Synthesis of bench-stable diarylmethylium tetrafluoroborates

Article abstract of DOI:10.1021/acs.joc.5b00418

A representative number of bench-stable nonsymmetric diarylcarbenium tetrafluoroborates have been isolated via the direct coupling of aryl (or heteroaryl) aldehydes and N-heteroarenes and fully characterized. They have proven to be highly stable in the pr

Full text of DOI:10.1021/acs.joc.5b00418

Note
pubs.acs.org/joc
Synthesis of Bench-Stable Diarylmethylium Tetrafluoroborates
Margherita Barbero,*,† Roberto Buscaino,† Silvano Cadamuro,† Stefano Dughera,† Andrea Gualandi,‡
Domenica Marabello,§ and Pier Giorgio Cozzi*,‡
†Dipartimento di Chimica, Universita
̀
di Torino, Via P. Giuria 7, 10125 Torino, Italy
di Bologna, Via Selmi 2, 40126 Bologna, Italy
‡Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum, Universita
̀
§Dipartimento di Chimica e CrisDi (Interdepartmental Center for Crystallography), Universita
Torino, Italy
̀
di Torino, Via P. Giuria 7, 10125
S
* Supporting Information
ABSTRACT: A representative number of bench-stable nonsymmetric
diarylcarbenium tetrafluoroborates have been isolated via the direct
coupling of aryl (or heteroaryl) aldehydes and N-heteroarenes and fully
characterized. They have proven to be highly stable in the presence of
both EDG and EWG substituents. An (E)-iminium vinylogous
substructure has been shown as the common cation scaffold by X-ray
analysis and by NOE determination.
he importance of carbocations as intermediates in organic
electron-rich aldehydes gave positive results, and it was not
Tchemistry does not need to be pointed out. Persistent possible to replace the indole ring with the pyrrole one.
Intrigued by the reasons behind such stabilities and in order
to find new stable structures of these normally highly reactive
intermediates, we examined the role of the anion and then,
once it was possible to replace it with an economically
convenient analogue, took the cationic scaffold into consid-
eration. We herein report the synthesis and structural
characterization of new nonsymmetric diarylmethylium tetra-
fluoroborates 3a−l (Table 1), ready to use and with long shelf
lives. We believe that the presence of the widespread indolyl
(or pyrrolyl) moiety, the high stability, and easy preparation
procedure make them a tool of great synthetic relevance in
organic chemistry (notably, enantioselective unsymmetrical
triarylmethanes synthesis,15 α-alkylation of carbonyl com-
pounds,16 2,3-disubstituted indoline synthesis,17 intramolecular
tandem reactions,18 aza-ene reactions,19 normally achieved via
in situ generated carbenium ions).
In order to examine anion relevance, we initially tested the
reaction between the electron-rich 4-methoxybenzaldehyde
(1a) and 2-methylindole (2a) in the presence of a
tetrafluoroboric acid diethyl ether complex, widely used in
organic synthesis as a source of a non-nucleophilic and quite
air-stable anion. The indole substitution in position 2 prevents
its acid-catalyzed polymerization through 2,3 linkages.20
We applied optimized conditions;13 a solution of 2a in
anhydrous MeCN was added dropwise to a solution of 1a and
types of these species can be formed, normally at low
temperature in superacidic systems,1 and immediately reacted
with suitable nucleophiles. Diarylmethyl carbocations, stable
enough to be spectroscopically studied, are often generated in
situ via diarylmethanols2 or halides3 ionization, the laser flash
induced photolysis of suitable precursors,4 benzhydryl ester or
halide solvolysis,5 DDQ-mediated oxidation, benzylic carbon−
hydrogen activation,6 and oxidative diarylmethane C−H bond
dissociation using anodic oxidation.7
A large number of symmetric and nonsymmetric diary-
lcarbenium salts have been studied by Mayr’s research group as
reference electrophiles toward a variety of neutral π-, n-, and σ-
nucleophiles or carbanion nucleophiles, leading to extensive
electrophilicity and nucleophilicity scales.8 Benzhydrylium
tetrafluoroborates have been prepared, but fully characterized,
in few cases;9 they have more often been generated and reacted
in situ as tetrafluoroborates, tetrachloroborates,10 or triflates.11
Meanwhile, Takekuma’s group has reported a class of mono-
and dicarbenium hexafluorophosphates (or tetrafluoroborates)
stabilized by the 3-guaiazulenyl group.12
Finally, we have reported a nonsymmetric diarylmethylium
salt synthesis via the direct coupling of aryl (or heteroaryl)
aldehydes and N-heteroarenes, which have been recently
employed in a direct organocatalyzed asymmetric alkylation
of aldehydes.13 Diarylmethylium o-benzenedisulfonimides,
which bear 2-methylindole or 1,2-dimethylindole, were isolated
as stable and long shelf life salts; the counteranion was chosen
because of its well-known non-nucleophilic character.14 Only
Received: February 27, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.5b00418
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The Journal of Organic Chemistry
Note
Table 1. Diarylmethylium Tetrafluoroborates 3a−l and Diarylmethane 4a−l Yields
a
b
c
Reaction conditions: molar ratio 1:2:HBF4Et2O = 1.2:1:1.2; anhydrous MeCN, rt. Yields refer to pure solid isolated products. Reaction
d
conditions: molar ratio 3:NaBH4 = 1:1; anhydrous MeCN, rt. Yields refer to purified products (column chromatography; eluent:PE/EE 6/4).
Scheme 1. Synthesis of Salts 3a−l
HBF4·Et2O in the same solvent (molar ratio 1a:2a:HBF4·Et2O
= 1.2:1:1.2), at rt in an open vessel. The immediate formation
of a bright red-colored solution and then the separation of a red
precipitate indicated the reaction success. Product 3a was
isolated in 95% yield as an easily handled, bench-stable, long-
life solid salt; the structure and purity were confirmed by
spectroscopic methods, elemental analysis, and chemical
reduction with NaBH4 in anhydrous MeCN to diarylmethane
4a in 94% yield.
In order to explore its applicability, we tested the reaction
between less stabilized aryl and heteroaryl aldehydes with 2-
methylindole (2a) or 1,2-dimethylindole (2b) (Scheme 1).
Unsubstituted benzaldehyde (1b), electron-poor 4-chloro-
benzaldehyde (1c), heteroaromatic indol-3-carbaldehyde (1d),
and acid-sensitive 5-methylfuran-2-carbaldehyde (1e) were
reacted with 2a. Aryl 2-methyl-3-indolylmethylium tetrafluor-
oborates 3b−e were isolated in higher yields than the
corresponding o-benzenedisulfonimide salts.
Encouraged by these positive results, we tested the reaction
on the very electron-poor 4-nitrobenzaldehyde (1f). We were
gratified by successful reactions with indoles 2a and 2b: salts 3f
and 3g were isolated in high yield and purity. Quite
surprisingly, aldehyde 1f did not give positive results in the
presence of the o-benzenedisulfonimide.13,21 Although we are
not currently able to account for these contrasting results, we
found that the solvent can be crucial for the successful
nucleophilic reaction of stabilized carbenium ion.13 Reactivity
vs stability of the obtained intermediates probably controls this
reaction, in which the insoluble carbenium ion salt separates
from the reaction mixture driving the reaction to completion.
With the aim of confirming the synthetic procedure relevance
and of understanding the need for the indole, we took the 4-
(N,N-dimethylamino)phenyl scaffold into consideration, as it is
a well-known stabilizing group in benzhydrylium ions.9 First,
we reacted 4-(N,N-dimethylamino)benzaldehyde (1g) with 1,2-
dimethylindole (2b): tetrafluoroborate 3h was isolated in a 97%
yield. Next, we tested the reaction protocol between 1g and
electron-rich arenes, other than indole (e.g., 1,2,4-trimethox-
ybenzene and 2-methylfuran), and between aldehyde 1a with
N,N-dimethylaniline, but, in both cases, without success. This
confirmed the decisive role played by the indole moiety in the
stabilization of the carbenium ion.
These results confirmed that the benzhydryl carbocation,
which is the intermediate in a Friedel−Crafts hydroxyalkylation
reaction,21 can be isolated from the reaction mixture as a bench-
stable salt with the counteranion of the tetrafluoroboric acid, a
cheap commercially available reagent and, therefore, an
economic alternative to the o-benzenedisulfonimide.14
B
DOI: 10.1021/acs.joc.5b00418
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The Journal of Organic Chemistry
Note
Figure 1. Asymmetric unit of compound 3i with atom labeling (displacement ellipsoids for all but hydrogen and fluorine atoms are drawn at 50%
probability) and relevant cation bond distances.
The X-ray analysis of one of the aforementioned
benzhydrylium salts, namely, 4-methoxyphenyl(2-methyl-3-
indolyl)methylium o-benzenedisulfonimide, showed that the
cation charge is mainly localized on the nitrogen atom in an
azafulvenium (iminium vinylogous) species, further stabilized
by H-bonds between indole and the counteranion.13 We,
therefore, decided to substitute the indole nucleus with the
pyrrole one, despite its well-known acid sensitivity.
The reaction of 4-methoxybenzaldehyde (1a) with 2,5-
dimethylpyrrole (2c) under the conditions optimized for indole
gave the tetrafluoroborate 3i in 70% yield. Although 3i was
obtained in a slightly lower yield than the corresponding indolyl
salt 3a, it confirmed our hypothesis on the stabilizing role of the
vinylogous iminium substructure.
In order to validate the synthetic procedure, we then tested
other reactions of 2c. The reaction with benzaldehyde (1b)
only furnished traces of the product, as confirmed by the GC−
MS analysis of the crude reaction mixture obtained via chemical
reduction of the scarce solid. Changes in the reaction
conditions with respect to acid amount and nature (HBF4
and o-benzenedisulfonimide) and dilution gave the same
unsatisfactory results. Differently to results obtained with 2a,
the halogen’s mesomeric effect did not prevail over the
inductive one in the reaction of 1c with 2c; the expected salt
was not isolated. A dark solid was isolated from the reaction
with aldehyde 1g, but the chemical reduction furnished only
traces of diarylmethane. Similarly, aldehyde 1e did not give the
expected salt; the carbocation is probably too unstable because
of the strong electron-withdrawing substituent effect. Positive
results were, however, obtained when reacting aldehyde 1d: 3j
was isolated in high yield and purity. Furthermore, we carried
out the reaction of 2c with 1-methylpyrrol-2-carbaldehyde (1h)
in order to obtain a dipyrrolylmethane core, which is of central
importance in chemical and biological research areas. Quite
unexpectedly, the tetrafluoroborate 3k was obtained in a very
high yield and purity.
In the light of these results, we can infer that, in the presence
of an azole moiety (whether isolated or benzene-fused),
nonsymmetric diarylcarbenium ions can be easily isolated as
tetrafluoroborates via direct coupling between aryl/heteroaryl
aldehydes and N-heteroarenes.
The intermediacy of vinylogous iminium species in reaction
mechanisms involving indole substructures (alkylideneindole-
ninium ions) has often been recognized in cases such as direct
coupling,22 DDQ-based 3-arylmethylindole dehydrogenation,23
and suitable leaving group elimination (hydroxyl, sulfonamide,
halides), for example, in the (1H-3-indolyl)(aryl)methanols
acid dehydration24 or acid-catalyzed N-tosyl-3-indolylbenzyl-
amines treatment.25 Sometimes, the presence of vinylogous
iminium intermediates has been demonstrated using spectro-
scopic methods.26 The chemistry of alkylideneindoleninium
ions and alkylideneindolenines has been recently reviewed.27
In most salts 3, the positive charge can be delocalized by
resonance to both diarylcarbenium rings. As previously
reported, X-ray analysis of 4-methoxyphenyl(2-methyl-3-
indolyl)methylium o-benzenedisulfonimide clearly showed an
almost planar cation, the two planar aromatic moieties form an
angle of 31°, and the positive charge is localized on the nitrogen
atom in a highly extended conjugated scaffold. A X-ray analysis
was needed to confirm our initial hypothesis on the feasibility
of indole ring substitution in the new salts. Therefore, X-ray
diffraction analysis was performed on salt 3i, which was chosen
in order to compare present and previous results.12,13
The asymmetric unit of 3i and some relevant bond distances
of the diarylmethylium ion are reported in Figure 1.
The diarylmethylium moiety is completely planar (mean
deviation from planarity = 0.055 Å), suggesting wide electron
density delocalization throughout the whole molecule.
However, typical double bond distances are located at the
C1−C2, C3−C4, and N5−C6 bonds. Furthermore, although
−
the BF4 ion is disordered in three positions, its distance from
the N5−H5 fragment is sufficiently short (1.971 Å av.) to
hypothesize that the positive charge is localized on the N5
atom, confirming the major significance of a vinylogous
iminium resonance structure. Moreover, the positive charge is
slightly transferred to the oxygen atom of the p-anisyl group in
a quinoid structure. This is demonstrated by the planarity of the
entire cation and by the C12−O15 bond distance, which is
shorter than a typical C−O single bond.
This fact could explain the very few tetrafluoroborates
obtainable from 2,5-dimethylpyrrole (3i−k) and the single one
from 2,4-dimethylpyrrole (3l) in comparison with the number
and variety of salts from indole (3a−h). In salts 3i−l, both parts
of the benzhydrylium cation are involved in charge
delocalization, meaning that particularly electron-rich (p-
Finally, we tested the reaction between 2,4-dimethylpyrrole
(2d) and selected aldehydes (1a, 1d, and 1g). Only 1d afforded
the tetrafluoroborate 3l in good yield and purity, whereas 1a
was ineffective and 1g gave a dark solid, which, by reduction,
furnished only traces of diarylmethane.
All the deeply colored salts proved to be stable to air and
moisture, storable at low temperatures for long periods, and
ready to use. Their identity and purity were confirmed by
means of NaBH4 chemical reduction in anhydrous MeCN;
diarylmethanes 4a−l were obtained in yields ranging from 83 to
99% (the single exception was 4i; changes in reducing agent
and solvent amounts were inefficient).
C
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Note
R1 was 0.0889 (I > 2σ(I)), and wR2 was 0.2891 (all data). All non-
hydrogen cation atoms were anisotropically refined. The BF4 anion is
disordered between three equivalent positions with a 0.33 occupancy
factor, and all of its atoms were isotropically refined. Hydrogen atom
positions were calculated and refined riding on the atom connected,
with Uiso = 1.2 or 1.5 Ueq of the corresponding bonded atoms. A
Gaussian absorption correction was applied. Software used:
CrysAlisPro28 (collection, integration), SHELXS (structure solution
and refinement),29 and OLEX2-molecular graphics.30 Crystal data
were deposited at CSD with code CCDC 1038486.
anisyl/indole/pyrrole) scaffolds are required in the aldehyde
moiety.
Furthermore, an E configuration was shown, as previously
observed.13 This result prompted us to confirm the stereo-
chemistry using NOE experiments (see the Supporting
Information).
Different selective excitations were performed on salts 3c and
3f. Selective excitation of methyl protons of 3f showed a
positive NOE on the benzylic proton, while selective excitation
of the phenyl ring protons in the meta position (with respect to
NO2) showed a positive NOE on the benzylic proton, the
phenyl protons in ortho (with respect to NO2) and the indole
proton in position 4. Finally, selective excitation of the indole
proton in 4 showed a positive NOE on the proton in 5 and on
the phenyl protons in the meta position (with respect to NO2).
Similar behavior was observed for 3c. Moreover, it was possible
to perform a selective excitation of the benzylic proton. A
positive NOE was observed on the protons in the meta position
(with respect to Cl) and on the methyl protons. On the basis of
this evidence, we can confirm that the phenyl moiety and
indole ring occupy the same side of the double bond.
In summary, we have reported a simple, direct synthesis and
spectral characterization of new stable, long shelf life, and ready
to use diarylcarbenium tetrafluoroborates via the direct
coupling of aryl or heteroaryl aldehydes and N-heteroarenes,
where the azole ring is the crucial framework for the high
stability.
Caution! Although easily handled, isolated product 3k was a respiratory
irritant.
General Procedures. General Procedure for Diarylmethylium
Tetrafluoroborates 3a−l Synthesis. A solution of aromatic compound
2 (3.0 mmol) in anhydrous MeCN (5 mL) was added dropwise at rt
and under stirring to a mixture of aldehyde 1 (3.6 mmol) and
tetrafluoroboric acid/ether complex (0.59 g, 3.6 mmol) in anhydrous
MeCN (15 mL) in an open vessel. The deeply colored solution
lightened, and a red or orange solid separated. After stirring at rt for 30
min, anhydrous Et2O was added to complete the separation, and the
solid was gathered on a Buchner funnel, washed with anhydrous Et2O,
and dried under reduced pressure.
(4-Methoxyphenyl)(2-methyl-3-indolyl)methylium Tetrafluoro-
borate (3a). (0.99 g, 98% yield); dp 211.0 °C (anhydrous MeCN/
1
anhydrous Et2O); H NMR (200 MHz, CDCl3−CF3COOD): δ =
2.89 (s, 3H), 3.97 (s, 3H), 7.13 (d, J = 8.8 Hz, 2H), 7.34−7.54 (2
overlapped m, 3H), 8.02 (d, J = 8.8 Hz, 2H), 8.14 (d, J = 7.2 Hz, 1H),
8.39 (s, 1H); 13C NMR (50 MHz, CDCl3−CF3COOD): δ = 12.8,
55.5, 114.8, 115.4 (2C), 122.4, 123.9 (q), 125.6 (q), 127.4 (q), 128.0,
129.7, 136.5 (2C), 139.5 (q), 158.3, 166.5 (q), 170.0 (q); IR (CHCl3)
νmax 3467, 3033, 3019, 1579, 1515, 1417, 1350; exact ESI full mass:
found m/z 250.26 (calcd for C17H16NO+, m/z 250.12); Anal. Calcd for
C17H16BF4NO: C, 60.57; H, 4.78; N, 4.15. Found C, 60.35; H, 4.76;
N, 4.13.
EXPERIMENTAL SECTION
■
General Information. All reactions were conducted in open air
vials using analytical grade reagents and were monitored by TLC, GC,
GC−MS, and NMR spectrometry. GC−MS spectra were recorded on
a mass selective detector connected to a GC with a cross-linked methyl
silicone capillary column. Mass spectra were recorded on a mass
spectrometer equipped with an electrospray ionization source (ESI).
Infrared (IR) data are presented as frequency of absorption (cm−1).
1H NMR and 13C NMR spectra were recorded in CDCl3 or CDCl3−
CF3COOD on a spectrometer at 200 and 50 MHz, respectively;
chemical shifts are given in ppm relative to CDCl3. Selective excitation
1H NMR spectra were recorded on a spectrometer at 400 MHz in a
mixture of CDCl3−TFA (10%), using a DPFGSE-NOE sequence with
a 50 Hz “rsnob” pulse and a mixing time of 1.5 s. Chemical shifts are
reported in ppm from TMS with the solvent resonance as the internal
standard (deuterochloroform: δ = 7.27 ppm). TLC were performed on
silica gel TLCPET foils GF 254, 2−25 μm, layer thickness 0.2 mm,
medium pore diameter 60 Å. Plates were visualized using UV light
(254 nm). Column chromatography was carried out using SiO2 (pore
size 70 Å, 70−230 mesh). Petroleum ether refers to the fraction
boiling in the range of 40−60 °C and is abbreviated as PE.
Commercially available reagents and solvents were used without
purification or distillation prior to use. Room temperature (20−25 °C)
(2-Methyl-3-indolyl)(phenyl)methylium Tetrafluoroborate (3b).
(0.80 g, 87% yield); dp 170.0 °C (anhydrous MeCN/anhydrous
1
Et2O); H NMR (200 MHz, CDCl3−CF3COOD): δ = 2.96 (s, 3H),
7.32−7.72 (m, 6H), 7.88−7.93 (m, 2H), 8.07 (d, J = 7.4 Hz, 1H), 8.53
(br s, 1H); 13C NMR (50 MHz, CDCl3−CF3COOD): δ = 13.3, 115.5,
123.1, 123.7 (q), 128.7, 129.5 (2C), 130.7, 131.0 (q), 131.8 (2C),
132.3 (q), 134.9, 140.0 (q), 158.1, 172.0 (q); IR (CHCl3) νmax 3470,
3018, 1714, 1612, 1572, 1459, 1339, 1231; exact ESI full mass: found
m/z 220.21 (calcd for C16H14N: M+, m/z 220.11); Anal. Calcd for
C16H14BF4N: C, 62.58; H, 4.60; N, 4.56. Found C, 62.38; H, 4.57; N,
4.53.
(4-Chlorophenyl)(2-methyl-3-indolyl)methylium Tetrafluoro-
borate (3c). (0.85 g, 83% yield); dp 190.0 °C (anhydrous MeCN/
1
anhydrous Et2O); H NMR (200 MHz, CDCl3−CF3COOD): δ =
2.96 (s, 3H), 7.30−7.50 (m, 1H), 7.52−7.64 (m, 4H), 7.87 (d, J = 8.6
Hz, 2H), 8.05 (d, J = 7.6 Hz, 1H), 8.46 (s, 1H); 13C NMR (50 MHz,
CDCl3−CF3COOD): δ = 13.2, 115.5, 123.0, 123.4 (q), 128.8, 129.9
(2C), 130.6 (q), 131.0, 131.3 (q), 132.8 (2C), 140.0 (q), 141.6 (q),
156.2, 173.0 (q); IR (CHCl3) νmax 3025, 3018, 2401, 1521, 1420,
1207; exact ESI full mass: found m/z 254.21 (calcd for C16H13ClN+:
M+, m/z 254.07); Anal. Calcd for C16H13BClF4N: C, 56.27; H, 3.84;
N, 4.10. Found C, 56.22; H, 3.80; N, 4.12.
1
is abbreviated as rt. Yields for pure (GC, GC−MS, TLC, H NMR)
(3-Indolyl)(2-methyl-3-indolyl)methylium Tetrafluoroborate (3d).
(0.95 g, 92% yield); dp 200 °C (anhydrous MeCN/anhydrous Et2O);
1H NMR (200 MHz, CDCl3−CF3COOD): δ = 2.82 (s, 3H), 7.35−
isolated products are listed in Table 1. The structure and purity of all
1
new products were determined by elemental analysis, ESI, H, 13C
NMR, and DEPT spectra. The structure and purity of known products
were confirmed by means of comparison of their physical and spectral
data (MS, 1H NMR, and 13C NMR) with those reported in the
literature.
Crystal Analysis. Reflections were collected at room temperature
on an X-ray diffractometer, with graphite monochromatized Mo Kα
radiation (λ = 0.71073 Å). Crystal data for C14H16BNOF4 (M =
301.09 g/mol): orthorhombic, space group Pbca (No. 61), a =
13.8025(13) Å, b = 12.8183(6) Å, c = 16.7453(8) Å, V = 2962.7(3) Å3,
Z = 8, T = 295 K, μ = 0.118 mm−1, Dcalc = 1.350 g/cm3, 6281
7.53 (m, 5H), 7.58−7.70 (m, 2H), 7.86−7.90 (m, 1H), 8.64 (s, 1H),
8.65 (s, 1H); 13C NMR (50 MHz, CDCl3−CF3COOD): δ = 12.3,
113.7 (2C), 117 (q), 120.0, 120.9 (q), 121.7, 124.2 (q), 125.1, 126.2,
126.3, 126.5 (q), 127.5, 137.3 (q), 138.1 (q), 141.2, 147.8, 162.7 (q);
IR (CHCl3) νmax 3467, 3027, 3024, 1576, 1426, 1218, 1024; exact ESI
full mass: found m/z 259.20 (calcd for C18H15N2: M+: m/z 259.12);
Anal. Calcd for C18H15BF4N2: C, 62.46; H, 4.37; N, 8.09. Found C,
62.30; H, 4.35; N, 8.06.
(5-Methyl-2-furyl)(2-methyl-3-indolyl)methylium Tetrafluoro-
reflections measured (4.866 ≤ 2Θ ≤ 49.426), 2486 unique (Rint
=
borate (3e). (0.85 g, 91% yield); dp 204 °C (anhydrous MeCN/
1
0.0387, Rsigma = 0.0580), which were used in all calculations. The final
anhydrous Et2O); H NMR (200 MHz, CDCl3−CF3COOD): δ =
D
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Note
2.74 (s, 3H), 2.84 (s, 3H), 6.69 (d, J = 3.8 Hz, 1H), 7.43−7.54 (m,
3H), 7.70 (d, J = 3.8, 1H), 7.85 (s, 1H), 8.59−8.64 (m, 1H); 13C
NMR (50 MHz, CDCl3−CF3COOD): δ = 12.5, 14.5, 114.2, 115.1,
122.0 (q), 123.8 (q), 124.5, 127.3, 128.8, 134.2, 137.5, 138.8 (q), 149.9
(q), 168.0 (q), 169.4 (q); IR (CHCl3) νmax 3032, 3012, 1577, 1427,
1238, 1198; exact ESI full mass: found m/z 224.16 (calcd for
C15H14NO: M+, m/z 224.11); Anal. Calcd for C15H14BF4NO: C,
57.91; H, 4.54; N, 4.50. Found C, 57.72; H, 4.52; N, 4.52.
(q), 142.8, 156.7 (q); IR (CHCl3) νmax 3331, 3026, 3017, 1621, 1509,
1403, 1225, 1218, 1209; exact ESI full mass: found m/z 187.12 (calcd
for C12H15N2: M+, m/z 187.12); Anal. Calcd for C12H15BF4N2: C,
52.59; H, 5.52; N, 10.22; Found C, 52.44; H, 5.50; N, 10.24.
(3,5-Dimethyl-2-pyrrolyl)(3-indolyl)methylium Tetrafluoroborate
(3l). (0.78 g, 84% yield); dp 216 °C (anhydrous MeCN/anhydrous
1
Et2O); H NMR (200 MHz, CDCl3−CF3COOD): δ = 2.46 (s, 3H),
2.54 (s, 3H), 6.40 (s, 1H), 7.35−7.42 (m, 2H), 7.45−7.55 (m, 1H),
7.77−7.85 (m, 1H), 7.99 (s, 1H), 8.50 (s, 1H); 13C NMR (50 MHz,
CDCl3−CF3COOD): δ = 11.5, 13.9, 113.2 (q), 113.4, 118.0, 120.1,
124.6, 125.8, 127.0 (q), 132.5, 133.1 (q), 136.7 (q), 137.8, 147.1 (q),
151.6 (q); IR (CHCl3) νmax 3032, 3012, 1583, 1237, 1198; exact ESI
full mass: found m/z 213.17 (calcd for C15H15N2: M+, m/z 223.12);
Anal. Calcd for C15H15BF4N2: C, 58.10; H, 4.88; N, 9.03; Found C,
58.15; H, 4.86; N, 9.04.
(2-Methyl-3-indolyl)(4-nitrophenyl)methylium Tetrafluoroborate
(3f). (0.89 g, 84% yield); dp 200 °C (anhydrous MeCN/anhydrous
1
Et2O); H NMR (200 MHz, CDCl3−CF3COOD): δ = 3.02 (s, 3H),
7.35−7.43 (m, 1H), 7.50−7.61 (m, 2H), 7.79 (d, J = 7.6 Hz, 1H), 8.03
(d, J = 8.8, 2H), 8.44 (d, J = 8.8, 2H), 8.56 (s, 1H); 13C NMR (50
MHz, CDCl3−CF3COOD): δ = 13.3, 116.0, 123.0 (q), 123.5, 124.3
(2C), 129.3, 131.3 (2C), 131.8, 134.3 (q), 138.4 (q), 140.4 (q), 149.5
(q), 152.9, 174.7 (q); IR (CHCl3) νmax 3033, 3012, 1640, 1235, 1197;
exact ESI full mass: found m/z 265.07 (calcd for C16H13N2O2: M+, m/
z 265.10); Anal. Calcd for C16H13BF4N2O2: C, 54.58; H, 3.72; N, 7.96;
Found C, 54.40; H, 3.71; N, 7.98.
General Procedure for Diarylmethylium Tetrafluoroborates 3a−l
Reduction. NaBH4 (1.0 mmol, 0.04 g) was added portionwise at rt
and under stirring to a solution of salt 3 (1.0 mmol) in anhydrous
MeCN (15 mL). The reaction was instantaneous, and the deeply
colored solution immediately faded. After stirring at rt for 10 min, the
reaction mixture was treated with Et2O/water (40 mL; 1:1). The
organic phase was separated, washed with brine (2 × 20 mL), and
evaporated under reduced pressure. The crude residue was the
(1,2-Dimethyl-3-indolyl)(4-nitrophenyl)methylium Tetrafluoro-
borate (3g). (1.04 g, 95% yield); dp 218 °C (anhydrous MeCN/
1
anhydrous Et2O); H NMR (200 MHz, CDCl3−CF3COOD): δ =
2.96 (s, 3H), 4.01 (s, 3H), 7.36−7.44 (m, 1H), 7.53−7.64 (m, 2H),
7.95 (d, J = 8.8, 2H), 8.39 (d, J = 8.8, 2H), 8.56 (s, 1H); 13C NMR (50
MHz, CDCl3−CF3COOD): δ = 12.1, 33.4, 113.8, 123.0 (q), 123.6,
124.3 (2C), 129.6, 131.2 (2C), 131.5, 133.8 (q), 138.6 (q), 143.0 (q),
149.4 (q), 152.2, 173.2 (q); IR (CHCl3) νmax 3032, 3012, 1534, 1238,
1198; exact ESI full mass: found m/z 279.11 (calcd for C17H15N2O2:
M+, m/z 279.11); Anal. Calcd for C17H15BF4N2O2: C, 55.77; H, 4.13;
N, 7.65; Found C, 55.69; H, 4.14; N, 7.62.
1
virtually pure reduction product 4 (GC, GC−MS, H NMR), which
was, however, purified via short column chromatography (eluent PE/
EE 6/4); yields of purified products are reported in Table 1. Structure
and purity of products 4a−e were confirmed by comparing physical
and spectral data (MS, 1H NMR, and 13C NMR) with previously
reported data;13 yields and physical and spectroscopic data are listed
below for new products 4f−l.
(4-Nitrophenyl)(2-methyl-3-indolyl)methane (4f).31 Yellow solid
(0.22 g, 83% yield); mp 123.0−124.0 °C (DCM−PE); 1H NMR (200
MHz, CDCl3): δ = 2.34 (s, 3H), 4.10 (s, 2H), 6.97−7.11 (m, 2H),
7.20−7.24 (m,2H), 7.29 (d, J = 8.8 Hz, 2H), 7.83 (br s, 1H), 8.03 (d, J
= 8.8 Hz, 2H); 13C NMR (50 MHz, CDCl3): δ = 11.6, 29.9, 108.7 (q),
110.2, 117.7, 119.4, 121.2, 123.4 (2C), 128.2 (q), 128.8 (2C), 131.8
(q), 135.1 (q), 146.0 (q), 149.3 (q); IR (CHCl3) νmax 3471, 3009,
2921, 1599, 1518, 1461, 1347, 1068; MS m/z (%): 266 [M+](100),
144 (85).
(4-N,N-Dimethylaminophenyl)(1,2-dimethyl-3-indolyl)methylium
Tetrafluoroborate (3h). (1.06 g, 97% yield); dp 178 °C (anhydrous
MeCN/anhydrous Et2O); 1H NMR (200 MHz, CDCl3−CF3COOD):
δ = 2.96 (s, 3H), 3.45 (s, 6H), 4.03 (s, 3H), 7.41−7.49 (m, 1H), 7.55−
7.68 (m, 2H), 7.83 (d, J = 8.8 Hz, 1H) overlapped with 7.80−7.88 (m,
1H), 8.05 (d, J = 8.6, 2H), 8.54 (s, 1H); 13C NMR (50 MHz, CDCl3−
CF3COOD): δ = 11.7, 33.0, 47.2 (2C), 113.6, 121.2 (2C), 122.8 (q),
123.4, 129.7, 131.5, 132.9 (2C), 133.4 (q), 135.2 (q), 142.9 (q), 144.3
(q), 152.1, 172.8 (q); IR (CHCl3) νmax 3026, 3016, 1615, 1574, 1513,
1376, 1327, 1174; exact ESI full mass: found m/z 277.22 (calcd for
C19H21N2: M+, m/z 277.17); Anal. Calcd for C19H21BF4N2: C, 62.66;
H, 5.81; N, 7.69. Found C, 62.70; H, 5.79; N, 7.71.
(2,5-Dimethyl-3-pyrrolyl)(4-methoxyphenyl)methylium Tetra-
fluoroborate (3i). (0.63 g, 70% yield); dp 182 °C (anhydrous
MeCN/anhydrous Et2O); 1H NMR (200 MHz, CDCl3−CF3COOD):
δ = 2.29 (s, 3H), 2.72 (s, 3H), 3.96 (s, 3H), 6.69 (br s, 1H), 7.11 (d, J
= 9.0, 2H), 7.98 (d, J = 9.0, 2H), 8.19 (s, 1H); 13C NMR (50 MHz,
CDCl3−CF3COOD): δ = 11.4, 11.7, 55.6, 108.8, 116.1 (2C), 126.6
(q), 130.3 (q), 138.4 (2C), 141.6 (q), 158.5, 164.9 (q), 167.9 (q); IR
(CHCl3) νmax 3026, 3018, 1582, 1512, 1328, 1275, 1233, 1212, 1200;
exact ESI full mass: found m/z 214.15 (calcd for C14H16NO: M+, m/z
214.12); Anal. Calcd for C14H16BF4NO: C, 55.85; H, 5.36; N, 4.65;
Found C, 55.71; H, 5.33; N, 4.66.
(4-Nitrophenyl)(1,2-dimethyl-3-indolyl)methane (4g). Yellow
solid (0.27 g, 93% yield); mp 114.0−115.0 °C (DCM−PE); 1H
NMR (200 MHz, CDCl3): δ = 2.34 (s, 3H), 3.65 (3H), 4.14 (s, 2H),
6.95−7.08 (m, 2H), 7.10−7.28 (m, 2H), 7.29 (d, J = 8.8 Hz, 2H), 8.03
(d, J = 8.8 Hz, 2H); 13C NMR (50 MHz, CDCl3): δ = 10.2, 29.5, 30.2,
107.9 (q), 108.6, 117.7, 119.0, 120.8, 123.4 (2C), 127.3 (q), 128.7
(2C), 133.7 (q), 136.5 (q), 146.0 (q), 149.6 (q); IR (CHCl3) νmax
3468, 3029, 2943, 1597, 1521, 1474, 1347, 1068; MS m/z (%): 280
[M+](100), 158 (100); Anal. Calcd for C17H16N2O2: C, 72.84; H,
5.75; N, 9.99; Found C, 72.70; H, 5.77; N, 9.96.
(4-N,N-Dimethylaminophenyl)(1,2-dimethyl-3-indolyl)methane
(4h). Light pink solid (0.24 g, 86% yield); mp 128.0−129.0 °C
(DCM−PE); 1H NMR (200 MHz, CDCl3): δ = 2.36 (s, 3H), 2.87 (s,
6H), 3.64 (s, 3H), 4.02 (s, 2H), 6.65 (d, J = 8.6 Hz, 2H), 7.01−7.15
(m, 2H) overlapped with 7.10 (d, J = 8.8 Hz, 2H), 7.17−7.28 (m, 1H),
7.46 (d, J = 7.6 Hz, 1H); 13C NMR (50 MHz, CDCl3): δ = 10.2, 29.2,
29.4, 40.8 (2C), 108.3, 110.3 (q), 112.9 (2C), 118.3, 118.6, 120.3,
127.9 (q), 128.6 (2C), 130.2 (q), 133.1 (q), 136.5 (q), 148.8 (q); IR
(CHCl3) νmax 3467, 3029, 3022, 3011, 2943, 1614, 1519, 1473, 1236,
1198, 1066; MS m/z (%): 278 [M+](100), 263 (100); Anal. Calcd for
C19H22N2: C, 81.97; H, 7.97; N, 10.06; Found C, 81.78; H, 7.95; N,
10.02.
(2,5-Dimethyl-3-pyrrolyl)(3-indolyl)methylium Tetrafluoroborate
(3j). (0.85 g, 91% yield); dp 174 °C (anhydrous MeCN/anhydrous
1
Et2O); H NMR (200 MHz, CDCl3−CF3COOD): δ = 2.21 (s, 3H),
2.58 (s, 3H), 7.31−77.45 (m, 1H), 7.48−7.54 (m, 2H), 7.80−7.85 (m,
1H), 8.30 (s, 1H), 8.46 (s, 1H); 13C NMR (50 MHz, CDCl3−
CF3COOD): δ = 11.4, 11.7, 107.7, 113.8, 118.0 (q), 118.3 (q), 125.0
(q), 125.5 (2C), 126.3, 137.4 (q), 137.6 (q), 148.2 (q), 155.8 (q),
156.0 (q); IR (CHCl3) νmax 3025, 3014, 1580, 1334, 1295, 1225; exact
ESI full mass: found m/z 213.16 (calcd for C15H15N2: M+, m/z
223.12); Anal. Calcd for C15H15BF4N2: C, 58.10; H, 4.88; N, 9.03;
Found C, 58.15; H, 4.85; N, 9.00.
(2,5-Dimethyl-3-pyrrolyl)(4-methoxyphenyl)methane (4i). Oil
(0.10 g, 47% yield); 1H NMR (200 MHz, CDCl3): δ = 2.17 (s,
3H), 2.19 (s, 3H), 3.69 (s, 2H), 3.80 (s, 3H), 5.60−5.65 (m, 1H), 6.84
(d, J = 8.8 Hz, 2H), 7.15 (d, J = 8.8 Hz, 2H), 7.48 (br s, 1H); 13C
NMR (50 MHz, CDCl3): δ = 10.9, 12.9, 31.3, 55.1, 106.9, 113.6 (2C),
118.3 (q), 121.8 (q), 124.9 (q), 129.2 (2C), 134.9 (q), 160.4 (q); IR
(CHCl3) νmax 3443, 3020, 3006, 2935, 1611, 1511, 1465, 1248, 1201,
1177; MS m/z (%): 215 [M+](100), 200 (75), 108 (60). Anal. Calcd
(2,5-Dimethyl-3-pyrrolyl)(1-methyl-2-pyrrolyl)methylium Tetra-
fluoroborate (3k). (0.82 g, quantitative yield); dp 208 °C (anhydrous
MeCN/anhydrous Et2O); 1H NMR (200 MHz, CDCl3−CF3COOD):
δ = 2.21 (s, 3H), 2.56 (s, 3H), 3.88 (s, 3H), 6.63 (br s, 1H), 7.48−7.62
(m, 2H), 7.81 (s, 1H); 13C NMR (50 MHz, CDCl3−CF3COOD): δ =
11.2, 11.7, 34.4, 117.8, 123.6 (q), 129.2, 133.2 (q), 137.8 (2C), 138.2
E
DOI: 10.1021/acs.joc.5b00418
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
for C14H17NO: C, 78.10; H, 7.96; N, 6.51; Found C, 77.91; H, 7.95;
N, 6.49.
(6) Benfatti, F.; Capdevila, M. G.; Zoli, L.; Benedetto, E.; Cozzi, P. G.
Chem. Commun. 2009, 5919 and references therein.
(2,5-Dimethyl-3-pyrrolyl)(3-indolyl)methane (4j). Light brown
solid (0.20 g, 91% yield); mp 113.0−114.0 °C (DCM−PE); 1H
NMR (200 MHz, CDCl3): δ = 2.16 (s, 6H), 3.80 (s, 2H), 5.68 (br s,
1H), 6.84 (s, 1H), 7.03−7.19 (m, 2H), 7.25−7.35 (m, 2H), 7.61 (d, J
= 7.2 Hz, 1H), 7.76 (br s, 1H); 13C NMR (50 MHz, CDCl3): δ = 10.9,
12.8, 21.6, 107.1, 110.8, 117.1 (q), 117.9 (q), 118.9 (2C), 121.6 (2C),
124.7 (q), 127.4 (q), 133.7 (q), 136.3 (q); IR (CHCl3) νmax 3470,
3003, 2920, 1606, 1455, 1418, 1338, 1085; MS m/z (%): 224
[M+](100), 209 (65), 107 (60); Anal. Calcd for C15H16N2: C, 80.32;
H, 7.19; N, 12.49; Found C, 80.15; H, 7.16; N, 12.44.
(7) (a) Okajima, M.; Soga, K.; Nokami, T.; Suga, S.; Yoshida, J. Org.
Lett. 2006, 8, 5005. (b) Nokami, T.; Watanabe, T.; Musya, N.;
Suehiro, T.; Morofuji, T.; Yoshida, J. Tetrahedron 2011, 67, 4664 and
references therein.
(8) Maji, B.; Joannesse, C.; Nigst, T. A.; Smith, A. D.; Mayr, H. J. Org.
Chem. 2011, 76, 5104 and references therein.
(9) (a) Mayr, H.; Bug, T.; Gotta, M. F.; Hering, N.; Irrgang, B.;
Janker, B.; Kempf, B.; Loos, R.; Ofial, A. R.; Remennikov, G.;
Schimmel, H. J. Am. Chem. Soc. 2001, 123, 9500. (b) Minegishi, S.;
Kobayashi, S.; Mayr, H. J. Am. Chem. Soc. 2004, 126, 5174. (c) Kempf,
(2,5-Dimethyl-3-pyrrolyl)(1-methyl-2-pyrrolyl)methane (4k).
B. Ph.D. Dissertation, Ludwig-Maximilians-Universitat Munchen,
̈
̈
Light brown solid (0.16 g, 85% yield); mp 84.0−85.0 °C (DCM−
Munich, Germany, 2003. Kempf, B. Chem. Abs. 2003, 143, 305735.
(10) (a) Mayr, H.; Schneider, R.; Schade, C.; Bartl, J.; Bederke, R. J.
Am. Chem. Soc. 1990, 112, 4446. (b) Mayr, H.; Schneider, R.; Irrgang,
B.; Schade, C. J. Am. Chem. Soc. 1990, 112, 4454.
(11) Dilman, A. D.; Ioffe, S. L.; Mayr, H. J. Org. Chem. 2001, 66,
3196.
1
PE); H NMR (200 MHz, CDCl3): δ = 2.09 (s, 3H), 2.15 (s, 3H),
3.48 (s, 3H), 3.61 (s, 2H), 5.54−5.58 (m, 1H), 5.80−5.85 (m, 1H),
5.98−6.10 (m, 1H), 6.48−6.53 (m, 1H), 7.26 (br s, 1H); 13C NMR
(50 MHz, CDCl3): δ = 10.7, 12.8, 23.4, 33.5, 106.1, 106.4, 106.9, 116.1
(q), 120.8, 121.6 (q), 124.8 (q), 133.0 (q); IR (CHCl3) νmax 3468,
3032, 2998, 1575, 1494, 1436, 1068; MS m/z (%): 188 [M+](100),
173 (45), 107 (90); Anal. Calcd for C12H16N2: C, 76.55; H, 8.57; N,
14.88; Found C, 76.40; H, 8.54; N, 14.82.
(12) Takekuma, S.; Tamura, M.; Minematsu, T.; Takekuma, H.
Tetrahedron 2007, 63, 12058 and references therein.
(13) (a) Barbero, M.; Cadamuro, S.; Cauda, F.; Dughera, S.;
Gervasio, G.; Venturello, P. J. Org. Chem. 2012, 77, 4278.
(b) Armenise, N.; Dughera, S.; Gualandi, A.; Mengozzi, L.; Barbero,
M.; Cozzi, P. G. Asian J. Org. Chem. 2015, 4, 337.
(14) Barbero, M.; Crisma, M.; Degani, I.; Fochi, R.; Perracino, P.
Synthesis 1998, 1171.
(15) Sun, F.-L.; Zheng, X.-J.; Gu, Q.; He, Q.-L.; You, S.-L. Eur. J. Org.
Chem. 2010, 47.
(16) (a) Sinisi, M.; Vita, V.; Gualandi, A.; Emer, E.; Cozzi, P. G.
Chem.Eur. J. 2011, 17, 7404. (b) Gualandi, A.; Cozzi, P. G. Synlett
2013, 24, 281.
(3,5-Dimethyl-2-pyrrolyl)(3-indolyl)methane (4l). Light brown
1
solid (0.19 g, 86% yield); mp 84.0−85.0 °C (DCM−PE); H NMR
(200 MHz, CDCl3): δ = 2.07 (s, 6H), 3.96 (s, 2H), 5.66 (br s, 1H),
6.91 (s, 1H), 7.03−7.17 (m, 2H), 7.32 (d, J = 8.6 Hz, 1H), 7.49 (d, J =
7.6 Hz, 1H), 7.90 (br s, 1H); 13C NMR (50 MHz, CDCl3): δ = 10.8,
12.7, 21.4, 107.5, 110.9, 113.6 (q), 114.1 (q), 118.8, 119.4, 122.0,
122.1, 124.7 (q), 124.9 (q), 127.2 (q), 136.3 (q); IR (CHCl3) νmax
3480, 3018, 1576, 1418, 1222, 1069; MS m/z (%): 224 [M+](100),
209 (70), 117 (45; Anal. Calcd for C15H16N2: C, 80.32; H, 7.19; N,
12.49; Found C, 80.12; H, 7.20; N, 12.51.
(17) Duan, Y.; Chen, M.-W.; Ye, Z.-S.; Wang, D.-S.; Chen, Q.-A.;
Zhou, Y.-G. Chem.Eur. J. 2011, 17, 7193.
ASSOCIATED CONTENT
* Supporting Information
■
(18) (a) Sun, F.-L.; Zeng, M.; Gu, Q.; You, S.-L. Chem.Eur. J.
2009, 15, 8709. (b) Suarez, A.; Garcia-Gracia, P.; Fernandez-
Rodriguez, M. A.; Sanz, R. Adv. Synth. Catal. 2014, 356, 374.
(19) Tan, W.; Du, B.-X.; Li, X.; Zhu, X.; Shi, F.; Tu, S.-J. J. Org. Chem.
2014, 79, 4635.
S
1H and 13C NMR spectra for compounds 3a−l and 4a−l;
selective excitation spectra for salts 3c and 3f; tables of full
bond lengths and angles and CIF file for compound 3i; and
ORTEP and full refinement data of 3i. This material is available
free of charge via the Internet at http://pubs.acs.org.
(20) (a) Smith, G. F. Adv. Heterocycl. Chem. 1963, 2, 287. (b) Jones,
R. A. In Comprehensive Heterocyclic Chemistry; Katritzky, A. R., Rees, C.
W., Eds.; Pergamon Press: Oxford, U.K., 1984; Vol. 4, pp 201−312.
(21) Barbero, M.; Cadamuro, S.; Dughera, S.; Magistris, C.;
Venturello, P. Org. Biomol. Chem. 2011, 8393 and references therein.
(22) Shiri, M.; Zolfigol, M. A.; Kruger, H. G.; Tanbakouchian, Z.
Chem. Rev. 2010, 110, 2250.
(23) Guo, C.; Song, J.; Luo, S.-W.; Gong, L.-Z. Angew. Chem. Int. Ed.
2010, 49, 5558.
(24) Guo, Q.-X.; Peng, Y.-G.; Zhang, J.-W.; Song, L.; Feng, Z.; Gong,
L.-Z. Org. Lett. 2009, 11, 4620.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: margherita.barbero@unito.it. Fax: +39-011-6707642.
Tel: +39-011-6707645 (M.B.).
*E-mail: piergiorgio.cozzi@unibo.it. Fax: +39-051-2099456.
Tel: +39-051-2099511 (P.G.C.).
■
Notes
(25) He, L.; Sun, F.-L.; Zheng, X.-J.; You, S.-L. Synlett 2009, 1111.
(26) Duan, Y.; Chen, M.-W.; Ye, Z.-S.; Wang, D.-S.; Chen, Q.-A.;
Zhou, Y.-G. Chem.Eur. J. 2011, 17, 7193.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Italian MIUR and by the
■
(27) (a) Wang, L.; Chen, Y.; Xiao, J. Asian J. Org. Chem. 2014, 3,
1036. (b) Palmieri, A.; Petrini, M.; Shaikh, R. R. Org. Biomol. Chem.
2010, 8, 1259.
Universita
̀
di Torino.
(28) CrysAlisPro, Version 1.171.37.31; Agilent Technologies UK Ltd.:
Oxford, U.K., 2014.
(29) Sheldrick, M. SHELXS-2014/7; University of Gottingen:
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DOI: 10.1021/acs.joc.5b00418
J. Org. Chem. XXXX, XXX, XXX−XXX