Activation of pentafluoropropane isomers at a nanoscopic aluminum chlorofluoride: Hydrodefluorination versus dehydrofluorination

Article abstract of DOI:10.3762/BJOC.16.213

The hydrofluorocarbon 245 isomers, 1,1,1,3,3-pentafluoropropane, 1,1,1,2,2- pentafluoropropane, and 1,1,1,2,3-pentafluoro-propane (HFC-245fa, HFC-245cb, and HFC-245eb) were activated through C-F bond activations using aluminium chlorofluoride (ACF) as a catalyst. The addition of the hydrogen source Et₃SiH is necessary for the activation of the secondary and tertiary C-F bonds. Multiple C-F bond activations such as hydrodefluorinations and dehydrofluorinations were observed, followed by hydroarylation and Friedel-Crafts-type reactions under mild conditions.

Full text of DOI:10.3762/BJOC.16.213

Activation of pentafluoropropane isomers at a nanoscopic
aluminum chlorofluoride: hydrodefluorination
versus dehydrofluorination
Maëva-Charlotte Kervarec, Thomas Braun*, Mike Ahrens and Erhard Kemnitz
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 2623–2635.
Department of Chemistry, Humboldt-Universität zu Berlin,
Brook-Taylor-Straße 2, 12489 Berlin, Germany
https://doi.org/10.3762/bjoc.16.213
Received: 27 July 2020
Email:
Accepted: 09 October 2020
Published: 23 October 2020
Thomas Braun* - thomas.braun@cms.hu-berlin.de
* Corresponding author
This article is part of the thematic issue "Organo-fluorine chemistry V".
Guest Editor: D. O'Hagan
Keywords:
aluminum fluoride; C–F bond activation; dehydrofluorination;
hydrodefluorination; hydrofluorocarbons
© 2020 Kervarec et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The hydrofluorocarbon 245 isomers, 1,1,1,3,3-pentafluoropropane, 1,1,1,2,2- pentafluoropropane, and 1,1,1,2,3-pentafluoro-
propane (HFC-245fa, HFC-245cb, and HFC-245eb) were activated through C–F bond activations using aluminium chlorofluoride
(ACF) as a catalyst. The addition of the hydrogen source Et3SiH is necessary for the activation of the secondary and tertiary C–F
bonds. Multiple C–F bond activations such as hydrodefluorinations and dehydrofluorinations were observed, followed by hydro-
arylation and Friedel–Crafts-type reactions under mild conditions.
Introduction
Hydrofluorocarbons (HFCs) have been intensively used in daily (HFOs), which have been regarded as the next generation of
life, mainly due to their excellent properties in refrigeration ap- refrigerants, exhibiting zero ozone depletion potential (ODP)
plications [1-3]. In the past, HFCs were considered as replace- and a negligible GWP [11-13]. A considerable amount of
ments that do not deplete ozone for chlorofluorocarbons (CFCs) studies has been carried out to synthesize HFOs under mild
and hydrochlorofluorocarbons (HCFCs), which have been conditions [11,14-16]. Among them are routes to access 2,3,3,3-
strictly regulated by the Montreal protocol [4-6]. However, due tetrafluoropropene and 1,3,3,3-tetrafluoropropene (HFO-1234yf
to the high global warming potential (GWP), HFCs have also and HFO-1234ze), for which numerous patents suggest synthe-
been included in the Montreal protocol in 2019 (Kigali amend- tic pathways and showcase the reactivity [12,13,15]. One possi-
ment) and have to be phased out [7-10].
bility for the preparation includes the conversion of pentafluoro-
propanes (HFC-245 isomers) using chromia-based catalysts, or
On the other hand, HFCs are valuable starting compounds or metal chloride/fluoride (AlF3, MgF2)-supported catalysts at
intermediate products for the synthesis of hydrofluoroolefins elevated temperatures (350 °C) [11,14,15,17,18]. The group of
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Lu recently reported the gas-phase transformation of 1,1,1,3,3- tivations was deeply investigated. For instance, the activation of
pentafluoropropane (HFC-245eb) into 1,3,3,3-tetrafluoro- fluoromethanes was observed at ACF in the presence of HSiEt3
propene (HFO-1234ze) using mesoporous nanoscopic alumi- as a hydrogen source to produce, in the presence of benzene as
num fluoride-based catalysts [19]. The catalysts were prepared the solvent, Friedel–Crafts products as main compounds [47]. In
via a sol–gel process in the presence of polyols, allowing for the contrast, the hydrodefluorination products were generated in the
evolution of a large surface area and improved acidic properties absence of benzene. Thermodynamically, the generation of
when compared to fluorinated Cr2O3 or traditional β-AlF3 cata- strong H–F, Al–F, or Si–F bonds can enforce an activation of
lysts. At a reaction temperature set at 280 °C, the conversion of C–F bonds under mild conditions, and hence the addition of the
1,1,1,2,2- pentafluoropropane (HFC-245fa) into the 1,3,3,3- silane HSiEt3 as a hydrogen source [27,48,49]. More recently,
tetrafluoropropene (HFO-1234ze) varied between 50 and 60%, ACF was shown to efficiently convert the fluoroalkenes HFO-
depending on the conditions used to synthesize the catalyst, 1234yf (1) and HFO-1234ze (4a) in the presence of the
reaching almost full selectivity. The harsh conditions are in part hydrogen source HSiEt3 into the hydrodefluorination or
needed due to the high dissociation energy of C–F bonds, and in Friedel–Crafts products (Scheme 1) [16].
general, C–F activation steps are considered to be challenging
[20-27].
The activation of fluoropentane was achieved using a modified
ACF, loaded with germane or silane [39]. When silane was
Solid Lewis acids with a high fluoride ion affinity as catalysts immobilized at the surface of ACF in the presence of benzene,
are useful tools for C–F bond activation reactions since the Friedel–Crafts products were again generated. In comparison,
Lewis acidic centers can induce dehydrofluorination reactions, when ACF was loaded with germane, dehydrofluorination prod-
involving the abstraction of a fluoride ion by heterolytic bond ucts were detected. Besides, 2-chloro-1,1,1,2-tetrafluoro-
cleavage [28-31]. AlF3-based catalysts are among the strongest propane (HCFC-244bb) was as well effectively activated at
Lewis acidic materials. They exhibit an effective activity in C–F ACF to yield the corresponding dehydrofluorination product
bond conversion reactions and are widely investigated 2-chloro-3,3,3-trifluoropropene (HFO-1233xf) without the use
[16,28,32-39]. Especially microporous aluminum chlorofluo- of any additional hydrogen source [50]. In the presence of
ride (ACF, AlClxF3−x; x = 0.05–0.3), which has a large surface silane and ACF, HFO-1233xf was further activated via allylic
area (>200 m2g−1) and was patented by Dupont in 1992, has hydrodefluorination.
been extensively studied [40-46]. It is an amorphous aluminum
fluoride doped with chlorine atoms which causes a distortion of In this paper, we report on the reactivity of ACF towards hydro-
the structure resulting in the amorphicity and high Lewis acidity fluorocarbons, and in particular, the pentafluoropropane isomers
of the compound. The reactivity of ACF towards C–F bond ac- (HFC-245). Effective hydrodefluorination and dehydrofluorina-
Scheme 1: Reactivity of tetrafluoropropanes HFO-1234yf (1) (top) and HFO-1234ze (4a) (bottom) in the presence of ACF as the catalyst and HSiEt3
as a hydrogen source [16].
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tion steps of pentafluoropropane isomers in the presence of Several patents cover the transformation of 10a by dehydrofluo-
Et3SiH as a hydrogen source at mild conditions are described. rination at chromia-based catalysts, but the reaction tempera-
tures were above 200 °C [17,51,52].
Results and Discussion
Activation of 1,1,1,2,3-pentafluoropropane
Mechanistically, an abstraction of a fluorine from the CH2F
group by the surface of ACF can occur, generating carbenium-
like species and surface fluorides (Scheme 3). Via HF elimina-
(HFC-245eb, 10a)
The treatment of 1,1,1,2,3-pentafluoropropane (10a) with ACF tion, the olefin 1 can be produced, followed by a refluorination
at 70 °C in C6D12 gave the dehydrofluorination product 2,3,3,3- of the double bond by the released HF, generating 10b
tetrafluoropropene (HFO-1234yf, 1) and the isomerization (Scheme 3, left). In the presence of C6D6, the hydroarylation
product 1,1,1,2,2-pentafluoropropane (HFC-245cb, 10b) in a product 12 can be generated from 1 at the ACF surface. Alter-
1:2 ratio (Scheme 2, top) with almost full conversion. The natively, the aromatic solvent can also attack the carbenium-like
group of Kemnitz previously showed that 1 and 10b can be in species, producing a zwitterionic Wheeland intermediate, which
an equilibrium when HF is present in the reaction mixture [33]. can release the Friedel–Crafts product 11 and DF to regenerate
It was demonstrated that starting from 2-chloro-3,3,3-trifluoro- the catalyst (Scheme 3, right).
propene (HFO-1233xf) in the presence of fluorinated Cr2O3 as
a catalyst and HF, 2,3,3,3-tetrafluoropropene (HFO-1234yf, 1) Although a hydrogen source was not needed to accomplish the
is generated by the replacement of the chlorine substituent with activation of 10a, it was of interest to introduce a silane,
a fluorine atom, and is further transformed by HF addition into because as mentioned above, recent reports showed that the ac-
1,1,1,2,2-pentafluoropropane (HFC-245cb, 10b) [33].
tivation of various substrates treated with ACF was indeed
promoted by the presence of silanes [16,39,47]. Thus, the ex-
When the aromatic solvent C6D6 was used instead of C6D12, periments were also conducted in the presence of HSiEt3, either
10a was once more transformed into 1 as the main compound, in a solvent (C6D6 or C6D12), or in neat silane under similar
together with traces of the Friedel–Crafts product conditions (all reactions were carried out at 70 °C and moni-
CF3CFHCH2C6D5 (11) and the hydroarylation product tored for 7 days).
CF3CFDCH2C6D5 (12) (Scheme 2, bottom), with only 22%
conversion. The low conversion in C6D6 could be a conse- The treatment of 10a and HSiEt3 in C6D12 generated 1 and 10b
quence of a possible interaction of the aromatic solvent with the again, in addition to traces of 1,1,2-trifluoropropene (2,
surface of ACF, which would result in the blocking of the acidic Scheme 4, top). The ratio between the olefin 1 and the refluori-
sites, and thus hamper the adsorption of the substrates. Indeed, nation product 10b observed in the presence of HSiEt3 was 3:1,
in a previous study, a pulse TA experiment suggested the pres- whereas, without the silane, a ratio of 1:2 was detected. This
ence of a strong interaction between benzene and the surface of difference in the ratio might relate to the amount of HF present
ACF [38]. This result was further confirmed by 1H MAS NMR in the reaction mixture, which would be lower in the presence
spectroscopy.
of silane because the latter can convert with HF into fluorosi-
lane and H2 [39,50,53]. Consequently, less refluorination takes
Note, that 10a was activated under mild conditions without the place, and a higher selectivity towards the formation of the
use of an additional hydrogen source, which often has been olefin 1 is observed. In C6D6, the activation of 10a gave compa-
added for the activation of C–F bonds at ACF [16,39,47]. rable results as when no HSiEt3 was introduced (Scheme 4,
Scheme 2: Reactivity of 10a in the presence of ACF as the catalyst in C6D12 (top) or C6D6 (bottom) as solvents.
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Scheme 3: Proposed catalytic cycle of the transformation of 10a in C6D12 and C6D6 in the presence of ACF as the catalyst.
Scheme 4: Reactivity of 10a in the presence of ACF as the catalyst and HSiEt3 as a hydrogen source in C6D12 (top) or C6D6 (middle) as solvents or
in neat silane (bottom).
middle). Compound 10a is transformed into 1, 11, and 12 with Note, that for the transformation in the presence of silane, the
the additional presence of traces of the Friedel–Crafts product conversions reached over 99% after 7 days at 70 °C, which
CF2=CFCH2C6D5 (3). However, in neat silane, 10a was con- underlines the significant role of the silane in the reaction mix-
verted into 1, 10b, 2, 1,1,1,2-tetrafluoropropane (13), 1,1-diflu- ture. The improved conversion can generally arise from an
oropropene (5), and 1,1,1-trifluoropropane (6, Scheme 4, interaction of silane with the surface of ACF, also competing
bottom). Thus, by having a large excess of silane, a consecutive with the above-mentioned benzene interaction. Furthermore, in
reactivity was observed, which led to the formation of the the presence of silane, additional mechanistic C–F activation
hydrodefluorination product 6 as the main compound. However, steps have to be considered (Scheme 5). Basically, the immobi-
the reaction is unselective, and various intermediates are still lization of silane at the Lewis-acidic surface might result in
present in considerable amounts (Scheme 4).
silylium-like species, which subsequently initiate the C–F bond
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Scheme 5: Proposed catalytic cycle for sylilium-mediated hydrodefluorinations and dehydrofluorinations from 10a at the surface of ACF.
activation at the primary carbon–fluorine bond in 10a, gener- benium species can be initially produced by an abstraction of a
ating FSiEt3, the corresponding carbenium-like species, and a fluoride ion from a fluorinated group by the surface of the cata-
surface-bound hydride. At this stage, either the hydrodefluori- lyst. Then, carbenium ions can react with silane to yield
nation product 13 can be generated, or dehydrofluorination hydrodefluorination products, or after dehydrofluorination, HF
occurs to furnish the olefin 1 and H2, both in the presence of that in the presence of silane, produces FSiEt3 and H2 [39,50].
silane. Additionally, 1 can further react with any silylium ion Furthermore, any intermediate carbenium species, generated
species at the surface of ACF, resulting in a C–F bond cleavage directly at the ACF surface or via interaction with a silylium
at the CF3 group, yielding once again a surface hydride and the species, can be engaged in Friedel–Crafts-like reactions to give
corresponding carbenium ion. Subsequently, the allylic with C6D6 11 or 3, which is consistent with previous studies
hydrodefluorination product 2 is formed. Allylic hydrodefluori- [16].
nation reactions were previously observed at ACF. Indeed, in
the presence of silane and ACF, the CF3 group in tetrafluoro- The formation of the products described above (Scheme 4)
propenes (HFO-1234yf, 1 and HFO-1234ze, 4) was trans- involves consecutive reaction steps, such as dehydrofluorina-
formed into an olefinic CF2 group (Scheme 1) [16]. Previous tion, hydrodefluorination, hydrofluorination, allylic defluorina-
MAS NMR studies also gave evidence for the existence of tion, hydroarylation, and Friedel–Crafts reactions. In the pres-
silylium species at an ACF surface [39,47]. In addition, silylium ence of an excess of silane, the dehydrofluorination product 1
species that are stabilized by weakly coordinating anions can and the hydrodefluorination intermediate 13 are generated si-
also catalyze hydrodefluorination reactions in a homogeneous multaneously to further lead to 2, 5, and 6 as the main com-
phase with silanes as hydrogen source [54-57]. In contrast, pounds. To get further insight, independent reactions were per-
silylium-mediated dehydrofluorination reactions have not been formed to elucidate reaction patterns and to demonstrate the
found in a homogeneous phase, but germylium ions can conceivable transformations between certain products, which
promote such reaction pathways [58]. Nevertheless, the forma- were observed in the activation of 10a in the presence of ACF
tion of the compounds 1, 10b, 11, and 12 can alternatively be and HSiEt3.
initiated by the Lewis acidity of ACF itself, as outlined above
without the presence of silane (see Scheme 3). Therefore, as an It turned out that the tetrafluoropropene 13 reacts in the pres-
alternative to the initial formation of the surface silylium ion ence of silane with ACF as the catalyst in C6D12 or neat silane
species at ACF, it is in principle also conceivable that car- to give 5 and 6 (ratio 1:2, Scheme 6, top). When C6D6 was used
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Scheme 6: Reactivity of 13 in the presence of ACF as the catalyst, with (top) or without (bottom) HSiEt3 as a hydrogen source.
as a solvent in the presence of silane, 5, 14, 9, and the C6D6 (Scheme 7, bottom). Indeed, in C6D12, allylic defluorina-
Friedel–Crafts product CF3CH2CH2C6D5 were generated. Note, tion to yield 5 was observed, together with 6 and traces of
that in this context, in the absence of silane and in C6D12, the 2-chloro-1,1,1-trifluoropropene (15). The chlorinated product
selective formation of 14 was detected (Scheme 6, bottom). In 15 could stem from a plausible HCl formation if the substrate is
benzene, 14 was also formed as the main compound, together fluorinating ACF [41]. It should be noted that the synthesis of
with the corresponding Friedel–Crafts and hydroarylation prod- ACF itself consists of the fluorination of AlCl3 by chlorofluo-
ucts, both observed in traces. This suggests that silane promotes roalkanes [40,41]. Nevertheless, in C6D6, the formation of 5
the generation of 5 and 6, but for the formation of 14 it is not and 6 and the Friedel–Crafts product CF2=CHCH2C6D5 (9) was
essential.
observed from 14.
Furthermore, another independent reaction by treating 5 with Based on these findings, a general scheme can be drafted to
HF was performed (Scheme 7, top), but the hydrofluorination illustrate the sequential generation of products starting from 10a
product 6 was only detected in small amounts. In accordance for the conversions in the presence of silane at ACF
with this result, the treatment of 6 in the presence of silane and (Scheme 8). Note, that the reaction of 1 in the presence of
ACF gave the dehydrofluorination product 5. The observations silane, ACF, and solvents was repeated under the same condi-
nevertheless suggest the presence of an equilibrium between 5 tions as for the activation of 10a, and the generation of 2 and 3
and 6, that depends on the amount of HF or HSiEt3 present in was confirmed similarly as reported (see above, Scheme 1)
the reaction mixture.
[16].
Additionally, it is conceivable that 14 can further react to form Overall, the reactivity study on 10a in the presence of ACF and
5 via allylic hydrodefluorination [16]. Therefore, the reactivity HSiEt3 suggests that the formation of 1 described in the top part
of 14 was tested in the presence of ACF and silane in C6D12 or of the mechanism is favored when no silane is present in the
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Scheme 7: Independent reactions starting from 5, 6, or 14 in the presence of ACF as the catalyst.
reaction mixture or when only a small amount of silane is
present (Scheme 8). In contrast, in neat silane, the bottom part
of Scheme 8 is preferred, leading to the formation of 6.
Activation of 1,1,1,3,3-pentafluoropropane
(HFC-245fa, 10c)
The reactivity of 1,1,1,3,3-pentafluoropropane (10c) at ACF
was compared with the one of the isomer 1,1,1,2,3-pentafluoro-
propane (10a), again to elucidate conceivable reaction path-
ways and to understand potential similarities in their reactivity.
In contrast to the findings for 10a, no conversion was observed
without the use of HSiEt3 as a hydrogen source, indicating that
for the activation of CHF2 groups, silane might be required.
When 10c was treated with 0.5 equivalents of HSiEt3 with
respect to the substrate in the presence of ACF at 70 °C, the
selective generation of the 1,3,3,3-tetrafluoropropenes (HFO-
1234ze, 4a and 4b) was detected with an E:Z ratio of 10:1 and
43% conversion (Scheme 9). The transformation of 10c into 4a
and 4b is remarkable since other catalytic conversions at
chromia-based catalysts require elevated temperatures
[11,19,59-62].
Scheme 8: Proposed reaction pathways starting from 10a in the pres-
ence of ACF and silane.
Scheme 9: Reactivity of 10c in the presence of ACF as the catalyst
and 0.5 equivalents of HSiEt3 as a hydrogen source in C6D12.
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The mechanisms for the C−F bond cleavage starting from 10c produced, and the conversion is driven by the HF reaction with
to yield the isomers 4 are similar to the ones proposed in the ac- HSiEt3 to give FSiEt3.
tivation of 10a in the presence of silane (see above, Scheme 5).
It is likely that silylium species are involved in the C–F activa- When the amount of silane was increased to one equivalent,
tion steps at the ACF surface since HSiEt3 is needed to initiate further reactivity was observed (Scheme 11). In C6D12, 6 was
any reactivity. On the one hand, an initial C–F bond activation generated as the main compound, with traces of 5 and 14
at 10c by some silylium ion species will produce FSiEt3 and the (Scheme 11, top). When C6D6 was used as the solvent, 5 as the
corresponding carbenium species (Scheme 10). The latter can main product, 6 as a minor product, and 14 in traces were again
generate 4a and 4b together with H2, leading to the regenera- generated, together with the Friedel–Crafts products
tion of the catalyst. When only 0.5 equivalents of silane are CF2=CHCH2C6D5 (9) and CF3CH=CHC6D5 (8, Scheme 11,
present, the reaction does not pursue further, as shown above middle). In neat silane, 5 and 6 were detected in a ratio of 1:4
(Scheme 9). On the other hand, as for 10a, an alternative mech- (Scheme 11, bottom).
anism can be proposed where the carbenium species is initially
generated by an abstraction of a fluoride ion at the CHF2 group Notably, for all conversions (C6D6 and silane, C6D12 and
by the surface of the catalyst. Via HF elimination, 4a and 4b are silane, or in neat silane), monitoring of the reaction by
Scheme 10: Proposed catalytic cycles for the transformation of 10c in C6D12 and in the presence of 0.5 equivalents of silane and ACF as the cata-
lyst.
Scheme 11: Reactivity of 10c in the presence of ACF as the catalyst and HSiEt3 as a hydrogen source in C6D12 (top) or C6D6 (middle) as solvents or
in neat silane (bottom).
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19F NMR spectroscopy led to the observation of the early gen- when only one equivalent of silane is used. Therefore, the rate
eration of 4a and 4b after 24 hours reaction time at 70 °C. After of the reaction from 10c to 6 might be increased in neat silane
3 days at 70 °C, the transformation into the different products favoring two subsequent hydrodefluorination steps from 10c to
described above could be detected. Moreover, the continuous yield 6.
formation of FSiEt3 over time was observed for all reactions,
which underlines the crucial role of the hydrogen source at each As observed in the study on the reactivity of 10a, the variety of
reaction step. As stated, it has been reported before that for the products for a larger amount of silane can be explained by
activation of 1,3,3,3-tetrafluoropropene (HFO-1234ze, 4) at several consecutive reactions, which include C–F bond activa-
ACF in neat HSiEt3 1,1-difluoropropene (5) and 1,1,1-trifluoro- tion steps (Scheme 12). Via hydrodefluorination, 10c can
propane (6) are furnished (Scheme 1) [16]. In the presence of produce the intermediate 1,1,1,3-tetrafluoropropane (not ob-
C6D6 and silane, the Friedel–Crafts products CF2=CHCH2C6D5 served). This intermediate could give 6 via a second hydrode-
(9) and CF3CH=CHC6D5 (8) were observed, together with the fluorination. Alternatively, 1,1,1,3-tetrafluoropropane could
hydroarylation product CF3CHDCH2C6D5 (7) [16]. To note, also produce 14, which was shown to give 6 via the intermedi-
in neat silane, a distinct selectivity was detected, depending ate 5 by independent reactions. However, pathways to yield 6
on the substrates used. Indeed, starting from 10c, the product are also required from the olefins 4, because they were detected
ratio between 5 and 6 was 1:4, whereas it was reported as intermediates. This implies HF addition to 4 or 5. In the pres-
to be 1:0.8 starting from 4a [16]. This observation might ence of benzene, 14 can also generate the Friedel–Crafts prod-
be the result of a hydrofluorination reaction from 5 to 6. More- uct 9; however, product 9 can also be generated from 4 by
over, in neat silane, 14 was not detected, whereas when less hydrodefluorination followed by a Friedel–Crafts reaction. The
silane and a solvent was present, this product was observed. Friedel–Crafts product 8 can as well be formed from 10c
Due to a large amount of silane present, a more considerable directly, via a Friedel–Crafts reaction followed by HF elimina-
amount of the silylium species can be generated compared to tion.
Scheme 12: Proposed reaction pathways starting from 10c in the presence of ACF and silane.
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Comparable to the reaction patterns of 10a, the reactivity study produce 9 via a Friedel–Crafts reaction, which again supports
on 10c reveals that with a small amount of silane, the dehydro- the pathway proposed to achieve 5.
fluorination to 4 is favored, and with more silane, the conver-
sions in Scheme 12 end up favorably in the formation of 6.
Overall, for the reactivity of the pentafluoropropane isomer
10b, two different pathways seem to compete. The upper part of
the reaction patterns in Scheme 14 leads to the formation of 2,
and the bottom part provides pathways to 5. Thus, the hydrode-
Activation of 1,1,1,2,2-pentafluoropropane
(HFC-245fa, 10b)
As observed for 10c, the isomeric 1,1,1,2,2-pentafluoropropane fluorination step at 10b to form 13 seems to be more difficult
(HFC-245fa, 10b) could also not be activated without the pres- than for the other isomers 10a and 10c.
ence of HSiEt3 as a hydrogen source. The treatment of 10b in
C6D12 in the presence of silane at 70 °C gave the allylic Conclusion
hydrodefluorination product 2 as well as 5 in a ratio of 1:1 with This study on the reactivity of pentafluoropropane isomers has
38% conversion (Scheme 13, top). When benzene was used as a revealed that ACF is a suitable catalyst for dehydrofluorination
solvent together with silane, the Friedel–Crafts products 3 and 9 and hydrodefluorination reactions of polyfluorinated com-
were observed in a ratio of 1:1, but the conversion reached only pounds under mild conditions. It was elaborated that the single
18% (Scheme 13, middle). In neat silane, 2 and 5 were again C–F bond in 10a does not require the use of HSiEt3 as a hydro-
observed, but this time the conversion did not exceed 10% gen source to be activated via dehydrofluorination. However,
(Scheme 13, bottom).
when silane was introduced, further reactivity was observed,
leading to the formation of subsequent defluorination products.
As for the activation of 10c, it is plausible that surface silylium In contrast, CF2, CHF2, and CF3 groups need the presence of a
ion species are formed at ACF. Therefore, a reaction pathway hydrogen source in order to promote the activation of at least
can be suggested, starting with the initial generation of 1, one C–F bond by the formation of the thermodynamically stable
FSiEt3, and H2. Subsequently, 2 can be generated via an allylic Si–F bond. This observation is consistent with a decrease of the
hydrodefluorination as it was observed in the study of tetrafluo- bond dissociation energies of C–F bonds from trifluoromethyl
ropropenes at ACF [16]. Additionally, 5 can stem from several via difluoromethyl to monofluoromethyl groups [63]. Addition-
C–F bond activations at 10b, starting with a hydrodefluorina- ally, the C–F bond at the CHF2 group in 10c was easier to acti-
tion to generate the intermediate 13, which further undergoes an vate than the C–F bond in the CF3CF2 group in 10c, resulting in
HF elimination, followed by an allylic hydrodefluorination to an order of reactivity of 10a > 10c > 10b. When only a small
give 5. In the presence of benzene, at 1, a Friedel–Crafts reac- amount of silane was introduced for the reaction of 10a or 10c,
tion can generate 3, which further supports the formation as an the major products are due to dehydrofluorination, whereas in
intermediate of 1. Moreover, the intermediate 14 can also neat silane, formally hydrodefluorination products are gener-
Scheme 13: Reactivity of 10b in the presence of ACF as the catalyst and HSiEt3 as a hydrogen source in C6D12 (top) or C6D6 (middle) as solvents or
in neat silane (bottom).
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Beilstein J. Org. Chem. 2020, 16, 2623–2635.
Scheme 14: Proposed reaction pathway starting from 10b in the presence of ACF and silane.
ated mainly. By using C6D6, various Friedel–Crafts products Sigma–Aldrich in a sure seal bottle and stored under argon.
can be further generated. For 10b, the conversions are very low, 1,1,1,2,3-Pentafluoropropane (HFC-245eb, 10a), 1,1,1,3,3-
but dehydrofluorination and hydrodefluorination pathways pentafluoropropane (HFC-245fa, 10c), and 1,1,1,2,2-pentafluo-
compete with each other.
ropropane (HFC-245cb, 10b) were gifted by Arkema and used
without further purification. 3,3,3-Trifluoropropene (14, 99%),
Mechanistically, the C–F bonds of the fluorinated substrates can 1,1,1-trifluoropropane (6), and 1,1,1,2-tetrafluoropropane (13)
be activated by Lewis-acidic sites at the ACF surface. In the were purchased from abcr and used without further purification.
presence of silane, it can be assumed that preferentially silylium 1,1-Difluoropropene (5) was bought from Apollo Scientific and
surface species initiate the C–F bond cleavage. For both, the used without further purification. ACF was synthesized accord-
generated carbenium species show further reactivity to result in ing to the literature and stored in a glove box. The number of
dehydrofluorination, hydrodefluorination, or Friedel–Crafts active sites (1 mmol acidic sites/g of catalyst) was determined
products. Notably, the conversion in neat silane was lower in by temperature programmed desorption of ammonia (NH3-
the case of 10c and 10b, possibly because of a certain blocking TPD) [34,65]. NMR spectra were recorded at room tempera-
of the acidic sites of ACF by silane. Note in that context that ture using a Bruker DPX 300 spectrometer. A capillary of tri-
there are reports showing that silylium species can interact with fluorotoluene was employed as an external standard for quan-
more silane to generate larger entities [56,58,64].
tification purposes. The 19F NMR spectra were referenced to
PhCF3 (δ = −63.5 ppm) and the chemical shifts in 1H NMR
were referenced to residual C6D5H (δ = 7.16 ppm) or C6D11H
(δ = 1.38 ppm).
Experimental
Material and methods
The reactions were carried out using Schlenk techniques as well
Procedure for reactions with gaseous
as JYoung NMR tubes. The solvents were purchased from
Eurisotop. C6D12 was dried over molecular sieves and purged substrates
with argon prior to use. C6D6 was dried with K-Solvona and A JYoung NMR tube was loaded with 25 mg of ACF inside a
distilled prior to use. Et3SiH (99%) was purchased from glovebox. In experiments involving a solvent (C6D6 or C6D12),
2633

Beilstein J. Org. Chem. 2020, 16, 2623–2635.
7. United Nations Environment Programme; Amendment to the Montreal
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0.4 mL of the solvent was added under Schlenk conditions,
together with the corresponding amount of HSiEt3. In the reac-
tions without solvent, 0.5 mL of HSiEt3 was added using
Schlenk techniques to the JYoung NMR tube loaded with ACF.
The gases were then condensed using a small glass bulb filled
with 0.5 atm of the corresponding gas (0.1 mmol). The reac-
tions were monitored by 1H and 19F NMR spectroscopy. The
tubes were kept at 70 °C for 7 days. PhCF3 was used as an
external standard in a closed capillary to calculate the conver-
sion based on the consumed substrate by the integration of the
19F NMR spectra.
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Supporting Information File 1
14.Fang, X.-X.; Wang, Y.; Jia, W.-Z.; Song, J.-D.; Wang, Y.-J.; Luo, M.-F.;
Lu, J.-Q. Appl. Catal., A 2019, 576, 39–46.
Analytical data and copies of spectra.
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Acknowledgements
Special thanks to Dr. Clara Patricia Marshall for the fruitful
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discussion on the topic.
Funding
Financial support by the CRC 1349 “Fluorine-Specific Interac-
tions” funded by the Deutsche Forschungsgemeinschaft
(gefördert durch die Deutsche Forschungsgemeinschaft
(DFG)–Projektnummer 387284271–SFB 1349) is gratefully ac-
knowledged.
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