Bipyridinium and Phenanthrolinium Dications for Metal-Free Hydrodefluorination: Distinctive Carbon-Based Reactivity

Article abstract of DOI:10.1002/chem.202101534

The development of novel Lewis acids derived from bipyridinium and phenanthrolinium dications is reported. Calculations of Hydride Ion Affinity (HIA) values indicate high carbon-based Lewis acidity at the ortho and para positions. This arises in part from extensive LUMO delocalization across the aromatic backbones. Species [C₁₀H₆R₂N₂CH₂CH₂]²⁺ (R=H [1 a]²⁺, Me [1 f]²⁺, tBu [1 g]²⁺), and [C₁₂H₄R₄N₂CH₂CH₂]²⁺ (R=H [2 a]²⁺, Me [2 b]²⁺) were prepared and evaluated for use in the initiation of hydrodefluorination (HDF) catalysis. Compound [2 a]²⁺ proved highly effective towards generating catalytically active silylium cations via Lewis acid-mediated hydride abstraction from silane. This enabled the HDF of a range of aryl- and alkyl- substituted sp³(C?F) bonds under mild conditions. The protocol was also adapted to effect the deuterodefluorination of cis-2,4,6-(CF₃)₃C₆H₉. The dications are shown to act as hydride acceptors with the isolation of neutral species C₁₆H₁₄N₂ (3 a) and C₁₆H₁₀Me₄N₂ (3 b) and monocationic species [C₁₄H₁₃N₂]⁺ ([4 a]⁺) and [C₁₈H₂₁N₂]+ ([4 b]⁺). Experimental and computational data provide further support that the dications are initiators in the generation of silylium cations.

Full text of DOI:10.1002/chem.202101534

Accepted Article
Accepted Article
Title: Bipyridinium and Phenanthrolinium Dications for Metal-Free
Hydrodefluorination: Distinctive Carbon-Based Reactivity
Authors: Douglas Wade Stephan, Katherine I. Burton, Iris Elser,
Alexander E. Waked, Tobias Wagener, Ryan J. Andrews, and
Frank Glorius
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.202101534
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1/2020

Chemistry - A European Journal
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Bipyridinium and Phenanthrolinium Dications for Metal-Free
Hydrodefluorination: Distinctive Carbon-Based Reactivity
Katherine I. Burton+[a], Iris Elser+[a], Alexander E. Waked , Tobias Wagener , Ryan J. Andrews ,
[a]
[b]
[a]
[
b]
[a]
Frank Glorius , Douglas W. Stephan
Dedicated to the memory of Prof. Dr. Suning Wang
[
a]
Katherine I. Burton, Dr. Iris Elser, Dr. Alexander E. Waked, Ryan J. Andrews, Prof. Dr. Douglas W. Stephan
Department of Chemistry, Davenport Research Laboratories
University of Toronto, Toronto, Ontario M5S 3H6 (Canada)
E-mail: douglas.stephan@utoronto.ca @FLPchemist
These authors contributed equally.
[
[
+]
b]
Tobias Wagener, Prof. Dr. Frank Glorius
Organisch-Chemisches Institut,
Westfälische Wilhelms-Universität Münster
Correnstraβe 40, 48149 Münster (Germany)
E-mail: glorius@uni-muenster.de
Electronic Supplementary Information (ESI) available: Synthetic and spectral data are deposited. X-ray crystallographic data can be obtained from the
CCDC 2080239-2080242.
Abstract: The development of novel Lewis acids derived from
cations with Lewis acids to achieve catalytic HDF. Most notably,
Ozerov and co-workers established a moisture-sensitive trityl
carborane as an initiator for HDF (Scheme 1). Since then, cationic
aluminum[13] and germanium[14] systems have been used as
initiators for the catalytic HDF of fluoroalkanes. Further, HDF has
bipyridinium and phenanthrolinium dications is reported. Calculations
of Hydride Ion Affinity (HIA) values indicate high carbon-based Lewis
acidity at the ortho and para positions. This arises in part from
extensive LUMO delocalization across the aromatic backbones.
2
+
2+
2+
2+
[15]
[16]
Species [C10
CH
evaluated for use in the initiation of hydrodefluorination (HDF)
H
6
R
2
N
2
CH
2
CH
2
]
(R = H [1a] , Me [1f] , tBu [1g] ), and
been reported using cationic lithium,
silicon
and
2
+
2+
2+
[17]
[
C H R N
12 4 4 2
2
CH
2
]
(R = H [2a] , Me [2b] ) were prepared and
phosphorus centers.
2
+
catalysis. Compound [2a]
proved highly effective towards
generating catalytically active silylium cations via Lewis acid-
mediated hydride abstraction from silane. This enabled the HDF of a
3
range of aryl- and alkyl- substituted sp (C-F) bonds under mild
conditions. The protocol was also adapted to effect the
deuterodefluorination of cis-2,4,6-(CF
3 3 6 9
) C H . The dications are
shown to act as hydride acceptors with the isolation of neutral species
C
16
H
14
N
2
(3a) and C16
H
10Me
4
N
2
(3b) and monocationic species
+
+
+
[
C
14
H
13
N
2
]
([4a] ) and [C18
H
21
2
N ]+ ([4b] ). Experimental and
computational data provide further support that the dications are
initiators in the generation of silylium cations.
Introduction
Catalytic C-F bond activation using transition metals has
received considerable attention over the past two decades.[1]
Following the initial isolation of a stable silyl cation by Lambert
and co-workers[2] in 1993, Milstein et al.[3] reported the preparation
3 2
of a Rh(I) silyl complex [L RhSiMe Ph] effective towards the
catalytic hydrodefluorination (HDF) of polyfluoroarenes. These
studies prompted a number of reports by the groups of
Whittlesey,[ Kakiuchi,[5] Lentz[6] and others,[7]] demonstrating
HDF as mediated by a transition metal and silane reductant.
Efforts to establish comparable reactivity using main-group
elements have also intensified. Accordingly, the collective
4]
contributions of Ozerov, Braun,[9] Kemnitz,[10] Müller,[11] and
others,[12] have established the utility of generating trivalent silicon
[8]
Scheme 1. The metal-free hydrodefluorination reaction; carbon-based Lewis
acids; the focus of this work.
1
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The use of carbon-based cations as Lewis acids has
progressed in recent years. For example, trityl cations have been
shown to exhibit robust functional group tolerance across a range
is diminished to 38 kcal/mol. The alkyl and methoxy-substituents
in [1b]2+ and [2b] , [2c] and [1d]2+ result in diminished HIA
2+
2+
2
+
2+
2+
values, while the tert-butyl substituents in [1c] , [2d] and [2e]
of organic substrates.[18] Further, electron-deficient allenes,[19]
borenium cation and N-methyl acridinium salts,[20] the Ru complex
PhB(C PPh
RuCl]+,[21] and trioxatriangulenium cations[22]
have all been exploited as Lewis acids in frustrated Lewis pair
FLP) chemistry. Similarly, C60/C70 fullerenes (Scheme 1) have
a
present elevated HIA values at the site of substitution. This
presumably reflects the reduction in steric congestion upon
rehybridization of the carbon atom following hydride addition.[27]
[
6
H
4
2
)
2
;
(
[
23]
been shown to form classical Lewis adducts with carbenes. In
our own work, a recent study of a hexacoordinate P(V) dication
revealed its Lewis acidity is centered on the carbon atom of the
terpyridine ligand para to the nitrogen atom.[17c] Thus, the utility of
this species in HDF was attributable to carbon-based Lewis
acidity. To probe this aspect further, herein we query the Lewis
acidity of simple organic dications. We establish the Lewis acidity
of dications derived by alkylation of bipyridine or phenanthroline
with dibromoethane which allows these dications to act as
initiators for hydrodefluorination catalysis of a range of mono- di-,
and trifluoromethyl-derivatives. The mechanism of these
reactions is probed both experimentally and computationally.
Results and Discussion
To begin we assessed the Lewis acidity of the bipyridinium
2
+
2+
dication, [C10 CH
H
8
N
2
2 2
CH ] [1a] and phenanthrolinium dication
]2+ [2a] via natural bond order (NBO) analysis
2+
[
C
12
H
8
N
2
CH CH
2 2
performed at M06-2X/Def2-TZVP level of theory.[24] The
calculations indicated extensive LUMO delocalization around
both aromatic rings of the bipyridinium scaffolds (Figure 1). This
+
situation is reminiscent to the pyridinium-based NAD /NADH
coenzyme, which is known to reversibly accept hydride at its para
carbon.[25] The hydride ion affinities (HIA)
[26]
of the series of
Figure 2. Bipyridinium and phenanthrolinium dications; HIA values in kcal/mol
relative to BEt are indicated for the ortho (blue) and para (red) positions. These
3
are calculated at HF/3-21G and M06-2X/6-311+G(d,p) levels of theory.
2
+
2+
2+
2+
dications [1a] -[1e] and [2a] -[2f] , (Figure 2) were computed
using density functional theory (DFT) at HF/3-21G and M06-2x/6-
311+G(d,p) levels of theory. The values reveal that the Lewis
acidity is dependent on the degree of ring substitution and extent
of delocalization. The unsubstituted dication [1a]2+ exhibits HIA
values of 48 kcal/mol at both ortho and para carbon positions. In
contrast, species [2a]2+ exhibits HIA values of 56 and 57 kcal/mol
at the ortho and para carbon atoms, respectively. Similarly, the
extended conjugation of species [1e]2+ results in a comparatively
high HIA value of 61 kcal/mol at the ortho carbon, while the HIA
at the para carbon
Figure 1. LUMO of (a) [1a]2+ and (b) [2a]2+ computed at M06-2X/Def2-TZVP
level of theory.
Scheme 2. Synthesis of bipyridinium and phenanthrolinium salts.
2
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Chemistry - A European Journal
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2
+
With these computations in hand, the bis-bromide salts [1x]
The bipyridinium salts, [1a][B(C , and [1f][B(C
6
F
5
)
4
]
2
6 5 4 2
F ) ] were
]2+ (R = H [1a] , Me [1f] , tBu [1g] ), and
2+
2+
2+
also tested. Specifically, [1a][B(C F ) ] led to 18% conversion of
[
C
10
H
6
+
R
2
N
2
CH
2
CH
CH
2
6 5 4 2
2+
2+
[
2x]2 [C12
H
4
R
4
N
2
2
CH
2
]2+ (R = H [2a] , Me [2b] ) were
S1 under the optimized reaction conditions at 40 °C for 4 hours in
oDFB and full conversion after 5 days at room temperature.
[
28]
synthesized following literature procedures involving alkylation
at 130 °C overnight (Scheme 2). The protocol afforded these salts
in isolated yields of 73-96%. The corresponding triflate and
tetrakis(pentafluorophenyl)borate salts were prepared via anion
Table 1. Hydrodefluorination of α,α,α-trifluorotoluene.
metathesis using Me
corresponding salts in 79-97% yield. Alternatively, [1g][B(C
was synthesized by reaction of [1g][Br] with in situ generated
SiEt .
tol][B(C in 38% yield (Scheme 2). Crystals of
1a][OTf] , [2b][OTf] and [2a][B(C were obtained via
recrystallization from acetonitrile/ pentane, acetonitrile and oDFB
solutions, respectively. The crystal structures confirmed the
dicationic nature of the species (Figure 3), although in the case of
3 3 6 5 4
SiOTf or [CPh ][B(C F ) ] providing the
6 5 4 2
F ) ]
2
consumpt.
[
[
3
6
F
5
)
4
]
cat.[a]
silane
solvent
T (°C)
time(h)
/ SiF form.
[%][
b]
2
2
6 5 4 2
F ) ]
[
1a]2+
3
Et SiH
3
Et SiH
3
Et SiH
3
Et SiH
3
Et SiH
3
Et SiH
3
Et SiH
C
C
6
H
H
4
F
2
2
2
rt
33
4
0 / 0
6
4
F
40
rt
18 / 6
99 / 69
0 / 0
CH
2
Cl
120
24
14
4
[
[
1f]2+
2a]2+
C
C
C
6
6
6
H
H
H
4
4
4
F
2
2
2
2
60
rt
[
6 5 4 2
2a][B(C F ) ] the cation was disordered. The metric parameters
F
F
99 / 61
99 / 99
99 / 99
0 / 0
were unexceptional.
40
rt
CH
2
Cl
48
4
iPr
Me
Me
3
SiH
Si)
HSiOSiMe
C
C
C
C
C
6
H
6
H
6
H
6
H
6
H
4
4
4
4
4
F
2
2
2
2
2
40
40
40
rt
(
3
3
SiH
F
F
F
F
4
11 / 5
99 / 41
94 / 73
86 / 79
2
3
0.5
33
13
[
2b]2+
Et
3
SiH
SiH
Et
3
40
[
a] [B(C
6
F
5
)
4
]
2
anions omitted for clarity. [b] Determined by 19F NMR
).
spectroscopy (internal standard C
6 6
F
Table 2. Hydrodefluorination of fluorinated substrates[a]
.
consumpt. / SiF form.
[
ent.
substrate
4-((HO) B)C
%][
a]
1
2
3
4
5
6
7
8
9
2
6
H
4
CF
3
>99 / 41
99 / 39
4-BrC
2-BrC
6
H
H
4
CF
3
3
6
4
CF
98 / 88
Figure 3. POV-ray depictions of the crystal structures of the cations of (a)
2,6-(CF
2-CF
CF
4-CF
2-MeC
3
)
2
-4-BrC
6
H
3
>99 / >99
30 / 30
2 6 5 4 2 2
[1a][OTf] (b) [2a][B(C F ) ] and (c) [2b][OTf] . C: black, N: blue. Hydrogen
3
C
6
H
4
C(O)C
6
H
5
atoms were omitted for clarity.
C
6
H
5
2
H
>99 / 38
>99 / 94
>99 / 86
>99 / 16
>99 / >99
>99 / 92
>99[b] / N/A
99 / 41[b]
3
C
6
H
4
CH
2
F
6
4 2
H CH F
Initial trials probing the utility of [2a][B(C
HDF reactions of α,α,α-trifluorotoluene (S1) in the presence of
Et
SiH were undertaken and the reaction monitored by 19F NMR
spectroscopy, using C as an internal standard. In CH Cl , use
of [2a][B(C furnished full consumption of S1 at room
6 5 4 2
F ) ] to initiate
C
C
C
C
6
H
H
11CF
3
10
11
12
13
6
11
F
3
10
H
15
F
6
F
6
2
2
6 5 2
H CCl H
6 5 4 2
F ) ]
3 3 6 9
2,4,6-(CF ) C H
temperature after 48 h. The comparatively sluggish reactivity was
attributed to the poor solubility of the phenanthrolinium species in
CH Cl . Accordingly, solvent screening (Table S1, S.I.) using a
2 2
range of halogenated and coordinating solvents led to superior
performance in ortho-difluorobenzene (oDFB), furnishing full
consumption in 14 h. Interestingly, despite improved solubility in
acetonitrile and pivalonitrile, no reaction was observed in these
solvents. Further optimization of the reaction temperature showed
reaction at 40 °C in oDFB decreased the reaction time to 4 h. The
[
a] Determined by 19F NMR spectroscopy (internal standard
Determined by 1H NMR spectroscopy (internal standard CH
Cl ). [c] Use of
Et SiD, 28 h.
6 6
C F ). [b]
2
2
3
6 5 4 2
Use of [1f][B(C F ) ] led to no reaction even under
prolonged reaction times and at higher reaction temperatures. In
contrast, the tetramethyl-substituted phenanthrolinium derivative
[
Interestingly, the observed order of reactivity of the dications
follows [2a] >[2b] >>[1a] >[1f] , and correlates with the
trends observed for the calculated HIA values. Using
[
variation in the degree of aryl ring substitution as inclusion of para-
boronic acid or ortho- or para-bromide substituents led to
6 5 4 2
2b][B(C F ) ] was effective in prompting the trial HDF (Table 1).
alternative silanes, iPr
considerably (0 and 11% conversion after 4 h at 40 °C). Although
use of Me HSiOSiMe afforded 99% conversion of α,α,α-
trifluorotoluene in 30 minutes at 40 °C, multiple SiF products were
observed. Thus, going forward, Et SiH was employed as the
hydride source due to ease of monitoring the reaction progress.
3 3 3
SiH and (Me Si) SiH, slowed the reaction
2
+
2+
2+
2+
2
3
2a][B(C
6
5
F )
4
]
2
the HDF of α,α,α-trifluorotoluene tolerated
3
3
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Chemistry - A European Journal
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FULL PAPER
complete HDF (Table 2 Entries 1-3). Similarly, a bromide meta to
It is also noteworthy that these HDF reaction mixtures
two CF
presence of 7.2 equivalents of silane (Table 2 Entry 4). In contrast, [2a][B(C
the ortho-CF derived benzophenone resulted in a markedly
diminished yield of 30%, attributed to competing hydrosilylation of
the carbonyl moiety (Table 2 Entry 5). In addition to aryl-CF
3
groups led to effective HDF of both CF
3
fragments in the
exhibited a bright pink color (Figure S39, S.I.). Heating
6
5
F )
4
]
2
at 40 °C in the presence of excess Et
3
SiH in oDFB
1
3
furnished a similar light pink coloration (Figure S41, S.I.). H NMR
spectroscopy of this mixture revealed a broadening of the
phenanthrolinium resonances, suggesting a reversible interaction
of the silane with the dication. Efforts to probe the nature of
hydride interaction with the dications prompted reactions of [2a]2+
3
derivatives, difluoro- and monofluoro-benzyl derivatives also
undergo complete HDF (Table 2 Entries 6, 7, 8). The cyclohexyl-
CF
3
and fluorides C
6
H
11CF
3
, C
6
H
11F as well as adamantyl fluoride
and [2b]2+ with the more reactive hydride donor LiAlH
reaction of the [2a][Br] and [2b][Br] precursors with two
equivalents of LiAlH in Et O yielded the doubly reduced bright
yellow, neutral species C16 (3a) and C16 (3b) in
4
. The
were readily converted to the corresponding alkanes (Table 2
Entries 9-11). In accordance with previous literature[8c] the chloro-
derivative
2
2
4
2
C
6
H
5
CCl
2
H
also
underwent
complete
H
14
N
2
4 2
H10Me N
hydrodechlorination (Table 2 Entry 12). As a final example, the
89% and 92% yield, respectively (Scheme 4). NMR data for 3a
suggested hydride addition to the carbons ortho to the nitrogen
atom. This hypothesis was further supported by HSQC data and
corresponding HDF reaction using the deuterated silane Et
3
SiD
29]
was applied to the cis-2,4,6-(CF ) C H
3 3 6 9
derivative (Scheme 3).[
Complete consumption of the precursor was observed by 19F
NMR spectroscopy; however, a reaction time of 28 hours was
required. In addition, 2H NMR spectroscopy shows a single H
NMR resonance attributable to methyl groups while high-
resolution GC-MS data were consistent with the formation of
is consistent with literature.
compound 3b showed only five resonances with no coupling to
CH groups, again suggesting hydride addition to the carbons
[30]
Similarly, H NMR data for
1
2
3
ortho to the nitrogen atoms. This was further confirmed via an X-
ray crystallographic analysis of crystals of 3b grown from n-
pentane at -30 °C (Figure 5).
3 3 6 9
2,4,6-(CD ) C H (Table 2 Entry 13; Scheme 3).
3 3 6 9.
Scheme 3. Deuterodefluorination of cis-2,4,6-(CF ) C H
To garner information about the mechanism, the HDF
reaction of α,α,α-trifluorotoluene mediated by [2a][B(C at
0 °C in oDFB was monitored by 19F NMR spectra in intervals of
minutes. These data exhibited an induction period of circa
0 minutes prior to the appearance of Et SiF. Thereafter, rapid
Figure 5. POV-ray depiction of 3b. C: black, N: blue, H: gray. Non-CH
2
hydrogen atoms were omitted for clarity.
6 5 4 2
F ) ]
4
5
4
3
consumption of the substrate occurred and was complete in under
5
minutes (Figure 4). This is consistent with the reports from
Ozerov et al.[
8b, 8c]
who described the auto-acceleration and
exothermal nature of analogous HDF reactions.
6 5 4 2
Scheme 4. Synthesis of 3a/3b and [4a]/[o-4b][B(C F ) ] .
Interestingly, compounds 3a and 3b degrade readily under
air generating intense red, but uncharacterized products.
Compounds 3a and 3b were also found to react within seconds
Figure 4. 19F NMR spectra monitoring the HDF of α,α,α-trifluorotoluene S1 after
at room temperature with [2a][B(C
6 5 4 2 6 5 4 2
F ) ] and [2b][B(C F ) ] ,
significant time intervals in the presence of [2a][X] ([X] = [B(C
6 5 4 2
F ) ] ) at 40 °C in
respectively, prompting the generation of intense purple solutions.
oDFB. For all unredacted NMR spectra see S.I. (Figure S37).
1
2+
The H NMR spectrum of 3/[2a] in CD
3
CN was more complex
1
than expected for a single product. H, COSY and NOESY NMR
spectra are consistent with the presence of two species tentatively
4
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Chemistry - A European Journal
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assigned to a mixture of an approximately 1:1 mixture of species
We have established that simple ammonium dications
derived from the facile alkylation of phenanthroline and bipyridine
derivatives act as carbon-based Lewis acids. These species have
been evaluated for their use in the initiation of silylium-catalyzed
HDF reactions of a range of aryl and alkyl substituted mono-, di-
and trifluoromethyl groups. In addition, we applied this protocol to
in which a hydride from 3a is delivered to either the ortho or the
para carbon of [2a]2+ affording a mixture of [o-4a] and [p-4a]
+
+
(
Scheme 4, see SI). This finding is consistent with the similar
2
+
computed HIAs for the ortho and para sites in [2a] (Figure 2). In
the case of the corresponding reaction of 3b/[2b] , the H NMR
2
+
1
spectrum exhibited ten resonances corresponding to the single
convert 2,4,6-(CF
3
)
3
C
6
H
9
3 3 6 9
to 2,4,6-(CD ) C H , a previously
+
dissymmetric product [o-4b] in which hydride is delivered to the
unknown isotopologue of trimethylcyclohexane. The activity of the
initiators correlates well with the calculated HIA values of the
dications. We are continuing to examine the chemistry of these
cations in Lewis acid chemistry.
+
carbon ortho to nitrogen (Scheme 4). Reaction of [4a] with one
equivalent of
[Et
3
Si(HSiEt
3
)][B(C
6
F
5
)
4
],
regenerates
2
+
[
2a][B(C F ) ] consistent with the notion that [2a] reacts
6 5 4 2
reversibly with silane (Scheme 5). This indicates that the dication
provides for the slow generation of silylium cation accounting for
the induction period to the onset of the comparatively fast silylium Experimental Section
mediated HDF reaction. This latter reaction involves a series of
C-F bond activations and subsequent hydride abstractions from
silane which provide the sequential replacement of the fluorine
atoms with hydrides, regenerating the active silylium cation each
time (Scheme 5). This mechanism is consistent with those
Methods and materials: Unless otherwise stated, all catalytic reactions
were conducted under an inert atmosphere of dry nitrogen, using Schlenk
techniques or an MBraun LABmaster SP glovebox, equipped with closed
loop circulation and a -35 °C freezer. Toluene, n-pentane, and diethyl ether
were collected from a Grubbs-type column system manufactured by
Innovative Technology, subsequently degassed under negative pressure,
[
8a, 17c]
previously proposed.
and stored over 4 Å molecular sieves. CH
CaH , followed by distillation, and degassing. Deuterated solvents were
distilled from CaH and stored over 4 Å molecular sieves.
2 2 3
Cl and CH CN were dried over
2
2
Nuclear Magnetic Resonance (NMR) spectral analysis was performed
using a Bruker Avance III 400 MHZ, and Agilent DD2 500 MHz
13
spectrometer equipped with a C-sensitive cryogenically cooled probe.
1
13
1
The H and C{ H} NMR data were referenced to residual solvent signals
while 19F{ H} NMR spectra were referenced to C
1
F
6 6
internal standard.
Chemical shifts (δ) are reported in ppm and the absolute values of the
coupling constants (J) are in Hz. CHN Elemental Analysis (EA) was
performed by the ANALEST facility on a Flash 2000 Analyzer. Mass
spectra were performed by the ANALEST facility on a Thermo TSQ 8000
EVO Triple Quadrupole GC-MS/MS equipped with a PTV inlet and
headspace with additional SPME autosampler capabilities. Crystals were
coated in Paratone-N oil in an N
Micromount, and placed under a N
2
filled glovebox, mounted on a MiTegen
stream to maintain a dry, O -free
2
2
environment for each crystal collection. The data were collected on a
Bruker Apex II diffractometer using a graphite monochromator with Mo Kα
radiation (λ = 0.71073 Å). The data were consistently collected at 150(2)
K for all crystals and further details are provided in the ESI. All
computational data was computed using Gaussian 09.[24a]
Scheme 5. Proposed initiation and catalytic cycle for the HDF of α,α,α-
trifluorotoluene.
The preparation of dibromide salts was performed using standard bench-
top techniques, according to established literature procedures. The
To further support the proposed reaction sequence
computations were undertaken at the M06-2X/Def2-SVP level of
following
chemical
species:
[(2,2’-bipyridyl)(CH
[Et
3
Si·tol][B(C
6
F
5
)
4
][2a]
),
,
],[8a]
[
Et
3
Si][Et
3
6 5
SiH][B(C F )
4
2
CH )][Br]
2
2
([1a][Br]
2
theory.[
G
monocation [1aH] in situ over an activation barrier of only G =
8.1 kcal/mol. This is consistent with the observations of room
temperature HDF in CH Cl (Table 1). While in principle,
24a]
Reaction of [1a] with Et
-39.9 kcal/mol) furnishing triethylsilylium and the
2+
SiH is shown to be exergonic
3
[
(4,4’-dimethyl-2,2’bipyridyl,)(CH
2
CH
2
)][Br]
2
([1f][Br]
),
), [(4,4’-di-tert-butyl-
2
(
=
2,2’bipyridyl,)(CH
2 2
CH )][Br]
2
([1g][Br]
2
[(1,10-
+
‡
phenanthroline)(CH
2
CH
2
)][Br]
)][Br]
2
2
([2a][Br]
([2b][Br]
2
), and [(3,4,7,8-tetramethyl-1,10-
) were prepared using literature
SiD was prepared by the method of
phenanthroline)(CH
2
CH
2
2
+
methods.[31] The compound Et
Oestreich and coworkers.[32]
3
2
2
subsequent hydride delivery could result from hydride abstraction
+
from [1aH] , this is computed to be energetically uphill by +50.2
Deposition Number(s) 2080239 (1a) 2080240 (3b), 2080241 (2b),
080242 (2a). contain(s) the supplementary crystallographic data for this
paper. These data are provided free of charge by the joint Cambridge
Crystallographic Data Centre.
kcal/mol. On the other hand, hydride abstraction from excess
silane is thermodynamically favored (G = -21.3 kcal/mol) and
regenerates the silylium cation. These findings further support the
role of [1a]2+ as an initiator, reminiscent of behavior of other Lewis
2
https://www.ccdc.cam.ac.uk/services/structures?id=doi:10.1002/chem.20
2101534
6 5 3 3
acids including B(C F ) and [Ph
C]+.
General protocol for synthesis of [B(C
temperature the respective bisbromide salt (4 mmol, 1 equiv) and
CPh ][B(C ] (8 mmol, 2 equiv) were charged into a 100 mL round
6 5 4 2
F ) ] salts: Method A: At room
Conclusion
[
3
6 5 4
F )
bottom flask and toluene (40 mL) was added. The mixture was stirred at
room temperature for 24 h. The suspension was filtered and the filter cake
5
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Chemistry - A European Journal
10.1002/chem.202101534
FULL PAPER
consisting mainly of the product [B(C
6
F
5
)]
2
salt was washed with toluene
. The remaining
yellow solid was dissolved in acetonitrile (6 mL) and again filtered to
remove insoluble minor residues of the [Br] starting material. The solvent
General protocol for synthesis of 3x: [2x][Br] (0.7 mmol, 1 equiv) and
2
LiAlH (1.4 mmol, 2 equiv) were suspended in Et O (3 mL each) and the
2
(
3x5 mL) and pentane (3x5 mL) to remove residual BrCPh
3
4
suspensions were combined. The mixture was stirred for 2 h at room
temperature upon which a bright yellow suspension was obtained. The
suspension was filtered and the filtrate evaporated to dryness. The
resulting residue was re-dissolved in n-pentane (5 mL) and filtered. The
filtrate was evaporated to dryness to afford 3x as a bright yellow powder.
2
was removed under vacuum and the resulting light-yellow oil was triturated
with n-pentane (approx. 3x5 mL, scratching with spatula) until a yellow
solid formed. The yellow solid was dried under vacuum (at least 1 h) to
remove residual solvent to afford analytically pure [B(C
Method B: At room temperature, freshly prepared [Et Si·tol][B(C
0.2 mmol, 1.90 equiv.) was slowly added to a suspension of [Br]
0.1 mmol, 1.00 equiv.) in fluorobenzene (2.5 mL) while stirring. The
6 5 4 2
F ) ] salts.
3
6 5
F )
4
]
3
J
C
NCH
a (89 % yield): H NMR (400 MHz, C
1
D
): δ 6.34 (s, 2H, C5/6H), 6.22 (dt,
6
6
(
(
2
salt
3
HH = 9.7, 4JHH = 1.7 Hz, 2H, p-CarH), 5.39 (dt, JHH = 9.7, 4.0 Hz, 2H, m-
arH), 3.40 (dd, 3JHH = 4.0, 4JHH = 1.7 Hz, 4H, o-CH
), 2.65 (s, 4H,
): δ 131.3 (Car), 127.7 (Car),
), 47.5 (NCH CH N).
3
2
mixture was stirred for 18 h, and all volatiles were removed in vacuo. The
residue was extracted with CH CN (0.8 mL) and the remaining solids were
filtered off over celite. The solvent was removed in vacuo and
N). 13C{ H] NMR (125 MHz, C
1
2
CH
2
D
6 6
3
1
22.1(Car), 121.1 (Car), 117.6 (Car), 50.6 (o-CH
2
2
2
6 5 4 2
subsequently triturated with n-pentane (7 mL) to give the target [B(C F ) ]
salt as a bright yellow powder.
3
b (92 % yield). Crystals of 3b for X-ray diffraction were grown from a
1
saturated solution of 3b in n-pentane. H NMR (400 MHz, C
D
6 6
): δ 6.77 (s,
N), 1.89 (s, 6H,
): δ 130.6 (Car), 125.3
ar), 124.6 (Car), 124.2 (Car), 114.2 (Calkene), 56.3 (o-CH ), 48.01
NCH CH N), 17.7 (CH ), 13.7 (CH ).
2
H, C5/6H), 3.28 (s, 4H, o-CH
). 13C NMR (125 MHz, C
6 6
CH ), 1.58 (s, 6H, CH D
2 2 2
), 2.88 (s, 4H, NCH CH
[
1a][B(C
6
F
5
)
4
3
]
J
2
(Method A, 82 % yield): 1H NMR (400 MHz, CD
3
CN):
3
3
δ 8.96 (dd,
HH = 6.0; 4 HH = 1.4 Hz, 2H), 8.82 (ddd, 3JHH = 8.2, 7.67;
J
(
(
C
2
4JHH = 1.4 Hz, 2H), 8.78 (dd, JHH = 8.2, 4JHH = 1.7 Hz, 2H), 8.32 (ddd, 2H,
3
2
2
3
3
3JHH = 7.7, 6.08; 4
MHz, CD
J
HH = 1.7 Hz), 5.09 (4H, CH
CH
2 2
). 13C{ H} NMR (125
1
3
CN): δ 150.4 (CarF), 149.4 (Car), 148.4 (Car), 147.9 (CarF), 140.7
General protocol for synthesis of [4x][B(C
6 5 4 6 5 4 2
F ) ]: [2x][B(C F ) ]
(
C
arF), 138.6 (CarF), 136.1 (CarF), 131.8 (Car), 129.6 (Car), 52.7 (CH
2
CH
2
).
(0.1 mmol, 1 equiv) and 3x (0.1 mmol, 1 equiv) were placed in a screw-
19
1
F{ H} NMR (376 MHz, CD
3
CN): δ -133.8 (s, 16F), -163.7 (t, 8F), -168.4
cap vial and oDFB (3 mL) was added. The mixture instantly turned dark
purple. After 1 h at room temperature the solvent was removed in vacuo
and the residue was co-evaporated with n-pentane (2x0.5 mL). The purple
1
(
t, 16F). 11B{ H} NMR (128 MHz, CD
3
CN): δ -16.7.
1
[
(
4
1f][B(C
6
F
5
)
4
]
2
(Method A, 82 % yield): H NMR (400 MHz, CD
3
CN): δ 8.75
6 5 4
residue was dried to afford [4x][B(C F ) ].
d, 3
J
HH = 6.3 Hz, 2H), 8.65 (d, 4JHH = 1.5 Hz, 2H), 8.08 (dd, 3JHH = 6.3,
1
J
HH = 1.0 Hz), 4.99 (s, 4H, CH
MHz, CD
arF), 139.8 (Car), 138.5 (CarF), 136.1 (CarF), 131.7 (Car), 129.8 (Car),
CH
2 2
), 2.79 (s, 6H, CH
3
). 13C{ H} NMR (125
] (90 % yield): 1H NMR (400 MHz, CDCl
), 8.65 (d, 3JHH = 8.6 Hz, 1H, C
HH = 8.3 Hz, 1H, C H, [p-4a][B(C
o-4a][B(C H, [p-4a][B(C ), 8.28
), 8.40 (d, 3JHH =5.3 Hz, 1H, C
d, 3JHH = 5.8 Hz, 1H, C ), 7.73 (dd, 3JHH = 8.3, 5.3 Hz,
H, [o-4a][B(C
), 7.65 (d, 3JHH = 8.4 Hz, 1H, C5or6H, A), 7.62 (d,
H, B), 7.59 (d, 3JHH = 8.4 Hz, 1H, C5or6H, A),
HH = 8.3 Hz, 1H, C5or6H, B), 7.40 (d, 3
HH = 8.3 Hz, 1H, C5or6H,
HH = 2.0 Hz, 1H, C H, [o-4a][B(C ), 6.04
HH = 10.0, 4.5 Hz, 1H, C H, [o-4a][B(C HH = 8.0,
), 6.01 (dt, 3
[
4a][B(C
J
6 5
F )
4
3
)**: δ 8.78 (d,
3
CN): δ 164.0 (Car), 150.3 (CarF), 147.8 (CarF), 147.0 (Car), 140.5
3
4
6 5
F )
4
]
2
4
H,
(
1
C
[
(
1
3
6 5
F )
4
]
2
2
6 5 4 2
F ) ]
19
1
2 2 3 3
25.1 (CarB), 52.7 (CH CH ), 22.7 (CH ). F{ H} NMR (376 MHz, CD CN):
δ -133.8 (m, 16F, Fortho), -164.0 (t, 3JFF = 20.7 Hz, 8F, Fpara), -168.4
2
6 5 4 2
F ) ]
3 6 5 4 2
H, C H, [p-4a][B(C F ) ]
1
(
pseudo t, 3
δ -16.7. Anal. Calc. for C62
Found: C 47.72 %, H 1.23%, N 2.18 %.
J
FF = 20.9 Hz, 16F, Fmeta). 11B{ H} NMR (128 MHz, CD
3
CN):
J
HH = 8.6 Hz, 5.8 Hz, 1H, C
3
H B F N
16 2 40 2
: C 47.42 %, H 1.03 %, N 1.78 %.
.48 (d, 3
7
J
J
B), 6.53 (dt, 3JHH = 10.0, 4
J
7
6 5 4 2
F ) ]
(
dt, 3
J
8
6
F
5
)
4
]
2
J
[
8
6
1g][B(C
6
F
J
5
)
4
]
2
(Method B, 38 % yield): 1H NMR (400 MHz, CD
3
CN): δ
4
3
J
HH = 1.7 Hz, 1H, C
H, [p-4a][B(C
8
H, [p-4a][B(C
6 5
F )
4
]
2
), 4.91 (dt, JHH = 8.0, 3.5 Hz, 1H,
), 4.82 (pseudo triplet, 2H, N CH
.90 (d, 3
.5; 4
HH = 6.5 Hz, 2H), 8.70 (d, 4JHH = 2.2 Hz, 2H), 8.30 (dd, 3JHH
=
+
C
8
6
F
5
)
4
]
2
2
, [p-4a][B(C
6 5
F )
4
]
2
),
HH = 4.5,
[p-
), 3.56
). 13C NMR (125 MHz, CD
CN):
), 147.7 (Car), 147.4 (Car), 147.0 (Car),
), 138.3 (br s, C ), 136.4 (br s, C ), 136.2
ar), 132.9 (Car), 132.9 (Car), 132.6 (Car), 131.4 (Car), 130.5 (Car), 130.0
(Car), 127.7 (Car), 126.2 (Car), 126.1 (Car), 126.1 (Car), 125.1 (Car), 124.0
(Car), 121.8 (Car), 121.6 (Car), 121.4 (Car), 117.7 (C=C-CH ), 100.5 (N-
), 51.0 (o-CH , [o-
, [p-4a][B(C ).
): δ 132.7 (s, 8F, o-F, C ), 162.4 (t, JFF
), 166.6 (br s, 8F, m-F, C ). Additional details on
NMR assignments are provided in the ESI.
1
J
HH = 2.2 Hz, 2H), 5.10 (s, 4H, CH
2
CH
2
). 13C{ H} NMR (125 MHz,
+
), 4.35 (dd, 3
4
J
.75 (pseudo triplet, 2H, N CH
2
, [o-4a][B(C
[o-4a][B(C
), 3.81 (pseudo triplet, 2H, NCH
pseudo triplet, 2H, NCH , [o-4a][B(C
δ 150.1 (br s, C ), 148.2 (br s, C
46.2 (Car), 140.3 (br s, C
6
F
5
)
4
]
2
J
3
CD CN): δ 175.1 (Car), 150.1 (CarF), 148.1 (CarF), 147.4 (Car), 140.3 (CarF),
4
HH = 2.0 Hz,
9 2
C H ,
6 5
F )
4
]
2
), 3.91 (br s, 2H,
C
7 2
H ,
1
38.2 (CarF), 136.3 (CarF), 131.1 (Car), 129.0 (Car), 128.3 (Car), 52.7
4a][B(C
6 5
F )
4
]
2
2 6 5
, [p-4a][B(C F )
4
]
2
1
(
CH
2 2
CH ), 38.4 (C-(Me)
3
), 30.0 (CH
3
). 19F{ H} NMR (376 MHz, CD
3
CN):
(
2
6 5
F )
4
]
2
3
3
δ -133.0 (s, 16F, Fortho), -164.0 (t, JFF = 19.7 Hz, 8F, Fpara), -168.4 (pseudo
t, 3JFF = 17.2 Hz, 16F, Fmeta). 11B{ H} NMR (128 MHz, CD
F
6 6
6 6
F
1
3
CN): δ 16.7 (s).
1
F
6 6
F
6 6
6 6
F
(
C
6 5 4 2
3
F ) ] CN): δ
(Method A, 89 % yield): 1H NMR (400 MHz, CD
[
9
2a][B(C
.43 (d, 3JHH = 8.3 Hz, 2H), 9.35 (d, JHH = 5.1 Hz, 2H), 8.66 (s, 2H), 8.55
3
2
(
(
C
(
pseudo t, 2H), 5.44 (s, 4H, CH
125 MHz, CD CN): δ 150.1 (br s, CarF and C
arF), 140.3 (m, CarF), 138.4 (m, CarF), 136.4 (m, CarF), 132.9 (C
), 130.1 (C ), 128.9 (C ), 125.1 (m, CarB), 53.4 (CH CH ). All signals
are broad; determination of 1JCF coupling constants was impossible.
CH
2 2
). All signals are broad. 13C{ H} NMR
), 149.3 (C ), 148.2 (br s,
), 131.0
1
C=C-CH
2
, [p-4a][B(C
), 45.6 (NCH
F{ H} NMR (376 MHz, CD
= 19.7 Hz, 4F, p-F, C
6 5
F )
4
]
2
), 55.7 (N CH
), 45.0 (NCH ), 28.1 (p-CH
Cl
+
2
), 54.5 (N CH
+
2
2
3
3
5
4a][B(C
6
F
5
)
4
]
2
2
2
2
6 5 4 2
F ) ]
19
1
3
1
3
3
6 6
F
C
4
6
7
2
2
F
6 5
6 6
F
1
3
1
9
1
F{ H} NMR (376 MHz, CD
3
CN): δ -133.8 (br s, 8F, Fortho), -163.9 (t,
FF = 19.7 Hz, 4F, Fpara), -168.4 (pseudo t, 3
FF = 17.4 Hz, 8F, Fmeta).
-16.7 (s). Anal. Calc. for
: C 47.54 %, H 0.77 %, N 1.79 %, B 1.38 %. Found: C
J
J
] (95 % yield): 1H NMR (400 MHz, CDCl
[
o-4b][B(C
6
F
5
)
4
3
): 8.12 (s, 1H,
HH = 8.9 Hz, C H, 1H),
), 3.51 (m, 2H,
), 2.08 (s, 3H, CH ), 1.92 (s,
CN): δ 156.3 (Car), 150.1 (br s, C ),
), 145.6 (Car), 140.3 (br s, C ), 138.3 (br s, C ), 136.4
6 5
F ), 135.4 (Car), 131.4 (Car), 130.5 (Car), 129.5 (Car), 126.6 (Car),
1
1
B{ H} NMR (128 MHz, CD
3
CN):
δ
H), 7.69 (d, 3JHH = 8.9 Hz, C
H, 1H), 7.53 (d, 3
C
9
6
J
5
C
62
H
B
12 2
F
40
N
2
+
4
.67 (pseudo triplet, 2H, N CH
NCH CH ), 2.78 (s, 3H, CH ), 2.51 (s, 3H, CH
H, CH
). 13C NMR (125 MHz, CD
148.2 (br s, C
(br s, C
125.7 (Car), 125.4 (Car), 124.5 (Car), 113.7 (Car), 55.7 (CH
46.0 (CH ), 18.0 (CH ), 17.4 (CH ), 16.5 (CH ), 13.6 (CH
(376 MHz, CD Cl ): δ -132.7 (br s, 8F, o-F, C
oDFB), -162.6 (t, 3JFF = 20.5 Hz, 4F, p-F, C
), -166.7 (br m, 8F, m-F,
). Additional details on NMR assignments are provided in the ESI.
2 2 2 2
CH ), 3.99 (s, 2H, C H
47.28 %, H 0.66 %, N 1.89 %.
2
2
3
3
3
3
3
3
6 5
F
[
2b][B(C
6
F
5
)
4
]
2
(Method A, 91 % yield): 1H NMR (500 MHz, CD
3
CN): δ
6
F
5
F
6 5
6 5
F
1
3
1
9
.16 (s, 2H), 8.73 (s, 2H), 5.33 (s, 4H), 3.05 (s, 6H), 2.77 (s, 6H). C{ H}
CN): δ 159.6 (Car), 150.1 (m, CarF), 148.6 (Car), 148.1
m, CarF), 140.2 (m, CarF), 138.6 (Car), 138.3 (m, CarF), 136.3 (m, CarF),
NMR (125 MHz, CD
3
2
), 53.9 (CH
2
),
). 19F{ H} NMR
1
(
2
3
3
3
3
1
31.1 (Car), 128.4 (Car), 126.9 (Car), 124.8 (m, CarB), 52.6 (CH
), 17.4 (CH
). All signals are broad; determination of 1JCF coupling
constants was impossible. F{ H} NMR (376 MHz, CD
2
CH
2
), 18.2
3
3
6 6
F ), -138.4 (m, 2F,
(
CH
3
3
6 6
F
19
1
3
CN): δ -133.8 (br
C
F
6 6
s, 16F, Fortho), -163.9 (t, 3
3JFF = 17.4 Hz, 16F, Fmeta). 11B{ H} NMR (128 MHz, CD
J
FF = 19.7 Hz, 8F, Fpara), -168.4 (pseudo t,
1
3
CN): δ -16.7.
6
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202101534
FULL PAPER
General protocol for HDF reactions: The substrate (1 equiv), Et
(
spectroscopy, approx. 3 – 10 mg) were dissolved in oDFB (0.6 mL) and H
and 19F NMR data were acquired in an air-tight NMR tube for reference.
The solution was added to the respective catalyst (solid, 0.05 equiv) and
the reaction mixture was transferred out of the glove box and heated at
3
SiH
[15] D. J. Sheldon, G. Coates, M. R. Crimmin, Chem. Commun. 2020,
56, 12929-12932.
[16] (a) A. Schulz, A. Villinger, Angew. Chem. Int. Ed. 2012, 51,
6 6
1.2 equiv per F atom in substrate) and C F
(internal standard for 19F NMR
1
4526-4528; (b) A. Hermannsdorfer, M. Driess, Angew. Chem.
Int. Ed. 2020, 59, 23132-23136; (c) S. Yoshida, K. Shimomori,
Y. Kim, T. Hosoya, Angew. Chem. Int. Ed. 2016, 55, 10406-
1
0409.
17] (a) J. T. Zhu, M. Pérez, D. W. Stephan, Angew. Chem. Int. Ed.
016, 55, 8448-8451; (b) S. Chitnis, F. Krischer, D. W. Stephan,
4
0°C in an oil bath for 4 hours. After 4 h the reaction mixture was cooled
[
to room temperature and H and 19F NMR data were collected.
1
2
Chem. Eur. J. 2018, 4, 6543 –6546; (c) A. E. Waked, S. S.
Chitnis, D. W. Stephan, Chem. Commun. 2019, 55, 8971-8974.
18] V. R. Naidu, S. Ni, J. Franzén, ChemCatChem 2015, 7, 1896-
[
[
Acknowledgements
1905.
19] (a) B. Inés, S. Holle, R. Goddard, M. Alcarazo, Angew. Chem.
Int. Ed. 2010, 49, 8389-8391; (b) D. Palomas, S. Holle, B. Ines,
H. Bruns, R. Goddard, M. Alcarazo, Dalton Trans. 2012, 41,
We thank the University of Toronto, the Natural Science and
Engineering Research Council (NSERC) for financial support.
D.W.S. also thanks the Guggenheim foundation for a 2020
Fellowship and the Killam Foundation for the 2021 Killam Prize in
Natural Science. F.G. gratefully acknowledges the Deutsche
Forschungsgemeinschaft (DFG) for presentation of the Leibniz
Award. I.E. gratefully acknowledges her DFG research stipend
9073-9082.
[
20] (a) E. R. Clark, M. J. Ingleson, Organometallics 2013, 32, 6712-
6717; (b) E. R. Clark, M. J. Ingleson, Angew. Chem. Int. Ed.
2
014, 53, 11306-11309.
21] M. P. Boone, D. W. Stephan, J. Am. Chem. Soc. 2013, 135,
508-8511.
[
[
[
8
22] A. C. Shaikh, J. M. Veleta, J. Moutet, T. L. Gianetti, Chem. Sci.
2021, 12, 4841-4849.
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G. C. Bazan, J. Am. Chem. Soc. 2011, 133, 12410-12413; (b) J.
Iglesias-Siguenza, M. Alcarazo, Angew. Chem. Int. Ed. 2012, 51,
(
EL 1045). R.J.A. thanks the support of an NSERC-CGS-D
scholarship. We thank Drs. Darcy Burns, Jack Sheng and Karl
Demmans for their advice regarding NMR experiments.
1523-1524.
Keywords: homogeneous catalysis • metal-free
hydrodefluorination • silanes • hydrogen transfer • carbon-based
Lewis acids
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Chemistry - A European Journal
10.1002/chem.202101534
FULL PAPER
Entry for the Table of Contents
The use of novel organic dications as carbon-based Lewis acids is reported. The Hydride Ion Affinity (HIA) of these bipyridinium and
phenanthrolinium salts is exploited to initiate hydrodefluorination catalysis. Further, the participation of these species enables
deuterodefluorination. Experimental and DFT analysis support the role of the dications in the initiation of silylium-mediated catalysis.
8
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