C?F Bond Activation by Silylium Cation/Phosphine Frustrated Lewis Pairs: Mono-Hydrodefluorination of PhCF3, PhCF2H and Ph2CF2

Article abstract of DOI:10.1002/chem.201705276

Single defluorination of aryl polyfluoromethyl functionalities is achieved by both intra- and intermolecular silylium cation/phosphine Lewis pairs. Phosphine-captured aryl fluoromethyl cations are then treated with Br?nsted base to complete the first mono

Full text of DOI:10.1002/chem.201705276

A Journal of
Accepted Article
Title: C-F Bond Activation by Silylium Cation/Phosphine Frustrated
Lewis Pairs: Mono-Hydrodefluorination of PhCF3, PhCF2H and
Ph2CF2
Authors: Ian Mallov, Adam Ruddy, Hui Zhu, Stefan Grimme, and
Douglas Wade Stephan
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201705276
Link to VoR: http://dx.doi.org/10.1002/chem.201705276
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C-F Bond Activation by Silylium Cation/Phosphine Frustrated Lewis
Pairs: Mono-Hydrodefluorination of PhCF3, PhCF2H and Ph2CF2
Ian Mallov, Adam J. Ruddy, Hui Zhu, Stefan Grimme* and Douglas W. Stephan*
Dedicated to the memory of Professor Samuel Mallov, State University of New York
Abstract: Single defluorination of aryl trifluoromethyl functionalities is
achieved by both intra- and intermolecular silylium cation/phosphine
Lewis pairs. Phosphine-captured aryl fluoromethyl cations are then
treated with Brønsted base to complete mono-hydrodefluorinations of
PhCF3, Ph2CF2, and PhCF2.
hydrodefluorination of C-F bonds catalyzed by silylium cations in
the presence of silane,[12] Reed and coworkers used silylium
cations with a carborane-based anion to isolate fluorinated
carbon-cations.[13] Müller and coworkers subsequently described
the use of disilyl-cations in hydrodefluorination reactions,[14] while
we described hydrodefluorinations of fluoroalkanes catalyzed by
[15]
B(C6F5)3
or electrophilic phosphonium cation salts such as
The electronegative element fluorine is currently included in
over 200 commercial pharmaceuticals including such block-
buster drugs as Crestor, Prozac and Celebrex. Fluorinated
fragments are bioisosteric units that are chemically resistant to
metabolic oxidation. While a growing range of protocols for
fluorination or trifluoromethylation have been established, the
inclusion of CF2 units is much less common.[1] Nonetheless, CF2H
units have recently been shown to be unique, biologically-relevant
hydrogen bond donors.[2]
[(C6F5)3PF][B(C6F5)4].[15-16] These latter catalysts have also been
used in Friedel-Crafts couplings, of alkyl or aryl-CF3 species or
benzyl fluorides.[17] In a related vein, Paquin et. al. have described
acid mediated Friedel-Crafts arylation of benzyl fluorides.[18]
One conceivable strategy to CF2H fragments is the
derivatization of CF3 units. However, the relatively strong nature
of C-F bonds (ca. De = 120-130 kcal/mol),[3] makes this approach
challenging and accounts for the long environmental lifetimes of
molecules containing CF3 fragments.[4] In 1964 Olah et al.
reported the first Friedel−Crafts alkylation of alkyl monofluorides
using BF3.[5] Subsequently, C-F bond activations using transition
metal species have been studied by the groups of Jones,[6]
Grushin,[7] Oestreich[8] and others;[9] these systems effect
stoichiometric activations of aryl or monofluoroalkyl C–F bonds.
Avenues to chemistry of CF3 units have also utilized strong
main group Lewis acids.[4a] The alkylation of CF3 species has also
been carried out via stoichiometric reactions of aluminum alkyl
species.[10] Kemnitz and coworkers described alkylation of
benzene with fluoromethanes using the heterogeneous catalyst,
aluminum-chlorofluoride.[11] While Ozerov described the
Scheme 1: Previous aryl-CF3 to aryl-CF2 conversions reported by
Lectka (A) and Hosoya (B).
Reactions achieving mono-defluorinations of CF3 fragments
are rare. Electrochemical methods,[19] or use of excess Mg[20] or
SmI,[21] have been used to form CF2-C derivatives. In 1997,
Lectka described
intramolecular reaction
a
cyclic CF2 linked diarene via the
of aryl-CF3 and diazonium
fragments.(Scheme 1A).[22] In 2016, Hosoya and co-workers
described reactions in which silylium cations ortho to CF3 groups
effected single fluoride abstraction permitting reaction with vinyl
silanes to generate new CF2-vinyl moieties with adjacent SiF units
(Scheme 1B).[23] Most recently, Ishikawa used CF3-derived
alkenes to effect difluoroallylation of arenes.[24] In this
communication, we describe our efforts to develop an FLP
[*]
I. Mallov, Dr. A. J. Ruddy Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
80 St. George St, Toronto Ontario M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
Dr. Hui Zhu, Prof. Dr. S. Grimme, Mulliken Center for Theoretical
Chemistry, Institut für Physikalische und Theoretische Chemie
Universität Bonn, Beringstr. 4, 53115 Bonn (Germany).
Email: grimme@thch.uni-bonn.de
strategy
to
effect
selective
stoichiometric
mono-
hydrodefluorination of di- and trifluoromethyl functionalities
without the requirement of C-F activation by an ortho-silylium
cation. Wedemonstrate the first conversions of CF3 to CF2H and
CF2H to CH2F.
[**]
The authors gratefully acknowledge the financial support of the
NSERC of Canada and the Deutsche Forschungsgemeinschaft in the
framework of the Leibniz prize. DWS is also grateful for the award of
a Canada Research Chair and an Einstein Fellowship at TU Berlin.
Dr. Timothy Johnstone is thanked for helpful discussion.
Supporting information for this article is given via a link at the end of
the document.
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In an initial effort, we targeted the activation of a CF3
fragment adjacent a phosphine donor. Using our previously-
reported synthesis of 1-(bispentafluorophenylphosphino)-8-
trifluoromethylnaphthalene, C10H6(1-P(C6F5)2)(8-CF3) 3 [25] based
on protocols developed by Hartwig,[26] we also prepared 1-
(3) Å). These parameters are consistent with the poor donor ability
of the phosphorus centre, and reflect the considerably
electrophilic nature of the carbon centre The formation of 5 arises
by trapping of the difluoro-carbocation generated by fluoride
abstraction from CF3. This can be viewed as the action of the Si+/P
FLP on the CF3 fragment. A critical feature in this reactivity is the
electron-poor nature of the phosphine which affords the FLP in
the presence of the highly electrophilic and fluorophilic silylium
cation. As such 5 is, to the best of our knowledge, the first case in
which a phosphine donor intercepts a difluoro-carbocation
affording a CF2-P linkage.
(diphenylphosphino)-8-trifluoromethylnaphthalene,
PPh2)(8-CF3) in 60% yield. Treatment of
C10H6(1-
with
2
2
[Et3Si(tol)][B(C6F5)4] did not lead to C-F activation. Rather than the
combined action of the Lewis acid and Lewis base on the CF3
fragment, a classical Lewis acid-base adduct was derived via
coordination of the phosphine to the [Et3Si] cation affording
[C10H6(1-PPh2SiEt3)(8-CF3)] [B(C6F5)4] 4 in 88% yield. This was
evidenced by a shift in the 31P NMR resonance from –2.5 ppm to
5.0 ppm.[27] Diminished through-space coupling between the 19F
nuclei of the CF3 group and the 31P nucleus from 110 Hz in the
free phosphine to 29 Hz is consistent with formation of the adduct
4.[28] P-Si satellites were not observed for 4. A previous report
reveals P-Si couplings in phosphine-silylium adduct to be of
similar magnititude to the observed P-F coupling.[29] Indeed, this
o
P-Si bond remained intact even upon heating to 100 C and no
evidence of C-F bond activation was observed (Scheme 2).
In a subsequent effort, the corresponding reaction of 3,
which incorporates a phosphine donor of reduced basicity, was
undertaken, anticipating that this would deter classical adduct
Figure 1: POV-ray depiction of the cation of 5. C: black, P: orange,
F: pink. H-atoms have been omitted for clarity.
formation. Treatment of
3
with 0.9 equivalents of
[Et3Si(tol)][B(C6F5)4] gave rise to a new species 5 in 91 % yield.
Extensive DFT calculations at the PW6B95-D3/def2-
QZVP + COSMO-RS(C6H5Br) // TPSS-D3/def2-TZVP +
DCOSMO-RS(CHCl3) level were performed to probe the
mechanism of this C-F activation of 3 by cation [Et3Si(tol)]+.[30]
Geometry optimization calculations were carried out at the TPSS-
D3/def2-TZVP + DCOSMO-RS(CHCl3) level of theory, which
included the solvation effects using CHCl3 (ε = 4.81) whose
dielectric constant is similar to the experimental solvent C6H5Br
(ε = 5.17). Frequency analysis was used to confirm the nature of
stationary points and to calculate thermal free energy corrections
Gc at 298 K and 1 atm. Single-point energies were computed at
the PW6B95-D3/def2-QZVP level of theory and using better
solvation free energies (Gsol) based on the COSMO-RS (C6H5Br)
calculations The final free energies at the highest PW6B95-D3
level including both thermal and solvation free energy corrections
are used in our discussion. Two energetically similar free energy
pathways for the formation of 5 were computed, with the
[Et3Si(tol)]+ cation complexes interacting with either the ortho- or
para-carbon atom (see SI). Interactions of the silylium ion Et3Si+
with the ortho- or para-fluorine atoms of the borate anion were
also considered but shown to be less stable than the silylium-
toluene adducts, consistent with the reported solid-state structure
showing Et3Si+ as the toluene adduct.[31] In all cases, Si-F bond
formations followed by ring-closure via P-C bond formation
afforded the products in overall exergonic reactions. The reaction
between [Et3Si(tol)]+ and 3 is 46.2 kcal/mol exergonic with a free
energy barrier of 19.7 kcal/mol to form product 5 together with
Et3SiF.(See SI).
This species exhibits a triplet in the 31P{1H} NMR spectrum at 11.5
2
ppm with a typical JP-F coupling constant of 108 Hz. The
corresponding 19F resonance was the expected doublet of
quintets at -84.7 ppm, with 5JF-F coupling to the four ortho-fluorine
atoms of 7 Hz. These data support the formulation of 5 as
[C10H6(1-P(C6F5)2)(8-CF2)][B(C6F5)4] (Scheme 2) in which the
phosphorus atom is bonded to a CF2 fragment generated by
fluoride ion abstraction by silylium cation. Monitoring the reaction
mixture by 19F NMR spectroscopy confirmed the formation of the
corresponding triethylsilylfluoride. This formulation of 5 was
further confirmed crystallographically (Figure 1).
In furthering this FLP approach to C-F activation, the
conversion of C6H5CF3 to C6H5CF2H was targeted. To this end,
the intramolecular FLP containing a phosphorus donor and a
silylium cation was prepared from the known species C6H4(1-
PPh2)(2-SiHPh2)[32] via reaction with [Ph3C][B(C6F5)4]. The salt
[C6H4(1-PPh2)(2-SiPh2)][B(C6F5)4] 6 was isolated in 77% yield.
This species gave a resonance in the 31P NMR spectrum at +11.6
ppm, consistent with an intramolecular interaction between P and
Scheme 2: Synthesis of 2-5.
The solid-state structure of 5 shows an unusually long P-
CCF2 bond of 1.915(2) Å to the difluoro-substituted carbon atom,
which is significantly longer than the P-C bonds to the fluoroarene
rings (1.788(2) and 1.797(2) Å). The corresponding C-F bonds are
on the short end of the spectrum for aliphatic C-F bonds (av. 1.358
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Si. Although minor impurities were persistent in syntheses of 6, a
29Si-1H HMBC spectrum revealed a 29Si NMR chemical shift at
+12.8 ppm. Despite an apparent dative interaction between P and
Si, ring strain of the four-membered ring is thought to provide
access to the open form of the FLP for reactivity. This situation is
directly analogous to the now classic Erker FLP system,
Mes2PCH2CH2B(C6F5)2 where the open-FLP is accessed as a
result of similar ring strain. Probing this aspect via reaction, freshly
prepared and isolated 6 was combined with a solution of C6H5CF3
in fluorobenzene. Stirring for 24 h at room temperature, followed
by NMR analysis of the reaction mixture indicated the formation
of a new product 7 in 52% yield. This species exhibited the
diagnostic triplet in the 31P{1H} NMR spectrum at 23.8 ppm with a
2JP-F coupling constant of 104 Hz. The corresponding doublet was
seen at –94.7 ppm in the 19F NMR spectrum. This species was
isolated by column chromatography. These data indicate the FLP
activation of the CF3 fragment affording difluoro-benzyl
doublets was observed at -184.9 ppm in the 19F NMR spectrum
2
2
with a JPF and JFH coupling of 81 Hz and 57 Hz, respectively
were consistent with the formation of [C6H4(1-P(CFHPh)Ph2)(2-
SiFPh2)]+[B(C6F5)4]- 9 (Scheme 3). This was generated in only
18% yield; nonetheless, subsequent treatment of 9 with a 0.40 M
solution of KOH in 50:50 H2O/THF mixture gave 8 in >95%
conversion from 9 and permitted the isolation of PhCH2F by
column chromatography.
Similarly, 31P and 19F NMR spectroscopy confirmed that
treatment of Ph2CF2 with one equivalent of 6 led to the formation
of the product [C6H4(1-P(CFPh2)Ph2)(2-SiFPh2)][B(C6F5)4] 10 in
80% yield (Scheme 3). In this case, the additional phenyl
substituent is postulated to lower the energy of the transition state
as well as stabilize the transient carbenium cation. Again,
treatment under the conditions described above resulted in the
isolation of mono-hydrodefluorinated product Ph2CHF in >95%
yield.
substituted
phosphonium
salt
[C6H4(1-P(CF2Ph)Ph2)(2-
SiFPh2)][B(C6F5)4] 7. It is thought that the proximity of the Lewis
acid and base centre in 6 is key to trapping the CF2Ph fragment
as efforts to effect analogous chemistry with intermolecular FLPs
were unsuccessful (eg [Et3Si(tol)]+ and P(C6F5)2Ph). It is also
noteworthy that this reaction was most favourable in
fluorobenzene as the generation of difluorocarbocations in more
electron-rich aromatic solvents has been shown to afford Friedel-
Crafts chemistry.
Scheme 3: Reactions of 6 with C6H5CF3, C6H5CF2H and Ph2CF2,
conversions as determined by NMR are provided.
It is important to note that previous reports using Lewis acids
to mediate hydrodefluorination converted CF3 to CH3 fragments
regardless of the stoichiometry. In the present report, the use of
an Si+/P FLP reagent is shown to facilitate a two-step process
involving initial activation of a C-F bond and subsequent
hydrolysis of the generated P-C bond to give the conversion of
PhCF3 to PhCF2H, PhCF2H to PhCFH2 and Ph2CF2 to Ph2CHF.
The yields of the initial C-F activations are not optimal, and
particularly the poorer yield of 9 relative to those of 7 and 10
suggests the FLP is more effective when capturing more stable
carbocations. Nonetheless, these findings demonstrate the
viability of this strategy CF3 derivatization.
In summary, the chemistry described herein exploits a FLP
derived from silylium cations and phosphine donors to activate C-
F bonds and capture the resulting carbocations. Subsequent
hydrolysis of the P-C bonds affording the first stoichiometric
protocol for mono-hydrodefluorinations. While this work
demonstrates that a FLP strategy is conceptually viable, we are
continuing to explore applications of other FLPs in CF3 chemistry,
specifically targeting more controlled stoichiometric and selective
reagents for derivatization or functionalization of CF3 moieties.
Figure 2: The computed reaction free energy profile (in kcal/mol) for
the formation of 5. (see SI for more details).
Similar DFT calculations reveal the Lewis acidic Si-centre of
cation 6+ abstracts fluoride from C6H5CF3 directly affording the Si-
P bond of 6+ opposite to the breaking C-F bond of C6H5CF3. This
step is 7.1 kcal/mol endergonic over a moderate barrier of 21.7
+
kcal/mol. In the computations, the transient solvated C6H5CF2
cation combines rapidly with the neutral acyclic C6H4(1-PPh2)(2-
SiFPh2) to form the product cation [C6H4(1-P(CF2Ph)Ph2)(2-
SiFPh2)]+ of 7 giving an overall reaction that is exergonic by −17.2
kcal/mol.
Treatment of phosphonium cations with Brønsted bases is
a well-established route[33] to effect P-C bond cleavage, with
liberation of the most apicophilic substituent.[34] Treatment of 7
with a 0.40 M solution of KOH in 50:50 water:THF affords the
liberation of C6H5CF2H (Scheme 3) in >95% yield, demonstrating
selective cleavage of the difluorinated substituent, with concurrent
formation of the phosphine oxide 8 while C6H5CF2H could be
separated by column chromatography for spectral confirmation.
The analogous reaction of PhCF2H with 6 was performed in
in fluorobenzene and allowed to react for 24 h. While the reaction
was far from complete, a doublet at +21.1 ppm in the 31P NMR
spectrum with a coupling constant of 81 Hz and a doublet of
Keywords: silylium cations • FLP • CF bond activation •
hydrodefluorination •
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10.1002/chem.201705276
Chemistry - A European Journal
COMMUNICATION
Layout 1:
COMMUNICATION
Selective
Hydrodefluorination
F
Ph CF2H
3
Reactions of CF3: Silylium /
phosphine- based FLPs are
shown to effect the selective
abstraction of fluoride ion
from aryl-CF3 and CF2
groups, ultimate converting
CF3 to CF2H to CFH2
Ian Mallov, Adam J. Ruddy, Hui Zhu, Stefan
Grimme* and Douglas W. Stephan*
Ph
Ph
C
Page No. – Page No.
Ph
FH
C
2
F H
C
2
C-F Bond Activation by Silylium/Phosphine
Frustrated Lewis Pairs: Mono-
Hydrodefluorination of PhCF3, PhCF2H and
Ph2CF2
Ph FH
2C
Ph
2C
F
2
+
fragments.
mediated
R
PR FLPs
3Si /
3
by
This article is protected by copyright. All rights reserved.