Room-temperature catalytic hydrodefluorination of C(sp3)-F bonds

Article abstract of DOI:10.1021/ja0426138

Room-temperature catalytic hydrodefluorination of the strong C(sp³)-F bonds in benzotrifluorides and fluoropentane is catalyzed by Et₃Si[B(C₆F₅)₄] and uses Et₃SiH as the source of H. Ar-CF<

Full text of DOI:10.1021/ja0426138

Published on Web 02/08/2005
Room-Temperature Catalytic Hydrodefluorination of C(sp3)-F Bonds
Valerie J. Scott, Remle C¸ elenligil-C¸ etin, and Oleg V. Ozerov*
Department of Chemistry, Brandeis UniVersity, MS015, 415 South Street, Waltham Massachusetts 02454
Received December 8, 2004; E-mail: ozerov@brandeis.edu
Scheme 1
Activation of carbon-fluorine bonds remains one of the major
challenges of modern organometallic chemistry. C-F activation is
both a fundamentally appealing task (C-F is the strongest single
bond to carbon)1 and one of practical relevance to the potential
remediation of atmospheric pollutants. Freons (CFCs) are a proven
hazard to the ozone layer,2 while CF4 and heavier perfluoroalkanes
are extremely potent and long-lived greenhouse gases.3 The simplest
transformation of a C-F bond is conversion to a C-H bond
(hydrodefluorination or HDF, eqs 1 and 2). Utilization of H2 in
HDF results in formation of the toxic and difficult-to-handle
hydrogen fluoride. Replacement of H2 with X3SiH (X ) alkyl, aryl)
as the source of hydride leads to the more benign X3SiF and makes
the overall reaction more thermodynamically favorable.4a
R-F + H-H catalyst8 R-H + H-F
(1)
B have proven difficult. Our catalytic cycle calls for excess Et3-
SiH, and B is made by abstraction of hydride from Et3SiH by Ph3C-
[B(C6F5)4] (A).9b,c We find that generation of B in situ via addition
of a catalytic amount of A to the mixture of substrate and Et3SiH
is preferable.
R-F + X3Si-H catalyst8 R-H + X3Si-F
(2)
Transition metal-catalyzed HDF has so far been largely limited
to select fluoroarene substrates,4 although stoichiometric HDF
reactions with aliphatic fluorides have been reported by Jones et
al. with the Cp*2ZrH2 reagent.5 Most designs for C-F bond
activation utilize electron-rich transition metal centers and depend
on C-F oxidative addition or one-electron transfer followed by
loss of fluoride as the key C-F bond-breaking steps.6,7 In this work
we present the first example of the room-temperature HDF of
aliphatic C(sp3)-F bonds.
We selected a series of benzotrifluorides as well as 1-fluoro-
pentane and perfluoromethylcyclohexane as our test substrates
(Scheme 1).13 We performed the reactions either “neat” (substrate
+ Et3SiH + catalytic (1-4%) amount of A or B) or in a solvent,
with monitoring by 19F NMR and eventually by GC-MS. The
results are summarized in Table 1. For substrates 1-7, products
resulting from complete replacement of aliphatic fluorines by
hydrogens were identified. The relative activity of the substituted
benzotrifluorides can be roughly correlated with standard substituent
effects.14 For the more reactive substrates 1-3 and 7, complete
consumption was observed in <2 h at 22 °C. The results are
consistent with the more facile F- abstraction from the more
electron-rich substrates that would generate more stable carboca-
tions. For the Ar-CF3 substrates we did not observe ArCHF2 or
ArCH2F intermediates (19F NMR in situ), which is consistent with
the slow abstraction of first F-. We have no evidence of the
formation of free silylium or alkyl cations in our reactions. However,
it is likely that highly electrophilic species approaching “free”
cations are generated,15 and qualitative considerations for the cations
should be applicable. The HDF reactions are tolerant of the aryl
halide functionality. Abstraction of Hal- (halide) from Ar-Hal is
expected to be more difficult because of the great instability of the
aryl cations. Lectka et al. documented abstraction of F- from Ar-
CF3 groups by aryl cations.16 The inertness of 8 probably reflects
the relative difficulty in generating a perfluoroalkyl cation under
the employed conditions. The reactions proceed more slowly in
p-C6H4BrF and in some neat reactions, likely because of the lower
polarity of the media. In the reactions in o-C6H4Cl2, we also
observed partial disproportionation of Et3SiF into Et4Si and Et2-
SiF2 (GC-MS and 19F NMR evidence). This redistribution is
undoubtedly catalyzed by one of the strong Lewis acids in solution
and is probably boosted by the higher solvent polarity.
We set out to explore a conceptually different approach to C-F
activation wherein the crucial C-F cleavage step occurs via
abstraction of F- by a Lewis acid. This clearly requires a Lewis
acid with exceptional affinity for F-, and we surmised that an X3-
Si+ cation should be an appropriate choice (Scheme 1). The high
affinity of Si for F is well-established. It is a matter of some debate
what should be considered a “free” X3Si+ cation in the condensed
phase.8,9 Regardless, it is clear that X3Si+ “salts” of weakly
coordinating anions (carboranes and tetrarylborates) are exception-
ally electrophilic and approach the properties and the reactivity of
the putative “free” X3Si+.8,9 Abstraction of F- from a C-F bond
in R-F would generate a carbocation R+. Conveniently, many X3-
Si+ “salts” are generated by abstraction of H- from X3SiH by
Ph3C+.8,9 It is reasonable to assume that a carbocation R+ more
reactive than the relatively stabilized Ph3C+ would be able to
abstract H- from X3SiH as well. The overall process is designed
to be a Si-H/C-F metathesis catalyzed by X3Si+ (Scheme 1).
Krause and Lampe in the 1970s studied the reactions between
+
+
CF4 and SiH3 (or between CF3 and SiH4) in the gas phase and
found that redistribution favoring F on Si and H on C occurred
readily.10 Attempts at preparation of Et3Si[B(C6H3(CF3)2)4] resulted
in the formation of Et3SiF, presumably via abstraction of F- from
the anion.11 The mechanism in Scheme 1 is also related to the
reduction of ethers and ketones, reported by Gevorgyan and Piers.12
We chose Et3Si[B(C6F5)4] (B)9b,c as a synthetic equivalent of
“X3Si+” and Et3SiH as the H source. Isolation and storage of pure
9
2852
J. AM. CHEM. SOC. 2005, 127, 2852-2853
10.1021/ja0426138 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. HDF Results after 24 h at 22 °C (Ar-CF3 f ArCH3)
In summary, we have demonstrated the first room-temperature
catalytic hydrodefluorination of aliphatic C-F bonds. Our process
utilizes an unconventional method of C-F bond activation that
relies on the abstraction of F- by an electrophilic silylium species.
Although activation of C-F bonds in perfluoroalkanes by this
method remains elusive, it may yet be achieved via utilization of
more robust counterions, such as halogenated carboranes,8,9 and
silanes other than Et3SiH.
Si
−
F conv,
C−
F conv,
a
b
no.
substrate
catalyst
solvent
%
%
TONc
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
A
A
A
A
A
A
B
B
A
neat
75d
65e
38
70
61
126
60
35
61
53
65
63
19
71
o-C6H4Cl2
p-C6H4BrF
p-ClC6H4Me
C6H5F
C6D5Br
o-C6H4Cl2
p-C6H4BrF
100
100
100
100
100
75
73e
22
Acknowledgment. This research was supported by Brandeis
University (start-up funds), Research Corporation (Research In-
novation Award to O.V.O.), and the Nathan and Bertha Richter &
Ernest Grunwald Undergraduate Award to V.J.S.
f
o-C6H4Cl2
81e
100
100
10
11
12
13
14
15
16
17
18
2
2
2
2
2
2
2
2
2
A
A
A
A
A
A
B
B
A
neat
47d
87
97
87
46
67
94
95
27
81
o-C6H4Cl2
p-C6H4BrF
p-ClC6H4Me
C6H5F
C6D5Br
o-C6H4Cl2
p-C6H4BrF
>95e
>95
53
Supporting Information Available: Experimental details and
characterization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
100
100
100
100
77
>95
>95e
31
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o-C6H4Cl2
93e
100
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