Transition metal-free hydrodefluorination of acid fluorides and organofluorines by Ph3GeH promoted by catalytic[Ph3C][B(C6F5)4]

Article abstract of DOI:10.1039/c9cc05075b

It has been shown that the germane Ph₃GeH converts aryl and aliphatic acid fluorides directly to their corresponding aldehydes without over-reduction via the conversion of Ph₃GeH to the germylium cation[Ph₃Ge]⁺ by a catalytic amount of the tritylium salt[Ph₃C][B(C₆F₅)₄]. Here, no transition metal catalyst is required and there is no decarbonylation of the acid fluoride, which are advantages over existing methods. The fluorine atoms can also be abstracted from organofluorine compounds using this method.

Full text of DOI:10.1039/c9cc05075b

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and S. Weinert, Chem. Commun., 2019, DOI: 10.1039/C9CC05075B.
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Transition metal-free hydrodefluorination of acid fluorides and
organofluorines by Ph3GeH promoted by catalytic [Ph3C][B(C6F5)4]
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Ardalan Hayatifar,a Alejandro Borrego,b David Bosek,b Matthew Czarnecki,b Gabriel Derocher,b
Adam Kuplicki,b Erik Lytle,b Jonas Padilla,b Charles Paroly,b Gillian Tubay,b Jackson Vyletel,b and
Charles S. Weinert*,a
It has been shown that the germane Ph3GeH converts aryl and typically proceeded in low yields and also resulted in the
aliphatic acid fluorides directly to their corresponding aldehydes formation of trifluorotoluene via decarbonylation of the acid
without over-reduction via the conversion of Ph3GeH to the fluoride, and the product distribution was also difficult to
germylium cation [Ph3Ge]+ by a catalytic amount of the tritylium control.20
salt [Ph3C][B(C6F5)4]. Here, no transition metal catalyst is required
Ogiwara, Sakai, et al. recently reported a versatile
and there is no decarbonylation of the acid fluoride, which are conversion of alkyl and aryl acid fluorides to aldehydes using
advantages over existing methods. The fluorine atoms can also be triethylsilane that was catalyzed by palladium(II) acetate in the
abstracted from organofluorine compounds using this method.
presence of a phosphine ligand.21 When PCy3 was used as the
phosphine the reaction proceeded without decarbonylation,
but decarbonylation of the acid fluoride to yield the
corresponding hydrocarbon could also be selectively achieved
by changing the ligand to a bidentate phosphine. These
findings will likely have a significant impact on the catalytic
synthesis of a variety of important molecules.22
Also of interest is the hydrodefluorination of aliphatic and
aromatic organofluorine compounds, with aliphatic carbon –
fluorine bonds being the most difficult to activate. Ozerov et
al. have reported the hydrodefluorination of a wide variety of
aliphatic organofluorine compounds by the conversion of
silanes to silylium cations promoted by tritylium salts of
halogenated carborane ions.23,24 The conversion of alkyl
fluorides to their corresponding alkanes was achieved using
Et3SiH and a catalytic amount of B(C6F5)3 as reported by
Stephan et al, which can also be achieved stoichiometrically
using FLP chemistry.25 The same R3SiH/B(C6F5)3 system also
was used by Piers et al. for the hydrosilation of carbon-oxygen
double bonds, converting aldehydes and ketones to silyl ethers
and esters to aldehydes in generally high yields,26 as well as
the reduction of alcohols to hydrocarbons,27 and the formation
of siloxanes.28-33
The preparation of aldehydes from acid chlorides can be
achieved using a variety of reagents, including transition metal
hydrides,1-3 tin hydrides in the presence of transition metals,4-6
aluminum hydrides,7-10 or boron hydrides.9 Furthermore, the
non-decarbonylative conversion of acid chlorides to aldehydes
using hydrosilanes has been known for some time, and these
processes are catalyzed by the use of transition metal
complexes.11-17 In some cases, these conversions can be
carried out for a variety of substrates under mild reaction
conditions11-12 with chemoselectivity and the tolerance of
different functional groups,11 and the transition metal catalysts
can be recovered and recycled.11, 13
Due to their ready availability from either carboxylic acids
or acid chlorides and their stability towards hydrolysis,18-19 acid
fluorides represent a useful class of carboxylic acid derivatives
for use in organic synthesis. One key synthetic transformation
of acid fluorides is their reduction to either aldehydes or their
reductive decarbonylation to hydrocarbons. However, only a
few reports of such conversions have appeared.
An initial report of such reactivity involved the palladium-
catalyzed conversion of 3-trifluoromethylbenzoyl fluoride to 3-
trifluoromethylbenzaldehyde, but the reactions employed
During our recent successful preparation of the branched
germanium fluoride compound (Ph3Ge)3GeF, it was observed
that germanium was highly fluorophilic and also that
germylium cations exhibited a tendency to abstract fluorine
from several substrates including the [BF4]- anion.34 Although
the germylium cation [(Ph3Ge)3Ge]+ that was used to obtain
(Ph3Ge)3GeF was found to be unstable with respect to
rearrangements, the simple germylium cation [Ph3Ge]+ does
a. Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma,
74078, USA. Email: charles.s.weinert@okstate.edu
b. University of Detroit Jesuit High School and Academy, 8400 S. Cambridge, Detroit,
MI, 48221, USA
†Electronic Supplementary Information (ESI) available: Experimental and synthetic
procedures, NMR spectra, GC/MS data, and computational details.
DOI: 10.1039/x0xx00000x
See
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not have this complication. The aforementioned use of Ph3GeF could be isolated in 75 % yield uVsiienwgArticcolelOunmlinne
silylium cations as fluorine abstraction reagents21-25 prompted chromatography on silica gel with benzene as the eluent.
DOI: 10.1039/C9CC05075B
us to investigate if this germylium cation would function in the
In order to verify that the [Ph3Ge]+ cation serves as the
same capacity. We wish to report that [Ph3Ge]+ can indeed fluorine abstraction agent, a neat mixture of benzoyl fluoride
serve to convert acid fluorides directly to aldehydes, and that and Ph3GeH was stirred in the absence of [Ph3C][B(C6F5)4] for
this process occurs in the absence of transition metal catalysts. 24 h, and also a mixture of benzoyl fluoride and 3 mol % of
This species can also abstract fluorine from aliphatic carbon [Ph3C][B(C6F5)4] in the absence of Ph3GeH was stirred for 24 h.
atoms.
Neither reaction resulted in conversion of the benzoyl fluoride
The conversion of benzoyl fluoride to benzaldehyde to benzaldehyde. Based on these findings, we propose that
without decarbonylation to generate benzene and also the reaction proceeds according to the pathway shown in
without over-reduction to benzyl alcohol was achieved using Scheme 1. The tritylium cation [Ph3C]+ abstracts the hydrogen
[Ph3Ge]+ as the fluorine abstraction agent. This species is atom from Ph3GeH to generate the active species [Ph3Ge]+,
readily generated from triphenylgermane Ph3GeH via hydride which in turn abstracts the fluorine atom from benzoyl fluoride
abstraction by the tritylium cation [Ph3C]+. The reaction can be to generate Ph3GeF and the benzoyl cation. The benzoyl
1
conveniently monitored using NMR spectroscopy since the H cation then proceeds to abstract a hydrogen from Ph3GeH to
NMR resonances for the hydrogen atoms in Ph3GeH and the yield benzaldehyde and regenerates the [Ph3Ge]+ cation.
Ph3CH formed during the course of the reaction are distinctive,
appearing at δ 5.81 and 5.44 ppm in C6D6 solvent, respectively.
In addition, 19F NMR spectroscopy in C6D6 can be used to
monitor the disappearance of the resonance at δ 17.9 ppm for
benzoyl fluoride and the appearance of the peak at δ – 202.4
ppm corresponding to Ph3GeF.
A mixture of benzoyl fluoride, 1.1 equiv. of Ph3GeH and 3
mol % of [Ph3C][B(C6F5)4] were combined and the mixture was
stirred at room temperature for 18 h under nitrogen as shown
in Equation 1. The product was obtained as a white solid that
1
was analyzed by H and 19F NMR spectroscopy in C6D6.
A
1
singlet at δ 9.69 ppm in the H NMR spectrum indicated the
presence of benzaldehyde, and the 19F NMR spectrum
exhibited a singlet at δ – 202.4 ppm corresponding to Ph3GeF
while no resonance was visible at δ 17.9 ppm indicating the
absence of benzoyl fluoride. Analysis of the reaction product
using GC/MS led to the observation of a peak with a retention
time of 6.95 minutes, and the mass spectrum for the eluted
material contained a peak at m/z = 105.00 corresponding to
the parent ion C6H5CO+. The GC/MS data for the reaction
mixture, including the fragmentation pattern in the mass
spectrum, is identical to that observed for an authentic sample
of benzaldehyde.
Scheme 1. Proposed reaction pathway for the hydrodefluorination of benzoyl fluoride.
During the course of the reaction, the germylium cation is
likely stabilized an interaction of the weakly coordinating anion
[B(C6F5)4]-, as has been determined for hydrodefluorination
processes involving silylium cations.35 The degree of
coordination of the anion can have an affect on the geometry
and reactivity of the cation. A minimal interaction of the anion
with the cation would result in an essentially trigonal planar
[Ph3Ge]+ cation, while more significant interactions would
1.1 equiv. Ph3GeH
3 mol % [Ph3C][B(C6F5)4]
O
O
+
H
Ph3GeF + Ph3CH
F
neat, r.t., 18 h
Equation 1. Hydrodefluorination of benzoyl fluoride using Ph3GeH.
result in pyramidalization and
a
decrease in the
electrophilicty.36 The structure of the [Ph3Ge]+ cation with no
interaction of the [B(C6F5)4]- anion by DFT can be found in the
ESI. Both the [Ph3Ge]+ and PhC(O)+ cations that are formed
during the reaction also likely interact with the PhC(O)F and
Ph3GeH reagents, respectively, as proposed in Scheme 1.37,38
The germylium cation [Ph3Ge]+ can also convert aliphatic
acid fluorides to aldehydes, as demonstrated by the reaction
of Ph3GeH with pentanoyl fluoride in the presence of
[Ph3C][B(C6F5)4] (Equation 2). After stirring the reaction
mixture in the absence of solvent at room temperature for 18
h the 19F NMR spectrum in CD2Cl2, which was used in place of
C6D6 due to the product being poorly soluble in C6D6, indicated
The same reaction was repeated in the absence of solvent
1
in order to isolate the benzaldehyde product. The H and 19F
NMR spectra as well as the GC/MS data of the product
obtained after stirring the reaction mixture at room
temperature for 18 h was identical to that observed for the
pervious reaction. The reaction was monitored using TLC, and
after the reaction was complete the benzaldehyde product
was extracted from the reaction mixture using water. The
benzaldehyde was then extracted from the water phase with
Et2O, and the solvent was then removed under vacuum to
provide benzaldehyde in 47 % yield. In a separate experiment
2 | J. Name., 2012, 00, 1-3
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the formation of Ph3GeF and the resonance at δ 45.1 ppm 1,1,1-trifluorotoluene and benzoyl fluoride werVeiewaAttrteicmle Opntleinde
corresponding to pentanoyl fluoride was absent, while a using B(C6F5)3 as the catalyst. DOInI: 10t.1h0i3s9/Cc9aCsCe0,507n5oB
1
singlet at δ 9.11 ppm in the H NMR spectrum indicated the hydrodefluorination of 1,1,1-trifluorotoluene was observed
presence of pentanal. The identity of the product was further and although the hydrodefluorination of benzoyl fluoride did
confirmed by GC/MS. The GC of the product mixture exhibited occur the conversion was only 18 % after 18 h.
a peak with a retention time of 12.18 min that is identical to
Since Ozerov et al. reported that hydrodefluorination could
the GC of an authentic sample, and the mass spectrum for the be achieved using Et3SiH in the presence catalytic amounts of
eluted material contained a peak at m/z = 85.10 corresponding tritylium halocarborane salts,23 the reaction of Ph3SiH with
to the parent ion C5H9O+.
1,1,1-trifluorotoluene and benzoyl fluoride in the presence of
[Ph3C][B(C6F5)] was attempted. Upon addition of
1.1 equiv. Ph3GeH
[Ph3C][B(C6F5)] to the mixture of the substrate and Ph3SiH, a
highly exothermic reaction was observed and partial
conversion to toluene or benzaldehyde occurred. The reaction
was also attempted using Ph3CH in place of Ph3GeH and,
unsurprisingly, no reaction was observed.
O
O
3 mol % [Ph3C][B(C6F5)4]
+
Ph3GeF + Ph3CH
neat, r.t., 18 h
H
F
Equation 2. Hydrodefluorination of pentanal using Ph3GeH.
In addition to acid fluorides, Ph3GeH also abstracts the
fluorine atoms from fluoroalkanes, as shown by its reaction
1.1 equiv. Ph3GeH
with
1-fluorooctane,
1-fluorocyclohexane,
1,1,1-
3 mol % [Ph3C][B(C6F5)4]
F
F
+
Ph3GeF + Ph3CH
neat, r.t., 1 h
trifluorotoluene, and 1,3-bis(trifluoromethyl)benzene (Scheme
2). In each case, one equivalent of Ph3GeH was used for each
fluorine atom present in the substrate. Both octane and
cyclohexane were formed from 1-fluorooctane and 1-
fluorocyclohexane, respectively, in over 99 % conversion. The
19F NMR signals at δ – 217.0 and – 173.0 ppm, respectively, for
these starting materials were absent after the reaction was
1.1 equiv. Ph3GeH
3 mol % [Ph3C][B(C6F5)4]
neat, r.t., 5 min
+
Ph3GeF + Ph3CH
3.1 equiv. Ph3GeH
3 mol % [Ph3C][B(C6F5)4]
CH3
CF3
+
Ph3GeF + Ph3CH
neat, r.t., 18 h
1
complete and the H NMR spectra and GC/MS of the products
were identical to those of authentic samples. These reactions
also proceed rapidly, as that for 1-fluorooctane was complete
within 1 h and that for fluorocyclohexane is complete in less
than 5 min.
CF3
6.1 equiv. Ph3GeH
CH3
3 mol % [Ph3C][B(C6F5)4]
+
Ph3GeF + Ph3CH
neat, r.t., 18 h
CF3
CH3
Scheme 2. Defluorination of alkyl fluorides using Ph3GeH.
The reactions of Ph3GeH with 1,1,1-trifluorotoluene and
1,3-bis(trifluoromethyl)benzene are significantly slower and
conversions of only 56 and 74 %, respectively, were achieved
after 18 h. The conversion in each case was determined by
integration of the 19F NMR resonances for the starting
materials and Ph3GeF. Furthermore, the [Ph3Ge]+ cation
appears to be selective for the abstraction of fluorine atoms
bound to aliphatic carbon atoms since no fluorine abstraction
was observed in the reactions of fluorobenzene and
octafluorotoluene with Ph3GeH in the presence of
[Ph3C][B(C6F5)4].
These reactions can also be carried out in solvent, provided
the solvent is not very Lewis basic. However, benzene cannot
be used as the solvent since the [Ph3Ge]+ cation is sufficiently
electrophilic to undergo Friedel-Krafts reactions with benzene,
as reported by Fu et al..39 The reactions shown in Equations 1
and 2 and Scheme 2 were carried out in hexane solvent and
identical products were obtained as those from the reactions
carried out in the absence of solvent, as shown by NMR
spectroscopy and GC/MS.
In summary, the hydrodefluorination of acid fluorides by
the [Ph3Ge]+ cation offers several advantages over previously
reported methods. The reaction occurs without the need for
a transition metal catalyst, the reaction is selective in that it
avoids decarbonylation of the acid fluoride to the hydrocarbon
and also over-reduction to the corresponding alcohol, and it is
sustainable since the Ph3GeF produced in the reaction can be
easily recovered by chromatography or recrystallization and
subsequently converted to Ph3GeH using lithium aluminum
hydride. This chemistry might serve for the remediation of
organofluorine compounds that are currently known to be
environmental hazards, such as the known carcinogen
perfluorooctanoic acid (PFOA) and its related derivatives. It
might also be possible to achieve partial hydrodefluorination
of organofluorine compounds, which would be highly useful
for the synthesis of pharmaceutical agents. Further
investigations into this chemistry are presently underway.
This work was supported by a grant from the National
Science Foundation (CHE-1464462) and is gratefully
acknowledged. CSW is grateful to Prof. Klaus Friedrich
(University of Detroit Mercy) for his assistance in obtaining
NMR data for this project.
The borane B(C6F5)3 was shown to activate alkyl carbon –
fluorine bonds both catalytically and stoichiometrically.25
Furthermore, this same species serves for the catalytic
hydrosilylation of carbon – oxygen double bonds.26 In order to
determine if a similar reactions to those described above
would proceed using a catalytic amount of B(C6F5)3 in the
presence of Ph3GeH, the hydrodefluorination reactions of
Conflicts of interest
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34 A. Hayatifar, F. A. Shumaker, S. P. Komanduri, S.VAiew. Article Online
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2018, 37, 1852-1859.
There are no conflicts to declare.
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