Evidence of Phosphonium-Carbenium Dication Formation in a Superacid: Precursor to Fluorinated Phosphine Oxides

Article abstract of DOI:10.1002/anie.201811032

Unambiguously confirmed by low-temperature in situ NMR experiments, X-ray diffraction and vibrational spectroscopy, phosphonium-carbenium superelectrophiles are shown to be generated in strong acidic conditions. Representing crucial intermediates, their exploitation allows for the synthesis of unprecedented fluorinated (cyclic) phosphine oxides.

Full text of DOI:10.1002/anie.201811032

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Accepted Article
Title: Evidence of Phosphonium-Carbenium Dications Formation in
Superacid: Key Precursors of Fluorinated Phosphine Oxides
Authors: Sebastien Thibaudeau, Ugo Castelli, Jean François Lohier,
Ines Drukenmüller, Agnès Mingot, Christian Bachman, Carole
Alayrac, Jérôme Marrot, Karin Stierstorfer, Andreas Kornath,
and Annie-Claude Gaumont
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811032
Angew. Chem. 10.1002/ange.201811032
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10.1002/anie.201811032
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Evidence of Phosphonium-Carbenium Dication Formation in a
Superacid: Precursor to Fluorinated Phosphine Oxides
Ugo Castellia, Jean-François Lohierb, Ines Drukenmüllerd, Agnès Mingota, Christian Bachmana, Carole
Alayracb, Jérôme Marrotc, Karin Stierstorferd, Andreas Kornath*d, Annie-Claude Gaumont*b and
Sébastien Thibaudeau*a
Abstract: Unambiguously confirmed by low-temperature in situ
NMR experiments, X-ray diffraction and vibrational spectroscopy,
phosphonium-carbenium superelectrophiles are shown to be
generated in strong acidic conditions. Representing crucial
intermediates, their exploitation allows for the synthesis of
unprecedented fluorinated (cyclic) phosphine oxides.
example of a hydrofluorination[16] and cyclization/fluorination of
unsaturated phosphine oxides. The involvement of phosphonium-
carbenium superelectrophiles is evidenced by X-ray
crystallography, low-temperature NMR spectroscopy and DFT
calculations (Scheme 1b).
Fluorinated compounds[1] are undeniably associated with the
development of catalysis,[2] agrochemicals and drugs.[3]
Organophosphorus compounds have found wide application in
various fields such as catalysis, medicinal chemistry, material
science and inorganic chemistry.[4] To the best of our knowledge,
the formation of fluorinated phosphines and their derivatives via
direct addition is limited to
trifluoromethylfluorosulfonylation[5],
a
few reactions: the
the
bromodifluoromethylation[6] of allyldiphenyl phosphine oxide, the
nucleophile mediated reaction of Ruppert-Prakash reagent (or
perfluoroalkyltrimethylsilanes) with phosphites, or LiC2F5 to
chlorinated phosphine[7] and the unimolecular radical nucleophilic
substitution reaction.[8] Although the addition of an element-
hydrogen pair to an unsaturated C-C bond is a standard direct
Scheme 1. a. Postulated and reported phosphorus containing polycations
formation in acidic conditions (literature); b. Exploitation of phosphonium-
carbenium superelectrophiles for the synthesis of fluorinated phosphine oxides
(this work).
strategy to functionalize
a
molecular skeleton, hydro-
This work aims to address two important challenges: 1) The
deactivation of the unsaturation after protonation of the P-
containing function, which could prevent substrates from any
reaction, needs to be outweighed by the high acidity of the media;
2) the impact of the protonated function on the electrophilicity of
the cation if a dication is formed. The reactivity of model allylic
derivatives 1a, 2a and 3a was evaluated (Scheme 2). After a
reaction at –20°C in HF/SbF5, phosphine oxide 1a was shown to
cyclize to phosphindoline 4a as a 2/3 mixture of syn/anti
diastereoisomers. The same substrate submitted to anhydrous
superacid HF (H0 = –15) or CF3SO3H (H0 = –14.1) remained
unreactive at -20°C.[17] This confirmed that the activation of the
substrate is strongly dependent on the acidity of the respective
medium; a result that is consistent with a superelectrophilic
activation process. Starting from allyldiphenylphosphine 2a,
similar results were obtained. With phosphine-borane analogue
3a, traces of the β-fluorinated phosphine oxide 5a[18] and the
reduced product 6a were formed beside the expected product 4a
under the determined reaction conditions. These results
confirmed that the phosphorus containing function strongly
influenced the reaction course.
(hetero)element addition to allylic or propargylic phosphorus
containing derivatives is rather limited.[9] The pH dependent
stability and the prototropic isomerization of the corresponding
phosphonium derivatives may be the reason for the difficulty to
selectively add any species to these derivatives under acidic
conditions.[10] To avoid undesired rearrangement, Berlin used
polyphosphoric acid (115 %) at elevated temperatures to perform
the intramolecular Friedel-Crafts reaction of allyltriphenyl
phosphonium salts.[11] In situ formation of a dication was
postulated in this seminal contribution, whose hypothesis was
recently reformulated by Vasilyev and Stankevic (Scheme 1a).[12]
Laali[13] and Klumpp[14] went a step beyond by respectively
showing the protonation of tetraphosphocubane and
ketophosphonium salts in superacids.[15] On this basis, we
postulated that the generation of a superelectrophile from an
unsaturated organophosphorus compound can be expected
under super acidic conditions and further exploited to produce
valuable and innovative (P,F)-derivatives. We report here the first
[a]
[b]
IC2MP-UMR CNRS 7582 - Université de Poitiers
4 rue Michel Brunet TSA 51106, F-86073 Poitiers cedex 9, France
E-mail: sebastien.thibaudeau@univ-poitiers.fr
Normandie Université, ENSICAEN, UNICAEN, CNRS
Laboratoire LCMT (UMR 6507 & FR 3038) 14000 Caen, France
E-mail: annie-claude.gaumont@ensicaen.fr
UMR CNRS 8180, 45 avenue des États-Unis, 78035 Versailles
Cedex, France
[c]
[d]
Department of Chemistry
Ludwig-Maximilian University of Munich
Scheme 2. Allylic derivatives reactions in HF/SbF5.
Butenandtstr. 5–13, D-81377 München, Germany
E-mail: akoch@cup.uni-muenchen.de>
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10.1002/anie.201811032
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To further explore this impact, the protonation of
(Scheme 4a). The electron withdrawing effect of the P-centered
cation on the generated carbenium ion increases its electrophilic
character, allowing its internal trapping with a weak nucleophilic
aryl group (substituted with a protonated phosphine oxide) or
external trapping with the “non-nucleophilic” solvated fluoride
ions.[32]
triphenylphosphine,
triphenylphosphine
oxide
and
triphenylphosphine borane in superacid was studied by in situ
low-temperature NMR experiments. Triphenylphosphine was first
added to the superacid HF/SbF5 at -40°C and the reaction was
followed by 31P NMR spectroscopy, revealing the unique
formation of phosphonium ion A (signal at 6.4 ppm to be
compared to signals at 3.89 ppm and 5.28 ppm for corresponding
triflate and triflimidate salts, Scheme 3).[19] Under the same
conditions, triphenylphosphine borane also led to A, alongside
with a vigorous gas evolution which is likely to arise from a
[20-21]
fluoride/hydride exchange followed by P-B bond cleavage.
Under the same conditions, triphenylphosphine oxide led to the
single cationic species B with a 31P NMR signal at 58.97 ppm,
more deshielded that analogous triflate and triflimidate salts[19]
(25.0 ppm for the neutral substrate).
Figure 1: Low temperature Infrared spectra of a) (CH3)3PO, b)
−
[(CH3)3POH]+[Sb2F11
vibrations).
]
and c) [(CH3)3POD]+[AsF6]− (* are marking the anions
If a superelectrophile, such as C, is formed in the superacid, it is
expected to act as a hydride abstracting agent and perform an
ionic hydrogenation process in the presence of cyclohexane.[33]
This was confirmed by the formation of deuterated product 6bD
from diethylmethallylphosphine oxide 1b after a reaction in
HF/SbF5 in the presence of cyclohexane-d12 (Scheme 4b).
Scheme 3. Phosphonium ions A and B generated from triphenylphosphine,
triphenylphosphine borane, and triphenylphosphine oxide in HF/SbF5 and
observed by low-temperature 31P NMR spectroscopy. Illustration of the B’ cation
(thermal ellipsoids with 50% probability) depicted according to the single crystal
-
X-ray structure analysis of [(CH3)3POH]+[Sb2F11
trimethylphosphine oxide in superacid HF/SbF5). [22-23]
]
(generated from
Single crystals grown from a solution of trimethylphosphine oxide
in HF/SbF5 were investigated by X-ray crystallography.[23-24] An
elongation of the P-O bond length (1.574(5) Å) of the
monoprotonated cation B’ was observed by means of a
comparison with the P-O bond in trimethylphosphinoxide
(1.489(6) Å) (Scheme 3). The P-O bond length is comparable with
that of phosphoric acid (1.548(4) Å),[25-26] but shorter than a typical
single P-O bond.[27-28] A possible explanation is that the highly
polarized P-O sigma-bond and the two p-type orbitals of the
oxygen atom undergo a negative hyperconjugation with the P-C
antibonding orbital.[29] The P-C bond length remains
approximately unchanged compared to that of the neutral
Scheme 4. a. Proposed mechanism for the hydrofluorination and cyclization of
alkenyl phosphine oxides. b. Ionic deuteration of a phosphonium-carbenium
dication.
The formation of a dication after the reaction of substrate 1b in
the superacid was finally confirmed by in situ NMR analysis
(Figure 2).
compound.[27]
The
infrared
spectra
of
(CH3)3PO(a),
[(CH3)3POD]+[AsF6]-(b), obtained in DF/AsF5 solution, and
[(CH3)3POH]+[Sb2F11]-(c), obtained in HF/SbF5 solution, are
illustrated in Figure 1. According to the Cs-symmetry of the cation,
39 fundamentals (22 A' + 17 A'') are expected, which all are active
in the Raman and IR spectra. The (OH) mode detected at
3618 cm–1 and the corresponding as(OD) observed at 2565 cm–1
are in good agreement with the Teller-Redlich rule for an H/D
isotopic effect.[30] The broad band shapes are typical of the OH
stretching modes that are involved in hydrogen bonds.[31] The PO
stretching mode is red-shifted from 1160 cm-1 in the oxide to
995 cm-1 in the protonated cation. This is in accordance with the
PO bond elongation due to the protonation, as observed in the
crystal structure. The vibrational data are comparable with
quantum chemically calculated frequencies (for details see
Appendix 1 in the Supporting information).[23] The P-O bond
elongation further confirmed the P-cationic character of the
protonated phosphine oxide. After a second protonation, ion B’’ is
expected to generate the phosphonium-carbenium dication C
350,00
C2D6CO
300,00
250,00
200,00
150,00
100,00
50,00
calcuted
experimental
0,00
P1
C2
C3
C4
C6
C7
C8
C9 C10
Figure 2. Generation and NMR spectra of dication C’ in HF/SbF5 (SbF5 mol% =
21.6) at –30°C and comparison of experimental and calculated 13C NMR and
31P NMR shifts.
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1
In the H NMR spectrum, methyl groups appear deshielded at
Encouraged by these findings, we reasoned that the
superelectrophilic character of these dications should allow their
efficient fluorination in HF/SbF5. It is known that for SbF5
2.80 ppm. The central methylene group signal is centered at 3.28
ppm. In addition, a singlet at 9.26 ppm, attributed to the
protonation of the phosphine oxide further confirmed the
formation of dication C’.[23] In the 13C NMR spectrum, beside
-
concentrations lower than 10 mol %, SbF6 almost is the only
anionic species in solution. Moreover, the nucleophilicity of the
fluoride ion source is likely to increase when the HF/SbF5 ratio
increases.[37] Substrate 1e was thus added to HF/SbF5 (%mol
SbF5 = 1.9). Under these conditions, allyldiethylphosphine oxide
1e led to the exclusive formation of the desired β-fluorinated
traces of
a fluorinated product, the carbocation of the
phosphonium-carbenium dication C’ was characterized by the
singlet located at 330.6 ppm. These values match the theoretical
NMR parameters deduced from calculations for the most stable
postulated intermediate (Figure 2). To note, the same reaction
conducted in TfOH led to the formation of the corresponding O-
protonated vinylphosphine oxide obtained after double bond
isomerization.[23] This result further confirms that it is necessary to
use strong superacid conditions to promote the reaction. To gain
further insight, the same substrate was submitted to less acidic
HF/SbF5 solutions (SbF5 mol % = 1.9) and to neat HF. No dication
formation could be observed and the ratio of the observed
protonated fluorinated phosphine oxide of type D over the O-
protonated starting material of type B’’ increased with acidity.[23]
Furthermore, this ratio increased through reaction time in these
conditions, confirming the mechanism depicted in scheme 4. This
phenomenon is not limited to ethyl-substituted phosphine oxide,
as diisopropylmethallylphosphine oxide also led to the formation
of a phosphonium-carbenium dication.[23]
product 5e in 95
% yield (Scheme 6). Similarly, the
hydrofluorination could be applied to methallyl substrates 1b and
1f. Under these “fluorinating conditions”, the cyclization could be
prevented, and the desired fluorinated product 5c was gratifyingly
obtained in 85 % yield from substrate 1c. An interesting preferred
gauche conformation in the solid state was evidenced by a single
crystal X-ray structure analysis (torsion angle Ø P-C-C-F =
64.23°).[38] When exploiting the conformational gauche
preference of β-fluorinated functionalized molecules[39] emerged
recently as a tool of choice in medicinal chemistry SAR studies or
catalysis,[40] this preferred gauche conformation of β-fluorinated
phosphine oxides offers future perspective in the field.[41]
Fluorinated δ-aminophosphine oxides 5g and 5g’, analogues of
the GABAB receptor agonist,[42] could also be directly generated
in reasonable yields from the unsaturated precursor 1g.
Having demonstrated that phosphonium-carbenium dications are
generated under these conditions, the impact of the substitution
of the phosphine oxide on their reactivity was evaluated (Scheme
5). Substrate 1c was efficiently converted to its trifluoromethylated
dihydrophosphindole 4c[34] and to tetrahydrophosphinoline oxide
congener 7c, confirming the possibility to trap the
superelectrophile with a weak nucleophile. The formation of a six-
membered ring product involving an anti-Markovnikov addition to
the olefin can be directly related to the formation of a hydrido-
bridged (carbonium ion structure) containing superelectrophile, as
already observed for ammonium-carbonium superelectrophilic
activation process.[35] Interestingly, despite the deactivation of the
unsaturation (inductive electron withdrawing effect of the second
phosphine oxide function), the cyclization still occurred for
substrate 1d, making it a relevant starting reaction for the
synthesis of ligand precursors.[36] Finally, we postulate that, if a
dication was generated in solution, any intermolecular trapping
with a weakly nucleophilic partner should occur. This was
confirmed with the formation of the arylated products 8eAr and
8eArF after the reaction of phosphine oxide 1e respectively with
benzene and fluorobenzene.
Scheme 6. Hydrofluorination of alkenyl phosphine oxides under superacid
conditions and X-ray image of 5c crystals.
Furthermore, the direct access to the -gem-difluorinated
analogues was evaluated. In a one-pot process, two consecutive
hydrofluorinations of propargylic phosphine oxide 1h successfully
delivered β-gem-difluorinated analogue 9h in 91 % yield (Scheme
7). By increasing the acidity of the media (%mol SbF5 = 21.6),
which must favor C-F bond activation,[43] the reaction became less
selective, generating a mixture of compounds with 22 % being
represented by the original fluorinated phosphindoline derivative
10h, which could be purified. The potential ability to perform a
tandem cyclization/fluorination process from diallylic substrates
was subsequently evaluated with substrate 1i in the superacidic
solution. Gratifyingly, after a reaction in HF/SbF5, the fluorinated
cyclic product 11i was generated in 63 % yield, opening further
perspectives for the synthesis of novel fluorinated cyclic
phosphine oxides.
Scheme 5. Evaluation of superelectrophilic activation starting from alkenyl
phosphine oxides.
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S. A. Pullakarkat, M. Ma, Y. Zhang, Y. Huang, Y. Li, A. Goel,
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8087.
Scheme 7. Consecutive hydrofluorination and cyclization/fluorination of
unsaturated phosphine oxides
In conclusion, using single-crystal X-ray diffraction studies,
vibrational spectroscopy and in situ low-temperature NMR
experiments, we have provided evidence for the formation of
phophonium carbenium dications from unsaturated phosphine
oxides protonation in superacid. The superelectrophilic character
of these dications was shown to be essential for the syntheses of
various novel fluorinated and cyclic phosphine oxides. Evidenced
by the direct generation of fluorinated analogues of ion-channel
receptors agonists and desymmetrization of prochiral dialkenyl
phosphine oxides, precursors of non-heterocyclic P-stereogenic
compounds, this work opens innovative directions toward the
synthesis of new (fluorinated) valuable organophosphorus
compounds.
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Acknowledgements
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The authors thank the Université de Poitiers, the CNRS, the
Ludwig-Maximilian University Munich (LMU) and the F-Select
GmbH for financial support. The French Fluorine Network is also
thanked for its support. The authors are thankful to the Labex
SynOrg (ANR-11-LABX- 0029) and the Région Normandie.
Normandie Université and CNRS (ACG, JFL, CA) are also
thanked for financial support.
Keywords: Superacid
• Superelectrophile • Fluorination •
Cyclization • Phosphine oxide
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This article is protected by copyright. All rights reserved.

10.1002/anie.201811032
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Phosphonium-carbenium superelectrophiles are evidenced
and exploited in superacid to generate innovative fluorinated
phosphine oxides.
Ugo Castelli, Jean François
Lohier, Ines Drukenmüller,
Agnès Mingot, Christian
Bachman, Carole Alayrac,
Jérôme Marrot, Karin
Stierstorfer, Andreas Kornath, *
Annie-Claude Gaumont *, and
Sébastien Thibaudeau*
Page No. – Page No.
Evidence of Phosphonium-
Carbenium Dication Formation
in a Superacid: Precursor to
Fluorinated Phosphine Oxides
This article is protected by copyright. All rights reserved.

10.1002/anie.201811032
Angewandte Chemie International Edition
COMMUNICATION
This article is protected by copyright. All rights reserved.