C-F activation reactions at germylium ions: Dehydrofluorination of fluoralkanes

Article abstract of DOI:10.1039/d0cc01420f

Reactions of the trityl cations with germanes afford the germylium ions [R₃Ge][B(C₆F₅)₄] (1a: R = Et, 1b: R = Ph, 1c: R = nBu). These compounds react with germane or fluorogermane to give polynuclear species, which are sources of the mononuclear ions, The latter convert with phosphines to yield the [R₃Ge-PR₃]⁺ (4a: R = Et, 4b: R = Ph) cations. Catalytic dehydrofluorination reactions were observed for the C-F bond activation of fluoroalkanes when using germanes as hydrogen source.

Full text of DOI:10.1039/d0cc01420f

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C–F activation reactions at germylium ions:
dehydrofluorination of fluoralkanes†
Cite this:DOI: 10.1039/d0cc01420f
Maria Talavera,
Gisa Meißner, Simon G. Rachor and Thomas Braun
*
Received 23rd February 2020,
Accepted 10th March 2020
DOI: 10.1039/d0cc01420f
rsc.li/chemcomm
Reactions of the trityl cations with germanes afford the germylium by Weinert and co-workers using in situ formed [Ph Ge][B(C F ) ]
3
6 5 4
6
3 6 5 4
ions [R Ge][B(C F ) ] (1a: R = Et, 1b: R = Ph, 1c: R = nBu). These ions in neat substrates. However, literature known trialkyl germy-
compounds react with germane or fluorogermane to give poly- lium ions are limited to [R Ge] (R = Me, Et) stabilized by carborane
nuclear species, which are sources of the mononuclear ions, The anions and no studies on bond activation reactions are known. In
+
3
5d,g
+
latter convert with phosphines to yield the [R
b: R = Ph) cations. Catalytic dehydrofluorination reactions were organic compounds are very scarce
observed for the C–F bond activation of fluoroalkanes when using C–F bond activation reactions have not been studied.
germanes as hydrogen source. Herein, we describe the generation and identification of
3 3
Ge-PR ] (4a: R = Et, general, investigations on the reactivity of germylium ions towards
5i,l,n,o,6
4
and so far, stoichiometric
+
À
the germylium ions [R Ge] with [B(C F ) ] as counteranion
3
6 5 4
Fluorinated compounds play an important role in everyday life (1a: R = Et, 1b: R = Ph, 1c: R = nBu) and their role in the C–F
in terms of agrochemicals, pharmaceuticals, liquid crystals and bond activation reactions of fluorinated alkanes. The reactions
1
cooling agents. Catalytic C–F activation reactions to access led not to hydrodefluorination reactions, but instead to unpre-
fluorinated building blocks open up opportunities in synthetic cedented dehydrofluorination reactions at molecular main
2
chemistry, but can often be considered as a major challenge,
which is frequently overcome by the formation of strong H–F,
group compounds.
Treatment of [Ph
3
C][B(C
6
F
5
)
4
] with one equivalent of triethyl-,
Ge][B(C ] (1a: R = Et,
2
,3
B–F, Al–F, Si–F or Ge–F bonds. In this regard, strong Lewis- triphenyl- or tributylgermane gave [R
acidic main-group compounds such as silylium ions were used 1b: R = Ph, 1c: R = nBu) as well as Ph CH (Scheme 1). NMR
3
6 5 4
F )
4
3
to induce C–F activation reactions resulting often in hydro- spectroscopic data confirmed the consumption of the corres-
defluorinations in the presence of hydrogen sources.
ponding germane due to the abstraction of a hydride by the trityl
have been obtained cation. In the case of the cation in 1a, the quartet for the CH
via different synthetic approaches. They often bear bulky sub- moiety appears Dd = 0.5 ppm shifted towards higher field with
stituents and are stabilized by weakly coordinating anions. One respect to the previously synthesized [Et Ge][CHB11 Br
of the most used methodologies consists of a hydride transfer germylium ion. LIFDI-TOF mass spectra show the isotopic
4
g,5
During the last years, germylium ions
2
3
H
5
6
]
5g
+
reaction at germanes to a trityl cation. Thus, in 2013 M u¨ ller pattern and molar masses of [R
et al. described the syntheses of sterically bulky germylium ions As it has been previously reported by Reed et al. that with
such as [Mes Ge] , where the latter is formed from Mes MeGeH carboranes as counteranions, digermylium ions might be gen-
3
Ge] (see ESI†).
+
3
2
4g
5g
by a rearrangement reaction. One year later, they synthesized a erated in the presence of an excess of germane. On treatment
hydrogen bridged naphtyldigermylium ion capable of performing of [R Ge][B(C ] (1a–c) with one equivalent of germane the
catalytic hydrodefluorination reactions at C(sp )–F bonds using formation of polynuclear species such as the digermylium ions
Et SiH as hydrogen source with comparable turnover numbers [R Ge–H–GeR ][B(C ] (2a–c) can be assumed (Scheme 2). Broad
3
6 5 4
F )
3
3
3
3
6 5 4
F )
5i
than those found for similar silylium ions as catalyst. Catalytic signals at d = 2.08 ppm (for R = Et), d = 5.40 ppm (for R = Ph)
hydrodefluorination reactions have also been recently published
Department of Chemistry, Humboldt-Universit a¨ t zu Berlin, Brook-Taylor-Str. 2,
1
2489 Berlin, Germany. E-mail: thomas.braun@cms.hu-berlin.de
†
Electronic supplementary information (ESI) available: Full characterization,
NMR spectra and X-ray data are provided. CCDC 1983059. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/d0cc01420f
Scheme 1 Formation of the germylium ions 1 (1a: R = Et, 1b: R = Ph,
1c: R = nBu).
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analogues. Additionally, the asymmetric unit shows a water
molecule which binds via a hydrogen bond to the coordinated
water molecule with a O–O separation of 2.492 Å.
In order to get more insight on the structure and reactivity of
1
a, it was also reacted with PEt or PPh in deuterated ortho-
3 3
dichlorobenzene. After 5 minutes, signals at 1.9 ppm and
.3 ppm for [Et Ge–PR ][B(C ] (R = Et (4a), Ph (4b)) in
2
3
3
6 5 4
F )
3
1
1
the P{ H} NMR spectra were observed (Scheme 2). Further
support for the presence of these compounds was provided by
the LIFDI-TOF spectrum of 4b, which reveals a peak at m/z 423
with the corresponding isotopic pattern for the cation (see ESI†).
Scheme 2 Reactivity of 1a. All reactions were performed in ortho-
dichlorobenzene-d
4
at room temperature for 5 min (4a: R = Et, 4b: R = Ph).
1
and d = 1.97 ppm (for R = nBu) in the H NMR spectra can be
The mixture of 1a and Et GeH was also treated with one
3
assigned to the proton bridging two germylium centres. In every equivalent of PEt (Scheme 2). Again, compound 4a was obtained
3
case, the resonance appears shielded with respect to R GeH together with Et GeH, GeEt and Et GeH in an approximate
3
3
4
2
2
(Dd = 1.88, 0.55 and 2 ppm for R = Et, R = Ph and R = nBu, ratio of 8 : 2.4 : 1. This again indicates the occurrence of rearran-
respectively), which is in accordance with data for the previously gement reactions, but also that a mixture of 1 and germanes
5i
described naphtyldibutyldigermylium cation. However, a DOSY shows a comparable reactivity to those of 1.
NMR experiment of a product mixture of compound 1a and
Treatment of [R Ge][B(C F ) ] (1a: R = Et, 1c: nBu) with
Et GeH indicated that more than one species are present. Note fluorocyclohexane in a 1 : 1 ratio in deuterated 1,2-dichloro-
that after adding more Et
3
6 5 4
3
3
GeH to the mixture of 1a and Et
3
GeH benzene gave the Friedel–Crafts product, 1,2-dichloro-4-cyclo-
1
10
a shift of the H NMR signal to lower field as well as the hexylbenzene-d . In addition, a broad peak at d = À200 ppm
3
1
9
formation of GeEt was observed, which suggest the occurrence in the F NMR spectrum was observed, which might be assigned
4
4g
of rearrangement reactions.
3 3 6 5 4
to polynuclear species such as [R Ge–F–GeR ][B(C F ) ] (5),
The germylium ions in 1 are highly water-sensitive. Thus, 1a although a second fluorine containing product was not detected.
1
9
reacts with water to yield [Et
3
Ge–OH
The H NMR spectrum of 3 reveals a very broad signal at varies over time or after addition of further fluorocyclohexane
.1 ppm due to the protons at the germanium bound water. suggesting the formation of various fluoride-bridge germylium
2 6 5 4
][B(C F ) ] (3) (Scheme 2). However, the chemical shift in the F NMR spectrum slightly
1
5
The molecular structure of compound 3 was determined by compounds. Additionally, the reaction of a mixture of 1a,c with
single-crystal X-ray diffraction analysis (Fig. 1). Compound 3 R GeH and fluorocyclohexane provided the same result (Scheme 3),
3
19
crystallizes in a distorted tetrahedral structure. The sum of the giving a signal in the F NMR spectrum of the product reaction
C–Ge–C angles is 347.261, which is consistent with literature- solution at d = À196.9 ppm (R = Et) or d = À201.6 ppm (R = nBu).
1
known structures for germanols or cations exhibiting hydroxo- Note that HD and H formation was observed in the H NMR
bridged germanium centres.
2
5
b,7
The Ge–O bond length in 3 of spectrum. The presence of fluoride-bridged cations is further
1
.923(2) Å is slightly longer than the distance in the hydroxo supported by a reaction of the compounds 1a,b with R GeF
3
+
5b
digermylium ion [(Me
around 0.15 Å longer than the Ge–O bond in Ph
3
Ge)
2
OH] (1.897(4) and 1.903(4) Å) and (R = Et, Ph), which resulted in resonances at d = À196.9 ppm for
7
3
GeOH or R = Et and d = À170.4 ppm for R = Ph, which appear at lower field
8
19
(Ph
3
Ge)
2
O. Similar differences have been found for silicon than the signals for R GeF in the F NMR spectra (Scheme 3).
3
The in situ generated ions 1 were then used as catalysts
(5 mol%) for the C–F bond activation of 1-fluorocyclohexane.
In a very different outcome, the presence of two equivalents
of Et GeH or nBu GeH in deuterated 1,2-dichlorobenzene as
3
3
solvent promoted the formation of the dehydrofluorinated
Fig. 1 ORTEP representation of the cation in 3ÁH
2
O; ellipsoids are drawn
at a 50% probability level. Selected bonds lengths (Å) and angles (1): Ge1–
O1 1.9247(14); Ge1–C1 1.932(2); Ge1–C3 1.9324(19); Ge1–C5 1.932(2);
C3–Ge1–C5 116.42(9); C1–Ge1–C5 113.97(9); C1–Ge1–C3 116.89(9);
C3–Ge1–O1 103.43(8); C5–Ge1–O1 101.47(8); C1–Ge1–O1 101.20(8).
Scheme 3 Formation of the germylium ions 5 (5a: R = Et, 5b: R = Ph,
5c: R = nBu).
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Table 1 C–F activation of fluorocyclohexane by germylium ions
Table 2 Dehydrofluorination of 1-fluoropentane
a
a
R
T (1C)
Time
Conversion (%)
Products ratio x (mol%)
T (1C)
Time
Conversion (%)
b
Et
Et
Et
nBu
nBu
rt
rt
100
rt
65
30 min
5 h
1 d
4 d
2 h
100
99
100
23
7 : 1
5
10
5
10
10
rt
rt
65
65
65
1 dc
62
73
99
100
99
b
c
12 : 1
10 : 1
1 : 0
1 d
6 h 30 min
3 h
4 h
d
100
3 : 1
a
a
19
Based on the consumption of fluorocyclohexane by integration of the
Based on consumption of fluoropentane by integration of the
F
19
b
c
b
c
d
F NMR spectra. 2.5 mol% of catalyst. Compound 4b as catalytic NMR spectra. 3 d, 66% conversion. 3 d, 77% conversion. nBu GeH
3
precursor.
used as hydrogen source.
product cyclohexene (Table 1) together with the corresponding
fluorogermane species and dihydrogen. The formation of small
amounts of cyclohexane could also be detected. A reaction of
Et GeH or nBu GeH with fluorocyclohexane in presence of
3
3
+
[
Ph C] without using a solvent did not lead to any different
3
outcome. The observations are in sharp contrast to the
mentioned report on catalytic hydrodefluorination reactions
at fluoroalkanes, which occur in the presence of HGePh
3
with-
Scheme 4 Simplified mechanism of the catalytic dehydrofluorination of
fluorocyclohexane by germylium ions (R = Et, nBu).
out solvent or by using a naphtyldigermylium ion and silanes as
5
i,6
hydrogen source. Note that when 1a was used as catalyst and
HSiEt as the hydrogen source, only hydrodefluorination of
fluorocyclohexane is observed, which is consistent with the and a carbenium-like ion. In the presence of germane, an olefin
3
4
a,b,e,i
reactivity pattern of silylium ions.
When Et GeH was used as hydrogen source a TON of 40 the germylium ion (Scheme 4). The formation of intermediate
could be obtained after five hours. In contrast, nBu GeH only carbenium-like ions as intermediates is also supported by
and dihydrogen are generated in addition to the regeneration of
3
3
gave a 23% of conversion at room temperature after four days, the formation of 2-pentene isomers instead of 1-pentene
however, heating at 65 1C reduced the reaction time to two when 1-fluoropentane is used as starting material. Note that a
hours with full conversion of fluorocyclohexane. Note that the mechanism via carbenium-like species based on silylium ions
triphenylgermylium ions led to reactivity towards the counter- was proposed for hydrodefluorination reactions by Ozerov and
4a,e,i
anion as well as to the generation of Friedel–Crafts products M u¨ ller,
but the presence of germanes in solution obviously
with the solvent. [Et Ge-PPh ][B(C F ) ] (4b) was also tested in favours a dehydrofluorination pathway.
3
3
6 5 4
the catalytic reaction, which results in a comparable outcome
as with [Et Ge][B(C ] (1a) when heating at 100 1C. Therefore, germanes in heterogeneous reactions using the Lewis-acid
triethylgermane was chosen as the best hydrogen source for the aluminum chlorofluoride ACF (AlCl 3Àx, x = 0.05–0.3) as
following studies, using the trityl cation as pre-catalyst for the catalyst, for which germylium-type intermediates were proposed,
in situ formation of the catalyst 1a (Table 2). and dehydrofluorination reactions were also found, although
The germylium ion reactivity resembles the behaviour of
3
6 5 4
F )
x
F
1
1
Treatment of a mixture of the trityl cation as catalytic 70 1C and 4 d were required. Note also that for homogeneous
precursor (10 mol%) and triethylgermane as hydrogen source reactions, only very few examples of transition metal complexes
with 1-fluoropentane gave at 65 1C in 3 hours 2-pentene isomers of Sc, Ti and Ce have been reported to perform dehydrofluorina-
in a ratio of the E- and Z-isomer of 3 : 1. Other fluorinated tion reactions via an initial C–H bond activation followed by
1
2
compounds such as 1-fluoroheptane and fluoroethane were also b-fluoride elimination.
tested. Whereas the former gave a mixture of E/Z-2-heptene In conclusion, the synthesis of germylium ions has been
isomers (3 : 1 ratio), Et GeF and dihydrogen, the latter converted achieved and their reactivity towards fluorinated alkanes was
into GeEt , ethane and Et GeF. Difluoroethane, trifluoroethane studied. Although the structure of the polygermylium ions
3
4
3
and 1,1,1,2,3,3-hexafluoropropane did not react, which suggest remains unclear, their reactivity towards phosphines demon-
that the activation of any CF2 or CF3 moieties is difficult. strate their applicability as sources of mononuclear species.
However, 1,1,1,3,3-pentafluorobutane was converted to 2,4,4,4- The germylium-type ions react with fluoroalkanes to form fluoro-
tetrafluoro-1-butene in 4.6% after three days at 65 1C.
germylium ions. When no excess of germane is present, Friedel–
A tentative mechanistic proposal for the catalytic dehydro- Crafts reactivity was observed. However, in the presence of
fluorination reactions involves the C–F activation of a fluoro- germane the carbenium-like ion reactivity is diminished.
alkene by a germylium ion-type species to give fluorogermane Remarkably, catalytic C–F activation at monofluorinated alkanes
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using Et
3
GeH or nBu
3
GeH promote dehydrofluorination reac-
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reactions routes for defluorination of fluorinated alkanes.
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