Synthesis of 2-mesityl-3-methylpyrrole via the Trofimov reaction for a new BODIPY with hindered internal rotation

Article abstract of DOI:10.1016/j.tet.2005.01.065

The reaction of E-ethylmesitylketoxime with acetylene in the system KOH/DMSO (the Trofimov reaction) (70-74°C, 3 h, atmosphere pressure) affords 2-mesityl-3-methylpyrrole (23%), 2-mesityl-3-methyl-1-vinylpyrrole (8%), Z- (5%) and E- (2%) isomers of O-vinylethylmesitylketoxime. Initial ethylmesitylketoxime was prepared in two ways: via very slow oximation of ethylmesitylketone in 30% yield after 8 months, and, more efficiently, by oximation of ethylmesitylketimine hydrochloride derived from bromomesitylene in several steps. 2-Mesityl-3-methylpyrrole was used for the synthesis of 4,4-difluoro-2,6-dimethyl-3,5,8-trimesityl-4-bora-3a,4a-diaza-s-indacene with mesityl substituents having hindered internal rotation and preventing π-stacking at high concentrations. The latter factor enables the fluorescence of crystals of the prepared BODIPY, a feature that was not previously documented for such molecules.

Full text of DOI:10.1016/j.tet.2005.01.065

Tetrahedron 61 (2005) 2683–2688
Synthesis of 2-mesityl-3-methylpyrrole via the Trofimov reaction
for a new BODIPY with hindered internal rotation
a
Alexey B. Zaitsev, Rachel Meallet-Renault, Elena Yu. Schmidt, Al’bina I. Mikhaleva,
b
a
a,
*
´
b
b
a
a
´
´
Sophie Badre, Cecile Dumas, Alexander M. Vasil’tsov, Nadezhda V. Zorina
and Robert B. Pansub
aA. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1 Favorsky Str.,
Irkutsk 664033, Russian Federation
b
´ ´
Laboratoire de Photophysique et Photochimie Supramoleculaires et Macromoleculaires (CNRS UMR 8531),
´ ´
Ecole Normale Superieure de Cachan, 61 Avenue du President Wilson, 94235 Cachan cedex, France
Received 2 September 2004; revised 21 December 2004; accepted 13 January 2005
Abstract—The reaction of E-ethylmesitylketoxime with acetylene in the system KOH/DMSO (the Trofimov reaction) (70–74 8C, 3 h,
atmosphere pressure) affords 2-mesityl-3-methylpyrrole (23%), 2-mesityl-3-methyl-1-vinylpyrrole (8%), Z- (5%) and E- (2%) isomers of
O-vinylethylmesitylketoxime. Initial ethylmesitylketoxime was prepared in two ways: via very slow oximation of ethylmesitylketone in 30%
yield after 8 months, and, more efficiently, by oximation of ethylmesitylketimine hydrochloride derived from bromomesitylene in several
steps. 2-Mesityl-3-methylpyrrole was used for the synthesis of 4,4-difluoro-2,6-dimethyl-3,5,8-trimesityl-4-bora-3a,4a-diaza-s-indacene
with mesityl substituents having hindered internal rotation and preventing p-stacking at high concentrations. The latter factor enables the
fluorescence of crystals of the prepared BODIPY, a feature that was not previously documented for such molecules.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Herein, we describe a synthesis of the first representatives of
boradiazaindacenes, which are fluorescent in a crystalline
form. The main idea was to develop an approach to
structures with bulky mesityl substituents attached to the
BODIPY core, distorting overall planarity and thus
hampering the p-stacking at high concentrations. Further-
more, the hindered internal rotation of mesityl rings reduces
non-radiative relaxation of excited states, hence decreasing
fluorescence quantum yields.7
For more than 30 years since their discovery1 BODIPY (4,4-
difluoro-4-bora-3a,4a-diaza-s-indacene) derivatives have
enjoyed wide popularity among experts in chemistry,2
physics,3 biology4 and related fields5 for their exceptional
optical properties.
During the practical realization of various devices based on
BODIPY dyes, when high concentrations are needed,
intermolecular p-stacking can cancel out their advantages
such as high fluorescence quantum yields and intensities.6
To the best of our knowledge, so far, no fluorescent crystals
of BODIPY have been reported, although the pigments
(solid dyes) based on them, due to their anticipated high
brightness and enhanced resistance to photobleaching,
might find extensive application. Therefore, the design of
new boradiazaindacenes with hindered p-stacking, which is
considered as the principal culprit behind the loss of
fluorescence in crystalline form, has remained a challenge.
2. Results and discussion
As a synthetic target 4,4-difluoro-2,6-dimethyl-3,5,8-tri-
mesityl-4-bora-3a,4a-diaza-s-indacene 1 was chosen
because its 2,6-methyl groups could additionally hinder
internal rotation and force benzene rings out of the
molecular plane more efficiently.
The disconnection of 1 (reverse to the synthesis of BODIPY
via the reaction of boron trifluoride etherate with dipyrro-
methenes derived from pyrroles and aldehydes8) leads to
dipyrromethene 2 and then to 2-mesityl-3-methylpyrrole 3
and 2,4,6-trimethylbenzaldehyde 4 (Scheme 1).
Keywords: 2-Mesitylpyrroles; Oximes; Acetylene; Superbases; BODIPY;
Hindered internal rotation; Fluorophores; p-Stacking.
* Corresponding author. Tel.: C7 3952 42 24 09/42 58 71; fax: C7 3952
41 93 46; e-mail: mikh@irioch.irk.ru
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.01.065

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A. B. Zaitsev et al. / Tetrahedron 61 (2005) 2683–2688
Scheme 1.
To accomplish this synthetic sequence, we had to develop
an approach to pyrrole 3. The Trofimov reaction (reaction of
ketoximes with acetylene in superbasic media affording
pyrroles and N-vinyl pyrroles)9,10 might be useful for
approaching 3 from ethylmesitylketoxime 6. However, until
the present work there were no examples of a successful
application of this reaction to highly hindered ketoximes
and this exploration could shed light on whether such
ketoximes are capable of affording pyrroles upon reaction
with acetylene.
To develop a better access to the ketoxime 6 we, there-
fore, attempted the oximation of ethylmesitylimine
hydrochloride 8 similar to that used previously for the
synthesis of some other mesityl oximes.13 This pro-
cedure, though multistep, allowed us to approach 6 from
bromomesitylene 7 in 39% yield in a matter of 2 days
(Scheme 3).
The reaction of the E-isomer of oxime 6 with acetylene in
the KOH/DMSO superbasic system (70–74 8C, 3 h, atmo-
spheric pressure) gave 2-mesityl-3-methylpyrrole 3 (23%),
2-mesityl-3-methyl-1-vinylpyrrole 9 (8%), Z- (5%) and
E- (2%) isomers of O-vinylethylmesitylketoxime 10
(Scheme 4).
Meanwhile, at the very beginning of the study we
encountered a serious problem of very low reactivity of
ethylmesitylketone 5 towards hydroxylamine. Oximation by
refluxing mixtures of ketones and hydroxylamine hydro-
chloride in pyridine, effective in the synthesis of some
hindered oximes,11 proved to be inefficient in the case of 5
(even when microwave activation was applied) and yielded
only trace amounts of 6. The ‘lethargic’ oximation, although
it gave mesitylmethylketoxime from mesitylmethylketone
in 98% yield in 28 days,12 after application to 5 afforded its
oxime 6 (mostly E-isomer) only in 30% yield after 8 months
(Scheme 2).
A short contact (70 8C, 5 min) of the E-isomer of 6 with
acetylene in the KOH/DMSO system under increased
acetylene pressure (initial pressure 17 atm) led to O-vinyl-
ketoxime 10 (23%) (E–Zw1:2) and pyrrole 3 (12%) (1H
NMR).
Interestingly, the E-isomer of oxime 6 was transformed
mainly to the Z-isomer of O-vinylketoxime 10. This is a
striking contrast to, for example, methylphenylketoxime,
which under similar conditions gave only the E-isomer of
O-vinylmethylphenylketoxime.14 Heating of the E-isomer
of the oxime 6 at 80 8C for 1 h in KOH/DMSO system
resulted in the formation of 1:1 mixture of its E- and
Z-isomers. Thus, the E–Z isomerization of 6 occurs under
the reaction conditions, and steric hindrances imparted on
the oxime hydroxyl by both mesityl and ethyl groups in 6
are similar. The specific behaviour of the oxime 6 can be
rationalized assuming twisting of mesityl ring out of the
C–C]N–O plane. When twisted, it strongly hinders the
C]N (C]O) carbon (as evident from the difficulty of
oximation of mesityl ketones) and, on the other hand, makes
Scheme 2.
It is obvious that the mesityl group blocks the carbonyl
carbon in ketone 5, and the blockage becomes much more
pronounced on passing from mesitylmethylketone to
ethylmesitylketone 5.
Scheme 3.

A. B. Zaitsev et al. / Tetrahedron 61 (2005) 2683–2688
2685
Scheme 4.
easier the approach of acetylene to the oximate anionic
centre generated under superbasic conditions.
in their E-isomers (syn disposition to oxime oxygen) are
shifted downfield (2.65 and 2.70 ppm for oxime and
O-vinyloxime, respectively) relative to their Z-isomers
(2.42 and 2.46 for oxime and O-vinyloxime, respectively).
As compared to other N-vinylpyrroles, the resonance of
HX proton in 9 residing in the shielding cone of the
twisted mesityl aromatic ring is shifted considerably upfield
(6.27 ppm).
The twisted mesityl group might also pre-organize acetyl-
ene via either p-stacking or p-hydrogen bonding (Fig. 1),15
thus making formation of Z-isomer of O-vinylketoxime 10
entropicaly more favourable.
Pyrrole 3 was introduced into the reaction with 2,4,6-tri-
methylbenzaldehyde 4 catalysed by trifluoroacetic acid
(TFA). Generated dipyrromethane 13 was further oxidized
in situ with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) to dipyrromethene 2. The reaction of 2 with boron
trifluoride etherate in the presence of diisopropylethylamine
afforded the target BODIPY 1 in 8% yield.
Figure 1.
Heating of a DMSO-d6 solution of O-vinylketoxime 10 at
120 8C (5 min) leads to its rearrangement to the pyrrole 3
(yield w50%, 1H NMR) in the absence of a base, similar to
the rearrangement of 5-(1-vinyloxyiminoethyl)[2,2]para-
cyclophane to the appropriate pyrrole.16
The isolation of the O-vinylketoxime 10 from the reac-
tion mixture is remarkable because so far only O-vinyl
(3-indolyl)ketoximes with an a-methylene group (different
from that constituting methyl) have been known to be stable
under the reaction conditions.17 Presumably, the stability of
the O-vinylketoximes with a-methylene group is due to
either increased electron donation of R1 substituent (e.g.,
3-indolyl) or a steric effect inhibiting the [1,3] hydrogen
shift (Scheme 5) assumed as a key step of the transformation
of O-vinylketoximes to pyrroles.10
Interestingly, the bulky mesityl substituent in aldehyde 4 did
not block its reaction with pyrrole 3, although the reaction
actually involves attack of secondary 2-pyrrolic nucleophile
at the hindered carbonyl carbon in 4.
Thus, the twisted mesityl group, due to its size, may
interfere with the [1,3] hydrogen shift in 10 leading to
O-vinylhydroxylamine 11 (R1ZMes, R2ZMe), as well as
[3,3]-sigmatropic rearrangement of the latter to iminoalde-
hyde 12 by impeding redistribution of electron density
during the rearrangement.
The optical absorption of the BODIPY 1, dissolved in
spectrometric grade dichloromethane (DCM), had the S0–S1
transition maximum at 543 nm with an extinction coef-
ficient 40,150 LMK1 cmK1. The emission maximum was
observed at 559 nm with a fluorescence quantum yield 95%
(Rhodamine in ethanol was used as a reference). The
fluorescence lifetime was 8.5 ns, which is longer than any
The distinction between the E- and Z-isomers of oxime 6
and O-vinyloxime 10 was made based on the difference in
1H NMR shifts of their methylene protons due to the
anisotropic influence of the oxime oxygen.18 These protons
Scheme 5.

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A. B. Zaitsev et al. / Tetrahedron 61 (2005) 2683–2688
reported lifetime for such molecules. Submicrometric
crystals of 1 obtained after evaporation of DCM exhibited
fluorescence with lifetime of 1.5 ns that is in striking
contrast to previously known boradiazaindacenes for which
fluorescence in crystals had not been reported. The use of
1,2-dichlorobenzene as a solvent gave slower evaporation
and smaller crystallized fluorescent particles with longer
lifetimes (3–4 ns). The detailed description of the study of
the fluorescent properties of the synthesized BODIPY 1 will
be described elsewhere.
solution of mesityl magnesium bromide prepared from
magnesium turnings (5.00 g, 205.8 mmol) and bromo-
mesitylene 7 (30.00 g, 150.7 mmol),20 a mixture of
propionitrile (8.25 g, 149.8 mmol) and diethyl ether
(10 mL) was added over 30 min and the obtained white
suspension was additionally stirred for 1 h at room
temperature. After that a 13% solution of hydrochloric
acid (100 mL) was added for 1 h to the stirred reaction
mixture cooled in a water bath with subsequent stirring for
1 h. The acid was neutralized with a solution of NaOH
(14.00 g, 350.1 mmol) in water (30 mL), and from the
resultant mixture ethylmesitylketone imine was extracted
with diethyl ether (5!40 mL). The combined extracts were
dried over K2CO3 and evaporated to the volume of 70 mL.
Dry hydrogen chloride was passed through the etheral
solution to precipitate the ketimine hydrochloride 8 which
was collected by filtration and dried. As a result, 8 (15.01 g,
47%) was obtained as a yellowish solid; mp 194–196 8C; 1H
NMR d (ppm) 13.73 (broad s, 1H, NHE), 13.36 (broad s, 1H,
3. Conclusion
An approach to a new BODIPY with bulky mesityl
substituents preventing the molecule from p-stacking at
high concentrations and possessing hindered internal
rotation decreasing non-radiative relaxation of excited
states was developed. The precursor of the boradiazainda-
cene 1, 2-mesityl-3-methylpyrrole 3, was prepared via the
Trofimov reaction. It was demonstrated that the mesityl
group imposes less steric hindrance on the nucleophilic
addition of ethylmesitylketoxime to acetylene than phenyl
group. However, the presence of mesityl in O-vinyl-
ethylmesitylketoxime inhibits its rearrangement to appro-
priate pyrrole. This was not the case for most known
O-vinylketoximes with a-methylene groups.
3
NHZ), 6.90 (s, 2H, Hm), 3.09 (q, 2H, JH1H2(Et)Z7.6 Hz,
3
CH2Me), 2.27 (s, 9H, Meo, Mep), 1.32 (t, 3H, JH1H2(Et)
Z
7.6 Hz, CH2Me); 13C NMR d (ppm) 196.3 (C]N), 141.3
(Ci), 133.7 (Co, Cp), 129.4 (Cm), 32.6 (CH2Me), 21.2 (Mep),
19.9 (Meo), 10.4 (CH2Me); IR (cmK1, KBr) 3372, 3250-
2250, 2017, 1687, 1611, 1556, 1456, 1382, 1297, 1187,
1120, 1020, 909, 854, 717.
4.1.3. Ethylmesitylketoxime (6). This procedure represents
a modified protocol.13 A mixture of 8 (10.00 g, 47.2 mmol),
NH2OH$HCl (6.56 g, 94.5 mmol), CH3COONa (9.68 g,
118.0 mmol) and 96% ethanol (160 mL) was refluxed for
8 h. After cooling, K2CO3 (10.00 g, 72.4 mmol) and water
(5 mL) were added to the solution with subsequent stirring
for 1 h to neutralize nascent acetic acid. Then diethyl ether
(200 mL) was added; the precipitate was filtered off and
washed with diethyl ether. The organic solutions were
combined and evaporated to dryness in vacuo to give
viscous amber coloured liquid (7.37 g, 82%) consisting of
the E- and Z-isomers of ethylmesitylketoxime 6 (E–Zw3:1)
(1H NMR) and crystallizing in several hours. The E-isomer
of 6 was isolated by column chromatography (basic Al2O3,
petroleum ether–diethyl ether).
4. Experimental
4.1. General
1H and 13C NMR spectra were recorded on a Bruker DPX
400 (400.13 and 100.61 MHz, respectively) and a Bruker
ultrashield 300 AC (300.1 and 75.4 MHz, respectively)
instruments in CDCl3 using HMDS as an internal standard.
IR spectra were recorded on a Bruker IFS 25 instrument.
Absorption spectra were obtained using a UV–vis Varian
CARY 500 spectrophotometer. Steady-state excitation and
emission spectra were measured on a SPEX Fluorolog-3
(Jobin-Yvon). A right-angle configuration was used. Optical
density of the samples was less than 0.1 to avoid
reabsorption artefacts. The fluorescence decay curves were
obtained with a time-correlated single-photon-counting
method using a titanium-sapphire laser pumped by an
argon ion laser. The Levenberg–Marquardt algorithm was
used for non-linear least squares fits.
4.1.4. E-isomer of 6. White crystals, mp 80–82 8C; 1H
NMR d (ppm) 9.0 (broad s, 1H, OH), 6.83 (s, 2H, Hm), 2.65
(q, 2H, 3JH1H2(Et)Z7.6 Hz, CH2Me), 2.26 (s, 3H, Mep), 2.18
(s, 6H, Meo), 0.93 (t, 3H, 3JH1H2(Et)Z7.6 Hz, CH2Me); 13
C
NMR d (ppm) 162.0 (C]N), 137.6 (Ci), 136.2 (Co), 132.8
(Cp), 128.3 (Cm), 22.8 (CH2Me), 21.1 (Mep), 19.8 (Meo), 9.7
(CH2Me); IR (cmK1, KBr) 3240, 2968, 2920, 1649, 1612,
1574, 1488, 1458, 1374, 1321, 1302, 1283, 1171, 1071,
1038, 972, 955, 892, 851, 764, 744, 726, 668, 592, 579.
4.1.1. Ethylmesitylketone (5). The ketone 5 was prepared
in 84% yield according to the procedure.19 Transparent
colourless liquid; bp 116–121 8C (5 mm); n2D2Z1.5093; H
1
3
NMR d (ppm) 6.80 (s, 2H, Hm), 2.67 (q, 2H, JH1H2(Et)
Z
7.4 Hz, CH2Me), 2.25 (s, 3H, Mep), 2.15 (s, 6H, Meo), 1.17
(t, 3H, CH2Me); 13C NMR d (ppm) 211.4 (C]O), 140.0
(Ci), 138.2 (Co), 132.5 (Cp), 128.5 (Cm), 38.0 (CH2Me), 21.1
(Mep), 19.1 (Meo), 7.7 (CH2Me); IR (cmK1, film) 2975,
2936, 2878, 2735, 1700, 1611, 1575, 1457, 1413, 1378,
1342, 1297, 1265, 1224, 1156, 1077, 1036, 1014, 961, 930,
851, 801, 740, 723, 592.
1
4.1.5. Z-isomer of 6. White crystals. H NMR d (ppm) 8.7
(broad s, 1H, OH), 6.86 (s, 2H, Hm), 2.43 (q, 2H,
3JH1H2(Et)Z7.3 Hz, CH2Me), 2.26 (s, 3H, Mep), 2.15 (s,
6H, Meo), 1.15 (t, 3H, CH2Me); 13C NMR d (ppm) 160.3
(C]N), 137.8 (Ci), 134.1 (Co), 132.1 (Cp), 128.2 (Cm), 28.6
(CH2Me), 21.1 (Mep), 19.5 (Meo), 10.3 (CH2Me)
4.1.2. Ethylmesitylketimine hydrochloride (8). This
The ‘lethargic’ oximation of 5 was carried out similarly to
protocol represents a modified method.13 To a stirred
mesitylmethylketone.12 However, 8 months were required

A. B. Zaitsev et al. / Tetrahedron 61 (2005) 2683–2688
2687
2
3
for the reaction to afford the oxime 6 (mainly E-isomer) in
only 30% preparative yield.
14.1 Hz, JABZ1.3 Hz, HB), 4.01 (dd, 1H, JAXZ6.7 Hz,
2JABZ1.3 Hz, HA), 2.46 (q, 2H, JH1H2(Et)Z7.5 Hz,
CH2Me), 2.27 (s, 3H, Mep), 2.13 (s, 6H, Meo), 1.15 (t,
3
3H, JH1H2(Et)Z7.5 Hz, CH2Me); 13C NMR d (ppm) 163.0
3
4.2. 2-Mesityl-3-methylpyrrole (3), 2-mesityl-3-methyl-
1-vinylpyrrole (9) and O-vinylethylmesitylketoxime (10)
(C]N), 152.5 (Ca), 137.7 (Cp), 133.7 (Co), 132.4 (Ci),
128.1 (Cm), 87.5 (Cb), 28.8 (CH2), 21.1 (Mep), 19.5 (Meo),
10.4 (CH2Me); IR (cmK1, film) 2975, 2922, 1643, 1620,
1576, 1459, 1380, 1306, 1187, 1152, 1088, 1055, 981, 949,
933, 873, 850, 833, 693, 604. Anal. calcd for C14H19NO: C
77.38, H 8.81, N 6.45; found: C 77.51, H 8.92, N 6.80.
A mixture of ethylmesitylketoxime 6 (1.00 g, 5.2 mmol),
fine-powdered KOH$0.5H2O (0.34 g, 5.2 mmol) and
DMSO (15 mL) was stirred under acetylene atmosphere at
70–74 8C for 3 h. After cooling to room temperature the
mixture was diluted with water (10 mL) and extracted with
diethyl ether (4!10 mL). The ether extracts were washed
with water (4!5 mL) and dried over anhydrous K2CO3.
The residue obtained after distilling off the solvent was
chromatographed on column (basic Al2O3, petroleum ether–
diethyl ether) to yield 2-mesityl-3-methylpyrrole 3 (23%),
2-mesityl-3-methyl-1-vinylpyrrole 9 (8%), Z- (5%) and E-
(2%) isomers of 10.
1
4.2.4. E-isomer of 10. A transparent colourless liquid; H
NMR d (ppm) 6.98 (dd, 1H, 3JAXZ6.8 Hz, 3JBXZ14.2 Hz,
HX), 6.86 (s, 2H, Hm), 4.58 (dd, 1H, 3JBXZ14.2 Hz, 2JAB
Z
3
2
1.4 Hz, HB), 4.09 (dd, 1H, JAXZ6.8 Hz, JABZ1.4 Hz,
HA), 2.70 (q, 2H, 3JH1H2(Et)Z7.6 Hz, CH2Me), 2.26 (s, 3H,
Mep), 2.20 (s, 6H, Meo), 0.96 (s, 3H, CH2Me); 13C NMR d
(ppm) 164.6 (C]N), 153.1 (Ca), 138.1 (Cp), 136.0 (Co),
132.0 (Ci), 128.5 (Cm), 87.4 (Cb), 24.1 (CH2Me), 21.1
(Mep), 19.9 (Meo), 9.9 (CH2Me).
The reaction of oxime 6 with acetylene under high pressure
was carried out as follows: ethylmesitylketoxime 6 (0.50 g,
2.6 mmol), KOH$0.5H2O (0.34 g, 5.2 mmol) and DMSO
(20 mL) were charged in a 250-mL steel rotating autoclave,
saturated with acetylene at room temperature (initial
acetylene pressure 17 atm), heated to 70 8C and kept at
this temperature for 5 min. After cooling and discharge, the
reaction mixture was diluted with 40 mL of water and
extracted with diethyl ether (5!10 mL). The extracts were
washed with water (4!5 mL) and dried over K2CO3. After
evaporation of the ether, a red liquid (0.50 g) containing (1H
NMR) pyrrole 3 (12%) and O-vinylketoxime 10 (E–Zw1:2)
(23%) was obtained.
4.3. Thermal E–Z isomerisation of ethylmesitylketoxime
(6)
A mixture of E-isomer of 6 (0.05 g, 0.3 mmol), fine-
powdered KOH$0.5H2O (0.05 g, 0.8 mmol) and DMSO
(5 mL) was stirred at 80 8C for 1 h. After cooling, the
mixture was neutralized with dry ice, diluted with water
(5 mL) and extracted with diethyl ether (3!10 mL). The
ether extracts were washed with water (3!3 mL) and dried
over K2CO3. After evaporation of the ether, a viscous mass
(0.04 g) consisting of E- and Z-isomers of 6 (E:Zw1:1) (1H
NMR) was obtained.
4.2.1. Compound 3. White crystals, mp 74–76 8C; 1H NMR
d (ppm) 7.65 (broad s, 1H, NH), 6.91 (s, 2H, Hm), 6.73 (t,
1H, JH1H4H5Z2.7 Hz, H5), 6.12 (t, 1H, JH1H4H5Z2.7 Hz,
H4), 2.30 (s, 3H, Mep), 2.03 (s, 6H, Meo), 1.86 (s, 3H,
MePyr); 13C NMR d (ppm) 139.2 (Cp), 137.7 (Co), 130.1
(Ci), 127.9 (Cm), 126.8 (C2), 116.2 (C3), 115.9 (C5), 109.8
(C4), 21.1 (Mep), 20.1 (Meo), 11.2 (MePyr); IR (cmK1, KBr)
3423, 2917, 2868, 1640, 1610, 1582, 1569, 1539, 1509,
1488, 1467, 1452, 1375, 1278, 1249, 1157, 1099, 1081,
1063, 1052, 1000, 897, 852, 821, 745, 719, 689, 631, 570,
554, 531, 494; EIMS: m/z calculated for C14H17N: 199.3,
found: 199 (MC).
4.4. Thermal rearrangement of Z-isomer of O-vinyl-
ethylmesitylketoxime (10) into 2-mesityl-3-
methylpyrrole (3)
An NMR ampule containing a solution of Z-isomer of 10
(w0.05 g) in DMSO-d6 (0.5 mL) was heated in the NMR
spectrometer at 120 8C for 10 min and kept at this
temperature until disappearance of the signals of the vinyl
group of 10 (5 min). During the heating the intensity of
the O-vinyl group signals decreased with a simultaneous
increase in the intensity of the signals of pyrrole 3. In
1
addition to signals of 3 (yield w50%, H NMR) heating
4.2.2. Compound 9. A transparent colourless liquid, n1D9Z
1.6815; 1H NMR d (ppm) 7.02 (d, 1H, 3JH4H5Z3.0 Hz, H5),
gave rise to unidentified singlets at 9.72 and 7.06 ppm with
half intensities of the pyrrole ring protons in 3 and singlet at
2.44 ppm with the threefold intensity of the latter.
3
3
6.91 (s, 2H, Hm), 6.27 (dd, 1H, HX, JAXZ9.0 Hz, JBX
Z
3
15.8 Hz), 6.15 (d, 1H, JH4H5Z3.0 Hz, H4), 4.90 (d, 1H,
3JBXZ15.8 Hz, HB), 4.33 (d, 1H, JAXZ9.0 Hz, HA), 2.31
3
4.4.1. 4,4-Difluoro-2,6-dimethyl-3,5,8-trimesityl-4-bora-
3a,4a-diaza-s-indacene (1). Pyrrole 3 (0.35 g, 1.8 mmol)
and 2,4,6-trimethylbenzaldehyde 4 (0.13 g, 0.9 mmol) were
dissolved in degassed CH2Cl2 (50 mL), then two drops of
TFA were added, and the obtained orange solution was
stirred for 24 h under argon at room temperature. TLC
analysis (SiO2 on aluminium plates, CH2Cl2–petroleum
etherZ1:1) of the reaction mixture showed the presence of
initial pyrrole 3, so an additional 0.025 g of aldehyde 4 and
two drops of TFA were added and stirring was continued
for some time until TLC showed no stain of pyrrole 3.
Then DDQ (0.19 g, 0.8 mmol) was added and the reaction
mixturewasstirredfor0.5 h. Afterthat, diisopropylethylamine
(s, 3H, Mep), 1.95 (s, 6H, Meo), 1.79 (s, 3H, MePyr); 13C
NMR d (ppm) 139.7 (Cp), 138.1 (Co), 131.2 (Ca), 128.9 (Ci),
128.1 (C2), 127.9 (Cm), 117.4 (C5), 115.0 (C3), 111.5 (C4),
95.2 (Cb), 22.8 (Mep), 21.2 (Meo), 14.2 (MePyr); IR (cmK1
,
film) 2922, 2856, 1640, 1613, 1582, 1491, 1472, 1419,
1388, 1370, 1305, 1229, 1201, 1160, 1073, 1034, 1015, 989,
966, 851, 761, 723, 693, 677, 593. EIMS: m/z calculated for
C16H19N: 225.3, found: 225 (MC).
4.2.3. Z-isomer of 10. A transparent colourless liquid, n1D9Z
1.5218; H NMR d (ppm) 6.86 (s, 2H, Hm), 6.80 (dd, 1H,
1
3
3
3JAXZ6.7 Hz, JBXZ14.1 Hz, HX), 4.52 (dd, 1H, JBX
Z

2688
A. B. Zaitsev et al. / Tetrahedron 61 (2005) 2683–2688
6. Shirai, K.; Matsuoka, M.; Fukunishi, K. Dyes Pigm. 1999, 42,
95–101.
´
7. (a) Meallet-Renault, R.; Pansu, R. B.; Amigoni-Gerbier, S.;
(2 mL, 1.48 g, 11.5 mmol) and boron trifluoride etherate
(2 mL, 2.24 g, 15.8 mmol) were added with subsequent
stirring of the obtained fluorescent solution for 0.5 h. After
evaporation of the solvent at ambient temperature in vacuo
the residue (was flash chromatographed under nitrogen on
silica (CH2Cl2–petroleum etherZ1:3) to afford 0.04 g (8%)
of BODIPY 1. A red powder, it does not melt but
Larpent, C. Chem. Commun. 2004, 2344–2345. (b) Chen, J.;
Burghart, A.; Derecsekei-Kovacs, A.; Burgess, K. J. Org.
Chem. 2000, 65, 2900–2906.
8. Montalban, A. G.; Herrera, A. J.; Johannsen, J.; Beck, J.;
Godet, T.; Vrettou, M.; White, A. J. P.; Williams, D. J.
Tetrahedron Lett. 2002, 43, 1751–1753.
1
decomposes above 280–300 8C; H NMR d (ppm) 7.00 (s,
00
2H, H3 ), 6.84 (s, 4H, H3 ), 6.42 (s, 2H, H1,7), 2.40 (s, 3H,
0
00
00
0
0
Me4 ), 2.26 (s, 6H, Me2 or Me4 ), 2.24 (s, 6H, Me4 or
9. (a) Gossauer, A. In Methoden der Organischen Chemie.
Hetarene I. Teil 1. Pyrrole. Georg Thieme: Stuttgart, 1994; pp
556–798. (b) Trofimov, B. A. In Advances in Heterocyclic
Chemistry; Katritzky, A. R., Ed.; Preparation of Pyrroles from
Ketoximes and Acetylenes; Academic: San Diego, 1990; Vol.
51, pp 177–301. (c) Trofimov, B. A. The chemistry of
heterocyclic compounds. Part 2. Pyrroles. In Jones, R. A., Ed.;
Vinylpyrroles; Wiley: New York, 1992; Vol. 51, pp 131–298.
(d) Tedeschi, R. J. In Encyclopedia of Physical Science and
Technology. Acetylene; Academic: San Diego, 1992; Vol. 1,
pp 27–65. (e) Mikhaleva, A. I.; Schmidt, E. Selected methods
for synthesis and modification of heterocycles. In Kartsev,
V. G., Ed.; Two-step Synthesis of Pyrroles from Ketones and
Acetylenes through the Trofimov Reaction; IBS: Moscow,
2002; Vol. 51, pp 331–352.
Me2 ), 2.06 (s, 12H, Me2 ), 1.65 (s, 6H, Me2,6); 13C NMR d
(ppm) 157.8, 141.6, 138.3, 137.4 (CHarom), 136.8, 134.2,
131.1, 129.0, 128.5, 128.1 (CHarom), 127.8 (CHarom), 127.0,
00
0
00
29.8 (Me4 ), 21.4 (Me4 ), 20.1 (Me2 or Me2 ), 20.0 (Me2
0
0
00
00
0
or Me2 ), 11.0 (Me2,6). EIMS: m/z calculated for
C38H41BF2N2: 574.6, found: 526 [MKBF2]C.
Acknowledgements
The authors are grateful to the Russian Foundation for Basic
Research (grants No. 03-03-32472 and No. 2241.2003.3),
the Presidium of the Russian Academy of Sciences (grant
No. 10002-251/II-25/155-305/200404-072) and the Centre
National de la Recherche Scientifique for financial support.
10. Trofimov, B. A.; Mikhaleva, A. I. N-Vinylpyrroles; Nauka:
Novosibirsk, 1984; p 264 (in Russian).
11. Bachmann, W. E.; Boatner, C. H. J. Am. Chem. Soc. 1936, 58,
2097–2101.
12. Pearson, D. E.; Keaton, O. D. J. Org. Chem. 1963, 68,
1557–1558.
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