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Synlett 2006(8): 1173-1176  
DOI: 10.1055/s-2006-926255
LETTER
© Georg Thieme Verlag Stuttgart · New York

Ruthenium-Catalyzed Rearrangement of cis-1-Ethynyl-2-vinyloxiranes to Substituted Phenols

Shambabu Joseph Maddirala, Arjan Odedra, Bhanu Pratap Taduri, Rai-Shung Liu*
Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan
e-Mail: rsliu@mx.nthu.edu.tw;
Further Information

Publication History

Received 19 August 2005
Publication Date:
10 March 2006 (online)

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Abstract

Catalytic cyclization of cis-1-ethynyl-2-vinyloxiranes was implemented with TpRuPPh3(CH3CN)2PF6 catalyst (10 mol%), to give 2,6-disubstituted phenols in reasonable yields. Under similar conditions, 1,1,2,2-tetrasubstituted oxirane gave the 2,3,6-trisubstituted phenol with skeleton reorganization. On the basis of 2H- and 13C-labeling results, we propose that the reaction mechanism involves electrocyclization of ruthenium-vinylidene intermediate with cleavage of the carbon-oxygen bond of the epoxide.

Key words

cis-1-ethynyl-2-vinyloxiranes - 2,6-disubstituted phenols - TpRuPPh3(CH3CN)2PF6 - ruthenium-vinylidine intermediate - endo-dig cyclization - 1,2-hydrogen shift

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Procedure for the Synthesis of Epoxide 5.
To a CH2Cl2 solution (60 mL) of species 1 (2.87 g, 13.64 mmol) was added MCPBA and the mixture was stirred at 28 °C for 4 h. The resulting suspension was quenched with H2O. The aqueous layer was extracted with CH2Cl2 twice. The combined organic layer was washed with H2O, dried over MgSO4 and the solvent was removed in vacuo. The resulting mass was eluted through a Et3N-pretreated silica column (EtOAc-hexane, 1:20) to get the epoxide 2 as a colorless oil (1.86 g, 8.22 mmol, 60%).
To a solution of oxalyl chloride (1.26 mL, 14.57 mmol) in CH2Cl2 (40 mL) maintained at -60 °C was added DMSO (2.09 mL, 29.14 mmol) with stirring. After 15 min a solution of 2 (1.65g, 7.30 mmol) in CH2Cl2 (5 mL) was added and the stirring continued for 1 h at -60 °C. Then, Et3N (8.83 mL, 63.35 mmol) was added and the reaction mixture was allowed to attain 28 °C over 2 h. Afterwards, H2O was added and the organic layer was separated. It was washed with brine, dried over MgSO4 and concentrated. The crude mass was eluted through Et3N-pretreated silica column (EtOAc-hexanes, 1:20) to get the epoxy ketone 3 as a colorless oil (1.55 g, 6.91 mmol, 92%).
To a solution of methyltriphenylphosphonium bromide (3.66 g, 10.24 mmol) in THF (45 mL) at -20 °C was added n-BuLi (3.77 mL, 9.42 mmol), and the mixture was stirred at that temperature for 1 h and then cooled to -78 °C. To the resulting solution was added a solution of 3 (1.15 g, 5.12 mmol) in THF (5 mL) over 10 min and the resulting reaction mixture was allowed to warm to -30 °C over 2 h. The reaction was quenched with sat. aq NH4Cl and concentrated in vacuo. The organic mass was extracted into Et2O. The ether layer was washed with brine, dried over MgSO4 and concentrated in vacuo. The residue was purified by column chromatography over a Et3N-pretreated silica column (hexane) to get the olefination product 4 as a colorless oil (0.80 g, 3.60 mmol, 70%).
To a solution of this silyl compound 4 in THF (5 mL) was added Bu4NF (1 M, 3.6 mL, 3.6 mmol) under ice conditions and allowed the resulting solution to warm to 26 °C over
4 h. Then, H2O (5 mL) was added and the solution was concentrated, extracted with Et2O. The ether layer was dried over MgSO4 and concentrated in vacuo. Elution of the crude enyne through a Et3N-pretreated silica column (hexane) gave 5 (0.50 g, 3.33 mmol, 93%).

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Typical Procedure for Cyclization of 2-Ethynyl-3-isopropenyl-2-propyl Oxirane ( 5) to 2-Methyl-6-propylphenol ( 6). A long tube containing TpRu(PPh3)(CH3CN)2PF6 (50.7 mg, 0.067 mmol) was dried in vacuo for 2 h. It was then charged with epoxide 5 (100 mg, 0.67 mmol) and freshly distilled toluene (0.6 mL). The mixture was heated at 100 °C for 16 h and then cooled to r.t. Concentration of the solution followed by elution through a silica column (hexane-Et2O = 5:1) afforded the phenol 6 (57 mg, 0.38 mmol, 57%) as a pale-yellow oil.
Spectral Data for 2-Ethynyl-3-(prop-1-en-2-yl)-2-propyloxirane ( 5).
IR (nujol): 3300 (m), 2235 (w), 1645 (w), 1210 (m) cm-1. 1H NMR (600 MHz, CDCl3): δ = 5.05 (dd, J = 2.98, 1.64 Hz, 2 H), 3.24 (s, 1 H), 2.29 (s, 1 H), 1.80 (s, 3 H), 1.61-1.58 (m, 1 H), 1.56-1.55 (m, 3 H), 0.96 (t, J = 7.0 Hz, 3 H). 13C NMR (150 MHz, CDCl3): δ = 138.6, 113.1, 80.1, 73.2, 65.7, 57.1, 38.9, 19.1, 18.8, 13.7. MS (75 eV): m/z = 150 [M+]. HRMS: m/z calcd for C10H14O: 150.1045; found: 150.1049.
Spectral Data for 2-Methyl-6-propylphenol ( 6).
IR (nujol): 3416 (s), 1620 (s), 1585 (m), 1230 (s) cm-1. 1H NMR (600 MHz, CDCl3): δ = 6.95 (d, J = 6.8 Hz, 2 H), 6.76 (t, J = 7.5 Hz, 1 H), 4.61 (s, 1 H), 2.55 (t, J = 7.6 Hz, 2 H), 2.23 (s, 3 H), 1.64-1.61 (m, 2 H), 0.96 (t, J = 7.4 Hz, 3 H). 13C NMR (150 MHz, CDCl3): δ = 151.8, 128.5, 127.9, 127.6, 123.0, 120.2, 32.1, 22.9, 15.9, 14.1. MS (75 eV): m/z = 150 [M+]. HRMS: m/z calcd for C10H14O: 150.1045; found: 150.1043.

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In a separate experiment, epoxide 12 (0.6 M) was heated alone in toluene (100 °C, 12 h), and we isolated the phenol 18 and aldehyde 21 in 4% and 10%, respectively, whereas the starting epoxide 12 was recovered in 51% (Scheme [10] ).