Friedel–Crafts Type Methylation with Dimethylhalonium Salts

Article abstract of DOI:10.1002/chem.202001457

The dimethylchloronium salt [Me₂Cl][Al(OTeF₅)₄] is used to methylate electron-deficient aromatic systems in Friedel–Crafts type reactions as shown by the synthesis of N-methylated cations, such as [MeNC₅F₅]⁺, [MeNC₅F₄I]⁺, and [MeN₃C₃F₃]⁺. To gain a better understanding of such fundamental Friedel–Crafts reactions, the role of the dimethylchloronium cation has been evaluated by quantum-chemical calculations.

Full text of DOI:10.1002/chem.202001457

Accepted Article
Accepted Article
Title: Friedel-Crafts type methylation with dimethylhalonium salts
Authors: Sebastian Hasenstab-Riedel, Sebastian Hämmerling, Patrick
Voßnacker, Simon Steinhauer, and Helmut Beckers
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To be cited as: Chem. Eur. J. 10.1002/chem.202001457
Link to VoR: https://doi.org/10.1002/chem.202001457
01/2020

10.1002/chem.202001457
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Friedel-Crafts type methylation with dimethylhalonium salts
Sebastian Hämmerling, Patrick Voßnacker, Simon Steinhauer, Helmut Beckers, Sebastian Riedel*
Dedication ((optional))
Abstract: The dimethylchloronium salt [Me2Cl][Al(OTeF5)4] is used to
methylate electron-deficient aromatic systems in Friedel-Crafts type
reactions as shown by the synthesis of N-methylated cations, such as
[MeNC5F5]+, [MeNC5F4I]+ and [MeN3C3F3]+. To gain a better under-
standing of such fundamental Friedel-Crafts reactions, the role of the
dimethylchloronium cation has been evaluated by quantum-chemical
calculations.
role of dialkylhalonium intermediates in this process and the rela-
tionship between these two intermediates has to our know-
+
Me
Me
-
+
AlCl3
MeCl
Al
ClMe](solv)
[Cl3
(s)
–
–
AlCl3
HCl
H
H
[AlCl4]–
Introduction
Me
+
+
2 MeCl AlEt3 4 HOTeF5
Cl
The Friedel-Crafts alkylation is a well-known and widely used but
also challenging synthetic tool.[1] The Friedel-Crafts alkylation re-
action is based on the polarization of an alkyl halide by a Lewis
acid like AlCl3 which further reacts as an electrophilic reagent e.g.
with an aromatic molecule to form an arenium ion (Wheland inter-
mediate, Scheme 1, top).[2] Rearomatization of the Wheland inter-
mediate occurs through elimination of HCl and recovers the cata-
lyst. Depending on the stabilizing effects of the alkyl group the
alkylating species has a distinct carbocationic character. The ten-
Me
–
–
HCl
HEt
–
]
[Al(OTeF5)4
1
Scheme 1. Friedel-Crafts alkylation of benzene with [Cl3Al−ClMe] (top) and for-
mation of the dimethylchloronium salt [Me2Cl][Al(OTeF5)4] (1, bottom).
ledge not been fully elucidated. Previous investigations on the
mechanism only considered the direct methylation of the aromat
by the Lewis acid-alkylhalide complex.[8]
dency to form a free CH3 cation[3] is low, therefore Olah sug-
+
gested the formation of the dimethylchloronium cation as an inter-
mediate for Friedel-Crafts reactions[4] and isolated [Me2Cl][SbF6][5]
as a thermally labile compound. The only other known anion
which is able to stabilize the dimethylchloronium cation so far is
the carborate anion [CHB11Cl11]−. The salt [Me2Cl][CHB11Cl11] is
considerably more thermally stable.[6] We recently synthesized
the easily accessible and room temperature stable dimethylchlo-
ronium salt [Me2Cl][Al(OTeF5)4] (1, see scheme 1 bottom).[7]
In this work we report on the role of the dimethylchloronium cation
in Friedel-Crafts type methylation reactions, especially in the sys-
tem MeCl-AlCl3 and the reaction of 1 with electron-deficient aro-
matic systems.
We have measured 27Al and H NMR spectra of a slurry of
1
aluminum trichloride in chloromethane. The solubility of aluminum
trichloride at room temperature in pressurized chloromethane is
low. However a broad resonance at δ = 108.6 ppm (FWHM =
357 Hz) can be detected in the 27Al NMR spectrum, which we as-
sign to a [Cl3Al−ClMe] complex. The signal is broadened by quad-
rupolar interactions and is in a typical range for donor stabilized
tetrahedral coordinated AlCl3.[9] This chemical shift is comparable
with that of the [AlCl4]– anion at δ = 104.2 ppm, which shows how-
ever a significantly lower linewidth of 3 Hz. The 1H NMR spectrum
of this solution shows only the signal of MeCl at δ = 3.35 ppm. The
addition of 1,2-difluorobenzene to this [Cl3Al−ClMe]/MeCl mixture
results in a slow methylation of the aromatic compound within
days at room temperature. For comparison, a solution of
[Me2Cl][Al(OTeF5)4] (1) in chloromethane revealed a distinct sig-
Results and Discussion
1. The role of the dimethylchloronium cation in Friedel-Crafts
type methylation reactions
The electrophilic intermediate in Friedel-Crafts type methylation
reactions is still controversial. While it seems well established that
this alkylation proceeds via an Lewis-acid activated alkylhalide
such as the [Cl3Al−ClMe] intermediate shown in scheme 1, the
1
nal of the cation at δ = 4.75 ppm in the H NMR spectrum. The
cross signals in an EXSY NMR spectrum indicates a slow ex-
change between [Me2Cl]+ and the free MeCl at room temperature.
We carried out quantum-chemical calculations at the RI-B3LYP-
D3/def2-TZVPP (COSMO, εR MeCl) level of theory to obtain fur-
ther information about the relationship between the two interme-
diates, the activated [Cl3Al−ClMe] complex and the dimethylchlo-
ronium ion [Me2Cl]+. These calculations support our NMR spec-
troscopic observation of a [Cl3Al−ClMe] complex (Figure 1). In the
presence of 1,2-difluorobenzene this complex is further stabilized
by 14.0 kJ·mol−1 with respect to the free educts. However, in con-
trast to common textbook knowledge these calculations disclose
that the methylation of 1,2-difluorobenzene does not take place
via the electrophilic [Cl3Al−ClMe] complex, but via the contact ion
[*]
M. Sc. S. Hämmerling, M. Sc. P. Voßnacker, Dr. S. Steinhauer, Prof.
Dr. S. Riedel
Freie Universität Berlin, Institut für Chemie und Biochemie
Fabeckstr.34/36, 14195 Berlin (Germany)
E-mail: s.riedel@fu-berlin.de
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.202001457
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pair [Me2Cl][AlCl4]. The contact ion pair is 22.6 kJ·mol−1 less sta-
ble and separated from the [Cl3Al−ClMe] complex by a barrier of
59.5 kJ·mol−1. Starting from the contact ion pair the methylation
reaction of 1,2-difluorobenzene has a barrier of 45.0 kJ·mol−1.
The direct methylation of 1,2-difluorobenzene with [Cl3Al−ClMe]
has a higher barrier of 71.0 kJ·mol−1 (see figure 1 and S39). The
two intermediates are linked by the equilibrium reaction (1).
F
F
F
F
F
[Me2Cl][Al(OTeF5)4],
r.t.,
min
H
H
F
F
F
F
F
ex.
Et
SO2,
30
2O
+
–
[H(OEt2)2]
[Al(OTeF5)4]–
Me
F
Me
F
[Al(OTeF5)4]–
Scheme 2. Reaction of [Me2Cl][Al(OTeF5)4] (1) with 1,2,3,4-tetrafluorobenzene.
–
+
MeCl
Al
[Cl3
1
( )
ClMe]
[Me2Cl][AlCl4]
The NMR spectroscopic analysis of the purified product mixture
in CD2Cl2 confirms the formation of 5-methyl-1,2,3,4-tetrafluoro-
benzene as well as an excess of the starting compound 1,2,3,4-
tetrafluorobenzene which is in agreement with the H+/Me+ ratio
described above. Increasing the reaction time from 30 min. to
three hours at room temperature yields a product mixture of
C6F4H2, C6F4HMe, and C6F4Me2 in the ratio 18:2.4:1. The for-
mation of a two times methylated product can be explained by a
fast equilibrium between methyltetrafluorobenzene and proto-
nated tetrafluorobenzene (see scheme 3). The more basic me-
thyltetrafluorobenzene (see proton affinities in table 1) reacts
We point out that the choice of the solvent model, or the disper-
sion correction as well as the selected functional does not change
the main conclusion drawn from the computed reaction scheme
(see figure S40). Therefore the [Me2Cl]+ cation, as part of a con-
tact ion pair, is a more reasonable intermediate for the Friedel-
Crafts methylation reaction in chloromethane.
faster with the dimethylchloronium cation than tetrafluorobenzene.
This observation is also supported by calculated transition state
energies, which is 7.2 kJ·mol−1 lower in energy for the methylation
of methyltetrafluorobenzene by [Me2Cl]+ than that for tetrafluoro-
benzene. For longer reaction times unspecific decomposition re-
actions occurred.
F
F
F
F
F
F
F
∆
H
H
=
G
+16.9
F
F
F
F
F
F
F
F
H
H
+
+
Me
Me
H
F
Cl]+
Cl]+
+
+
[Me2
=
[Me2
=
Figure 1. Reaction profiles for the methylation of 1,2-difluorobenzene with the
AlCl3-MeCl system. Energies (ZPE corrected) in kJ·mol−1 on the RI-B3LYP-
D3/def2-TZVPP level of theory with COSMO (εR MeCl). The final steps (re-
aromatization and catalyst recovery) are omitted for clarity, and energy differ-
ences between linked energy levels are indicated.
‡
‡
∆
∆
E
58.1
E
50.9
Scheme 3. Proposed equilibrium between protonated arene intermediates in
the reaction mixture of [Me2Cl][Al(OTeF5)4] (1) with 1,2,3,4-tetrafluorobenzene;
Calculated relative transition state energies for the methylation reactions (not
shown) are given in kJ·mol−1 on the RI-B3LYP-D3/def2-TZVPP level of theory
using the program COSMO (εR SO2).
2. Methylation of electron-deficient aromatic systems
To further investigate the reactivity of the [Me2Cl]+ cation we
treated a series of deactivated aromatic compounds with
[Me2Cl][Al(OTeF5)4] (1). Upon addition of a twofold excess of
1,2,3,4-tetrafluorobenzene to a solution of [Me2Cl][Al(OTeF5)4] (1)
in SO2 (scheme 2) a slow color change from colorless to a pale
yellow color is observed within 30 minutes at room temperature.
The yellowish color of the reaction mixture is typical for fluorinated
arenium cations.[10] After adding a slight excess of diethyl ether to
the reaction mixture the mixture decolorizes immediately, and the
formation of protonated and methylated diethyl ether ([H(OEt2)2]+
and [Me(OEt2)]+ (ratio 1:9)) is confirmed by 1H NMR spectroscopy
(Scheme 2). Attempts to isolate the Wheland intermediates were
so far unsuccessful. In contrast to Friedel-Crafts methylation us-
ing the [Cl3Al−ClMe]/MeCl system, where rearomatization of the
Wheland intermediate takes places instantaneously during the re-
action by liberation of HCl, the rearomatization of the arene inter-
mediate formed in Scheme 2 occurs by the addition of the ether.
Table 1. Experimental and calculated[a] proton affinities (PAs) and methyl
cation affinities (MCAs)[b] in kJ·mol−1
.
Compound
PA
MCA
MeCl
647.3[11]
260[12], 279.2[7]
260[12], 279.2[7]
310.0
MeBr
647.3[11]
1,2,3,4-tetrafluorobenzene
MeI
700.4[11], 714.9
691.7[11]
323.7[7]
methyl-1,2,3,4-tetrafluorobenzene
1,2,3-trifluorobenzene
746,1
338.3
724.3[11], 738.0
361.4
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not react with 1,2,3-trifluorobenzene or 1,2-difluorobenzene dur-
ing 24 hours at room temperature or during two hours at 50 °C.
While this observation disagrees with calculated (gas-phase) me-
thyl cation affinity (MCA) values (see table 1) it is in agreement
with our computed transition state energies on the RI-B3LYP-
D3/def2-TZVPP level of theory using the COSMO solvent model
(εR SO2). These calculations revel increasing transition state en-
ergies for the corresponding methylation reaction of 1,2-difluoro-
benzene from 51.7 kJ·mol−1 (Me2Cl+) to 56.6 kJ·mol−1 (Me2Br+)
and 73.6 kJ·mol−1 (Me2I+) (see figure S41 and S42).
1,2-difluorobenzene
731.2[11], 742.9
756.0
369.0
373.8
397.6
4-methyl-1,2,3-trifluorobenzene
4-methyl-1,2-difluorobenzene
775.3
[a] Values in italics are calculated at the RI‐B3LYP‐D3/def2‐TZVPP level of
theory. [b] MCA=−ΔH0 for the reaction B + Me+  BMe+ (B = base).
1,2,3-trifluorobenzene and 1,2-difluorobenzene react within 30
minutes under quantitative consumption of 1. After adding diethyl
ether to the reaction mixtures, the intense yellow color of the so-
lution vanishes and a quantitative formation of protonated ether is
4. Methylation of weak Nitrogen bases
1
proved H NMR spectroscopically. The methylation takes place
As recently shown, methylation of the weak base PF3 with the di-
methylchloronium salt (1) yields highly electrophilic [MePF3]+
which readily reacts with the weakly coordinating anion
preferentially in para position to a fluorine atom (table 2). This is
in agreement with our quantum-chemical calculations on the RI-
B3LYP-D3/def2-TZVPP level of theory with COSMO (εR SO2)
where the transition state for the methylation of 1,2-difluoroben-
zene with [Me2Cl]+ in 4-position is by 3.1 kJ·mol−1 lower in energy
than in 3-position. All multi-methylated isomers up to 4,5,6-trime-
thyl-1,2,3-trifluorobenzene and 3,4,5,6-tetramethyl-1,2-difluoro-
benzene are identified by GC/MS and for 1,2,3-trifluorobenzene
also by NMR spectroscopy. Detailed analysis of the spin systems
of most methylation products were performed and are given in the
supporting information.
[Al(OTeF5)4]− [7] We wanted to expand the scope of methylation
.
reactions using (1) to weakly basic N-heteroaromatic compounds.
Pentafluoropyridine is known to be a very weak base. The pen-
tafluoropyridinium cation could so far only be isolated as
[HNC5F5][EF6] (E = As, Sb)[13] salts. We found that 1 reacts in a
fast and quantitative reaction with the fluorinated pyridines
NC5F4X (X = F, I) under formation of the N-methylated products,
[MeNC5F4X][Al(OTeF5)4] (2F/2I) (equation 2). The products, iso-
lated as off-white solids, are, unlike 1, stable in dichloromethane
solution (see below). The 1H NMR spectrum of 2F shows a triplet
of doublets at δ(1H) = 4.35 ppm with couplings to the fluorine at-
oms in 2-, 6- and 4-position (4J(19F,1H) = 3.3 Hz, 6J(19F,1H) =
Table 2. Main products for the methylation with 1 after 30 minutes of reaction
time at room temperature.
1
1.2 Hz) and 13C satellites with J(13C,1H) = 152.7 Hz. The reso-
nance of 2I is detected in the 1H NMR spectrum at δ(1H) =
4.25 ppm and is split into a triplet (4J(19F,1H) = 3.3 Hz, 1J(13C,1H)
= 152.1 Hz). The 19F NMR spectra are of higher order featuring
A3MM’SXX’(2F) and A3MM’XX’ (2I) spin systems (M, S, X = F; A
substrate
consumption of 1
main products
(percentage in product mixture)
10 %
= H)[14]
.
(>90%)
(73 %)
Me
N
F
F
N
X
F
F
r.t.
SO2,
F
F
F
F
[Al(OTeF5)4]–
+
+
2
( )
MeCl
100 %
100 %
[Me2Cl][Al(OTeF5)4]
30 min.
X
=
X
F
2F
2I
I
Colorless crystals of 2I were grown by slowly cooling a dichloro-
methane solution to −80 °C. It crystallizes in the triclinic space
(60%)
(20 %)
�
group P1(see figure 2). The shortest contacts between the cation
and the anion are a F-C contact (d(F7’-C1) = 307.2(4) pm, ∢(F7’-
C1-N1) = 170.2(2)°) and a halogen bond between the iodine and
one of the fluorine atoms (F5) of the OTeF5 group (d(F5-I1) =
320.6(2) pm, ∢(C4-I1-F5) = 172.2(1)°). The normalized contact
(observed distance divided by sum of van der Waals radii)[15] is
with 0.92 quite large, indicating a weak interaction. For compari-
son, in the solid state structure of tetrafluoro-para-iodopyridine a
rather strong I-N interaction with a normalized contact of 0.80 is
found.[16] No cocrystals were obtained when 2I was crystallized
from dichloromethane solution at −80 °C in the presence of pen-
tafluoropyridine.
3. Reactivity of dimethylbromonium and dimethyliodonium
salts
To compare the reactivity of the dimethylchloronium cation with
that of the heavier homologues we treated 1,2,3-trifluorobenzene
with the corresponding dimethylbromonium and the dimethylio-
donium salts. The dimethylbromonium salt [Me2Br][Al(OTeF5)4]
reacts slower with 1,2,3-trifluorobenzene than the dimethylchloro-
nium salt 1. The addition of diethyl ether to the reaction mixture
after one-hour reaction time yields protonated and methylated di-
ethyl ether in the ratio 1:2.3. The ratio of the product isomers is
similar to that for the reaction of 1 with 1,2,3-trifluorobenzene after
30 minutes. The dimethyliodonium salt [Me2I][Al(OTeF5)4] does
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From
a
solution of
3
in dichloromethane crystals of
[MeN3C3F(OTeF5)2][Al(OTeF5)4] were grown by slowly cooling a
dichloromethane solution to −40 °C. The compound crystallizes in
monoclinic space group P21/c (see figure 3). At room temperature
a colorless solution of 3 in dichloromethane decomposes within
one day to a two phase system with a dark and oily lower phase.
This decomposition shows the highly Lewis acidic character of the
[MeN3C3F3]+ cation.
Figure 2. Molecular structure of 2I in the solid state. Thermal ellipsoids set at
50ꢀ% probability. Selected bond lengths [pm] and angles [°]: C1-N1 150.1(4),
N1-C2 134.4(3), C2-C3 136.6(4), C3-C4 138.5(4), C4-C5 138.6(4), C5-C6
137.0(4), C6-N1 134.9(3), C2-F3 131.4(3), C3-F4 132.8(3), C4-I1 205.9(3), C5-
F2 132.9(3), C6-F1 130.7(3), I1-F5 320.6(2), C1-F7’ 307.2(4); C4-I1-F5
172.2(1), C2-N1-C6 118.5(2), C3-C4-C5 116.9(3), F7’-C1-N1 170.2(2).
An even less basic and therefore more challenging substrate is
cyanuric fluoride[17], which is methylated with 1 by formation of
[MeN3C3F3][Al(OTeF5)4] (3) (equation 3). This is confirmed by the
observed triplet of doublets with 13C satellites in the 1H NMR spec-
trum at δ(1H)= 4.34 ppm (4J(19F,1H) = 1.6 Hz, 6J(19F,1H) = 0.9 Hz,
1J(13C,1H) = 153.0 Hz). The 15N NMR signals of all heterocycles
shift downfield upon methylation (see table 3).
Figure 3. Molecular structure of [MeN3C3F(OTeF5)2][Al(OTeF5)4] in the solid
state. Thermal ellipsoids are shown at 50ꢀ% probability. Selected bond lengths
[pm] and angles [°]: C1-N1 148.7(8), N1-C2 135.6(8), C2-N2 131.3(8), N2-C4
131.7(9), C4-N3 131.6(8), N3-C3 130.8(8), C3-N1 137.3(9), C2-O1 131.3(9),
C4-F1 129.9(7), C3-O2 129.8(8), O1-Te1 193.8(5), O2-Te2 194.0(5); C2-N1-C3
116.1(6), N1-C2-N2 123.7(6), C2-N2-C4 114.0(6), N2-C4-N3 129.0(6), C4-N3-
C3 114.2(6), N3-C3-N1 123.1(6), C2-O1-Te1 127.9(5), C3-O2-Te2 126.9(5),
N2-C2-O1-Te1 9.3(9), N3-C3-O2-Te2 4.0(9).
Me
F
N
F
F
r.t.
SO2,
F
N
F
–
+
+
3
( )
MeCl
[Me2Cl][Al(OTeF5)4]
[Al(OTeF5)4
]
N
N
30 min.
N
N
F
Also the dimethylchloronium salt 1 decomposes in SO2 solution
within days at room temperature under formation of MeOTeF5.[7]
In contrast to this, the addition of dichloromethane at room tem-
perature to solid 1 results in an immediate decomposition to a
dark brown suspension. At −40 °C the [Me2Cl]+ salt 1 has a poor
solubility in dichloromethane and decomposes upon slow warm-
ing with a quantitative consumption of the weakly coordinating an-
ion [Al(OTeF5)4]− to a yellow solution. Using solvent suppression
pulse sequences CH2Cl(OTeF5) and CH2(OTeF5)2 (ratio 10:1,
1000-fold excess of CH2Cl2) can be identified NMR spectroscopi-
cally as the only pentafluoro-orthotellurate containing species
(see equation 4 and 5).
Table 3. Experimental and calculated[a] Proton affinities (PAs) and methyl
cation affinities (MCAs)[a] in kJ·mol−1 as well as 15N NMR chemical shifts (in
ppm) of neutral (δ 15N, educt) and methylated (δ 15N, Me+) compounds.
compound
CH2Cl2
PA
MCA]
δ 15N, educt
δ 15N, Me+
628 ± 8[18]
,
265.4
-
-
643.1
CH2Cl(OTeF5)
MeCl
683.0
267.3
279.2[7]
358.5
-
-
-
647.3[11]
758.5
-
N3C3F3
−166.3[d]
−220.0
(N1)[d]
Me
Cl
CH2Cl2
NC5F5
764.9[11]
781.6
,
376.7
400.0
−145.5[b]
−131.4[b]
−213.2[c]
H
H
"
+
+
CH
4
[Me2Cl][Al(OTeF5)4]
C Cl
2Cl(OTeF5)
"Al(OTeF5)3
"Al(OTeF5)3
(
(
)
)
–
–
–
MeCl
MeCl
MeCl
[Al(OTeF5)4]–
NC5F4I
807.4
−208.0[c]
Me
Cl
CH
CH2Cl2
2Cl(OTeF5)
H
"
+
CH
2(OTeF5)2
5
C
OTeF5
H
–
MeCl
[a] Values in italics are calculated at the RI‐B3LYP‐D3/def2‐TZVPP level of
theory. [b] CDCl3, [c] CD2Cl2, [d] SO2, ext. [D6]acetone.
[Me2Cl][Al(OTeF5)4]
[Al(OTeF5)4]–
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acid was prepared according to literature[20] as well as cyanuric fluoride[21]
These species are likely formed via the intermediates
[MeCl…CH2Cl]+ and [MeCl…CH2OTeF5]+, which both can be de-
scribed as carbenium cations coordinated by a MeCl molecule
(see figure 5). These highly reactive carbenium cations are much
less stabilized than the methyl groups in the dimethylchloronium
ion. They exhibit considerably longer C-Cl distances than [Me2Cl]+
and less deviation from a planar geometry at the carbon atom as
expressed by their higher angular sums. According to an NBO
analysis[19] the [CH2OTeF5]+ cation is described as a CH2O moiety
coordinating to a TeF5+ group. Thus, the formation of such highly
electrophilic carbenium ions ([MeCl…CH2OTeF5]+) can probably
explain the fast decomposition of the [Me2Cl]+ salt 1 in dichloro-
methane.
and tetrafluoro-para-iodopyridine[16]
.
The salts [Me2Cl][Al(OTeF5)4],
[Me2Br][Al(OTeF5)4] and [Me2I][Al(OTeF5)4] where synthesized as already
described[7]. The salt [NEt4][AlCl4] was synthesized according to the litera-
ture with MeCl as a solvent instead of thionyl chloride[22]. IR spectra were
recorded on a Bruker ALPHA FTIR spectrometer inside a glovebox
equipped with a diamond ATR attachment (resolution 4 cm−1). Raman
spectra were recorded on a Bruker MultiRAM II equipped with a low-tem-
perature Ge detector (1064 nm, 30-80 mW, resolution 2 cm−1). NMR spec-
tra were recorded on a JEOL 400 MHz ECS, 400 MHz ECZ or 600 MHz
ECZ spectrometer or on a Bruker 700 MHz AVANCE700. For strongly cou-
pled spin systems all chemical shifts and coupling constants are reported
as simulated in gNMR.[14] Spin-spin coupling constants calculated with
Gauge-Independent Atomic Orbital method (GIAO)[23] on B3LYP/aug-cc-
pVTZ-J[24] and literature data for non-methylated species[25] provided a
reasonable first guess, signs of coupling constants were used directly from
these sources. All reported chemical shifts are referenced to the Ξ values
given in IUPAC recommendations of 2008[26] using the 2H signal of the
deuterated solvent as internal reference. For 14N/15N MeNO2 is used as
reference. For external locking acetone-d6 was flame sealed in a glass
capillary and the lock oscillator frequency was adjusted to give δ(1H) = 7.26
ppm for a CHCl3 sample. Mass spectra were recorded on an Advion Com-
pact mass spectrometer expression L with a quadrupole mass filter. Sam-
ples were dissolved in a dry solvent (CH3CN or CH2Cl2) for ESI. GC-MS
were measured on a Saturn 2100 GC/MS system from Varian Inc.
equipped with a “HP-5ms Ultra Inert” (length 30 m) column, injection vol-
ume 1 µL, split 100. The following temperature program was used: 50 °C
for 0.5 min, ramp with 20 °C·min−1 to 80 °C, hold for 1 minute, ramp with
10 °C·min−1 to 120 °C, ramp with 20 °C·min−1 to 250 °C, constant helium
gas flow of 280 L·min−1. Ionization voltage for EI: 80 eV. Crystal data were
collected on a Bruker D8 Venture diffractometer with a Photon 100 CMOS
area detector with Mo-Kα radiation. Using Olex2[27], the structures were
solved with the ShelXT[28] structure solution program by intrinsic phasing
and refined with ShelXL[29] refinement package using least square minimi-
Figure 5. Calculated structures of [Me2Cl]+, [MeCl…CH2Cl]+ and
[MeCl…CH2OTeF5]+ on the RI-B3LYP-D3/def2-TZVPP level of theory; values in
italics are with COSMO (εR CH2Cl2); and Lewis structures of underlying [CH3]+,
[CH2Cl]+ and [CH2OTeF5]+ according to the NBO analysis.
zation. Crystal structures were visualized with Diamond[30]
.
For density functional calculations the program package TURBOMOLE[31]
was used with its implementations of RI[32], MARI-J[33], B3LYP[34], Grimme-
D3[35] together with the basis set def2-TZVPP[36]. SCF energies were cor-
rected with chemical potential taken from TURBOMOLE implemented in
the freeh script to get free enthalpies. For single point calculations the func-
tionals m06[37] and B2-PLYP[38] were used as implemented in TURBO-
MOLE. For COSMO[39] optimized structures vibrational spectra were cal-
culated numerically. For MeCl εR = 10[40] and for SO2 εR = 17.6[41] were
used as 20 °C near values. NBO analysis was performed with NBO 7.0[42]
executed from Gaussian 16[43] as well as GIAO calculations.
Conclusion
In conclusion we were able to show that dialkylhalonium ions are
the key intermediates during the classical Friedel-Crafts methyla-
tion reactions. In addition we reported on the methylation of
weakly basic oligofluorobenzenes with the dimethylchloronium
salt [Me2Cl][Al(OTeF5)4] (1)[7], where the electrophilic attack, i.e.
the methylation step, and the rearomatization are separated in
contrast to typical Friedel-Crafts reactions using the
[Cl3Al−ClMe]/MeCl system.
Caution: Chloromethane and SO2 give a pressure of 4.9 bar and 3.3 bar,
respectively, at room temperature; care must be taken that reaction ves-
sels resist this pressure.
The reaction of 1 with weakly basic fluorinated nitrogen containing
heterocycles leads to the formation of N-methylated products. It
has been shown that the rapid decomposition reaction of 1 in di-
chloromethane results in the formation of CH2Cl(OTeF5) and
CH2(OTeF5)2. Further investigations on methylation reactions as
well as attempts to isolate Wheland intermediates are continuing
in our group.
Chloromethane and SO2 were treated as ideal gases and measured via
their pressure in a known volume. When cooled to −70 °C liquefied SO2
can be easily transferred using inert PFA or PTFE tubes, however, a small
amount of the solvent evaporates by cooling the tube so concentrations
are changing. This is not possible with MeCl.
Experimental Section
The experiments were performed under exclusion of air and moisture us-
ing standard Schlenk techniques. The solvent SO2 was dried over CaH2.
The oligofluorobenzenes, CH2Cl2 and CD2Cl2 were dried over Sicapent®
while diethyl ether was dried over Solvona®. MeCl (purchased from abcr)
was used without further purification. Triethylaluminium was purchased
from abcr and handled in a glovebox under a dry argon atmosphere. Teflic
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10.1002/chem.202001457
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Methylation of oligofluorobenzenes – general procedure
19F NMR (377 MHz, CD2Cl2, 20 °C): δMM’ = −144.6 ppm (F1/F4), δBB’
−162.7 ppm (F2/F3), JMX = JM’X’
3J(19F,19F) = −21.79 Hz, JMX’ = JM’X
4J(19F,19F) = 1.61 Hz, JMM’ 5J(19F,19F) = 12.68 Hz, JXX’
6J(19F,1H) = 1.43 Hz, JMA’ = JM’A
=
=
=
=
=
3J(19F,19F) =
=
4J(19F,1H) =
To a solution of [Me2Cl][Al(OTeF5)4] (1) in SO2 a slight excess of the sub-
strate is added at −30 °C. The initially colorless reaction mixture changes
the color to intense yellow while stirring at room temperature for 30 minutes.
Afterwards, diethyl ether is added at −30 °C resulting in a decolorization.
The [H(OEt2)2]+ and [MeOEt2]+ ratio is determined from this solution by 1H
NMR spectroscopy. All volatiles are condensed into a second flask. SO2 is
carefully removed under reduced pressure at −20 °C. The resulting clear
colorless liquid is diluted with CD2Cl2 and analyzed by NMR spectroscopy.
−19.30 Hz, JXA = JX’A’
=
2.48 Hz.
GC-MS: tR = 4.25 min, m/z = 178.1 (calc: 178.0 [M]•+).
1,2,3-Trifluorobenzene
The following amounts were used for a 30 minutes reaction: 1: 495 mg,
0.473 mmol, SO2: 27 mmol, approx. 1.3 mL, 1,2,3-trifluorobenzene:
0.05 mL, 0.485 mmol.
[MeOEt2][Al(OTeF5)4]
1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 4.70 (q, 4H, CH2, 3J(1H,1H) =
7.1 Hz), 4.24 (s, 3H, OCH3), 1.68 (t, 6H, CH2CH3, 3J(1H,1H) = 7.1 Hz) ppm.
Main product: 4-methyl-1,2,3-trifluorobenzene
ABM3SVZ spin system
1H NMR (400 MHz, SO2, ext. [D6]acetone, 20 °C): δ = 5.84 (q, 4H, CH2,
3J(1H,1H) = 7.2 Hz), 5.37 (s, 3H, OCH3), 2.74 (t, 6H, CH2CH3, 3J(1H,1H) =
7.2 Hz) ppm/ppm.
1H NMR (600 MHz, CD2Cl2, 20 °C): δ = 6.89 (m, 1H, H5, HB), 6.86 (m, 1H,
H6, HA), 2.21 (m, 3H, CH3, HM) ppm.
1H,13C-HMQC NMR (400 MHz/101 MHz, CD2Cl2, 20 °C): δ = 4.70/88.7
(CH2/CH2),
(CH2CH3/CH2CH3) ppm/ppm.
4.24/70.3
(OCH3/OCH3),
1.68/11.7
19F NMR (565 MHz, CD2Cl2, 20 °C): δ = −139.0 (m, 1F, F3, FS), −139.8 (m,
1F, F1, FV), −163.0 (m, 1F, F2, FZ) ppm.
1H,13C-HMBC NMR (400 MHz/101 MHz, CD2Cl2, 20 °C): δ = 4.70/11.7
(CH2, CH2CH3), 4.24/88.7 (OCH3/CH2), 1.68/88.7 (CH2CH3/CH2) ppm.
Coupling constants: JVZ
−19.85 Hz, JSV
4J(19F,19F) = 5.62 Hz, JAV
5J(19F,1H) = −2.53 Hz, JAS = 4J(19F,1H) = 7.92 Hz, JBV = 3J(19F,1H) = 9.89 Hz,
=
3J(19F,19F) = −20.19 Hz, JSZ
=
3J(19F,19F) =
=
=
4J(19F,1H) = 5.63 Hz, JAZ
=
JBZ = 4J(19F,1H) = 7.24 Hz, JBS = J(19F,1H) = −2.40 Hz, JAB = J(1H,1H) =
8.62 Hz, JAM
4J(1H,1H) = −1.00 Hz, JBM 5J(1H,1H) = 0.40 Hz, JSM
4J(19F,1H) = 2.32 Hz, JVM = 6J(19F,1H) 1.33 Hz.
5
3
[H(OEt2)2][Al(OTeF5)4]
1H NMR (400 MHz, SO2, ext. [D6]acetone, 20 °C): δ = 17.38 (s br, 1H, H),
5.23 (q, 8H, CH2, 3J(1H,1H) = 7.1 Hz), 2.55 (t, 12H, CH3, 3J(1H,1H) = 7.1 Hz).
=
=
=
GC-MS: tR = 2.78 min, m/z = 145.1 (calc: 145.0 [M − H]•+).
1,2,3,4-Tetrafluorobenzene
The following amounts were used for a 30 minutes reaction: 1: 372 mg,
0.356 mmol, SO2: 22 mmol, approx. 1.0 mL, 1,2,3,4-tetrafluorobenzene:
0.05 mL, 0.467 mmol.
1,2-Difluorobenzene
The following amounts were used for a 30 minutes reaction: 1: 1.75 g,
1.67 mmol, SO2: 110 mmol, approx. 5 mL, 1,2-difluorobenzene: 0.17 mL,
1.71 mmol.
The following amounts were used for a three hours reaction: 1: 401 mg,
0.383 mmol, SO2: 22 mmol, approx. 1.0 mL, 1,2,3,4-tetrafluorobenzene:
0.05 mL, 0.467 mmol.
4-methyl-1,2-difluorobenzene
ABCM3SX spin system
5-methyl-1,2,3,4-tetrafluorobenzene
1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 6.81 (m, 1H, ArH, 3J(19F,1H) =
10.65 Hz, 4J(19F,1H) = 8.11 Hz (F2), 5J(19F,1H) = −2.61 Hz, 4J(19F,1H) =
6.46 Hz (F4)), 2.23 (m, 3H, CH3, 4J(19F,1H) = 2.41 Hz, 6J(19F,1H) = 1.38 Hz)
ppm.
1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 7.01 (m, 1H, H6, HA), 6.95 (m, 1H,
H3, HB), 6.85 (m, 1H, H5, HC), 2.27 (m, 3H, CH3, HM) ppm.
19F NMR (377 MHz, CD2Cl2, 20 °C): δ = −140.1 (m, 1F, F2, FS), −144.4 (m,
1F, F1, FX) ppm.
19F NMR (377 MHz, CD2Cl2, 20 °C): δ = −142.0 (dddd, 1F, F1, 3J(19F,19F)
= −21.02 Hz, 4J(19F,19F) = −1.87 Hz, 5J(19F,19F) = 12.49 Hz, 3J(19F,1H) =
10.65 Hz), −143.9 (dddtd, 1F, F4, 3J(19F,19F) = −20.15 Hz, 4J(19F,19F) =
−1.67 Hz, 5J(19F,19F) = 12.49 Hz, 4J(19F,1H) = 6.46 Hz (H5), 4J(19F,1H) =
2.41 Hz (CH3)), −158.3 (dddd, 1F, F3, 3J(19F,19F) = −20.15 Hz (F4),
3J(19F,19F) = −19.5 Hz Hz (F2), 4J(19F,19F) = −1.87 Hz, 5J(19F,1H) =
−2.61 Hz), −161.3 (ddddt, 1F, F2, 3J(19F,19F) = −21.02 Hz (F1), 3J(19F,19F)
= −19.45 Hz (F3), 4J(19F,19F) = −1.67 Hz, 4J(19F,1H) = 8.11 Hz, 6J(19F,1H)
= 1.38 Hz) ppm.
3
Coupling constants: JSX = 3J(19F,19F) = −21.17, JAS = J(19F,1H) = 10.60 Hz,
4
JAX
11.58 Hz, JCS
6J(19F,1H) = 1.33 Hz, JAC = 3J(1H,1H) = 8.92 Hz, JAB = 5J(1H,1H) = 0.30 Hz,
JBC
4J(1H,1H) = 2.10 Hz, JBM 4J(1H,1H) = 0.75 Hz, JCM 3J(1H,1H) =
−0.75 Hz.
=
4J(19F,1H) = 8.36 Hz, JBS = J(19F,1H) = 7.73 Hz, JBX
=
3J(19F,1H) =
=
4J(19F,1H) = 4.18 Hz, JCX
=
5J(19F,1H) = −1.44 Hz, JMS
=
=
=
=
GC-MS: tR = 2.82 min, m/z = 127.1 (calc: 127.0 [M − H]•+).
GC-MS: tR = 2.72 min, m/z = 163.1 (calc: 163.0 [M − H]•+).
4,5-dimethyl-1,2-difluorobenzene
AA’M3M3’XX’ spin system
dimethyl-1,2,3,4-tetrafluorobenzene
A3A’3MM’XX’ spin system
1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 6.90 (m, 2H, ArH, HA/HA’), 2.16 (m,
6H, CH3, HM/HM’) ppm.
1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 2.15 (m, 6H, CH3, HA/HA’) ppm.
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19F NMR (377 MHz, CD2Cl2, 20 °C): δ = −144.7 (m, 2F, ArF, FX/FX’) ppm.
27Al{19F} NMR (104 MHz, CD2Cl2, 20 °C):
δ = 46.8 (s, 73.2 %,
[Al(OTeF5)4]–; d, 22.2 %, [Al(OTeF5)3(O125TeF5)]–, 2J(125Te,27Al) = 73.2 Hz;
d, 2.8 %, [Al(OTeF5)3(O123TeF5)]–, 2J(123Te,27Al) = 61.2 Hz; t, 2.6 %,
Coupling constants: JXX’ = 3J(19F,19F) = −20.00 Hz, JAX = JA’X’ = 3J(19F,1H)
[Al(OTeF5)2(O125TeF5)2]–,
2J(125Te,27Al) = 73.2 Hz;
t,
0.04 %,
= 9.90 Hz, JAX’ = JA’X
= =
4J(19F,1H) = 9.90 Hz, JMX’ = JM’X 6J(19F,1H) =
[Al(OTeF5)2(O123TeF5)2]–, 2J(123Te,27Al) = 61.3 Hz) ppm.
1.00 Hz, JAA’ = 5J(1H,1H) = 1.20 Hz.
GC-MS: tR = 4.28 min, m/z = 142.1 (calc: 142.1 [M]•+).
1,2,3-Trifluorobenzene with [Me2Br]+
IR (ATR, 25 °C): ꢀ� = 1657 (m), 1587 (vw), 1527 (m), 1489 (vw), 1438 (w),
1315 (w), 1285 (w), 1135 (vw), 987 (sh), 945 (sh), 928 (s, ν(Al-O)), 818 (m,
ν(C-I)), 687 (vs, ν(Te-F)), 641 (w), 580 (w), 543 (m) cm−1
.
FT-Raman (25 °C): ꢀ� = 2992 (m), 2964 (w), 1660 (s), 1439 (m), 1387 (m),
1286 (m), 989 (m), 821 (w), 719 (w), 697 (vs), 647 (s), 584 (m), 515 (m),
457 (m), 421 (m), 375 (w), 354 (w), 335 (m), 302 (m), 205 (w), 134 (w)
The following amounts were used for
a 60 minutes reaction:
[Me2Br][Al(OTeF5)4]: 332 mg, 0.304 mmol, SO2: 22 mmol, approx. 1.0 mL,
1,2,3-trifluorobenzene: 0.05 mL, 0.485 mmol. Product ratio according to
NMR identical to the activation with 1. However [MeOEt2]+/[H(OEt2)2]+ ratio
in residual solid 2.3/1.
cm−1
.
[MeNC5F5][Al(OTeF5)4] (2F)
Methylation attempts with [Me2I]+
Pentafluoropyridine (0.06 mL, 0.547 mmol) is added to a solution of 1
(476 mg, 0.455 mmol) in SO2 (44 mmol, approx. 2.0 mL) at –30 °C. The
reaction mixture is allowed to warm to room temperature and stirred for 30
minutes. All volatiles are removed under reduced pressure at room tem-
perature to yield [MeNC5F5][Al(OTeF5)4] (2F, 530 mg, 0.455 mmol) as an
off-white powder.
The following amounts were used for
a
24 hours reaction:
[Me2I][Al(OTeF5)4]: 407 mg, 0.357 mmol, SO2: 27 mmol, approx. 1.3 mL,
1,2,3-trifluorobenzene: 0.05 mL, 0.485 mmol. No color change was ob-
served. No methylation was observed by NMR spectroscopy. To a sample
in a Young NMR tube an excess of 1,2-difluorobenzene is added. No color
change is observed. No methylation is observed by NMR spectroscopy.
1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 4.35 (td, 98.9 %, N12CH3, 4J(19F,1H)
= 3.32 Hz, 6J(19F,1H) = 1.24 Hz; dtd, 1.1 %, N13CH3, 1J(13C,1H) = 152.7 Hz,
4J(19F,1H) = 3.32 Hz, 6J(19F,1H) = 1.24 Hz) ppm.
The following amounts were used for 2 hours reaction at 50 °C. Caution:
SO2 has
a vapor pressure of approximately 8 bar at 50 °C!
[Me2I][Al(OTeF5)4]: 201 mg, 0.177 mmol, SO2: 22 mmol, approx. 1.0 mL,
1,2,3-trifluorobenzene: 0.05 mL, 0.485 mmol. No color change is observed.
No methylation is observed by NMR spectroscopy. After addition of diethyl
ether only [MeOEt2]+ is detected.
1H NMR (400 MHz, SO2, ext. [D6]acetone, 20 °C): δ = 5.68 (td, 98.9 %,
N12CH3, 4J(19F,1H) = 3.3 Hz, 6J(19F,1H) = 1.2 Hz; dtd, 1.1 %, N13CH3,
1J(13C,1H) = 152.7 Hz, 4J(19F,1H) = 3.3 Hz, 6J(19F,1H) = 1.2 Hz) ppm.
13C{19F,1H} NMR (101 MHz, CD2Cl2, 20 °C): δ = 155.9 (C4), 146.3 (C2/C6),
136.5 (C3/C5), 37.0 (CH3) ppm.
[MeNC5F4I][Al(OTeF5)4] (2I)
Tetrafluoro-para-iodopyridine (103 mg, 0.372 mmol, 1.05 eq) is sublimed
in vacuum onto a frozen solution of 1 (370 mg, 0.354 mmol) in SO2 (33
mmol, approx. 1.5 mL) . The reaction mixture is allowed to melt and stirred
at room temperature for 30 minutes. Removal of all volatiles under reduced
pressure at room temperature yields [MeNC5F4I][Al(OTeF5)4] (2I, 451 mg,
0.354 mmol) as an off-white powder.
1H,15
4.35 ppm/−213.2 ppm.
N
HMBC NMR (400 MHz/41 MHz, CD2Cl2, 20 °C):
δ
=
19F NMR (377 MHz, CD2Cl2, 20 °C): cation: A3MM’SXX’ spin system,
δMM‘ = −93.3 (F2/F6) ppm, δS
= −103.5 (F4) ppm, δXX‘ = −150.8
(F3/F5) ppm, JMM’
=
4J(19F,19F) = −20.00 Hz, JMS = JM’S
=
4J(19F,19F) =
5
25.59 Hz, JMX = JM’X’ = 3J(19F,19F) = −11.72 Hz, JM’X = JMX’ = J(19F,19F) =
1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 4.25 (t, 98.9 %, N12CH3, 4J(19F,1H)
= 3.27 Hz; dt, 1.1 %, N13CH3, 1J(13C,1H) = 152.1 Hz, 4J(19F,1H) =
3.27 Hz) ppm.
12.69 Hz, JSX = JSX’ = J(19F,19F) −22.95 Hz, JXX’ = J(19F,19F) = 4.42 Hz,
3
4
JMA = JM’A
=
4J(19F,1H) = 3.32 Hz, JSA 6J(19F,1H) = 1.24 Hz; anion: δ =
=
−38.6 (m, AB4X, 1F, 2J(19F,19F) = 187.4 Hz, 1J(125Te,19F) = 3332.0 Hz),
−46.3 (m, AB4X, 4F, 2J(19F,19F) = 187.4 Hz, 1J(125Te,19F) = 3470.0 Hz) ppm.
1H NMR (400 MHz, SO2, ext. [D6]acetone, 20 °C): δ = 5.59 (t, 98.9 %,
N12CH3, 4J(19F,1H) = 3.3 Hz; dt, 1.1 %, N13CH3, 1J(13C,1H) = 152.1 Hz,
4J(19F,1H) = 3.3 Hz) ppm.
27Al{19F} NMR (104 MHz, CD2Cl2, 20 °C):
[Al(OTeF5)4]–; d, 22.2 %, [Al(OTeF5)3(O125TeF5)]–, 2J(125Te,27Al) = 73.4 Hz;
d, 2.8 %, [Al(OTeF5)3(O123TeF5)]–, 2J(123Te,27Al) = 61.9 Hz; t, 2.6 %,
[Al(OTeF5)2(O125TeF5)2]–,
[Al(OTeF5)2(O123TeF5)2]–, 2J(123Te,27Al) = 61.3 Hz) ppm.
δ = 46.8 (s, 73.4 %,
13C{19F,1H} NMR (101 MHz, CD2Cl2, 20 °C): δ = 146.7 (C3/C5), 142.6
(C2/C6), 107.2 (C4), 37.1 (CH3) ppm.
2J(125Te,27Al) = 73.4 Hz;
t,
0.04 %,
1H,15
4.25 ppm/−208 ppm.
N
HMBC NMR (400 MHz/41 MHz, CD2Cl2, 20 °C):
δ
=
IR (ATR, 25 °C): ꢀ� = 1683 (m), 1605 (w), 1548 (s), 1401 (w), 1352 (w),
1289 (w), 1152 (m), 981 (m), 930 (s, ν(Al-O)), 688 (vs, ν(Te-F)), 641 (m),
614 (w), 550 (s), 450 (w) cm−1.
19F NMR (377 MHz, CD2Cl2, 20 °C): cation: A3MM’XX’ spin system. δMM‘
=
−99.7 (F2/F6) ppm, δXX‘ =−112.2 (F3/F5) ppm, JMM’
=
4J(19F,19F) =
ESI-MS (acetonitrile, positive mode): m/z = 184.0 ([MeNC5F5]+, calc:
184.1).
−18.26 Hz, JMX = JM’X’ = J(19F,19F) = 16.64 Hz, JM’X = JMX’ = 5J(19F,19F) =
3
−12.73, JXX’
= =
4J(19F,19F) = −2.16 Hz, JMA = JM’A 4J(19F,1H) = 3.27 Hz;
anion: δ = −38.5 (m, AB4X, 1F, 2J(19F,19F) = 187.8 Hz, 1J(125Te,19F) =
3366.0 Hz), −46.1 (m, AB4X, 4F, 2J(19F,19F) = 187.8 Hz, 1J(125Te,19F) =
3478.0 Hz) ppm.
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[MeN3C3F3][Al(OTeF5)4]
1H,13C HMQC NMR (400 MHz, CH2Cl2, ext. [D6]acetone, 20 °C): δ =
5.99 ppm/77.2 ppm.
To a solution of 1 (392 mg, 0.375 mmol) in SO2 (44 mmol, approx. 2.0 mL)
cyanuric fluoride (0.07 mL, 0.816 mmol, 2.2 eq) is added at –30 °C. The
reaction mixture is allowed to reach room temperature and stirred for 30
minutes. All volatiles are removed under reduced pressure at room tem-
perature. The resulting yellowish oil is dissolved in 0.5 mL CH2Cl2. 5.0 mL
n-pentane are added quickly at room temperature to precipitate the salt.
The solution is separated by filtration to leave [MeN3C3F3][Al(OTeF5)4] af-
ter drying in vacuum as a white powder (396 mg, 0.350 mmol).
19F NMR (377 MHz, CH2Cl2, ext. [D6]acetone, 20 °C): δ = −43.7 (m, AB4X,
1F, 2J(19F,19F) = 181.2 Hz, 4J(19F,1H) = 0.6 Hz, 1J(125Te,19F) = 3502 Hz),
−49.4 (m, AB4X, 1F, 2J(19F,19F)
= = 2.7 Hz,
181.2 Hz, 4J(19F,1H)
1J(125Te,19F) = 3765 Hz, 1J(123Te,19F) = 3123 Hz) ppm.
CH2(OTeF5)2
1H NMR (400 MHz, CH2Cl2, ext. [D6]acetone, 20 °C): δ = 6.12 (nonet-t,
83 %, 4J(19F,1H) = 2.5 Hz, 4J(19F,1H)=0.4 Hz; d-nonet-t, 14 %, 3J(125Te,1H)
= 207.0 Hz, 4J(19F,1H) = 2.5 Hz, 4J(19F,1H)=0.4 Hz; d-nonet-t, 2 %,
3J(125Te,1H) = 172.0 Hz, 4J(19F,1H) = 2.5 Hz, 4J(19F,1H)=0.4 Hz) ppm.
Cooling a solution of [MeN3C3F3][Al(OTeF5)4] in dichloromethane to −40 °C
yields crystals of [MeN3C3F(OTeF5)2][Al(OTeF5)4] that are suitable for X-
Ray diffraction.
19F NMR (377 MHz, CH2Cl2, ext. [D6]acetone, 20 °C): δ = −44.6 (m, AB4X,
1F, 2J(19F,19F) = 181.2 Hz, 4J(19F,1H) = 0.4 Hz, 1J(125Te,19F) = 3540 Hz),
1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 4.34 (td, 98.9 %, N12CH3, 4J(19F,1H)
= 1.6 Hz, 6J(19F,1H) = 0.9 Hz; dtd, 1.1 %, N13CH3, 1J(13C,1H) = 153.0 Hz,
4J(19F,1H) = 1.6 Hz, 6J(19F,1H) = 0.9 Hz) ppm.
−49.2 (m, AB4X, 1F, 2J(19F,19F)
= = 2.5 Hz,
181.2 Hz, 4J(19F,1H)
1J(125Te,19F) = 3749 Hz, 1J(123Te,19F) = 3110 Hz) ppm.
1H NMR (400 MHz, SO2, ext. [D6]acetone, 20 °C): δ = 5.62 (s br, 98.9 %,
N12CH3; d br, 1.1 %, N13CH3, 1J(13C,1H) = 153.0 Hz) ppm.
Acknowledgements
13C{19F,1H} NMR (101 MHz, SO2, ext. [D6]acetone, 20 °C): δ = 177.7 (C4),
166.1 (C2/C6), 37.7 (CH3) ppm.
We gratefully acknowledge the DFG research training network
1582 “Fluorine as a key element” for financial support, Solvay
Fluor GmbH for donating chemicals, and the Zentraleinrichtung
für Datenverarbeitung (ZEDAT) of the Freie Universität Berlin for
computational resources and support. Gefördert durch die Deut-
14N NMR (29 MHz, SO2, ext. [D6]acetone, 20 °C): δ = −167.4 (3N/5N),
−220.0 (1N).
1H,15
4.34 ppm/−220.3 ppm.
N
HMBC NMR (400 MHz/41 MHz, CD2Cl2, 20 °C):
δ
=
sche Forschungsgemeinschaft (DFG)
–
Projektnummer
387284271 – SFB 1349. Funded by the Deutsche Forschungsge-
meinschaft (DFG, German Research Foundation) – Project-ID
387284271 – SFB 1349. We thank Dr. Carsten Müller for helpful
discussions.
19F NMR (377 MHz, CD2Cl2, 20 °C): cation: δ = 0.9 (t br, F4, 4J(19F,19F) =
16.0 Hz), −26.9 (d br, F2/F6, 4J(19F,19F) = 16.0 Hz); anion: δ = −38.3 (m,
AB4X, 1F, 2J(19F,19F) = 187.4 Hz, 1J(125Te,19F) = 3350.0 Hz), −46.0 (m,
AB4X, 4F, 2J(19F,19F) = 187.4 Hz, 1J(125Te,19F) = 3462.0 Hz) ppm.
Keywords: methylation • weakly coordinating anion • reaction
27Al{19F} NMR (104 MHz, CD2Cl2, 20 °C):
δ = 46.8 (s, 72.9 %,
mechanism • halonium ions • electrophilic substitution
[Al(OTeF5)4]–; d, 22.2 %, [Al(OTeF5)3(O125TeF5)]–, 2J(125Te,27Al) = 72.9 Hz;
d, 2.8 %, [Al(OTeF5)3(O123TeF5)]–, 2J(123Te,27Al) = 61.4 Hz; t, 2.6 %,
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[Al(OTeF5)2(O123TeF5)2]–, 2J(123Te,27Al) = 61.3 Hz) ppm.
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=
2.7 Hz, 4J(19F,1H)=0.6 Hz; d-quintet-d, 7.1 %,
3J(125Te,1H) = 214.7 Hz, 4J(19F,1H) = 2.7 Hz, 4J(19F,1H)=0.6 Hz; d-quintet-
d, 1%, 3J(125Te,1H) = 180.2 Hz, 4J(19F,1H) = 2.7 Hz, 4J(19F,1H) =
0.6 Hz) ppm.
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COMMUNICATION
The
dimethylchloronium
salt
Sebastian Hämmerling, Patrick Voßna-
cker, Simon Steinhauer, Helmut Be-
ckers, Sebastian Riedel*
[Me2Cl][Al(OTeF5)4] is used to meth-
ylate electron-deficient aromatic sys-
tems in Friedel-Crafts type reactions.
The role of the dimethylchloronium
cation in a classical Friedel-Crafts re-
action is evaluated by quantum-
chemical calculations. Furthermore
the
[MeNC5F5]+,
[MeN3C3F3]+ are presented.
Page No. – Page No.
Methylation of electron-deficient ar-
omatic systems by dimethyl-
halonium salts
N-methylated
cations
and
[MeNC5F4I]+
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