlled by employing either a germane or a silane as the hydrogen source. Unique transformations were designed to accomplish consecutive hydrogermylation and C?F activation steps. This allowed for an unprecedented transformation of an olefinic C?F bond into a C?H bond by heterogeneous catalysis. These reactions are catalyzed by nanoscopic aluminum chlorofluoride (ACF) under very mild conditions.

Full text of DOI:10.1002/anie.201707759

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Accepted Article
Title: Consecutive Transformations of Tetrafluoropropenes:
Hydrogermylation and Catalytic C-F Activation Steps at a Lewis-
acidic Aluminium Fluoride
Authors: Thomas Braun, Gisa Meißner, Erhard Kemnitz, and Konrad
Kretschmar
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707759
Angew. Chem. 10.1002/ange.201707759
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10.1002/anie.201707759
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Consecutive
Transformations
of
Tetrafluoropropenes:
Hydrogermylation and Catalytic CF Activation Steps at a Lewis-
acidic Aluminium Fluoride
Gisa Meißner, Konrad Kretschmar, Thomas Braun,* and Erhard Kemnitz*
was reported, for instance with silylium ions as catalyst.[9]
Abstract: Functionalization reactions of the refrigerants
The polyfluorinated olefins HFO-1234yf (
1, 2,3,3,3-
HFO-1234yf (2,3,3,3-tetrafluoropropene) and HFO-1234ze
(1,3,3,3-tetrafluoropropene) were developed. The selectivity and
reactivity towards CF3 groups of CF activation reactions can be
controlled by either employing a germane or silane as a
hydrogen source. Unique transformations were designed to
accomplish consecutive hydrogermylation and CF activation
steps. This allows for an unprecedented transformation of an
olefinic CF bond into a CH bond by heterogeneous catalysis.
The conversions are catalyzed by nanoscopic aluminium
chlorofluoride (ACF) under very mild conditions.
tetrafluoropropene) and HFO-1234ze ( , 1,3,3,3-tetrafluoro-
2
propene) represent very interesting substrates for the
synthesis of fluorinated building blocks of higher value.
Because of their low global warming potential, they are
useful substitutes for HFO-134a (1,1,1,2-tetrafluoroethane)
as refrigerants.[10] However, also the decomposition of
HFO-1234yf (1) in the atmosphere or at high temperatures
is a matter of discussion.[11] Scarce reactivity studies of
1
include reactions towards alkoxides,[12] alcohols[13] and
oxidative Heck reactions with arylboronic acids.[14]
HFO-1234ze (2), which is an isomer of HFO-1234yf (1),
The combination of CF bond activation and a concomitant
functionalization step can be considered as an ongoing
challenge.[1] The synthetic strategy opens up new reaction
routes to access fluorinated building blocks by minimizing the
number of reaction steps, and therefore can also reduce the
expenditure for fluorinated pharmaceuticals, agrochemicals or in
materials.[2] A CF bond cleavage can be challenging, in part
due to the high CF dissociation energy,[3] but also selectivity is
frequently an important issue.[1] A strategy to overcome the high
bond dissociation energy often involves the generation of
another thermodynamically favorable bond, such as BF, AlF,
Si−F or Ge−F bonds.[3]
Catalytic routes for the selective functionalization of per-
and polyfluorinated propenes via CF bond functionalization are
limited.[1,4] Heterogeneous catalysts give usually only products of
dehydrofluorination under very harsh reaction conditions.[5] CF
transformations of fluoroalkanes under moderate reaction
conditions impart reactions at nanoscopic aluminium
chlorofluoride AlClxF3-x (ACF, x ≈ 0.05-0.3)[6] as a catalyst in the
presence of Et3GeH or Et3SiH.[7] ACF is an amorphous,
was only applied in hydrosilylation reactions with HSiCl3,[15]
and patents cover the addition of HF, Cl2 or H2.[16]
Herein we report the distinct reactivity of tertiary germanes
and Et3SiH towards HFO-1234yf (1) and HFO-1234ze (2) in the
presence of ACF as catalyst under very moderate reaction
conditions. With Et3SiH rare monodefluorinations at
trifluoromethyl groups were achieved. On using germanes the
CF3 groups remain untouched and transformations were
developed to access trifluoroalkylgermanes by consecutive
hydrogermylation and CF activation steps. The strategy
involves a one-pot transformation of a C(sp2)F into a C(sp3)F
bond by hydrogermylation and a subsequent CF activation to
yield an alkene functionality, which then can even undergo
further hydrogermylation.
microporous, Lewis-acidic material with
structure and
large surface area (>100 m2/g).[5d,6d,8]
Trifluoromethyl groups, such as in CF3H or CF3CH3, react with
silane at ACF resulting in complete defluorination by
a highly distorted
a
a
hydrodefluorination or in benzene by Friedel-Crafts-like
reactions. With germanes dehydrofluorination of mono- and
difluoroalkanes are possible.[7b] Olefinic or aromatic CF bonds
do not react at ACF, neither in the presence of silane nor
germanes. In the homogeneous phase, the catalytic
hydrodefluorination of C(sp3)F bonds at fluorinated alkanes
Scheme 1. ACF catalyzed activation of 1 and 2 with silane after 16 h at 70°C.
Treatment of HFO-1234yf (1) with neat Et3SiH in the
presence of ACF gave the fluorinated propene 3 by selective
allylic monohydrodefluorination of only one CF bond, whereas
in C6D6 as solvent the Friedel-Crafts-product 4 and HD were
formed (Scheme 1). Reactions in C6D12 or 1,2-dichlorobenzene-
d4 as solvent yielded only traces of 3. A mechanistic proposal
G. Meißner, K. Kretschmar, Prof. Dr. T. Braun, Prof. Dr. E. Kemnitz
Department of Chemistry
Humboldt-Universität zu Berlin
Brook-Taylor-Str. 2, 12489 Berlin (Germany)
E-mail: thomas.braun@chemie.hu-berlin.de
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201707759
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involves the F2C(sp3)F activation at the ACF to generate allyl
carbenium-like surface species which subsequently react with
the silane as a hydrogen source.[7] Alternatively, silylium-like
species might be formed initially and then react with the
fluorinated substrate at the surface. In benzene the carbenium-
like species can react further by electrophilic attack yielding
Friedel-Crafts products.[4h,17] Remarkably, with 1, unlike to
polyfluoroalkanes,[7] the ACF/silane system does not result in a
full defluorination of the substrate, but a very selective activation
reaction was performed under photolytic conditions (311 nm)
with two equivalents of germane in the presence of ACF at 70°C,
the generation of the trifluoropropylgermane 10a was observed.
Similar transformations were found with the germanes nBu3GeH
and Ph3GeH to yield 10b and 10c, respectively (Scheme 2,
Table 1, entries 2, 4, 6). For comparison, the reactivity of the
germanes was also tested towards 2. The reactions are less
selective and the generation of 10a-c and 11a-c was observed
(Scheme 3, Table 2, entries 2, 4, 6). Apparently, the
of only one F2C(sp3)F bond. Examples for
transformation of the CF3 group into CF2 group by
a
selective
transformations on the generation of 10a-c involve
a
a
combination of hydrogermylation and CF bond cleavage
reactions, although at 2 the generation of 10a-c account for less
than 25% of the products. Mechanistic investigations by studying
conceivable key-steps stoichiometrically revealed that the
conversions involve three consecutive reactions, which are
described below.
monodefluorination are scarce and less selective, but provide
access to fluorinated building blocks.[4h,18] However, the
reactions of HFO-1234ze (2) in neat silane (Scheme 1) led to
the generation of 5 by two subsequent CF activations, and also
to 6 possibly by HF addition at 5.[7b] In benzene with silane, 2
converts into 7, 8 and 9 (5 : 1.6 : 1). Mechanistically, for 7 and 8
Irradiation of HFO-1234yf (1) in the presence of the tertiary
germanes gave after four hours at 31 °C the fluorinated
germanes 12a-c by hydrogermylation (Scheme 2). Note, that the
presence of ACF in the reaction mixtures has no influence on
the hydrogermylation step.[21] The addition of germanes to the
double bond of HFO-1234ze (2) gave the isomeric products
11a-c and 13a-c (Scheme 3). The reactions are thus slower
than the comparable hydrogermylation step to olefin 1. A
photolytic hydrosilylation of 1 or 2 with Et3SiH did not take place.
It is likely, that the conversions with germanes follow a radical
reaction pathway. This assumption is supported by adding
TEMPO as a radical scavenger or AIBN as radical initiator in the
absence of light to the reactions with nBu3GeH and 14. TEMPO
did inhibit the reaction, whereas for the latter experiment 70%
conversion was obtained after three days at room temperature.
Note that in general hydrogermylation reactions[22] are often
a
hydroarylation and a subsequent hydrodefluorination or
dehydrofluorination might occur, whereas for 9 a CF3 activation
followed by a Friedel-Crafts type reaction takes place.[19] In
addition, 3,3,3-trifluoropropene is formed in traces.
Scheme 2. Reaction steps of 1 for the consecutive transformations by hydro-
germylation and ACF catalyzed CF activation; a: R = Et, b: nBu, c: R = Ph.
initialized photochemically or thermally following
a radical
reaction pathway, but usually are not selective.[23] Although
transition-metal mediated hydro-germylations of olefins and
alkynes were described,[24] the combination of CF bond
activation with functionalization by hydrogermylation are
unprecedented. Recently, Blanchard reported on stereoselective
hydrogermylations of trifluoromethylalkynes to yield alkenes.[25]
Hydrogermylation reactions of fluorinated double bonds were so
far unknown, but rhodium-catalyzed hydrogermylations of
3,3,3-trifluoropropene (14) were established.[24e]
Table 1. Consecutive reactions of HFO-1234yf (1) with tertiary germanes.
Entry
Hydride
source
Equiv.
R3GeH
Product ratio
12 : 14 : 10
TON[a]
1
2
3
4
5
6
Et3GeH
1
2
1
2
1
2
1 : 20 : 3.5
1 : 5 : 86
1 : 4 : 0.8
1 : 0 : 1.6
1 : 4 : 0.1
1 : 0 : 3.5
34
37
13
9
Et3GeH
nBu3GeH
nBu3GeH
Ph3GeH
Ph3GeH
12
11
All samples were irradiated at 311 nm for 4 h. Afterwards, the samples were
warmed at 70 °C for 48 h (entries 1, 2), 72 h (entries 3, 4) or 91 h (entries
5, 6). Samples with 2 equiv. germane were again irradiated at 311 nm for 48 h
(entries 2, 4) or 4 h (entry 6). [a] TONs were based on the CF activation steps
at ACF and therefore calculated for the formation of 14 from 12a-c and
assuming 1 g of ACF contains roughly 1 mmol of catalytically active sites[8b,20]
and were calculated by integration of 19F NMR spectra on using PhCF3
(capillary) as standard; in all entries full conversions of 1 were observed.
Scheme 3. Reaction steps of 2 for consecutive transformations by hydro-
germylation and ACF catalyzed CF activation; a: R = Et, b: nBu, c: R = Ph.
When the mixtures containing 12a-c and ACF were
warmed up to 70 °C, a CF bond activation occurred selectively
at the 2-position to yield fluoropropene 14 and the corresponding
Treatment of 1 with Et3GeH in the presence of ACF in
benzene did not lead to any conversion. However, when the
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10.1002/anie.201707759
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COMMUNICATION
fluorogermane (Scheme 2). The CF activation step proceeds
only in the presence of ACF and the observed catalytic
conversions differ for the various germanes, presumably
because of steric reasons. In contrast, the tetrafluorogermanes
11a-c did not undergo any CF cleavage, presumably because
of the stronger CF bond in a CF3 group,[3] and because of the
geminal arrangement of the germyl group and the other fluorine
atom (Scheme 3). Note also that heating solutions of 12a or
11a/13a in toluene-d8 without the presence of ACF did not lead
to any fluorogermane elimination. The compounds 11a-c can be
isolated from the reaction mixture, because 13a-c is transformed
into gaseous propene 14.
In general, after the first hydrogermylation and subsequent
CF activation step the consecutive transformations at 1 or 2
can be stopped to yield 14 from 1 or 14 and 11a-c from 2,
respectively. In the presence of only one equivalent of germane,
the reaction of 1 in the presence of ACF gave 14 as main
product, although 10 was also already formed (see Table 1).
The photochemical hydrogermylations were more efficient with
Ph3GeH than with nBu3GeH or even Et3GeH. For the CF
activation at ACF, a reverse order of activity was observed.
In conclusion, two distinct and exceptional CF activation
processes were developed to derivatize the fluoroolefins 1 or 2
at ACF on using Et3SiH or tertiary germanes as additional
hydrogen sources. When Et3SiH is used as a hydrogen source,
1 reacts by CF activation of the CF3 group which is transformed
into a olefinic CF2 group. In contrast, with germanes the CF3
groups stay intact, but one-pot consecutive reactions to access
germylated CF3 building blocks were performed. This results in
an unprecedented reactivity pattern of an olefinic CF bond at
Table 2. Consecutive reactions of HFO-1234ze (2) with tertiary germanes.
Entry
Hydride
source
Equiv.
R3GeH
Product ratio
11 : 13 : 14 : 10
Conv.
[%][a]
1
2
3
4
5
6
Et3GeH
1
2
1
2
1
2
11.5 : 0.2 : 1 : 2.5
30 : 0.4 : 0.15 : 10
21 : 0.8 : 1 : 2
32 : 2 : 1 : 6
100
100
84
ACF and
a functionalization of the starting material by
germylation. Thus, the hydrogermylations have two roles: to
generate the C(sp3)F bond and to functionalize the olefin after
the CF bond activation step. The transformation of olefinic CF
bonds induced by Lewis-acids, e.g. silylium ions, was so far
elusive and was only described at transition metal centres.[1,4,9,30]
Et3GeH
nBu3GeH
nBu3GeH
Ph3GeH
Ph3GeH
76
2.5 : 0.5 : 1 : 1
5.5 : 1 : 0 : 3
76
72
Acknowledgements
All samples were irradiated at 311 nm for 30 h (entries 1, 3, 5) or 4 h (entries
2, 4, 6) and all samples were warmed at 70 °C for 24 h. Samples with 2
equiv. germane were again irradiated at 311 nm for 24 h (entries 2, 4, 6).
[a] Conversions were calculated by integration of 19F NMR spectra based on
converted 2 using PhCF3 (capillary) as standard.
We acknowledge support from the Graduate School
SALSA (School of Analytical Sciences Adlershof) as well as the
Cluster of Excellence UniCat (Unifying Concepts in Catalysis)
both funded by the DFG.
Although the CF activations reactions at 12a-c and 13a-c
resemble dehydrofluorination reactions, the described fluoro-
germane eliminations are unprecedented. Note that while
-fluorine elimination reactions are well known at transition metal
complexes,[26] they are less described at main-group
compounds.[27] Mechanistically, we suggest an initial polarization
of the CF bond at the Lewis-acidic ACF surface.[28] Afterwards,
-fluorine elimination occurs, which yields 14 and the
fluorogermanes. -Germylcarbenium-like surface species might
play a role and are presumably stabilized by the -effect, which
is well known for such species.[29] It is furthermore remarkable
that, in contrast to the conversion with Et3SiH, the reactions of 1
and 2 with germanes are independent from the used solvent
C6D6 or C6D12.
Keywords: CF activation • Hydrogermylation • Germanes •
Silanes • Heterogeneous catalysis
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10.1002/anie.201707759
Angewandte Chemie International Edition
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Entry for the Table of Contents
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G. Meißner, K. Kretschmar, T. Braun*
and E. Kemnitz*
Page No. – Page No.
Consecutive Transformations of
Tetrafluoropropenes:
Hydrogermylation and Catalytic CF
Activation Steps at a Lewis-acidic
Aluminium Fluoride
Functionalizations of tetrafluoropropenes such as HFO-1234yf were developed.
The transfer of the CF3 group into an olefinic CF2 group at nanoscopic aluminum
chlorofluoride (ACF) with silane was achieved. Consecutive hydrogermylation and
CF activation steps allow a transformation of an olefinic CF into a CH bond.
This article is protected by copyright. All rights reserved.