Activation of alkyl C-F Bonds by B(C6F5)3: Stoichiometric and catalytic transformations

Article abstract of DOI:10.1021/om200885c

The Lewis acid B(C₆F₅)₃ is shown to activate a series of alkyl fluorides. In stoichiometric reactions, treatment of sterically demanding phosphines with B(C₆F₅) ₃/alkyl fluorides gives phosphonium fluoroborate salts while treatment of B(C₆F₅)₃/alkyl fluorides with the salts [tBu₃PX][XB(C₆F₅)₃] (X = H, PhS) gives the alkane and the salt byproduct [tBu₃PX][FB(C ₆F₅)₃]. These fluoroalkanes are also catalytically converted to the corresponding alkanes by reaction of the fluoroalkane and Et₃SiH using B(C6F5)3 as the catalyst.

Full text of DOI:10.1021/om200885c

Communication
pubs.acs.org/Organometallics
Activation of Alkyl C−F Bonds by B(C F ) : Stoichiometric and
6
5 3
Catalytic Transformations
Christopher B. Caputo and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
S Supporting Information
*
ABSTRACT: The Lewis acid B(C F ) is shown to activate a series of alkyl fluorides. In
6
5 3
stoichiometric reactions, treatment of sterically demanding phosphines with B(C F ) /alkyl
6
5 3
fluorides gives phosphonium fluoroborate salts while treatment of B(C F ) /alkyl fluorides with
6
5 3
the salts [tBu PX][XB(C F ) ] (X = H, PhS) gives the alkane and the salt byproduct
3
6 5 3
[
tBu PX][FB(C F ) ]. These fluoroalkanes are also catalytically converted to the corresponding
3 6 5 3
alkanes by reaction of the fluoroalkane and Et SiH using B(C F ) as the catalyst.
3
6
5 3
C−F bonds are renowned for their stability. Indeed, they are
among the most inert bonds in all of organic chemistry, with a
bond energy of 490 kJ/mol. While transition-metal systems
To examine the reactivity of P/B FLPs with fluoroalkanes,
B(C F ) , tBu P, and 1-fluoropentane, 1,3-difluoropropane, 1-
6
5 3
3
fluoroadamantane, or 1,1,1,3,3,3-hexafluoro-2-(fluoromethoxy)-
propane were combined in a 1:1:1 ratio in CH Cl . In all cases,
1
−17
have been employed to effect C−F activation,
this can also
2
2
be achieved by interaction with strong Lewis acids. It was in
this resulted in the rapid formation of the new species 1−4,
1
964 when Olah et al. first reported Friedel−Crafts-type
respectively, which were isolated in yields of 88−94%. All the
18,19
11
alkylation of alkyl fluorides using BF .
have exploited BF3 and related Al reagents to effect
alkylation of a variety of substrates. Alkyl fluorides react
Since then, others
products exhibit a B signal at ca. −0.6 ppm, with B−F
3
20
21
coupling of about 65 Hz consistent with a direct B−F bond.
These chemical shifts, together with the comparatively small
gap between the 19F resonances attributable to the meta and
stoichiometrically with alane derivatives of the form R Al(X),
2
affording the formation of C−X bonds (X = Cl, C, H, O, S, Se,
para aryl fluorine atoms, support the presence of the
2
2
−
Te, N). Similarly, defluorinative allylic alkylation was achieved
fluoroborate anion [FB(C
6
F
5
)
3
] . The BF fragments give rise
19
31
by reaction of difluorohomoallyl alcohols with AlR followed by
to F resonances between −185 and −192 ppm. The P NMR
spectra for these species gave rise to shifts ranging from 45 to
49 ppm consistent with the alkylation of the phosphine, a view
3
2
3
24
25
hydrolysis. More recently, carbo and silyl cations have
been employed to abstract fluorides from organofluorine
1
compounds. The Mu
the Lewis acidity of carbonium and disilylium as well as
silylium and alumenium cations
̈
ller and Ozerov groups have exploited
further supported by H NMR data. These data were thus
24
26
consistent with the formulation of these products as [tBu P-
3
2
7,28
to activate C−F bonds and
(CH
(2), [tBu
[tBu PCH OCH(CF
2
)
4
CH
3
][FB(C
P(Adamantyl)][FB(C
] [FB(C
6
F
5
)
3
] (1), [tBu
3
P(CH
2
)
)
3
F][FB(C
F ) ]
6 5 3
effect catalytic hydrodefluorination or alkylative defluorination.
The driving force for these latter systems is the fluorophilicity
of Al and Si cations. In our own work, we have utilized the
Lewis acidity of B-based Lewis acids to generate frustrated
3
6
F
5
3
] (3), and
)
F
)
] (4), respectively
3
2
3
2
6
5
3
(Scheme 1). These formulations were further supported via a
crystallographic study of 2 (Figure 1a). The metric parameters
of this salt are typical and unexceptional. The analogous
29−32
Lewis pairs (FLPs).
We and others have demonstrated the
reaction of B(C F ) , 1,3-difluoropropane, and tBu PH
unique reactivity of FLPs, activating a variety of small
molecules, including olefins, alkynes, CO , and N O among
6
5
3
2
afforded [tBu PH(CH ) F][FB(C F ) ] (5). This species
2
2
3
6 5 3
2
2
exhibited similar spectral parameters for the fluoroborate
others. Perhaps most remarkably, FLPs provide a facile avenue
to the heterolytic cleavage of the strong H−H bond (bond
energy 426 kJ/mol) of H . While the mechanism of H
31
anion in addition to a P signal at 48.3 ppm with a P−H
coupling of 458 Hz and a 19F resonance at −221.13 ppm
consistent with the retention of one C−F bond. The
formulation was also unambiguously confirmed via crystallog-
raphy (Scheme 1, Figure 1b).
2
2
activation by phosphine/borane FLPs is not fully resolved, it
is clear that interaction with the borane Lewis acid plays a key
3
3,34
role. Gabbai and co-workers
have employed the fluorophi-
The corresponding reaction of B(C F ) , tBu P, and
licity of a variety of B derivatives to design highly sensitive
fluoride sensors. Herein, we exploit the fluorophilicity of the B-
based Lewis acid B(C F ) to activate C−F bonds for
6
5
3
3
fluorocyclohexane also led to a similarly rapid reaction.
1
However, in this case, H NMR data revealed the formation
6
5 3
of an olefinic resonance at 5.66 ppm attributable to cyclohexene
with the concurrent emergence of a PH doublet at 5.43 ppm
stoichiometric reactions with FLPs. This strategy for the
activation of alkyl fluorides by B(C F ) is further applied to
6
5 3
stoichiometric reactions with [tBu PH][HB(C F ) ] or by
3
6 5 3
catalytic reactions with silanes to give the corresponding
alkanes.
Received: September 21, 2011
Published: December 14, 2011
©
2011 American Chemical Society
27
dx.doi.org/10.1021/om200885c | Organometallics 2012, 31, 27−30

Organometallics
Communication
Scheme 1. Stoichiometric Reactions of Phosphine/Borane
FLPs and Fluoroalkanes
fluoroarenes were unsuccessful, suggesting the possibility that
only alkyl monofluorides are activated by borane. Although this
is expected to be a weak interaction, it is noteworthy that we
have previously documented the weak van der Waals
39
interaction between olefin and borane.
Expanding the reactivity via Lewis acid C−F activation, the
reaction with a hydridoborate anion as an alternative
nucleophile was probed. To a 1:1 mixture of B(C F ) and 1-
6
5 3
fluoropentane was added 1 equiv of the salt [tBu PH][HB-
3
40
(
C F ) ]. This results in the formation of pentane and a
6 5 3
mixture of 6 and B(C F ) . It is noteworthy that the salt
6
5 3
[
tBu PH][HB(C F ) ] does not react on its own with 1-
3 6 5 3
fluoropentane, implying that borane activation of the C−F
bond is required for the hydride attack. In the same vein,
treatment of B(C F ) and 1-fluoroadamantane with the salt
6
5 3
[
tBu PH][HB(C F ) ] yielded clean stoichiometric production
3 6 5 3
of adamantane and a mixture of the salt 6 and B(C F )
6
5 3
(
Scheme 2).
Scheme 2. Stoichiometric Reactions of Lewis Acid Activation
of Fluoroalkanes and [tBu PX][XB(C F ) ] (X = H, SPh)
3
6 5 3
In a similar fashion, the salt [tBu PSPh][PhSB(C F ) ] is
3
6 5 3
readily generated via reaction of (SPh) with tBu P and
2
3
41
B(C F ) . Addition of a mixture of fluoropentane and
6
5 3
B(C F ) results in the anion of the disulfide salt acting as a
nucleophile toward the Lewis acid activated fluoroalkane. This
6
5 3
Figure 1. POV-ray depictions of the salts (a) 2 and (b) 5: C, black; B,
yellow-green; F, pink; P, orange; H, gray. All hydrogen atoms except
P−H are omitted for clarity.
results in the formation of the salt [tBu PSPh][FB(C F ) ] (7)
3
6 5 3
and the thioether C H SPh. The salt 7 gives rise to the
5
11
11
expected 31P, F, and B NMR peaks, while the H NMR
19
41
1
(1
JH−P = 442 Hz) and a 31P signal at 57.1 ppm. The
42
data for the thioether were consistent with literature data.
19
11
corresponding F and B NMR data showed resonances at
As the above reactions demonstrate the utility of borane C−
F activation in stoichiometric transformations, extension of this
chemistry to a catalytic process was sought. To that end, 5 mol
% of B(C F ) was combined with Et SiH and one of the
1
−
187.3 and −0.5 ppm ( J = 70 Hz), respectively, consistent
B−F
with the presence of the fluoroborate anion. These data confirm
35
the formation of the salt [tBu PH][FB(C F ) ] (6) via
3
6 5 3
6
5 3
3
dehydrofluorination of fluorocyclohexane, giving cyclohexene.
This alternative reaction pathway presumably results from a
combination of the steric and electronic effects associated with
following fluorocarbons: 1-fluoropentane, 1,3-difluoropropane,
1-fluorocyclohexane, and 1-fluoroadamantane. In these cases
the H and F NMR data revealed the complete conversion of
1
19
the transition state. The basicity of tBu P undoubtedly plays a
the fluorocarbon used to the corresponding alkane and Et SiF
3
3
role in the deprotonation of the Lewis acid activated
fluorocyclohexane. Previous work has demonstrated the ability
in a quantitative yield in about 5 min (Scheme 3). In the case of
1
,1,1,3,3,3-hexafluoro-2-(fluoromethoxy)propane, the reaction
36,37
was much slower and required warming to 60 °C for 24 h,
resulting in the selective reaction of the fluoromethoxy
fragment, affording CH OCH(CF ) in 72% yield (Scheme
of tBu P/B(C F ) to deprotonate terminal alkynes.
3
6 5 3
The formation of 1−5 results from the interactions of the
C−F bond with the Lewis acid, which makes the C susceptible
to nucleophilic attack by phosphine. Since the combination of
phosphine and borane employed is a FLP, borane−phosphine
adduct formation does not occur, thereby allowing alkylation of
P to proceed. In this regard, a single example of C−F bond
activation by a carbon-based FLP was previously described by
3
3 2
3
). The slower reaction in this case is attributed to the presence
of the ethereal oxygen rendering the C−F bond less polar,
while offering the potential of a weak donor−acceptor
interaction with the Lewis acid, thereby sequestering some
portion of the Lewis acid catalyst. Conceptually, this finding is
38
43
44
Alcazaro and co-workers. Attempts to effect analogous
related to the pioneering work from the Piers, Gevorgyan,
45
46−50
reactions of tBu P/B(C F ) with polyfluorinated alkanes or
Rubinstajn, and Brook groups
in the use of B(C F ) in
3
6
5 3
6 5 3
2
8
dx.doi.org/10.1021/om200885c | Organometallics 2012, 31, 27−30

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a
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(
In conclusion, an approach to C−F bond activation
(
employing the commercially available Lewis acid B(C F )
has been reported. Stoichiometric reactions of alkyl fluorides
with FLPs afford salts of the general formula
6
5 3
[R PR′][FB(C F ) ]. Alternatively, activation of alkyl fluorides
3 6 5 3
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−
[XB(C F ) ] (X = H, SPh) are effective for X/F exchange.
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ASSOCIATED CONTENT
Supporting Information
■
(
*
S
Text, figures, and CIF files giving experimental, computational,
and crystallographic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
(
2
(
Hastie, J. J.; Geier, S. J.; Farrell, J. M.; Brown, C. C.; Heiden, Z. M.;
Welch, G. C.; Ullrich, M. Inorg. Chem. 2011, 50, 12338−12348.
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AUTHOR INFORMATION
Corresponding Author
E-mail: dstephan@chem.utoronto.ca.
■
*
(
(
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ACKNOWLEDGMENTS
■
D.W.S. gratefully acknowledges the financial support of the
NSERC of Canada, the award of a Canada Research Chair, and
a Killam Research Fellowship. C.B.C. is grateful for the support
of an NSERC-CGS Scholarship.
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dx.doi.org/10.1021/om200885c | Organometallics 2012, 31, 27−30