Tandem pinacol coupling-rearrangement of aromatic aldehydes with hydrogen catalyzed by a combination of a platinum complex and a polyoxometalate

Article abstract of DOI:10.1039/b710100g

Together with a strongly oxidizing polyoxometalate, H₅PV ₂Mo₁₀O₄₀, Pt^II(N-(2,6- diisopropylphenyl)pyrazin-2-ylmethanimine)Cl₂ forms a combined catalyst that was active in the tandem pinacol coupling-rearrangement of aryl aldehydes to give mostly the corresponding diarylacetaldehyde in high yields using molecular hydrogen as the reducing agent. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710100g

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Tandem Pinacol Coupling/Rearrangement of Aromatic Aldehydes with Hydrogen Catalyzed by a
Combination of a Platinum Complex and a Polyoxometalate
Olena Branytska,a Linda J. W. Shimonb and Ronny Neumanna*
(a) Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100
(b) Chemical Research Support Unit, Weizmann Institute of Science, Rehovot, Israel
76100
Electronic Supporting Information
Detailed Experimental Section
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Instrumentation. H and C NMR spectra were measured on a Brucker Avance 400 instrument
with TMS as standard. IR spectra were measured on a Nicolet 460 FTIR spectrophotometer.
ESI-MS were measured using Micromass Platform LCZ 4000 with an aqueous sample infused at
5µl/min. Elemental analysis C, H, N was with a Thermoelectron EA112 CHNO Elemental
Analyzer. Reactions were quantified by GLC-FID (HP 6890) using a 30 m 5% phenylmethyl
silicone capillary column with an ID 0.32 mm and 0.25 µm coating (Restek 5MS) and products
were identified by use of reference standards and by GC-MS (HP 5973) using the same column.
Synthesis of metallorganic complexes. Methyl pyrazinecarboxylate1 was prepared by dropwise
addition of thionyl chloride (13.07 ml, 0.18 mmol) to dry methanol (240 mL) over 1 h at 4-6 °C
under argon. Pyrazinecarboxylic acid (20 g, 0.16 mmol) was added at 9 °C and the mixture was
heated for 2 h at 60 °C. After the reaction mixture was cooled to room temperature, a solution of
sodium hydrogen carbonate (29 g, 0.35 mmol) in 280 mL of water was slowly added. The
methanol was distilled off on a rotary evaporator at 180 mbar and a bath temperature of 45 °C.
The residue was extracted three times with dichloromethane (80 mL, 50 mL and 50 mL).
Concentration of the organic phase provided 18.34 g of crude product. The residual solid was
recrystallized from diisopropyl ether. The yield of methyl pyrazinecarboxylate was 14.6 g (66
%). 1H NMR (250 MHz, CDCl3) δ: 3.98 (s, 3H), 8.66 (d, J = 2.5 Hz, 1H), 8.72 (d, J = 2.5 Hz,
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1H), 9.25 (s, 1H) ppm. C NMR (100 MHz, CDCl3) δ: 53.07, 143.13, 144.25, 146.14, 147.63,
164.25 ppm.
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Pyrazine-2-carboxaldehyde2 was prepared by dissolving methyl pyrazinecarboxylate (10 g, 70
mmol) in 200 mL of dry tetrahydrofuran and the mixture was cooled to -70 °C under argon.
While maintaining the temperature at -68 °C to -73 °C, a 1 M solution of LiAlH4 in dry
tetrahydrofuran (36 mL, 36 mmol) was added with stirring over a period of 30 min. After stirring
for an additional 15 min at -75 °C, the reaction was stopped by slow addition of 10 mL of glacial
acetic acid. The solvent was evaporated on a rotary evaporator at 260 mbar and a bath
temperature of 40 °C. The residue was dissolved in a mixture of 88 mL of 2 N hydrochloric acid
and 40 mL of chloroform. The aqueous layer was extracted eight times with 20 mL portions of
chloroform. The combined extracts were stirred with 20 mL of water and 10 g of sodium
bicarbonate until neutral pH was obtained. The chloroform layer was filtered off, dried with
anhydrous sodium sulfate and concentrated under reduced pressure. The yield of pyrazine-2-
carboxaldehyde was 3.51g, (41 %), as a light yellow liquid. 1H NMR (250 MHz, CDCl3) δ: 8.61
(d, J = 2.5 Hz, 1H, 5-H), 8.65 (d, J = 2.5 Hz, 1H, 6-H), 8.97 (s, 1H, 3-H), 9.96 (s, 1H, Hald)
ppm. 13C NMR (100 MHz, CDCl3) δ: 143.25, 144.74, 146.71, 148.34, 192.28 ppm.
N-(2,6-diisopropylphenyl)pyrazin-2-ylmethanimine was prepared by dissolving pyrazine-2-
carboxaldehyde (0.97 g, 8 mmol) in 15 mL of trimethyl orthoformate. 2,6-Diisopropylaniline
(1.41 g, 8 mmol) was added and the mixture was refluxed for 15 h. The mixture was cooled to
room temperature and solvent was evaporated on a rotary evaporator at 100 mbar and a bath
temperature of 40 °C. The residue was purified by flash chromatography using a
methanol/chloroform (1:1) eluent mixture. The yield of N-(2,6-diisopropylphenyl)pyrazin-2-
ylmethanimine was 0.99 g, (46 %) as pale yellow crystals. 1H NMR (250 MHz, CDCl3) δ: 1.18
(d, J = 5 Hz, 12H, CH3), 2.95 (quint, J = 7.5 Hz, 2H, CH), 7.18 (m, 3H, Ar-H), 8.33 (s, 1H,
HC=N), 8.69 (quad, J1 = 2.5 Hz, J2 = 5 Hz, 2H, Hpyrazine-5,6), 9.48 (s, 1H, Hpyrazine-3) ppm. 13C
NMR (100 MHz, CDCl3) δ: 23.43, 27.98, 123.14, 124.88, 137.09, 143.59, 144.19, 145.80,
147.96, 149.21, 161.15 ppm.
PtII(N-(2,6-diisopropylphenyl)pyrazin-2-ylmethanimine)Cl2 was prepared by dissolving
Pt(Cl)2(DMSO)2 (0.42 g, 1 mmol) in 5 mL of dry methanol. N-(2,6-diisopropylphenyl)pyrazin-
2-ylmethanimine (0.26 g, 1 mmol) separately dissolved in 5 mL of dry methanol was added to the
solution. The mixture was stirred at room temperature overnight. The orange precipitation that
was formed was filtered off, washed with methanol and dried under high vacuum. The yield of
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the title compound was 0.41 g (80 %). Crystals of the complex for X-ray analysis were grown
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from a chloroform solution by slow diffusion of diethyl ether. H NMR (400 MHz, CDCl3) δ:
1.13 (d, J = 8 Hz, 6H, CH3), 1.33 (d, J = 8 Hz, 6H, CH3), 3.15 (quint, J = 12 Hz, 2H, CH), 7.26
(d, J = 8 Hz, 2H, Ar-H), 7.39 (t, J = 8 Hz, 1H, Ar-H), 8.89 (s, 1H, HC=N), 9.11 (d, J = 4 Hz,
1H, Hpyrazine-5), 9.17 (s, 1H, Hpyrazine-3), 10.00 (d, J = 4 Hz, 1H, Hpyrazine-6) ppm. 13C NMR (100
MHz, CDCl3) δ: 22.96, 24.58, 28.22, 123.59, 129.65, 141.26, 142.63, 143.32, 147.97, 150.62,
152.44, 167.06 ppm. MS (ESI) m/z 556.18 [M + Na]+ 100%, 1101.41 [2M + Na]+. IR (KBr) ν
= 757, 800, 1058, 1184, 1363, 1411, 1463, 1521, 1600, 2869, 2964, 3064, 3093 cm-1. Elemental
analysis: experimental (calculated): C – 38.138 (38.28), H – 3.812 (3.97), N – 7.198 (7.88).
PdII(N-(2,6-diisopropylphenyl)pyrazin-2-ylmethanimine)Cl2 was prepared by dissolving
Pd(Cl)2(DMSO)2 (0.033 g, 0.1 mmol) in 15 ml of dry methanol followed by addition 2,6-bis(1-
methylethyl)-N-(2-pyrazinylmethylene)phenylamine (0.026 g, 0.1 mmol) that was separately
dissolved in 3 mL of dry methanol. The mixture was stirred at room temperature for 24 h. The
amount of solvent was reduced under a steady stream of argon. A portion by portion addition of
ether completed the precipitation, which was filtered off and dried under high vacuum. The yield
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of the title compound was 0.011 g (25 %). H NMR (250 MHz, CDCl3) δ: 1.15 (d, J = 5 Hz,
6H, CH3), 1.38 (d, J = 5 Hz, 6H, CH3), 3.22 (quint, J = 7.5 Hz, 2H, CH), 7.21 (d, J = 7.5 Hz,
2H, Ar-H), 7.37 (t, J = 7.5 Hz, 1H, Ar-H), 8.26 (s, 1H, HC=N), 9.15 (d, J = 2.5 Hz, 1H,
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Hpyrazine-5), 9.20 (s, 1H, Hpyrazine-3), 9.51 (d, J = 2.5Hz, 1H, Hpyrazine-6) ppm. C NMR (100
MHz, DMSO-d6) δ: 23.23, 24.51, 26.36, 123.54, 128.88, 140.92, 143.31, 143.96, 149.43,
150.33, 152.11, 173.39 ppm. MS (ESI) m/z 468.11 [M + Na]+ 100%. IR (KBr) ν = 755, 798,
1056, 1189, 1363, 1457, 1608, 2867, 2925, 2960, 3006, 3021, 3062, 3102 cm-1. Elemental
analysis: experimental (calculated): C - 45.91 (45.92), H- 4.33 (4.76), N - 9.27 (9.45).
Crystallographic Data Collection and Structure Determination of PtII(N-(2,6-
diisopropylphenyl)pyrazin-2-ylmethanimine)Cl2. The data was collected on orange triclinic plates,
with a size of 0.1 × 0.1 × 0.05 mm3, using a Nonius-Kappa CCD diffractometer using graphite-
monochromated Mo-Kα (λ = 0.71073 Å) radiation. 49905 (11399 independent, R(int) = 0.046)
reflections were collected over a range of θ = 2.74 – 27.42 ° with limiting indices of -14 ≤ h ≤ 14,
-19 ≤ k ≤ 19, 0 ≤ l ≤ 19. The data were processed with Denzo-Scalepack. The structures were
solved by direct methods with SHELXS-97. Full-matrix least-squares refinement was based on
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F2 with SHELX-97. Refinement of the data on 11399 unique reflections was with 524 parameters
with 0 restraints. The goodness of fit on F2 was 1.076. The final R indices for I > 2σ(I) were R1
= 0.0280 and Rw = 0.0660 and R indices for all data were R1 = 0.0366 and Rw = 0.0697. The
largest difference peak between peaks and hole were 1.500 and –1.218 e/Å3. The crystallographic
data are presented in the caption of the figure below and the cif file is to be found in a separate
file also as supplemental information.
General procedure for the reductive coupling-rearrangement of aromatic aldehydes. Reactions
were carried out in 15 mL glass pressure tubes. In a typical procedure for aromatic aldehydes 2
mmol of substrate, 0.063 g (0.03 mmol) H5PV2Mo10O40•34 H2O,3 and Pt(N-(2,6-
diisopropylphenyl)pyrazin-2-ylmethanimine)Cl2 (0.0053 g, 0.01 mmol) were placed in a glass
pressure tube. The air was replaced with argon. Hydrogen, 2.5 atm, was introduced to the
pressure tube by two consecutive pump-thaw cycles on Schlenk line. The tubes were placed in a
thermostated oil bath at 100 °C and reactions were carried out for 18 h. The reaction mixtures
were then cooled and 1 mL of dichloromethane was added; conversions and selectivity were
measured by GLC with both FID and MS detectors.
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