A New Entry to Sonochemical/Efficient Agitation Switching: Alkene Formation through Epoxide Deoxygenation

Abstract

‘Sonochemical switching’ whereby the pathway of a reaction is changed under sonication conditions was first reported by Ando, and in fact could just be the consequence of efficient agitation. We herein report a new addition to this switching phenomenon in which epoxides are converted to cyclic carbonates by addition of CO₂ under standard heating/electrolysis conditions but are ‘switched’ to alkenes under deoxygenating sonochemical conditions.

Key words

sonochemical - alkene - epoxide - cyclic carbonate - carbon dioxide

• References and Notes

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See for some recent examples:
• 7a Mahesh M, Murphy JA, Wessel HP. J. Org. Chem. 2005; 70: 4118
• 7b Choi KH, Choi KI, Kim JH, Yoon CM, Yoo BW. Bull. Korean Chem. Soc. 2005; 26: 1495
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• 8b Mitsudome T, Noujima A, Mikami Y, Mizugaki T, Jitsukawa K, Kaneda K. Angew. Chem. Int. Ed. 2010; 49: 5545
• 8c Noujima A, Mitsudome T, Mizugaki T, Jitsukawa K, Kaneda K. Angew. Chem. Int. Ed. 2011; 50: 2986
• 8d Ni J, He L, Liu Y.-M, Cao Y, He H.-Y, Fan K.-N. Chem. Commun. 2011; 47: 812
• 9 Huang J.-M, Lin Z.-Q, Chen D.-S. Org. Lett. 2012; 14: 22
• 10 Representative Procedure for the Standard Synthesis of Phenyl Ethylene Carbonate (Ref. 2): Styrene oxide (1; 0.12 g, 1.0 mmol) and CO₂ (balloon) in MeCN (150 mL) were electrolysed (constant current: 60 mA) for 6 h in a single compartment cell [Mg anode and copper(0) cathode] containing Bu₄NBr (0.32 g, 1.0 mmol) as supporting electrolyte at 50 °C. On completion the reaction mixture was washed with aq 0.1 M HCl (50 mL) followed by extraction with Et₂O (3 × 35 mL). The combined organic extracts were then dried over MgSO₄ and evaporated under reduced pressure to afford an amber oil, which was suspended in EtOAc (100 mL). After 1 h the precipitated Bu₄NBr (0.30 g, 95%) was removed by filtration and the solvent evaporated under reduced pressure to afford an amber oil. This crude material was purified by column chromatography on silica gel eluting with EtOAc–light petroleum to yield a colourless solid (conversion: 99%; yield: 0.12 g, 96%); mp 49–51 °C. IR (CH₂Cl₂): 1167 (C–O), 1789 (C=O) cm^–1. ¹H NMR (400 MHz, CDCl₃/Me₄Si): δ = 4.35 (dd, J = 7.9, 8.4 Hz, 1 H), 4.80 (dd, J = 8.5 Hz, 1 H), 5.68 (dd, J = 7.99 Hz, 1 H), 7.35–7.45 (m, 5 H). ¹³C NMR (100 MHz, CDCl₃/Me₄Si): δ = 71.2, 78.0, 125.9, 129.3, 129.8, 135.8, 154.8.
• 11 Representative Procedure for the Sonochemical Synthesis of Styrene (2): Styrene oxide (1; 0.12 g, 1.0 mmol) and CO₂ (balloon) in MeCN (150 mL) were electrolysed (constant current: 60mA) for 6 h in a single compartment cell [Mg anode and copper(0) cathode] containing Bu₄NBr (0.32 g, 1.0 mmol) as supporting electrolyte. The reaction vessel was submerged in a sonication bath at 50–60 Hz. On completion the reaction mixture was washed with aq 0.1 M HCl (50 mL) followed by extraction with Et₂O (3 × 35 mL). The combined organic extracts were then dried over MgSO₄ and evaporated under reduced pressure to afford an amber oil, which was suspended in EtOAc (100 mL). After 1 h the precipitated Bu₄NBr (0.30 g, 95%) was removed by filtration and the solvent evaporated under reduced pressure to afford an amber oil. This crude material was purified by column chromatography on silica gel eluting with EtOAc–light petroleum to yield a colourless liquid (0.094 g, 90%). IR (CH₂Cl₂): 1442 (C=C), 1610 (C=C) cm^–1. ¹H NMR (400 MHz, CDCl₃/Me₄Si): δ = 5.25 (dd, J = 0.8, 11.0 Hz, 1 H), 5.75 (dd, J = 1.2, 17.5 Hz, 1 H), 6.72 (dd, J = 11.0, 17.5 Hz, 1 H), 7.24–7.41 (m, 5 H). ¹³C NMR (100 MHz, CDCl₃/Me₄Si): δ = 113.7, 126.2, 127.8, 128.5, 137.0, 137.7.
• 12 General Procedure for Cyclic Carbamate Formation: The aziridine (2.0 mmol) in MeCN (20 mL) was added to a solution of supporting electrolyte Bu₄NBr (0.64 g, 2.0 mmol) in MeCN (130 mL), and the resulting solution was flushed with CO₂ for 1 h, followed by heated electrolysis at 50 ºC with constant stirring at a constant current of 60 mA for 6 h under a CO₂ balloon, in a single compartment cell containing a magnesium anode and copper cathode. On completion the reaction mixture was filtered to remove the precipitated MgCO₃ and evaporated to dryness followed by addition of EtOAc (50 mL). After 1 h the precipitated Bu₄NBr (ca. 70% recovered) was removed by filtration and the solvent evaporated under reduced pressure. The crude carbamate was then analysed by ¹H NMR spectroscopy to determine the regioselectivity and then purified by column chromatography on silica gel eluting with EtOAc–light petroleum.
• 13 3-Benzyl-4-methyloxazolidin-2-one (3): colourless oil, mixture of regioisomers observed in a 95:5 ratio in favour of the title compound (conversion: 98%; yield: 0.34 g, 87%). IR (CH₂Cl₂): 1181 (C–N), 1255 (C–O), 1416 (C=C), 1721 (C=O) cm^–1. ¹H NMR (400 MHz, CDCl₃/Me₄Si): δ = 1.21 (d, J = 6.0 Hz, 3 H), 2.97 (dd, J = 6.8, 8.6 Hz, 1 H), 3.50 (dd, J = 8.4 Hz, 1 H), 4.37–4.46 (m, 2 H), 4.57–4.67 (m, 1 H), 7.26–7.33 (m, 5 H). ¹³C NMR (100 MHz, CDCl₃/Me₄Si): δ = 18.3, 46.7, 50.8, 64.7, 127.8, 127.9, 128.3, 128.8, 158.3. 3-Benzyl-5-phenyloxazolidin-2-one (4): colourless solid, single regioisomer observed (conversion: 51%; yield: 0.162 g, 32%); mp 61–63 °C. IR (CH₂Cl₂): 1157 (C–N), 1250 (C–O), 1454 (C=C), 1752 (C=O) cm^–1. ¹H NMR (400 MHz, CDCl₃/Me₄Si): δ = 3.30 (dd, J = 7.5, 8.8 Hz, 1 H), 3.75 (dd, J = 8.8 Hz, 1 H), 4.41–4.56 (m, 2 H), 5.45 (dd, J = 8.3 Hz, 1 H), 7.21–7.40 (m, 10 H). ¹³C NMR (100 MHz, CDCl₃/Me₄Si): δ = 45.9, 48.5, 74.6, 125.5, 126.3, 128.1, 128.2, 128.8, 128.9, 135.7, 138.6, 158.0.
• 14 Clegg W, Harrington RW, North M, Pasquale R. Chem. Eur. J. 2010; 16: 6828
• 15 This was confirmed by subjecting phenyl ethylene carbonate instead of styrene oxide to the reaction conditions reported in reference 11. Only phenyl ethylene carbonate and supporting electrolyte was observed in the crude reaction ¹H NMR spectrum.