Lewis acidity of organofluorophosphonium salts: Hydrodefluorination by a saturated acceptor

Article abstract of DOI:10.1126/science.1241764

Prototypical Lewis acids, such as boranes, derive their reactivity from electronic unsaturation. Here, we report the Lewis acidity and catalytic application of electronically saturated phosphorus-centered electrophilic acceptors. Organofluorophosphonium salts of the formula [(C₆F ₅)_3-xPh_xPF][B(C₆F₅) ₄] (x = 0 or 1; Ph, phenyl) are shown to form adducts with neutral Lewis bases and to react rapidly with fluoroalkanes to produce difluorophosphoranes. In the presence of hydrosilane, the cation [(C ₆F₅)₃PF]⁺ is shown to catalyze the hydrodefluorination of fluoroalkanes, affording alkanes and fluorosilane. The mechanism demonstrates the impressive fluoride ion affinity of this highly electron-deficient phosphonium center.

Full text of DOI:10.1126/science.1241764

Lewis Acidity of Organofluorophosphonium Salts:
Hydrodefluorination by a Saturated Acceptor
Christopher B. Caputo et al.
Science 341, 1374 (2013);
DOI: 10.1126/science.1241764
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a Deutsche Forschungsgemeinschaft (DFG) grant within project
SPP 1369 (E.U.M.), and DFG grant SFB TRR 102. A description
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Supplementary Materials
www.sciencemag.org/content/341/6152/1371/suppl/DC1
Materials and Methods
Supplementary Text
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Acknowledgments: We thank the Leibniz Institute of Polymer
Research Dresden (IPF), especially P. Uhlamnn and A. Lederer,
9 April 2013; accepted 14 August 2013
10.1126/science.1238950
ions are electronically saturated and contain a
P center that is sterically shielded by a pseudo-
tetrahedral arrangement of substituents. As a con-
sequence, most phosphonium salts are unreactive
toward electron donors. Herein, the incorpora-
tion of electrophilic substituents at P is shown to
afford highly Lewis acidic phosphonium centers.
As a demonstration of their particularly strong
fluorophilicity, these organofluorophosphonium
cations are shown to directly activate C–F bonds
and to effect the catalytic hydrodefluorination
(HDF) of fluoroalkanes.
Lewis Acidity of Organofluorophosphonium
Salts: Hydrodefluorination by a
Saturated Acceptor
Christopher B. Caputo, Lindsay J. Hounjet, Roman Dobrovetsky, Douglas W. Stephan*
Prototypical Lewis acids, such as boranes, derive their reactivity from electronic unsaturation.
Here, we report the Lewis acidity and catalytic application of electronically saturated
phosphorus-centered electrophilic acceptors. Organofluorophosphonium salts of the formula
[(C6F5)3–xPhxPF][B(C6F5)4] (x = 0 or 1; Ph, phenyl) are shown to form adducts with neutral Lewis
The electron-deficient phosphines (C6F5)2PhP
and (C6F5)3P are cleanly oxidized with XeF2 to
bases and to react rapidly with fluoroalkanes to produce difluorophosphoranes. In the presence of give difluorophosphoranes (C6F5)2PhPF2 (1) and
hydrosilane, the cation [(C6F5)3PF]+ is shown to catalyze the hydrodefluorination of fluoroalkanes, (C6F5)3PF2 (2) in quantitative yields. These
affording alkanes and fluorosilane. The mechanism demonstrates the impressive fluoride ion
affinity of this highly electron-deficient phosphonium center.
species exhibit triplet resonances in their 31P{1H}
nuclear magnetic resonance (NMR) spectra at
chemical shifts [d/parts per million (ppm)] –54.8
hosphorus(III) Lewis bases are widely ex- recently exploited the acceptor capabilities of and –48.0, respectively, with a one-bond coupling
ploited as ligands in transition metal co- phosphonium cations, in tandem with boranes, constant (1JPF) of ~695 Hz. In contrast to the more
ordination and organometallic chemistry; to develop a series of fluoride ion sensors (8). electron-rich difluorophosphorane (C6F5)Ph2PF2
P
however, the electrophilic nature of phosphorus Whereas these findings reveal the Lewis acidity (13), neither 1 nor 2 undergoes fluoride ion
centers has garnered lesser attention. P(III) phos- of P cations, the chemistry of highly electrophilic abstraction by the Lewis acids B(C6F5)3 or
phenium cations have been explored by Gudat, phosphonium salts remains unexplored. Target- Me3SiOTf (OTf, trifluoromethanesulfonate)
Burford, and Ragogna, among others (1, 2). In ing such systems, we reported nucleophilic at- (9, 10), leading to our inference that the targeted
recent computational work, phosphenium cations tack of a phosphine donor at an electron-deficient fluorophosphonium cations should exhibit com-
have been predicted to exhibit fluorophilicities organofluorophosphonium salt (9, 10), leading to paratively stronger Lewis acidity. Nevertheless,
comparable to those of known neutral Lewis acids, the phosphonium-difluorophosphorane product 1 and 2 both undergo fluoride ion abstraction by
but substantially weaker than those of electro- [Ph3P(1,4-C6F4)Ph2PF2][O3SCF3] (Ph, phenyl) Al(C6F5)3·C7H8 (14) or [Et3Si][B(C6F5)4]·2C7H8
philic cations such as [Me3Si]+ (Me, methyl) (3). (9). This behavior is reminiscent of the reac- (Et, ethyl) (15, 16) to generate the corresponding
Although P(V) Lewis acidity has been explored tivity of B(C6F5)3 (11), suggesting that [(C6F5) fluorophosphonium salts [(C6F5)2PhPF]X {X =
less (4), it is noteworthy that the P(V) centers in Ph2PF]+ and B(C6F5)3 have similar Lewis [F(Al(C6F5)3)2], 3;[B(C6F5)4], 4}and[(C6F5)3PF]X {X =
ylide reagents account for the classic Wittig re- acidity. Nonetheless, in contrast to the electro- [F(Al(C6F5)3)2], 5; [B(C6F5)4], 6}, respectively
actions with ketones (5). Similarly, phosphonium philic nature of borane species, which is derived (Fig. 1). Products 5 and 6 were easily identified
cations have been used to facilitate additions to from the presence of a vacant p orbital, the Lewis by distinctive doublet resonances in their 31P{1H}
polar unsaturates (6) and Diels-Alder reactions acidity of organofluorophosphonium cations re- NMR spectra at d 77.7 (1JPF = 1042 Hz) and 67.8
(7). In related efforts, Hudnall et al. have also sides in the s* acceptor orbital oriented opposite (1JPF = 1062 Hz) and in their 19F NMR spectra at
the fluoride substituent (12). Further, the trigonal d –121.9 (doublet of pentets, 1JPF = 1042 Hz) and
1
planar, electronically unsaturated nature of borane –120.5 (doublet of septets, JPF = 1062 Hz),
Lewis acids make them reactive toward most respectively. The x-ray structure of 3 (Fig. 2A)
donor molecules. In contrast, phosphonium cat- clearly reveals the tetrahedral geometry of the
Department of Chemistry, University of Toronto, 80 St. George
Street, Toronto, Ontario M5S 3H6, Canada.
*Corresponding author. E-mail: dstephan@chem.utoronto.ca
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fluorophosphonium cation, with its P–F bond (7) of the phosphonium cation and stands in con- space opposite the P–F bond and is sheltered by
length of 1.533(2) Å. The anion contains an trast to simple alkyl- or aryl-phosphonium spe- the arene rings. Smaller components of the LUMO
approximately linear Al(1)–F–Al(2) angle of cies. Efforts to employ more conventional Lewis reside at the ortho and para positions of these
171.84(11)°, with Al(1)–F and Al(2)–F bond acidity tests were also undertaken. Addition of groups and at the P-bound F atom.
lengths of 1.788(2) and 1.780(2) Å, respectively. crotonaldehyde to 6 as prescribed by Childs’
Another demonstration of the Lewis acidity
The cation and anion pack in the solid state such method (17) resulted in a mixture of unidentified of 6 is derived from its reaction with Ph3CF.
that the P–F(2) bond is oriented along the Al products. In contrast, following the Gutmann pro- Combining 6 with Ph3CF (19) in a 1:1 ratio in
(1)–F–Al(2) vector with an F(2)···Al(1) inter- tocol (18), the combination of 6 and Et3PO was CD2Cl2 results in an instantaneous coloration
atomic separation of 3.677(2) Å, well within monitored by 31P NMR spectroscopy. Adding 1 of the reagents to produce a yellow-orange so-
the sum of the van der Waals radii of these nu- equivalent of Et3PO to a CD2Cl2 solution of 6 lution. The resulting 1H NMR spectrum shows
clei (3.98 Å). This interesting feature may be sug- at ambient temperature generated the adduct conversion of Ph3CF to [Ph3C][B(C6F5)4], whereas
gestive of p-stacking interactions between arenes [(Et3PO)(C6F5)3PF][B(C6F5)4] (8) (Fig. 1), as 19F and 31P{1H} NMR spectra demonstrate the
of the cation and anion and/or of a weak dative evidenced by two new doublet-of-doublet signals formation of 2 (see supplementary materials). Re-
F(2)→Al(1) attraction, consistent with the well- in the 31P{1H} NMR spectrum at dP –51.3 (1JPF
known hypervalency of Al.
674 Hz, 2JPP = 66 Hz) and 91.1 (2JPP = 66 Hz, to systems that generate stable carbocations.
=
actions of 6 with fluoroalkanes are not limited
Recognizing that 5 and 6 should incorpo- 3JPF = 7 Hz). The latter resonance was attributed Indeed, 6 was also shown to react immediately
rate the most electron-deficient P centers, efforts to the coordinated Et3PO unit and is shifted down- with 1-fluoropentane, producing 2, as confirmed
were made to gauge their Lewis acidity. Treatment field from that of free Et3PO (dP 50.7). This shift by x-ray analysis of crystals isolated from the
of 6 with excess N,N-dimethylformamide (DMF) (D = 40.4 ppm) is considerably larger than that reaction mixture (13). The reactive n-pentyl cat-
in CD2Cl2 solution gave rise to a new high-field observed when B(C6F5)3 and Et3PO are com- ion generated by this process gave rise to un-
doublet in the 31P{1H} NMR spectrum at d –46.7 bined (D = 26.6 ppm), suggesting that the cation identifiable side products arising from its reaction
with 1JPF = 705 Hz. The 19F NMR spectrum of this of 6 is ~1.5 times more Lewis acidic than B with [B(C6F5)4]–, as has been observed previously
sample shows a downfield shift of the correspond- (C6F5)3 on the Gutmann scale. To further probe (20). In a similar fashion, the addition of excess
ing P–F signal to d –3.6. These data demonstrate the Lewis acidic nature of 5 and 6, the geometry a,a,a-trifluorotoluene to solid 6 showed a more
DMF coordination to the cation of 6, affording of the cation was optimized at the wB97XD/def2- gradual conversion to 2, along with slower deg-
the salt [(Me2NC(O)H)(C6F5)3PF][B(C6F5)4] (7). TZVPP level of theory (see supplementary ma- radation of the [B(C6F5)4]– anion, as noted above.
Although 7 is generated in the presence of excess terials). The lowest unoccupied molecular orbital
The aforementioned reactivity was subse-
DMF, it was not isolable. Nonetheless, coordi- (LUMO) of 5 and 6 is concentrated on the P quently exploited for HDF reactions using the
nation of DMF demonstrates the Lewis acidity center (Fig. 2B). The major component occupies phosphonium salt 6 as a catalyst. Fluoroalkanes
were combined with Et3SiH in the presence of
1 mole percent (mol %) of 6 (Table 1) at room
temperature. In this fashion, 1-fluoroadmantane,
1-fluoropentane, fluorocyclohexane, and a,a,a-
trifluorotoluene are catalytically converted to
the corresponding hydrocarbons within 3 hours,
whereas 1,4-bis(difluoromethyl)benzene required
24 hours, as evidenced by disappearance of the
C–F resonances and appearance of the Et3SiF
signal in the 19F NMR spectra (see supplemen-
tary materials). In some cases, additional sub-
stituent redistribution affords small amounts of
Et2SiF2 and Et4Si, similar to that previously ob-
served (21). Interestingly, octafluorotoluene could
be converted exclusively to pentafluorotoluene
(C6F5CH3) and related products (22), demon-
strating selective activation of the C(sp3)–F groups,
and retention of C(sp2)–F groups. The substrates
1-bromo-3,5-bis(trifluoromethyl)benzene and
Fig. 1. Synthesis of compounds 1 to 8.
1,3,5-tris(trifluoromethyl)benzene also undergo
Fig. 2. Crystallography
and electronic structure
of fluorophosphonium
salts. (A) Persistence of
Vision Raytracer depiction
of 3. C, black; Al, blue-gray;
F, pink; P, orange. All hy-
drogen atoms are omitted
for clarity. (B) Graphical
presentation of the LUMO
of [(C6F5)3PF]+ calculated at
the wB97XD/def2-TZVPP
level of theory (surface
iso-value = 0.05).
www.sciencemag.org SCIENCE VOL 341 20 SEPTEMBER 2013
1375

REPORTS
HDF in this manner, although higher catalyst con cations (R3Si+), carbocations, alumenium ions deficient organofluorophosphonium salts exploits
loadings (5 mol %) were required, an observa- (R2Al+), organoaluminum species, and B(C6F5)3 electron-withdrawing substituents to generate
tion that is consistent with the greater number of (33). Though the silylium catalysts show greater enhanced Lewis acidity derived from a s* orbital
C–F bonds present in these substrates. Never- activity (20), the present development of electron- of an electronically saturated species. The result-
theless, using 10 mol % of 6 resulted in complete
defluorination of these substrates after 24 hours.
Previous reports have described C–F bond cleavage Table 1. Catalytic hydrodefluorination of fluoroalkanes. R, alkyl; TON, turnover number.
and catalytic HDF of fluoroalkanes by conven-
tional, main-group Lewis acids such as B(C6F5)3
(23), silylium (20), carbenium (24), and alumenium
cations (25), though not by phosphonium salts.
We probed the mechanism of the HDF re-
action. Combining HSiEt3 and 6 resulted in no
reaction, even on standing for several weeks as
evidenced by 31P{1H} and 19F NMR spectra. This
observation is contrary to a mechanism involving
hydride abstraction from silane by 6, generating
silylium cation, a known HDF catalyst (26). In
sharp contrast, reactions of fluoroalkanes with the
Lewis acidic phosphonium ion 6 lead to imme-
diate C–F bond activation to produce interme-
diate difluorophosphorane 2 and a carbocation.
In the catalytic process, the carbocation is quenched
by the hydridosilane to afford an alkane and a
transient silylium ion, which abstracts fluoride
from phosphorane 2 to regenerate active catalyst
6 (Fig. 3). Further support for this proposition
was obtained by a direct-competition experiment.
Adding 1 equivalent of [Et3Si][B(C6F5)4]·2C7H8
to a 1:1 mixture of 2 and C6F5CF3 in C6H5Br led
to the immediate precipitation of 6 with no spec-
*Relative to fluoroalkane substrate.
†Calculated from the proportion of C–F bonds originally present relative to Si–F
troscopic evidence for C–F bond activation after
10 min (see supplementary materials). This ob-
servation demonstrates that the fluoride ion in 2 is
more labile than the alkyl C–F bond, supporting
the proposition that catalyst 6 is regenerated by
fluoride ion abstraction from 2.
bonds formed.
‡Calculated from the proportion of C–F bonds consumed after time t (in hours). Conversions were
determined by 19F NMR spectroscopy using fluorobenzene as an internal standard.
Additional support for this mechanism was
acquired computationally at the wB97XD/def2-
TZVPP level of theory (gas phase) (27, 28). We
considered two conceivable reaction pathways
and found hydride abstraction from Me3SiH by
[(C6F5)3PF]+ (+30.2 kcal·mol−1) to be much more
endothermic than fluoride abstraction from tBuF
(+12.1 kcal·mol−1; t, tert). In either case, regener-
ation of catalyst 6 with production of alkane and
fluorosilane is substantially exothermic, with a
change in enthalpy DH value of –44.9 kcal·mol−1.
These computed energies support a reaction mech-
anism whereby the cation of 6 catalyzes HDF
by directly activating the C–F bond of the fluoro-
alkane (see supplementary materials).
The ability of 6 to activate strong C–F bonds
(bond energy = ~490 kJ/mol) (29) and effect HDF
catalysis results from the fluorophilicity of the
organofluorophosphonium cation. Such reac-
tivity is potentially important, as chlorofluoro-
carbons, hydrofluorocarbons, and perfluorocarbons
(30) are persistent greenhouse gases. Indeed, tran-
sition metal reagents and catalysts have been de-
veloped to effect stoichiometric cleavage of C–F
bonds and to bring about HDF catalysis (31, 32).
Metal-free strategies to HDF catalysis have also
been pursued, exploiting highly Lewis acidic,
Fig. 3. Proposed reaction mechanism for phosphonium-catalyzed HDF reactions. Energies are
calculated at the wB97XD/def2-TZVPP level of theory. Energy values are given in units of kilocalories
per mol relative to the energy of the [(C6F5)3PF]+, Me3SiH, and tBuF compounds; Gibbs free energies
electronically unsaturated species, including sili- are given in parentheses.
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Supplementary Materials
www.sciencemag.org/content/341/6152/1374/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S16
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samples consisted of fully densified “rocks” of
isostatically hot pressed polycrystalline Mg2GeO4
(Ge-olivine) containing minor amounts of Ge-
pyroxene (<5 vol %) (11). We used the germanate
analog of Mg2SiO4 olivine because transforma-
tion into its denser polymorph can be reached
at pressures routinely achievable in the deforma-
tion apparatus. Stress, transformation progress,
and strain were measured in situ by using x-ray
powder diffraction (XRD) and radiographic im-
aging, respectively. AEs were recorded continuous-
ly on six channels. Description of the set-up is
given in the supplementary materials (fig. S1) (12).
Differential stress, strain, and acoustic activ-
ity for sample D1247 evolved as a function of
time (Fig. 1). The sample was first pressurized
to 4 GPa at room temperature then deformed at
a constant temperature of 973 K with a strain rate
Deep-Focus Earthquake Analogs
Recorded at High Pressure
and Temperature in the Laboratory
Alexandre Schubnel,1* Fabrice Brunet,2 Nadège Hilairet,3† Julien Gasc,3
Yanbin Wang,3 Harry W. Green II4
Phase transformations of metastable olivine might trigger deep-focus earthquakes (400 to
700 kilometers) in cold subducting lithosphere. To explore the feasibility of this mechanism,
we performed laboratory deformation experiments on germanium olivine (Mg2GeO4) under
differential stress at high pressure (P = 2 to 5 gigapascals) and within a narrow temperature
range (T = 1000 to 1250 kelvin). We found that fractures nucleate at the onset of the
olivine-to-spinel transition. These fractures propagate dynamically (at a nonnegligible fraction of
the shear wave velocity) so that intense acoustic emissions are generated. Similar to deep-focus
earthquakes, these acoustic emissions arise from pure shear sources and obey the Gutenberg-Richter
law without following Omori’s law. Microstructural observations prove that dynamic weakening
likely involves superplasticity of the nanocrystalline spinel reaction product at seismic strain rates.
he origin of deep-focus earthquakes fun- regime so that rocks yield by creep or flow rather
damentally differs from that of shallow than by brittle fracturing (4). Polymorphic phase
T
(<100 km) earthquakes (1), for which transitions in olivine have provided an attractive
theories of rock fracture rely on the properties alternative mechanism for deep-focus earthquakes
of coalescing cracks and friction (2–4). As pres- (5, 6). For instance, transformation of olivine to its
sure and temperature increase with depth, intra- high-pressure polymorphs could induce faulting in
crystalline plasticity dominates the deformation polycrystalline Mg2GeO4 olivine (7, 8). This was
further confirmed on silicate olivine, (Mg,Fe)2SiO4,
1Laboratoire de Géologie, CNRS UMR 8538, Ecole Normale
Supérieure, 75005 Paris, France. 2Institut des Sciences de la
Terre, CNRS, Université de Grenoble 1, 73370 Grenoble, France.
3GeoSoilEnviroCARS, University of Chicago, Argonne, IL 60439,
USA. Department of Earth Sciences, University of California at
Riverside, CA 92507, USA.
during the olivine-wadsleyite transition (9). Ad-
ditional experiments demonstrated that the mech-
anism produced acoustic emissions (AEs) (10).
In total, we performed eight experiments on
both powdered and sintered Ge-olivine samples
in the stability field of the spinel polymorph, at
confining pressures from 2 to 5 GPa and temper-
4
Fig. 1. Stress, strain and acoustic emission.
Evolution of temperature, differential stress, strain,
and AE rate during experiment D1247 performed
*Corresponding author. E-mail: aschubnel@geologie.ens.fr
†Present address: Unité Matériaux et Transformations, CNRS
UMR 8207, Université Lille 1, 59655 Villeneuve d’Ascq, France. atures between 973 and 1573 K (fig. S2). Sintered at 4 GPa effective mean stress.
www.sciencemag.org SCIENCE VOL 341 20 SEPTEMBER 2013
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