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Synlett 2005(14): 2141-2146  
DOI: 10.1055/s-2005-872266
LETTER
© Georg Thieme Verlag Stuttgart · New York

Solvent-Free Synthesis of 2-Pyrones from Alkynes and Carbon Dioxide Catalyzed by Ni(1,5-cyclooctadiene)2/Trialkylphosphine Catalysts

Yasuhisa Kishimotoa, Ikuko Mitani*b
a The General Environmental Technos Co., Ltd., 3-3-1 Higashi-Kuraji, Katano, Osaka 576-0061, Japan
b Environmental Research Center, The Kansai Electric Power Company, Inc., 1-7 Hikaridai, Seika-cho, Souraku-gun, Kyoto 619-0237, Japan
Fax: +81(774)932894; e-Mail: mitani.ikuko@b2.kepco.co.jp;
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Publication History

Received 13 April 2005
Publication Date:
03 August 2005 (online)

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Abstract

Solvent-free, efficient, and mild 2-pyrone synthesis by the cycloaddition of alkynes with CO2 has been achieved under compressed CO2 with a Ni(cod)2/P(C4H9)3 or Ni(cod)2/P(C8H17)3 catalyst; both high yields (up to 98%) and selectivities (up to 99%) are attained. One attractive aspect of these catalysts is their high efficiency at low CO2 pressures (4 MPa). The applicability of this reaction to a broad range of alkyne substrates and the ease of handling the phosphine ligands are additional merits of the present new catalytic systems in comparison to the previously reported Ni(cod)2/P(CH3)3 catalyst in supercritical CO2.

Key words

2-pyrone derivatives - alkynes - carbon dioxide - Ni(0) complex - solvent-free

    References

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11

Typical procedure for the Ni(0)-catalyzed cycloaddition of alkynes with CO 2 : All manipulations were carried out under a nitrogen or argon atmosphere. Alkyne 1a (2 mmol), phosphine 3b (0.4 mmol), and o-xylene (40 µL, internal standard for GC analysis) were mixed in a Schlenk tube. Ni(cod)2 (0.2 mmol) was placed in a stainless steel autoclave containing a magnetic stirring bar, and, to this, was added the above mixture via a dry syringe. CO2 (ca. 5 MPa) was quickly introduced, and the autoclave was immersed in an oil bath at 120 °C. When the temperature of the reactor reached the desired temperature (typically after about 10 min), more CO2 was added to achieve the desired pressure of 15 MPa. The time of the second addition of CO2 was considered to be the start of the reaction. The mixture was allowed to stir for 20 h. Then, the vessel was removed from the oil bath, immersed in an ice bath for ca. 1 h, and allowed to cool to ca. 0 °C. Excess CO2 was then slowly vented at ca. 0 °C. The remaining organic liquid was diluted with acetone. The GC analysis of the solution indicated the conversion of 1a and the yield of 2a were 100% and 95%, respectively. Acetone was removed by evaporation, and the residue was chromatographed on silica gel (hexanes-EtOAc, 15:1) to give analytically pure 2a in 91% isolated yield. The product was identified by 1H NMR and GC-MS analyses. The 1H NMR spectrum was identical to the spectral data reported in ref. 1a.

12

CAUTION: Certain alkynes turn immediately brown when they are mixed with trialkylphosphines and the use of such alkynes does not give desired results. The reason for this is unclear at this time, but we recommend the use of the alkynes that cause no color change.

13

The product yield was determined by GC analysis on the basis of the alkyne 1 employed. Thus, the yield (%) of 2-pyrone 2 = 100 × [(mmol of alkyne component in 2) / (mmol of 1)] = 100 × [(2 × mmol of 2) / (mmol of 1)]; and the yield (%) of alkyne trimer 4 or 5 = 100 × [(mmol of alkyne component in 4 or 5) / (mmol of 1)] = 100 × [(3 × mmol of 4 or 5) / (mmol of 1)].

The selectivity for 2 is defined as the mol% of 2 in all of the reaction products and is calculated from the yields of 2, 4, and 5 by using the above equations. Thus, the selectivity (%) for 2 = 100 × [(mmol of 2) / (mmol of 2 + mmol of 4 + mmol of 5)] = 100 × [(% yield of 2) / 2] / {[(% yield of 2) / 2] + [(% yield of 4) / 3] + [(% yield of 5) / 3]}.

16

Selected data for compound 2c: 1H NMR (CDCl3, 270 MHz): δ = 2.36-2.49 (m, 6 H, CH2), 2.24-2.29 (t, J = 7.3 Hz, 2 H, CH2), 1.59-1.70 (m, 2 H, CH2), 1.30-1.45 (m, 14 H, CH2), 0.91-1.00 (m, 12 H, CH3). 13C NMR (CDCl3, 67.5 MHz): δ = 163.7 (C=O), 158.1 (Cvinyl), 154.5 (Cvinyl), 123.2 (Cvinyl), 115.4 (Cvinyl), 33.4 (CH2), 32.1 (CH2), 31.2 (CH2), 30.7 (CH2), 30.1 (CH2), 29.3 (CH2), 27.1 (CH2), 26.6 (CH2), 23.3 (CH2), 23.1 (CH2), 23.0 (CH2), 22.6 (CH2), 14.1 (CH3), 13.93 (CH3), 13.90 (CH3), 13.8 (CH3). HRMS (EI): m/z calcd for C21H36O2: 320.2715. Found: 320.2729.

17

Through careful column chromatography on silica gel (hexanes-EtOAc, 30:1), analytically pure 2d′ could be isolated from the reaction mixture as a colorless oil (15 mg, 8%). 1H NMR (CDCl3, 270 MHz): δ = 2.54 (t, J = 7.6 Hz, 2 H, CH2), 2.53 (t, J = 7.6 Hz, 2 H, CH2), 2.09 (s, 3 H, CH3), 1.94 (s, 3 H, CH3), 1.19 (t, J = 7.6 Hz, 3 H, CH3), 1.07 (t, J = 7.6 Hz, 3 H, CH3). 13C NMR (CDCl3, 67.5 MHz): δ = 163.4 (C=O), 157.9 (Cvinyl), 150.8 (Cvinyl), 124.4 (Cvinyl), 110.8 (Cvinyl), 24.6 (CH2), 20.5 (CH2), 16.3 (CH3), 13.0 (CH3), 12.8 (CH3), 12.1 (CH3). 1H NMR: NOE enhancement (2.8%) between the methyl protons at 1.94 ppm and the methyl protons at 2.09 ppm was observed. HRMS (EI): m/z calcd for C11H16O2: 180.1146. Found: 180.1144.

18

Selected data for compound 2e: 1H NMR (CDCl3, 270 MHz): δ = 4.47 (s, 2 H, CH2O), 4.45 (s, 2 H, CH2O), 4.37 (s, 2 H, CH2O), 4.33 (s, 2 H, CH2O), 3.42 (s, 3 H, CH3O), 3.41 (s, 3 H, CH3O), 3.40 (s, 3 H, CH3O), 3.37 (s, 3 H, CH3O). 13C NMR (CDCl3, 67.5 MHz): δ = 162.1 (C=O), 158.6 (Cvinyl), 152.1 (Cvinyl), 123.7 (Cvinyl), 115.4 (Cvinyl), 68.6 (CH2O), 67.0 (CH2O), 65.6 (CH2O), 65.1 (CH2O). HRMS (EI): m/z calcd for C13H20O6: 272.1260. Found: 272.1257.

19

Selected data for compound 5e: 1H NMR (CDCl3, 270 MHz): δ = 4.60 (s, 12 H, CH2O), 3.41 (s, 18 H, CH3O). MS (EI): m/z (%) = 310 (38) [M+ - 32], 295 (100), 265 (42), 233 (30), 203 (23).