Cooperative Lewis acidity in borane-substituted fluorophosphonium cations

Article abstract of DOI:10.1039/c6cc02607a

A series of aryl-difluorophosphoranes are converted to give fluorophosphonium salts [Ph₂PF(o-C₆X₄BR₂)]⁺ (X = H, F; R = Cy, Mes). The proximity of the two weakly Lewis acidic fluorophosphonium and borane moieties results in enhanced Lewis acid catalytic reactivity.

Full text of DOI:10.1039/c6cc02607a

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Cooperative Lewis acidity in borane-substituted
fluorophosphonium cations†
Cite this:DOI: 10.1039/c6cc02607a
Juri Mobus,‡ Thorsten vom Stein‡ and Douglas W. Stephan*
¨
Received 27th March 2016,
Accepted 13th April 2016
DOI: 10.1039/c6cc02607a
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A series of aryl-difluorophosphoranes are converted to give fluoro-
phosphonium salts [Ph2PF(o-C6X4BR2)]+ (X = H, F; R = Cy, Mes). The
proximity of the two weakly Lewis acidic fluorophosphonium and
borane moieties results in enhanced Lewis acid catalytic reactivity.
Since the articulation of the concept of frustrated Lewis pairs
(FLPs),1 there has been considerable interest in the chemistry
and development of potent Lewis acidic species.2 While a
number of investigations have focused on group 13 compounds
as well as some examples of group 14 Lewis acids,3 we have
focused recent attention on a range of group 15 Lewis acids,
exploiting the low energy s* orbital of electrophilic phosphonium
cations (EPCs) as Lewis acids.4 These systems have been employed
in a series of catalyses including hydrodefluorination,4a dehydro-
coupling,5 hydrosilylation,6 FLP hydrogenations,7 ketone deoxy-
genations8 and phosphine-oxide reductions.9 Since this work,
we have probed strategies to enhance the Lewis acidity at
phosphorus. While the incorporation of electron deficient sub-
stituents is an obvious approach, the introduction of positively
charged centers has also shown to improve Lewis acidity.10 In a
Fig. 1 A Gabbai fluoride ion sensor.
of aryl-fluorophosphonium cations in which a neutral borane
fragment is placed ortho- to the phosphonium fragment. The
nature of the impact of proximity of the two Lewis acidic centers
on the catalytic chemistry is probed and discussed in comparison
with known systems.
The species Ph2P(o-C6H4BCy2) 2 was prepared as a colorless oil
in 64% isolated yield via lithiation of the commercially available
Ph2P(o-C6H4Br) 112 in THF at À30 1C followed by quenching with
ClBCy2 (Scheme 1). Subsequent oxidation of 2 with XeF2 in
CH2Cl2 generates the corresponding pure difluorophosphorane
Ph2PF2(o-C6H4BCy2) 3 which was crystallized from pentane
¨
related sense, the pioneering work of Gabbaı and others
showed that the introduction of two Lewis acidic moieties into
one molecule showed significantly increased binding constants
for hydride and fluoride ions.11 Using this strategy, Gabbaı
¨
developed systems in which the proximity of a phosphonium
cation enhanced fluoride binding to a neutral boron center
yielding highly effective fluoride ion sensors A (Fig. 1).11h, j,k
We sought to employ a similar approach to enhance Lewis
acidity for catalytic applications. To this end, we have recently
described the facile conversion of common bidentate phosphines
to bis-fluorophosphonium dication Lewis acid catalysts.10b–d
Exploring an alternative approach, we now report the synthesis
Department of Chemistry, University of Toronto, 80 St. George St., Toronto,
Ontario, M5S 3H6, Canada. E-mail: dstephan@chem.utoronto.ca
† Electronic supplementary information (ESI) available: Experimental details and
spectral data have been deposited. CCDC 1470769–1470772. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c6cc02607a
‡ These authors contributed equally.
Scheme 1 Synthesis of 2–5 and 7–8. (a) 1.1 eq. n-BuLi, THF, À30 1C, 1 h,
then 1 eq. R2BCl (R = Cy, Mes), À30 1C to RT, 20 h. (b) XeF2, CH2Cl2, RT,
20 h. (c) 1.1 eq. i-PrMgCl LiCl, Et2O, À60 1C, 4 h, then 1 eq. Cy2BCl, À60 1C
to RT, 20 h.
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solution at À30 1C in 82% yield. In a similar fashion, the
analogous species Ph2P(o-C6H4BMes2) 4 was prepared in 87%
yield and converted to Ph2PF2(o-C6H4BMes2) 5 (Scheme 1). In
addition, magnesiation of 1,2-dibromotetrafluorobenzene employing
Knochel’s ‘‘Turbo-Grignard’’ reagent13 followed by quenching
with ClPPh2 gave the known species Ph2P(o-C6F4Br) 614 in
84% yield. This product was subsequently converted to the
phosphine–borane 7 in 90% yield. Oxidation with XeF2 proceeds
smoothly at À35 1C affording the difluorophosphorane
Ph2PF2(o-C6F4BCy2) 8 in 95% yield (Scheme 1).
In the case of 3, the room temperature 11B NMR spectrum
shows a resonance at d11B +29.7 (n1/2 B 710 Hz) clearly
indicating an interaction of a fluorine atom with the boron
center. This is also evident in the corresponding broad 19F NMR
resonances at d À76.0 and À76.8 (see ESI†). Cooling to À80 1C,
the 19F NMR spectrum shows two distinct doublets at d À75.3
(1JPF = 725 Hz) and d À83.1 (1JPF = 267 Hz), while the 31P NMR
spectrum showed the corresponding doublet of doublets, at d
À9.7 (see ESI†). By comparison, the room temperature spectra
of compound 8 (d11B +30.7) exhibit a sharp doublet of doublets
resonance in the 19F NMR spectrum at d19F À66.6 (1JPF = 547 Hz,
JFF = 71 Hz) and a corresponding triplet resonance in the 31P NMR
spectrum (d À9.8), typical for a difluorophosphorane unit. Upon
cooling, the NMR spectra of compound 8 exhibit similar features
to those of the system 3. These data are consistent with slow
rotation about the P–C bond to the aryl linker thus differentiating
the two P–F bonds as a result of a weak interaction of one of the
fluorine atoms with the boron center.15 The corresponding
11B NMR resonances were not observed due to temperature
induced broadening of the signals at low temperatures.
X-ray crystallographic analyses confirmed the structures of
3 and 8 as difluorophosphoranes with ortho-BCy2 fragments
(Fig. 2). The coordination geometries about the P-atoms are
distorted trigonal bipyramidal with the sum of the C–P–C
angles being 358.0(1)1 in 3 and 358.4(1)1 in 8, while the F–P–F
angles are 179.0(1)1 and 178.76(8)1 in 3 and 8 respectively. The
boron centers approach tetrahedral geometry with the sum of
the C–B–C angles being 346.0(1)1 in 3 and 350.5(1)1 in 8. In each
case, one of the fluorine atoms on P interacts with the proximal
B atom, accounting for the distortion from trigonal planarity.
The P–F bond proximal to boron is significantly elongated in
comparison to the trans-P–F bond. In compound 3, the P–F(2)
distance is 1.9381(8) Å while the other P–F distance amounts to
1.6206(9) Å. In comparison, in 8 the corresponding distances
are slightly different at 1.854(1) Å and 1.609(1) Å. The corres-
ponding B–F contacts are 1.640(2) Å and 1.665(2) Å in 3 and 8,
respectively.
Fig. 2 Solid-state structures of the difluorophosphoranes (a) 3 and (b) 8,
H-atoms are omitted for clarity. Carbon: black; fluorine: pink; phosphorus:
orange; boron: yellow-green. Selected bond lengths (Å) and angles (1): 3:
P–F(1): 1.6206(9), P(1)–F(2): 1.9381(8), B–F(2): 1.640(2), F(1)–P–F(2): 178.96(4),
P–F(2)–B: 119.43(7), 8: P–F(1): 1.609(1), P–F(2): 1.854(1), B–F(2): 1.665(2),
F(1)–P–F(2): 178.76(8), P–F(2)–B: 122.2(1).
Scheme 2 Preparation of 9–12: (a) 0.9–0.95 eq. [Et3Si][B(C6F5)4], À35 1C
to RT, 1–3 h, toluene; (b) 1.0 eq. 4-DMAP, CD2Cl2, RT, 1 h.
for the unsubstituted analogs.16 The 11B NMR shifts for 9–11
Subsequent reaction of 3, 5 and 8 with [Et3Si][B(C6F5)4] range from 73.5–84.8 ppm consistent with three coordinate
(Scheme 2) resulted in fluoride abstraction and the ortho-boryl boron centers.
substituted fluorophosphonium compounds [Ph2PF(o-C6H4BR2)]-
Single crystals of compounds 9 and 11 were characterized by
[B(C6F5)4] (R = Cy 9, Mes 10) and [Ph2PF(o-C6F4BCy2)][B(C6F5)4] 11 X-ray diffraction. Both solid-state structures (Fig. 3) show a
were obtained in 76%, 82% and 93% yield. NMR data for distorted tetrahedral environment around phosphorus with
compounds 9–11 showed the expected, significant increase in P–F bond distances of 1.554(2) Å and 1.555(2) Å in 9 and 11,
the 1JPF coupling constants (9: 980 Hz, 10: 1000 Hz and 11: 977 Hz) respectively. These values are typical for fluorophosphonium
in comparison to the precursor difluorophosphoranes. Interest- cations.17 In the solid state, geometries at boron are distorted
ingly these coupling constants are lower than those observed trigonal planar with the sum of the angles about boron being
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Scheme 3 Catalytic reactivity of 9–11. Reaction conditions: 0.1 mmol
substrate, 0.5 ml solvent. Conversion determined by 1H NMR integration.
a15% of Ph2CHOSiEt3 was observed. b8% of Ph2CHOSiEt3 were observed.
cConversion determined by 19F NMR integration.
11 affords 495% conversion to Et3SiCH2CHPh2 in 48 h at 100 1C.
The deoxygenation of benzophenone to diphenylmethane in the
presence of two equivalents of triethylsilane and 2 mol% catalyst
(9, 10, 11) proceeds with 80%, 0% and 495% conversion (48 h,
50 1C). Lastly, in the case of hydrodefluorination of 1-fluoropentane,
the catalysts 9–11 gave rise to 7%, 9% and 495% conversion of
starting material (Scheme 3).
Fig. 3 Solid state structures of compounds (a) 9 and (b) 11. Hydrogen
atoms and the [B(C6F5)4]Àcounterions are omitted for clarity. Carbon: black;
fluorine: pink; phosphorus: orange; boron: yellow-green. Selected bond
lengths (Å) and angles (1): 9: P(1)–F(1): 1.555(2), B(1)–F(1): 2.639(1). 11:
P(1)–F(1): 1.555(2), B(1)–F(1): 2.491(1).
As a further point of comparison, the P-methylated phos-
358.8(3)1 in 9 and 359.9(2)1 in 11. The BCCC plane is oriented phonium compound [Ph2PMe(o-C6H4BCy2)][B(C6F5)4] 13 was
almost perpendicular to the plane of the bridging aryl substi- prepared by alkylation of 2 with MeOTf and subsequent anion
tuent (9: y 86.3(1)1; 11: y 82.2(1)1). The F-atoms on phosphorus exchange with [Et3Si][B(C6F5)4] (see ESI†). This species shows
are in the corresponding PCCB planes oriented towards the no catalytic activity in all of the above reactions described for
empty pz-orbital at boron with B–F distances of 2.637(1) Å and 9–11 (Scheme 3). This finding suggests that the site of Lewis
2.491(1) Å in 9 and 11, respectively.
acidic reactivity of compounds 9–11 is the fluorophosphonium
Upon addition of a stoichiometric amount of 4-dimethyl- center, not the borane moiety.
aminopyridine (4-DMAP) to 9, formation of the nitrogen–boron
The above catalyst efficiencies were also compared to those
adduct 12 (1JPF = 1025 Hz) was observed.18 Interestingly, this derived from [Ph3PF][B(C6F5)4] 14, [Ph2(C6F5)PF][B(C6F5)4] 15 and
adduct exhibits an increase in the 1JPF coupling constant which [Ph(C6F5)2PF][B(C6F5)4] 16. In the hydrosilylation of 1,1-diphenyl-
is in the range of fluorophosphonium compounds where boron ethylene 14–16 gave 0%, 60% and 495% yields respectively.
substituents are absent. This suggests that the proximal tri- Comparing 11 and 15 after a reaction time of 12 h at 100 1C showed
coordinate boron centers in compound 9 influence the P–F conversions of 55% (11) and 5% (15). This clearly illustrates a
bond and the resulting Lewis acidity at P.
reactivity enhancement which results from the presence of the
To probe this further, the utility of compounds 9–11 in a ortho-boryl substituent. Compound 16 showed virtually full conver-
series of reactions, typically catalyzed by fluorophosphonium sion at 12 h, consistent with previous observation that reactivity of
compounds,4a was assessed (Scheme 3). In the reaction with the phosphonium cations increases significantly with each addi-
1,1-diphenylethylene, 2 mol% of 9 afforded a 50% yield of the tional C6F5 group. The reactivity difference between 9 and 11 follows
Friedel–Crafts dimer in 72 h at 50 1C. In contrast, utilizing com- this trend. For the other reactions depicted in Scheme 3, similar
pound 10 as catalyst resulted in no reaction whereas 11 gives observations of reactivity were made for compounds 9–11 and 14–16
quantitative conversion to the dimer under the same condi- (see the ESI†).
tions. For the hydrosilylation of 1,1-diphenylethylene, a similar
It is worth noting that the bulky substituents at boron in
trend was observed. Again, with 2 mol% catalyst, compound 9 compound 10 basically shut down reactivity, despite the presence
gave 46% of the hydrosilylation product while 10 is inactive and of a more electron deficient B center. This implies that the impact
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1348–1349; (c) J. M. Bayne and D. W. Stephan, Chem. Soc. Rev., 2016,
45, 765–774.
of the borane center on the reactivity of the adjacent phosphonium
center is not merely inductive but rather it plays a cooperative role
in enhancing Lewis acid catalyst activity. Indeed, the structural data
for the above fluorophosphonium cations support the notion
that presence of the borane does not impact the Lewis acidity
of the fluorophosphonium cations by direct interaction with
fluoride. Rather the boron center stabilize transiently generated
phosphorane-type intermediates analogous to those previously
proposed for the reaction mechanisms.4a In this way, the
combination of two relatively weak Lewis acids leads to increased
catalytic activity. It is interesting to note that use of even stronger
boron Lewis acids is not expected to improve reactivity further as
the stronger B-Lewis acid binds fluoride irreversibly leading to
the formation of zwitterionic phosphonium-fluoroborates. This
notion has been previously illustrated with the isolation of
Ph2PFC(tol)QC(C6F5)BF(C6F5)2.19
While there is no doubt that the presence of electron-
withdrawing substituents enhances the Lewis acidity of electro-
philic phosphonium cations, this strategy offers limited options
for tailoring these main group Lewis acids. Herein, we have
presented a conceptual alternative that enhances catalytic reac-
tivity of fluorophosphonium compounds. The incorporation of
weakly acidic boron centers enhance the reactivity at the P-based
s*-orbital by providing an avenue to stabilize hypervalent phos-
phoranes reaction intermediates via an intramolecular P–F–B
´
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¨
interaction. This finding is an interesting twist on Gabbaı’s
strategy in which a proximal phosphonium center enhanced
fluoride binding at boron. In our case, a proximal boron center
enhances the reactivity at an adjacent fluorophosphonium
center. Current efforts are directed towards exploiting this new
strategy to libraries of group 15 Lewis acid catalysts en route to a
range of new synthetic applications.
D. W. S. gratefully acknowledges the financial support of the
NSERC of Canada and the award of a Canada Research Chair.
J. M. is grateful to the Deutsche Forschungsgemeinschaft for a
postdoctoral fellowship. T. v. S. thanks the Alexander von Humboldt
Foundation for a Feodor Lynen Research Fellowship. J. M. is grateful
to the DFG for a postdoctoral fellowship.
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