Revisiting the Perfluorinated Trityl Cation

Article abstract of DOI:10.1002/chem.201806110

Although ultimately not isolable for X-ray structural characterization, the free perfluorinated trityl cation was shown to be observable in neat triflic acid, which represents milder conditions than previous reports of this cation in “magic acid” or oleum. A triflate-bound species could be generated in organic solvents using stoichiometric amounts of triflic acid and was shown to be synthetically viable for hydride abstraction from Et₃SiH. It was demonstrated that the para-position on the -C₆F₅ rings is the primary point of attack for decomposition of the cation.

Full text of DOI:10.1002/chem.201806110

A Journal of
Accepted Article
Title: Revisiting the perfluorinated trityl cation
Authors: Jason Laurence Dutton, Eoghan G. Delany, Satnam Kaur,
Steven Cummings, Kristoffer Basse, and David J. D. Wilson
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10.1002/chem.201806110
Chemistry - A European Journal
Revisiting the perfluorinated trityl cation
a
Eoghan G. Delany, Satnam Kaur, Steven Cummings, Kristoffer Basse, David J. D. Wilson, and
a
c
b
a
a
Jason L. Dutton*
a
Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe
University, Melbourne, Victoria, Australia, 3086
b
La Trobe University Comprehensive Proteomics Platform, Melbourne, Victoria, Australia, 3086
c
Department of Chemistry, Howard University, 525 College St, NW, Washington, DC 20059.
ABSTRACT
While ultimately not isolable for X-ray structural characterization, the free perfluorinated trityl
cation is shown to be observable in neat triflic acid, which represents milder conditions than
previous reports of this cation in “magic acid” or oleum. A triflate-bound species can be generated
in organic solvents using stoichiometric amounts of triflic acid and is shown to be synthetically
viable for hydride abstraction from Et3SiH. It is demonstrated that the para position on the –C6F5
rings is the primary point of attack for decomposition of the cation.
1
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INTRODUCTION
+
The perfluorinated analogue of the trityl cation (1 ) was first observed in superacidic media
1
2
independently by Filler and by Olah in the 1960s, generated by protonation of an alcohol precursor
3
+
(1OH, Scheme 1). The stability of 1 was limited in oleum, with a half life of 2 minutes at room
2
temperature. In “magic acid” (HSO3F-SbF5), F NMR spectra could be obtained at -60°C, but there
19
3
was no details on attempts at isolation. Filler was able to demonstrate that the cation can be used as
4
a one-electron oxidant in 100% H2SO4 and that the corresponding neutral radical is relatively stable.
+
Scheme 1. Generation of trityl cation 1 under superacidic conditions.
The electrophilicities of partially fluorinated trityl cations have been studied recently by Mayr and
5,6
co-workers, but the fully fluorinated analogue was not included. Given the explosion in utility
7,8
observed for the boron counterpart, trispentafluorophenyl borane, and the availability of new
9,10
counterions for reactive cations we proposed in a theoretical paper in 2014 that it was time to
11
revisit the fully fluorinated analogue of the trityl cation. We predicted that its fluoride and hydride
affinities would be very high, much higher than those of trityl cation and trispentafluorophenyl
borane. Stephan and co-workers also theoretically considered the perfluorinated trityl cation as an
extreme Lewis acid showing good correlation between predicted Global Electrophilicity Indexes
12
and Lewis acidity.
+
Herein we report on our efforts to observe and isolate cation 1 as a triflate or carborane salt in non-
superacidic media along with an initial exploration of its chemistry as a potential hydride acceptor.
2
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RESULTS AND DISCUSSION
Our initial goal was to prepare a halo derivative of 1OH for use in halide abstraction routes to form
+
1 . The starting alcohol 1OH was initially synthesized by Filler et al. by reacting LiC6F5 with
2
decafluorobenzophenone. They reported being unable to synthesize a halogenated derivative. We
repeated their synthesis of 1OH and further verified its structure by X-ray (Figure 1). Caution: in
the course of a routine 2 g synthesis, we experienced an explosion from LiC6F5 despite the mixture
being held in a dry-ice/EtOH bath. We subsequently found that replacing the lithium reagent with
the Grignard reagent BrMg-C6F5, generated from Mg metal and Br-C6F5, provides a suitable
alternative route to 1OH, albeit with a somewhat lower yield.
Figure 1. Solid-state structure of 1OH. Thermal ellipsoids are depicted at the 50% probability
level.
In an attempt to form the bromo derivative, 1OH was reacted with neat acetyl bromide, following
13
the reported procedure for the formation of trityl bromide from trityl alcohol. Fluorine-19 NMR of
3
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the reaction mixture and isolated material gave the expected three resonances, shifted from those in
1OH. However, X-ray studies revealed the presence of the acetate derivative, 1OAc, rather than
1
the targeted bromide (Figure 2). The H NMR spectrum of the isolated powder contained a singlet,
confirming identity of the bulk material with the single crystal.
Figure 2. Solid-state structure of 1OAc. Thermal ellipsoids are depicted at the 50% probability
level.
We next used SOCl2 to form the chloro derivative (Scheme 2). Reaction in p-xylene at 142 ºC
19
consistently resulted in a crude product showing nine F NMR signals. The mixture could be
chromatographically separated into two compounds, one showing six signals, the other showing the
4
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expected three signals from symmetric -C6F5 groups. X-ray diffraction revealed the latter structure to
19
be the targeted tris(perfluorophenyl) chloride 1Cl (Figure 3). The former compound, with six F
3
signals, was revealed to be chlorinated at the para-position forming an sp hybridized carbon, 2
(Figure 4). Heating 1Cl to high temperatures results in transformation to 2, presumably by attack
from chlorine onto the para-position, which is known to be a reactive site on -C6F5 rings with respect
14
to nucleophilic attack. Efforts to minimize formation of this para-substituted isomer through
various synthetic modifications (lower reaction temperatures, Vilsmeier-Haack chemistry or
alternative chlorinating agents) failed to give better yields of the desired 1Cl. Indeed, 2 often
became the sole product. Nevertheless, under milder reaction temperatures (< 80 ºC approx.) 2
shows significantly reduced solubility relative to the desired tris(perfluorophenyl) chloride,
allowing isolation of 1Cl in reasonable purity.
Scheme 2. Generation of chloro-derivatives 1Cl and 2.
Examination of the solid-state contacts in 1Cl shows that the intermolecular contact for the chloride
atom and the para-carbon position of an adjacent molecule is 3.35 Å, less than the sum of their Van
der Waals radii of ~3.5 Å. These are the closest contacts for these atoms, indicative of the Lewis
basic nature of the terminal chloride and more importantly, the Lewis acidic nature of the para-
carbon atom.
5
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Figure 3. Solid-state structure of 1Cl. Thermal ellipsoids are depicted at the 50% probability level.
Selected bond distances (Å): C(1)-Cl(1) 1.845(2), C(1)-C(4) 1.538(3).
6
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Figure 4. Solid-state structure of 2. Thermal ellipsoids are depicted at the 50% probability level.
Selected bond distances (Å): C(5)-Cl(1A) 1.727(3), C(5)-F(1) 1.415(5), C(4)-C(5) 1.484(2), C(3)-
C(4) 1.327(2), C(2)-C(3) 1.466(2), C(1)-C(2) 1.355(3).
With the requisite tris(perfluorophenyl) chloride in hand, we set about investigating its reaction
19
with various halide abstraction agents. F NMR analysis of a 1:1 mixture of 1Cl and silver triflate
(AgOTf) after stirring at ambient temperature for 20 hours in acetonitrile indicated formation of a
new species having similar characteristics to 1OH. However, concentration of the colourless
19
reaction mixture resulted in isolation of a new red solid whose F NMR showed five new
resonances. X-ray analysis revealed this compound to be the trienone 6 (Scheme 3, Figure 5),
15
previously reported from quenching reactions between 1OH and H2SO4 with water. Although
employing stringent anhydrous conditions, we initially surmised that trace amounts of water lead to
7
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its formation. However, despite all reasonable efforts to exclude water from the system, we
continued to isolate 6 as the sole reaction product, suggesting water does not represent the source of
the new ketonic oxygen atom. The same outcome was observed when employing a stoichiometric
amount of trimethylsilyl triflate (TMSOTf) instead of AgOTf. Attempts at using Ag2[B12Cl12] as a
2- 16
halide abstraction agent introducing the weakly coordinating [B12Cl12] anion resulted in no reaction.
A plausible reaction pathway to rationalize this reproducible formation of 6 is proposed in Scheme
3. Attack of an oxygen-containing nucleophile at the para position of one perfluorinated aryl ring
-
results in formation of triene 4 through loss of leaving group X. Subsequent elimination of F yields
the more stable conjugated trienone 6 via 5.
Scheme 3. Proposed mechanism for the generation of ketone 6.
8
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Figure 5. Solid-state structure of 6. Thermal ellipsoids are depicted at the 50% probability level.
Selected bond distances (Å): C(5)-O(1) 1.214(3), C(4)-C(5) 1.460(2), C(3)-C(4) 1.329(2), C(2)-
C(3) 1.462(2), C(1)-C(2) 1.357(3), C(1)-C(9) 1.494(2).
The addition of neat triflic acid to solid 1OH at -30 °C resulted in immediate generation of a deep
19
maroon solution (Scheme 4). F NMR spectroscopy of the solution with the NMR probe set at -30
o
C revealed three signals at -115, -128, and -157 ppm. The strongly downfield shift of the para and
+
ortho fluorine atoms is consistent with the observations of Olah and co-workers for ionized 1 in
3
13
strongly acidic media. The C NMR spectrum of the mixture shows a peak at 171 ppm that can be
assigned to the central carbon. This is well downfield from there the signal for the central carbon
3
appears in 1OH, consistent with the change from sp to sp hybridization in the cation. The chemical
2
17
shift for the parent trityl cation in chlorosulfonic acid solution is ~ 200 ppm. The upfield shift for
the perfluorinated derivative can be rationalized on the basis of increased π donation to the central
carbon by the fluorine atoms. This is consistent with theoretical calculations of the two cations, with
9
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13
the predicted C NMR shift of the trityl cation at 213 ppm and the predicted shift for 1 at 175 ppm.
+
o
While this cation is stable for at least two hours at -30 C, warming to room temperature results in
decomposition, with ketone 6 being the main product after a sample was held overnight. This is an
improvement over previous methods, as oleum cannot be used at low temperature, and “magic
acid” is far less convenient/accessible then triflic acid, and triflic acid allows the cation to be
observed at warmer temperatures than magic acid.
+
Scheme 4. Generation of 1 as a triflate salt in neat HOTf and decomposition to 6 upon warming.
Neat triflic acid though remains a harsh medium and likely precludes the synthetic use of 1+. We
therefore explored its formation in organic solvents. Reacting two equivalents of HOTf (one to
protonate the alcohol, and the second to sequester the resulting equivalent of H2O) with 1OH in
ortho-dichlorobenzene (ODCB) resulted in the immediate formation of a dark red colored solution
19
(Scheme 5). The F NMR spectrum of the reaction mixture at -10 C was relatively clean, with three
o
19
F signals at -142.5, -153.6, and -162.8 ppm. These signals are distinctly different from those of
1OH in the same solvent, but far upfield to what is consistent with a free cation, as observed with
neat triflic acid. Warming to room temperature resulted in decomposition of the mixture to signals
consistent with 6 as the main product after 48 hours. The data is consistent with the formation of
13
1OTf with the triflate fragment remaining bound to the carbon centre. The C NMR chemical shift
3
provides support for this notion, the shift of the central carbon is 71 ppm, consistent with an sp
hybridized centre. This can be compared to a shift of 57 ppm in 1Cl. Theoretical calculations of the
13
C NMR spectrum are again consistent with the observed results, with a shift of 90 ppm predicted
for the central carbon in the triflate bound species 1OTf, far upfield of what is predicted and
observed for the free cation in strong acid above. The parent trityl cation remains ionized in ODCB
13
solution, with an observed C NMR shift of 209 ppm. If the triflate remained bound to the central
10
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13
carbon for the parent trityl cation a C NMR shift of 110 ppm is predicted. The difference can be
rationalized with the greater Lewis acidity of the perfluorinated analogue.
To confirm our previous hypothesis that the presence of trace water in the reaction mixture does not
result in formation of trienone 6 (see Scheme 3), we further reacted two equivalents of HOTf with
1OH in ODCB. Subsequent addition of five equivalents of H2O results only in regeneration of
19
1OH via F NMR analysis of the crude reaction mixture.
Scheme 5. Generation of 1OTf and decomposition to 6 upon warming.
To lend support to the proposed formation of 1OTf and determine its potential synthetic utility, we
explored its ability to act as a hydride acceptor, which would be expected behavior if 1OTf had
indeed been generated. Addition of Et3SiH as a hydride donor to a solution of 1OTf prepared from
1OH and two equivalents of HOTf at -10 °C in ODCB (Scheme 6), resulted in an NMR spectrum
consistent with the presence of two species, one the regenerated alcohol 1OH, and the other as
18
tris(pentafluorophenyl)methane 1H. Performing the reaction in toluene at low temperature allowed
for simpler isolation of 1H in a pure form, because toluene is more easily removed under vacuum.
We hypothesize that the competing reactions shown in Scheme 6 account for this distribution of
19
products. Nevertheless, the generation of 1H is supportive of the generation of 1OTf with the F
NMR suggesting the triflate anion remains bound as it is too strongly coordinating to allow
+
ionization to 1 in low dielectric media.
11
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Scheme 6. Reaction pathways of 1OTf with hydride donor Et3SiH.
Carborane anions are much less coordinating than triflate and often allow isolation of extremely
9
reactive cations. To determine if the closo-carborane anion [HCB11Cl11] might allow isolation of
-
+
“free” 1 in non-super acidic media, solid alcohol 1OH was mixed with two equivalents of solid
[C6H7][HCB11Cl11], the protonated benzene salt of the carborane. IR spectroscopy indicated that
mixing the two solids did not result in any reaction but the addition of a drop of benzene resulted in
the immediate generation of a maroon slurry, which was followed by immediate evaporation. IR
spectroscopy of the resulting solid indicated loss of 1OH. The vibrations associated with stretching
-1
-1
of Ccentral-Cipso bonds are shifted to lower frequencies (1604-1611 cm ) compared to 1OH (1653 cm ).
For comparison, calculated vibration frequencies using B3LYP-D3(BJ)/def2-TZVPP give 1671-
-1
1678 cm for 1OH and 1667 cm for 1 . For comparison, the IR spectrum for the trityl salt of the
-1
+
-1
same carborane anion shows stretches associated with the central-ipso C-C bonds at 1577 cm while
-1
the computationally predicted vibration for the trityl cation is found at 1630 cm . The differences in
12
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the experimental/computationally predicted vibrations for the central-ispo C-C bonds in 1OH, the
+
targeted 1 and trityl cation are in good agreement. The blue shift for the perfluorinated compound
is likely a result of a stronger C-C bond induced by the π donor ability of the fluorine atoms,
13
consistent with the differences in the C NMR chemical shifts discussed above.
+
Despite this evidence for the existence of 1 in the solid state, it does not appear that the cation is
stable in low polarity solvents. Addition of 2 equivalents of [C6H7][CHB11Cl11] to a solution of 1OH
in a variety of arene solvents (toluene, benzene, ODCB) did result in the immediate formation of
19
red solutions, they underwent decolorization within minutes and we were unable to identify any F
NMR signals consistent with formation of 1. Attempted dissolution of the solid resulting from the
slurry in the IR experiment in arene solvents also resulted in rapid decolorization of the solution.
CONCLUSIONS
We have demonstrated that the perfluorinated trityl cation is relatively stable in neat triflic acid. In
less polar organic solvents the triflate is bound but nevertheless shows reactivity as a hydride
acceptor with silanes. The free cation appears to exist in the solid state when the counterion is
switched to an ultra-inert carborane but intrinsic decomposition prevents its full characterization. It
is apparent that the para-carbon position of the –C6F5 ring is the Achilles heel of this reactive
species.
ASSOCIATED CONTENT
1
19
Supporting Information. Experimental details with associated H, F, IR spectra of synthesized
compounds and reaction mixtures. Computational details and Cartesian coordinates for calculated
species. X-ray crystallographic details in .cif format.
AUTHOR INFORMATION
13
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Corresponding Author
*j.dutton@latrobe.edu.au
ACKNOWLEDGMENT
We thank The La Trobe Institute for Molecular Science for their generous funding of this project.
This work was also supported by an ARC Future Fellowship (JLD, FT16010007).
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(2)
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TOC Graphic and Abstract
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F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
O
S CF3
O
2 HOTf
Et3SiH
F
F
OH
F
F
F
F
O
H
F
F
F
F
F
F
F
F
F
F
F
F F
F F
F
F F
F
F
F
F
F
F
Accessable synthon for the perfluorinated trityl cation
Blast from the past. A triflate bound derivative of the perfluorinated trityl cation can be generated
from triflic acid in organic solvent at -20°C and used as a hydride acceptor. The free cation can also
be observed in neat triflic acid. Attack on the –para carbon by nucleophiles is the primary mode of
decomposition of the cation.
15
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