2,5-Dimethyl-3,6-bis[(2,6-diisopropylphenylimino)methyl]- and 2,6-bis[(2,6-diisopropylphenylimino)methyl]pyrazine: Two new chelating ligands for transition metal complexes

Article abstract of DOI:10.1515/znb-2005-0104

The reaction of tetramethylpyrazine with SeO₂ yields 2,5-dimethylpyrazine-3,6-dicarboxaldehyde (1) from which the 2,5-dimethyl-3,6-bis[(2,6-diisopropylphenylimino)methyl]pyrazine (2) was synthesized by treatment with 2 equivalents of 2,6-diisop

Full text of DOI:10.1515/znb-2005-0104

2,5-Dimethyl-3,6-bis[(2,6-diisopropylphenylimino)methyl]- and
2,6-Bis[(2,6-diisopropylphenylimino)methyl]pyrazine:
Two New Chelating Ligands for Transition Metal Complexes
Herbert Schumann and He-Kuan Luo
Institut fu¨r Chemie, Technische Universita¨t Berlin, Straße des 17. Juni 135, D-10623 Berlin,
Germany
Reprint requests to H. Schumann. E-mail: Schumann@chem.tu-berlin.de
Z. Naturforsch. 60b, 22 – 24 (2005); received August 26, 2004
The reaction of tetramethylpyrazine with SeO2 yields 2,5-dimethylpyrazine-3,6-dicarboxaldehyde
(1) from which the 2,5-dimethyl-3,6-bis[(2,6-diisopropylphenylimino)methyl]pyrazine (2) was syn-
thesized by treatment with 2 equivalents of 2,6-diisopropylaniline. 2,6-Dimethylpyrazine reacts
with benzaldehyde to give 2,6-distyrylpyrazine (3). Ozonolysis of 3, followed by treatment with
Na2SO3 and 2,6-diisopropylaniline resulted in the formation of 2,6-bis[(2,6-diisopropylphenyl-
imino)methyl]pyrazine (5) together with [(2,6-diisopropylphenylimino)methyl]benzene (6).
Key words: Diimine, Dialdehyde, Pyrazine, Chelating Ligands
Introduction
in the formation of 2,5-dimethylpyrazine-3,6-dicarb-
oxaldehyde (1) in 26% yield as a yellow pow-
der. This product reacts with 2 equivalents of 2,6-
diisopropylaniline yielding yellow 2,5-dimethyl-3,6-
bis[(2,6-diisopropylphenylimino)methyl]pyrazine (2)
in 70% yield (Scheme 1).
Nitrogen containing organic compounds, in partic-
ular pyridine and/or imine derivatives, are very im-
portant ligands widely used in various organometallic
complexes for olefin oligomerization and polymeriza-
tion [1]. In general, the performance of the catalysts
is regulated by the structure of the ligands. Therefore
the synthesis of new ligands is one of the keys for the
development of new catalysts.
First attempts to prepare 2,6-bis[(2,6-diisopropyl-
phenylimino)methyl]pyrazinein analogy to 2 via SeO2
oxidation of 2,6-dimethylpyrazine failed. The reac-
◦
tion temperature of more than 100 C does not allow
Usually, imine containing ligands are prepared via
condensation of carbonyl compounds with aniline
derivatives bearing bulky substituents [1]. Because of
the smoothly proceeding condensation of the carbonyl
group with aniline under mild conditions [2], the key
to the preparation of such ligands is the synthesis of
starting materials containing suitable carbonyl groups.
Widely used methods to prepare aldehydes are
the oxidation of α-methyl groups or of unsaturated
compounds by SeO2 [3], O3 [4], and other oxi-
dants [5]. In this paper we describe the synthesis of
two new chelating ligand systems derived from 2,5-
dimethylpyrazine-3,6-dicarboxaldehyde and pyrazine-
3,6-dicarboxaldehyde.
the isolation of pyrazine-2,6-dicarboxaldehyde, which
decomposes quickly after generation. Therefore we
chose another route which can be used for the synthe-
sis of thermally and oxidatively labile aldehydes, vic.
the oxidation of olefins with ozone, the reduction of the
resulting ozonides with Na2SO3, followed by conden-
sation with 2,6-diisopropylaniline [4].
Using a modification of the method successfully
employed for the synthesis of 2,5-distyrylpyrazine
[4b, 6], 2,6-distyrylpyrazine (3) was prepared in high
yield by condensation of 2,6-dimethylpyrazine with
benzaldehyde (Scheme 2).
◦
Treatment of 3 with ozone at −50 C, followed
by reduction of the ozonide formed with Na2SO3
◦
in methanol at −50 C and condensation of the di-
Results and Discussion
aldehyde 4 with 2,6-diisopropylaniline at room tem-
perature resulted in 2,6-bis[(2,6-diisopropylphenyl-
imino)methyl]pyrazine(5), and [(2,6-diisopropylphen-
The reaction of 2,3,5,6-tetramethylpyrazine with
a tenfold amount of SeO2 in wet dioxane results
c
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H. Schumann – H.-K. Luo · 2,5-Dimethyl-3,6-bis[(2,6-diisopropylphenylimino)methyl]pyrazine
23
Scheme 1.
Scheme 2.
ylimino)methyl]benzene (6) which were separated by
column chromatography (Scheme 3). The diozonide as
well as the dialdehyde 4 were not isolated and char-
acterized, but used in the “one-pot” reaction without
further purification.
In conclusion, two new poly-N-functional Schiff-
base ligands were synthesized via the dialdehydes.
Each of them has two imine groups and two nitro-
gen atoms in the aromatic pyrazine system. Obviously,
both are very good potential ligands, of which 2 can
be used as a bis-chelating ligand which can coordi-
nate with two metal centers, and 5 as a tridendate lig-
and to prepare various transition metal complexes for
catalysis.
Scheme 3.
◦
tion. Yield: 0,60 g (26%). M. p. 140 – 142 C. – 1H NMR
(200 MHz): δ = 2.86 (s, 6 H, CH3), 10.16 (s, 2 H, CHO). –
13C NMR (50.32 MHz): δ = 21.36 (CH3), 144.65 (CCH3),
151.95 (CCHO), 193.56 (CCHO).
Experimental Section
All operations were carried out in an atmosphere of dry,
oxygen-free nitrogen using standard Schlenk techniques.
Solvents were dried over sodium, purified and saturated with
nitrogen prior to use. Elemental analyses were performed
on a Perkin-Elmer 240C elemental analyzer. Mass spectra
(EI, 70 eV) were obtained using a Varian MAT 311A instru-
2,5-Dimethyl-3,6-bis[(2,6-diisopropylphenylmino)me-
thyl]pyrazine (2): A mixture of 1 (0,58 g, 3.5 mmol),
2,6-diisopropylaniline (1,4 ml) and acetic acid (0,1 ml) in
ethanol (8 ml) was refluxed for 15 h with stirring. Filtration
of the reaction mixture at room temperature and washing of
the resulting yellow powder with ethanol (5 ml) yielded 2
(1.19 g, 70%). M. p. 220 – 222 ◦C. – 1H NMR (200 MHz):
δ = 1.26 [d, 3J = 6.9 Hz, 24 H, CH(CH3)2], 3.08 [sept,
3J = 6.9 Hz, 4 H, CH(CH3)2], 3.11 [s, 6 H, CH3(pyr)],
7.21 – 7.27 (m, 6 H, Ph), 8.57 (s, 2 H, CH=N). – 13C NMR
(50.32 MHz): δ = 23.24 [CH3(py)], 23.45 (CHCH3), 28.02
(CHCH3), 123.01 (Ph), 124.63 (Ph), 136.72 (Ph), 145.32
(C-pyr), 149.02 (Ph), 151.24 (C-pyr), 163.53 (CH=N). –
C32H42N4 (482.71): calcd. C 79.62, H 8.77, N 11.61; found
C 79.68, H 8.53, N 11.78.
1
ment. H and 13C NMR spectra were recorded on a Bruker
ARX 200 (200 and 50.32 MHz, respectively) and a Bruker
ARX 400 (400 and 100.64 MHz, respectively) spectrometer
in CDCl3 unless otherwise stated.
2,5-Dimethylpyrazine-3,6-dicarboxaldehyde (1): Tetra-
methylpyrazine (1.9 g, 14 mmol) and SeO2 (15.4 g,
140 mmol) were dissolved in a mixture of dioxane (240 ml)
and water (10 ml) and refluxed for 5 h. After filtration of
the black powder received, the clear filtrate was concentrated
nearly to dryness. Toluene (120 ml) was added, the mixture
was refluxed for 30 min, decanted from the remaining residue
and the solvents were evaporated to leave a volume of 10 ml.
2,6-Distyrylpyrazine (3): A mixture of 2,6-dimethylpyraz-
Yellow needles of 1 separated and were collected by filtra- ine (2.1 g, 20 mmol), ZnCl2 (2.8 g, 20 mmol) and benzalde-
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24
H. Schumann – H.-K. Luo · 2,5-Dimethyl-3,6-bis[(2,6-diisopropylphenylimino)methyl]pyrazine
1
hyde (30 ml) was heated to 155 ◦C for 8 h with stirring. Then
5: M. p. 133 – 134 ◦C. – H NMR (400 MHz): δ = 1.26
3
3
ethanol (30 ml) was added, the reaction mixture refluxed for (d, J = 6.9 Hz, 24 H, CH3), 3.04 [sept, J = 6.9 Hz, 4 H,
30 min and cooled to room temperature and filtered. The re- CH(CH3)2], 7.21 – 7.27 (m, 6 H, Ph), 8.47 (s, 2 H, CH=N),
maining yellow powder was washed with ethanol (20 ml) 9.65 [s, 2 H, H(pyr)]. – 13C NMR (100,64 MHz): δ = 23.39
and dried in vacuum yielding 5.45 g (100%) of 3. M.p. 229 – (CH3), 27.98 (CHCH3), 123.17 (Ph), 124.99 (Ph), 137.03
230 ◦C.
(Ph), 144.25 (C-pyr), 147.94 (Ph), 148.49 (C-pyr), 160.97
2,6-Bis[(2,6-diisopropylphenylimino)methyl]pyrazine (5) (CH=N). – C30H38N4 (445.66): calcd. C 79.25, H 8.42,
and [(2,6-Diisopropylphenylimino)methyl]benzene (6): A N 12.32; found C 79.09, H 8.58, N 12.31.
◦
1
solution of 3 (2.1 g, 20 mmol) in methanol (200 ml) was
6: M. p. 123 – 124 C. – H-NMR (400 MHz): δ = 1.24
◦
3
3
cooled in a CO2/isopropanol bath to −50 C and O3 was (d, J = 6.8 Hz, 12 H, CH3), 3.06 [sept, J = 6.8 Hz, 2 H,
bubbled through the stirred solution until complete dissolua- CH(CH3)2], 7.18 – 7.24 (m, 3 H, Phm,p), 7.98 – 8.00 (m, 2 H,
tion of 3 and appearance of the pale blue colour of dissolved Pho), 8.27 (s, 1 H, CH=N). – 13C NMR (100,64 MHz): δ =
O3 (6 – 8 h). Excess of O3 was then displaced by bubbling 23.42 (CH3), 27.86 (CHCH3), 123.02 (C6H3-3,5), 124.13
N2 through the solution until the pale blue colour disap- (C6H3-4), 128.59 (C6H5-3,5), 128.83 (C6H5-2,6), 131.44
peared. Finely ground Na2SO3 (8.25◦g) was added and the (C6H5-4), 136.01 (C6H5-1), 137.60 (C6H3-2,6), 149.24
solution was stirred for 5 h at −50 C. The reaction mix- (C6H3-1), 162.03 (CH=N).
ture was warmed to room temperature and a white solid was
filtered off. 2,6-Diisopropylaniline (6,2 ml) and acetic acid
Acknowledgements
We thank the Alexander von Humboldt Foundation, the
Deutsche Forschungsgemeinschaft (Graduiertenkolleg “Syn-
thetische, mechanistische und reaktionstechnische Aspekte
von Metallkatalysatoren”) and the Fonds der Chemischen In-
dustrie for support of this work.
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were isolated by column chromatography (toluene).
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