Catalytic Hydrodefluorination of C?F Bonds by an Air-Stable PIII Lewis Acid

Article abstract of DOI:10.1002/chem.201801305

Catalytic hydrodefluorination (HDF) of unactivated fluoroalkanes or CF₃-substituted aryl species is performed using the P^III Lewis acids, [(bipy)PPh]²⁺ (1²⁺) and [(terpy)PPh]²⁺ (2²⁺) under

Full text of DOI:10.1002/chem.201801305

A Journal of
Accepted Article
Title: Catalytic hydrodefluorination of C-F bonds by an air-stable P(III)
Lewis acid
Authors: Saurabh Chitnis, Felix Krischer, and Douglas Wade Stephan
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To be cited as: Chem. Eur. J. 10.1002/chem.201801305
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Catalytic hydrodefluorination of C-F bonds by an air-stable P(III)
Lewis acid
Saurabh S. Chitnis, Felix Krischer and Douglas W. Stephan*[a]
Dedication ((optional))
Abstract: Catalytic hydrodefluorination (HDF) of unactivated
fluoroalkanes or CF3-substituted aryl species is performed using the
P(III) Lewis acids, [(bipy)PPh]2+ (12+)and [(terpy)PPh]2+ (22+) under
mild conditions (25 oC or 50 oC). Mechanistic studies indicate that
activation of C-F bond by the P(III) center is key. Particularly
noteworthy is that the catalyst 2[B(C6F5)4]2 is air-stable and readily
accessible from bench-stable, commercially available reagents in
one-step and can be used without isolation.
species can stoichiometrically activate strong E-H (E = H, C, P)
bonds.[7] In a related sense, Gudat has pioneered the reactivity of
the masked phosphenium cations derived from N-heterocyclic
phosphines.[8] Recently the Vidovic group showed that P(III)
dications exhibit stoichiometric activation of C-F bonds at 100 oC,
albeit with a limited scope of substrates (only α-phenyl C-F
bonds).[9] The same group also showed abstraction of hydrides
from silanes and amine/pyridyl-boranes under ambient
conditions.[10] Despite the above Lewis acidic behavior,
applications of electrophilic P(III) compounds in catalysis have
been limited to N-heterocyclic phosphines (NHPs) and
triaminophosphines.[11]
The high thermodynamic and kinetic stability of C-F σ-bonds
underpins the numerous applications of fluorocarbons as small
molecules and materials. On the other hand, this stability limits
opportunities for functionalization leading to accumulation of
organofluorine compounds in the environment, posing toxicity and
global warming concerns. Thus, developing new synthetic tools
for C-F bond functionalization is not only fundamentally
interesting but relevant to environmental remediation efforts. A
number of stoichiometric and catalytic protocols have emerged for
the modification of C-F bonds.[1] Several transition metal and
lanthanide complexes effect catalytic hydrodefluorination (HDF)
offering an attractive strategy for reduction of C-F groups, with
most examples involving C(sp2)-F bonds (aryl or vinyl fluorides).[2]
HDF catalysis with p-block Lewis acids has emerged as a
complementary strategy that selectively targets C(sp3)-F bonds.
A variety of boron, silicon, aluminum, and antimony electrophiles
have been reported as catalysts or precatalysts for the HDF of
fluoroalkanes (Figure 1, I-V).[1b, 3] Our group has systematically
evolved electrophilic phosphonium, [XPR3]+ (X
= electron
withdrawing group) and diphosphonium, [R3PPR3]2+, cations, as
tunable catalysts for hydrosilylation, transfer hydrogenation,
Figure 1. Main group Lewis acids for catalytic HDF of
fluoroalkanes.
hydroarylation,
hydrothiolation,
hydrodeoxygenation
and
fluoroalkane HDF (Figure 1, VI and VII).[4] The Lewis acidity of
these P(V) cations is derived from an energetically accessible P-
X σ*-antibonding orbital.
P(III) compounds (e.g. phosphines) are typically recognized
as Lewis bases, acting as electron donors in coordination
chemistry and Lewis base catalysis.[5] Nevertheless, a rich
coordination and reaction chemistry has been described for
neutral P(III) halides and phosphenium cations.[6] Burford has
systematically studied P(III) acceptors, highlighting the
electrophilicity of bipyridine-stabilized P(III) polycations. Such
In targeting applications of electrophilic P(III) compounds for
HDF catalysis, we have explored the reactivity of P(III) dications
featuring bipyridine (bipy) or terpyridine (terpy) ligands,
[(bipy)PPh]2+, 12+, and [(terpy)PPh]2+, 22+ (Figure 1). Herein, we
demonstrated that despite the formally hypervalent environment
around the phosphorus atom, the air stable P(III) species,
2[B(C6F5)4]2, is sufficiently electrophilic to realize catalytic HDF of
primary, secondary, and tertiary alkyl C-F bonds.
Compounds 1[X]2 (X = OTf and B(C6F5)4) were prepared
according to Scheme 1a. The 31P NMR spectra of the cations
showed singlets at δP = 131 ppm and 144 ppm, respectively, for
the triflate and borate salts, and these values are consistent with
values for other tricoordinate P(III) polycations.[7] The structure of
1[OTf]2 was determined in the solid state by single-crystal
diffraction confirming the pyramidal environment around the
phosphorus atom (Figure 1a). The structure of the cation shows
two sets of inter-ion contacts, one of which is trans to the P-C
bond and the other trans to one of the P-N bonds. The compound
[a]
Dr. S. S. Chitnis, Mr. F. Krischer, and Professor Dr. D. W. Stephan
Department of Chemistry,
University of Toronto,
80 St. George St., Toronto, Ontario, Canada M5S3H6
E-mail: dstephan@chem.utoronto.ca
Supporting information for this article is given via a link at the end of
the document. CCDC # 1817740-1817742
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1[B(C6F5)4]2 exhibits a high Lewis acidity as suggested by the
Gutmann-Beckett acceptor number (AN) of 93 (Figure S1, ESI).
This is notably greater than that of B(C6F5)3 (AN 82) and slightly
less than that of the electrophilic phosphonium cation [FP(C6F5)3]+
(AN 102). DFT calculations revealed that the LUMO of 12+ also
exhibits a prominent lobe at the phosphorus atom (Figure 1b)
suggesting the potential for Lewis acid behavior at the central
atom. Consistent with its exceptional Lewis acidity, 1[B(C6F5)4]2
decomposes quantitatively and rapidly (< 1 h) to a complex
mixture of products upon exposure to air.
2[B(C6F5)4]2 left open to air for 24 h also retained its yellow colour
and showed <10 % decomposition by 31P NMR spectroscopy.
Despite its apparent air-stability, 1[B(C6F5)4]2 exhibits a Lewis
acidity that is greater than B(OMe)3 as evidenced by the
Gutmann-Beckett test (Figure S2, ESI) which revealed acceptor
numbers of 27 and 23,[12] respectively, for the two species.
Figure 2. a) Crystallographic structure of the cation in 1[OTf]2, b)
calculated LUMO of 12+, c) crystallographic structure of the cation
in 2[OTf]2∙EtCN, d) calculated LUMO of 22+. Hydrogen atoms and
noninteracting triflate and solvent molecules are omitted.
DFT calculations (PBE1-D3/cc-pVTZ) were carried out to
assess the hypervalent electronic structure of cation 22+ in the gas
phase. The optimized structure revealed very similar bond lengths
and angles to those observed in 2[OTf]2∙EtCN and confirmed the
asymmetry of the axial and equatorial P-N interactions. The
Wiberg Bond Indices for the P-N bonds (WBIs, equatorial P-N:
0.62 axial P-N: 0.56) are consistent with the dative N→P donor-
acceptor interactions. The LUMO of 22+ is delocalized over the
terpy ligand framework but with a prominent lobe at the
phosphorus atom that is antibonding with respect to the P-N and
P-C interactions (Figure 2d). Due to the mutually trans disposition
of two axial P-N interactions, the sites opposite to the central P-N
bond or the P-C bond are expected to interact with incoming
nucleophiles. This view is supported by contact with the triflate
anion trans to the P-C bond in the solid-state structure of
2[OTf]2∙EtCN, which also evidence a retention of Lewis acidity
despite the hypervalent electronic structure of the cation.
The potential for catalytic HDF using 1[B(C6F5)4]2 was
probed (Figure S4-S7, ESI). Quantitative reduction of C-F bonds
was observed for primary, secondary, or tertiary fluoroalkanes
(Table 1, entries 1-3) in the presence of Et3SiH as a hydride donor.
The CF3 groups in c-C6H11CF3 and PhCF3 were converted at room
temperature to CH3 groups (entries 4, 5) with no evidence of CHF2
or CH2F containing intermediates even when limiting amount of
Et3SiH was used. Very slow conversion was observed for PhOCF3
even at elevated temperature (entry 6).
Scheme 1. Synthesis of 1[X]2 and 2[X]2 (X = OTf, B(C6F)4].
Combination of terpyridine (terpy), PhPCl2, and either
KB(C6F5)4 or Me3SiOTf in DCM at room temperature (Scheme 1a)
for 5 minutes yielded salts formulated as [(terpy)PPh][X]2, 2[X]2 (X
= OTf or B(C6F5)4), in >80 % isolated yield (Scheme 1b). The 31
P
NMR spectra of the two salts showed peaks at 34 ppm and 38
ppm for the OTf and the B(C6F5)4 salt, respectively. These values
are ca. 100 ppm upfield compared to those for salts 1[X]2. Single
crystals of 2[OTf]2∙EtCN grown from a mixture of EtCN and Et2O
were studied by X-ray diffraction (Figure 1c). The nitrile of
crystallization shows no interactions with the cation. The
geometry at P is best described as a seesaw geometry, typical of
four coordinate P(III) centers with a stereochemically active lone
pair. The two axial P-N interactions are nearly identical (2.002(3)
and 1.971(2) Å) and, notably, approximately 0.2 Å longer than the
equatorial P-N bond (1.820(3) Å). The three aromatic rings of the
terpy ligand deviate slightly from coplanarity due to the tridentate
interaction while the phenyl substituent is angled at 102.7(2)o with
respect to the (N)3P plane. Two inter-ion P-O contacts of 3.268(2)
and 3.309(2) Å, are similar to the sum of the P, O van der Waals
radii (3.32 Å) and occur trans to the P-C bond. To the best of our
knowledge, cation 22+ represents the first terpyridine complex of
phosphorus.
A solid sample of 2[B(C6F5)4]2 left in air for 3 days retained
its characteristic bright yellow colour, while NMR data obtained
upon redissolution were identical to those of a sample stored
under nitrogen (Figure S3, ESI). Similarly, a DCM solution of
The independent reaction of 1[B(C6F5)4]2 with a ten-fold
excess of fluoroadamantane proceeds quantitatively in 30
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minutes (Figure S15, ESI). Monitoring by 31P and 19F NMR
spectroscopy revealed resonances attributable to species
containing PF and PF2 fragments as well as PhPF2 (δP = 100 ppm,
Compound 2[B(C6F5)4]2 was also found to mediate the
catalytic HDF of C-F bonds. Using 5-10 mol% catalyst loading,
nearly quantitative conversion of unactivated primary, secondary,
and tertiary fluoroalkanes was observed at ambient temperature
(Table 1, entries 7-13). Runs involving CF3-containing substrates
showed no evidence of partial HDF products (R-CF2H or R-CFH2).
Interestingly, reduction of CF3 groups occurred selectively even in
the presence of an olefin with no evidence of hydrosilylation
products (entry 11). The more electron-deficient trifluoromethyl
group in PhOCF3 reacted very slowly at room temperature but
excellent conversion was observed upon heating to 50 oC for 96
hours (entry 15). However, negligible conversion was observed
for the most electron-deficient CF3 group in C6F5CF3 (entry 16).
1
δF = -107 ppm, JPF = 1223 Hz).[13] These observations indicate
fluoride ion abstraction from the fluoroalkane by the P(III) dication.
Compound 1[B(C6F5)4]2 also reacts with excess Et3SiH over
20 h to give complete conversion to a species exhibiting a
1
resonance at δ = -97 ppm with a JPH of 347 Hz in the 31P NMR
spectrum (Figure S16). This species was identified as the bis-
silylated cation [PhP(SiEt3)2H]+ (3+) by independent synthesis of
3[B(C6F5)4] from the reaction of PhP(H)SiEt3 and
[Et3Si(PhMe)][B(C6F5)4] (Scheme 2). This implies interception of
transiently formed phenylphosphinidene [PPh] by a silylium cation
(generated by hydride transfer from Et3SiH to 12+), affording the
phosphenium cation [Et3SiPPh]+ (Scheme 2), which reacts further
with silane to give [PhP(SiEt3)2H]+. Repeating the above reaction
in MeCN yielded crystals of [Et3Si(NCMe)][B(C6F5)4], confirming
hydride abstraction from the silane (Figure S18). Interestingly, the
reaction of 1[OTf]2 with Et3SiH, led to the quantitative formation of
the cyclic polyphosphines (PhP)x (x = 4, 5), supporting the
intermediacy of a phosphinidene (Figure S16, ESI). Apparently,
the in-situ formed Et3SiOTf is not sufficiently Lewis acidic to trap
phosphinidene, allowing oligomerization to proceed. The 1H NMR
spectrum (Figure S17) of the reaction between 1[B(C6F5)4]2 and
Et3SiH also indicated significant amounts of the protonated
bipyridine cation, 4+ suggesting abstraction of hydride from silane
by 12+ is followed by reductive elimination, affording protonated
bipyridine. Related reductive elimination of protonated bipyridine
has been observed by Burford in reactions of FLPs derived from
a P(III) trication / tBu3P with H2.[7] In addition, this redox process
is reminiscent of elimination of C6F5H from the reactions of
[Sb(C6F5)4]+ with silane described by Gabbai et al.[3h] The reaction
of 12+ with fluoroadamantane suggests that 12+ behaves as a
catalyst. On the other hand, the reaction with silane is consistent
with hydride abstraction, raising the possibility that 12+ acts as an
initiator generating a silylium cation that catalyses HDF. The
present data do not enable a definitive determination of the
mechanistic role of 12+.
Table 1. HDF Catalysis using 12+ or 22+ as the catalyst.
#
Cat
%
cat
10
10
R-F
t (h)
T
%Yield %Conv.
(oC) (Si-F) (C-F)
1
2
3
4
5
6
7
8
12+
12+
12+
12+
12+
12+
22+
22+
22+
22+
22+
n-C5H11F
c-C6H11F
8
5
25
25
95
93
97
76
50
4
>99
>99
>99
>99
90
10 Fluoroadamantane 0.25 25
5
5
c-C6H11CF3
C6H5CF3
C6H5OCF3
n-C5H11F
16 25
36 25
48 50
1.5 25
4
4
4
5
9
10
10
5
5
5
97
95
80
74
>99
97
93
c-C6H11F
25
25
25
9
10
11
Fluoroadamantane
c-C6H11CF3
4-isopropenyl-
C6H4CF3
>99
24 25
20 25
20 25
36 25
96 50
20 25
20 25
95
73
70
9
95
>99
>99
10
12
13
14
15
16
22+
22+
22+
22+
22+
5
5
5
5
5
5
C6H5CF3
4-Br-C6H4CF3
C6H5OCF3
C6H5OCF3
C6F5CF3
80
1
94
2
17 “[Et3Si]+”
C6F5CF3
55
75
Conditions: DCM solutions, 0.10 M fluoroalkane, 1.2 eq silane per C-
F bond. Conversion and yield determined by 19F NMR integration
using fluorobenzene or 1,2-difluorobenzene as internal standards.
The reaction of 2[B(C6F5)4]2 with
a ten-fold excess of
fluoroadamantane was examined by 31P and 19F NMR
spectroscopy, indicating fluoride transfer from the alkane to the
P(III) dication. This afforded species containing PF and PF2
fragments as well as PhPF2 (Figure S15). The corresponding
reaction of 2[B(C6F5)4]2 with a ten-fold excess of Et3SiH showed
no reaction after 20 h (Figure S16). The ability of 2[B(C6F5)4]2 to
directly activate C-F bonds and the absence of its reactivity
towards Et3SiH support its role as a catalyst, rather than an
activator and suggest a mechanism involving fluoride abstraction
by 22+ to give 2-F+ as an elementary step during catalysis
(Scheme 3). Moreover, the combination of 2[B(C6F5)4]2 and
Et3SiH is inactive in the catalytic HDF of C6F5CF3 (Table 1, entry
16), whereas use of in-situ generated of [Et3Si][B(C6F5)4] effected
HDF catalysis of C6F5CF3 (entry 17), indicating that C-F activation
by silylium species makes a negligible contribution to catalysis
involving 2[B(C6F5)4]2. Efforts to intercept the product of fluoride
Scheme 2. Reaction of 1[OTf]2 and 1[B(C6F5)4]2 with Et3SiH.
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COMMUNICATION
abstraction, 2-F+, from reaction of 2[B(C6F5)4]2 with CsF or NBu4F
were unsuccessful, showing only PhPF2 and a broadened peak in
the same region as 22+ (Figure S19). Despite this inability to
isolate 2-F+, computations reveal that it would adopt a pseudo
square-based pyramidal geometry with 12-valence electrons
about the P(III) center (Figure S20) and that subsequent reaction
with a silylium ion to regenerate 22+ is thermodynamically downhill
by 6.8 kcal/mol.
J. Fluorine Chem. 2015, 179, 14-22; c) H. Amii, K. Uneyama, Chem. Rev.
2009, 109, 2119-2183.
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R. P. Hughes), Springer International Publishing, Cham, 2015, pp. 143-
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22+
3C-F
R
R
3Si-F
-6.8
-22.9
Si]+
2-F+
C]+
[R3
R
[R3
-13.4
[4]
a) C. B. Caputo, L. J. Hounjet, R. Dobrovetsky, D. W. Stephan, Science
2013, 341, 1374-1377; b) J. Zhu, M. Pérez, C. B. Caputo, D. W. Stephan,
Angew. Chem. Int. Ed. 2016, 55, 1417-1421; c) J. M. Bayne, M. H.
Holthausen, D. W. Stephan, Dalton Trans. 2016, 45, 5949-5957; d) M. H.
Holthausen, R. R. Hiranandani, D. W. Stephan, Chem. Sci. 2015, 6,
2016-2021; e) J. M. Bayne, D. W. Stephan, Chem. Soc. Rev. 2016, 45,
765-774.
3C-H
R
3Si-H
Scheme 3. Proposed mechanisms for HDF catalyzed by 22+ with
R = Me. Numbers indicate calculated (PBE1-D3/cc-pVTZ) values
of ΔGrxn for each step (kcal/mol).
The use of 2[B(C6F5)4]2 in HDF catalysis offers practical
advantages over known electron-deficient (e.g. borane, silylium
and alumenium cations) or electron-precise species (e.g. silane,
stibonium or phosphonium cations).[3-4] In contrast to such
previous systems, this salt is air-stable and easily synthesized
from commercially-available, bench-stable reagents within
minutes with no need for purification. Indeed, catalytic HDF was
also achieved by the generation of 10 mol% 2[B(C6F5)4]2 in-situ
from terpy, PhPCl2, and K[B(C6F5)4] and subsequent addition to a
mixture of 1-fluoropentane and Et3SiH in dry DCM. In addition, 22+
is also unique in featuring an electron-rich (hypervalent)
environment and a lone pair at the reactive electrophilic site.
In summary, we have exploited C-F bond activation by a
readily accessible and air-stable salt 2[B(C6F5)4]2 to achieve the
first example of catalytic HDF by P(III) Lewis acids. The use of
terpy as a stabilizing, yet potentially hemilabile ligand (Figure S20),
suggests new strategies for designing more robust versions of
other p-block Lewis acid catalysts. These findings bode well for
wider adoption of main-group catalyzed C-F bond reduction
methodologies by eliminating the challenging synthetic protocols
and requirements for strictly anhydrous conditions and highly
reactive intermediates that currently characterize this field.
[5]
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[13] P. Švec, P. Novák, M. Nádvorník, Z. Padělková, I. Císařová, L. Kolářová,
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Acknowledgements
D.W.S. gratefully acknowledges the financial support from
NSERC Canada and the award of Canada Research Chair.
D.W.S. is also grateful for the award of an Einstein Visiting
Fellowship at TU Berlin.
Keywords: phosphorus, Lewis acids, catalysis, dications,
hydrodefluorination
[1]
a) T. Ahrens, J. Kohlmann, M. Ahrens, T. Braun, Chem. Rev. 2015, 115,
931-972; b) Q. Shen, Y.-G. Huang, C. Liu, J.-C. Xiao, Q.-Y. Chen, Y. Guo,
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10.1002/chem.201801305
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Catalytic
2+
Hydrodefluorination
free
Catalytic
Saurabh S. Chitnis, Felix
Krischer, and Douglas
W. Stephan*
-
metal
R
+
F
hydrodefluorination of
C-F bonds with an air-
stable P(III) Lewis acid
We report an air-stable
and readily-accessible
P(III) Lewis acid,
-
air-stable catalyst
N
Et
3SiH
3SiF
-
acid
Lewis
electron-rich
P(III)
P
-
Ph
Et
N
- commercial
Page No. – Page No.
starting materials
N
- <
5 mins
synthesis
Catalytic
R
H
o
2o 3o alkyl
,
-
-
1
C-F
,
hydrodefluorination of
C-F bonds with an air-
stable P(III) Lewis acid
2[B(C6F5)4]2, as a
catalyst for
CF
alkyl/aryl
3
hydrodefluorination
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