Chemoselective and Metal-Free Synthesis of Aryl Esters from the Corresponding Benzylic Alcohols in Aqueous Medium Using TBHP/TBAI as an Efficient Catalytic System

Abstract

A novel and transition-metal-free strategy has been developed for the synthesis of aryl esters starting from corresponding benzylic primary alcohols as the exclusive substrates using tert-butyl hydroperoxide (TBHP) as a terminal oxidant in the presence of catalytic amount of tetrabutylammonium iodide (TBAI) and imidazole, where the aliphatic alcohols remained unaffected. These reactions are highly chemoselective and associated with high yield and wide applicability accommodating a wide range of substituents. Excellent chemoselectivity has also been demonstrated through intramolecular competition experiments. This protocol can be considered as an important analogue of Tishchenko reaction using benzylic alcohols as the substrates instead of benzaldehydes.

Key words

aqueous reaction - chemoselectivity - green chemistry - esters - oxidation

• References and Notes

• 1 Verma S. Baig RB. N. Han C. Nadagouda MN. Varma RS. Green Chem. 2016; 18: 251
• 2 Miyamura H. Yasukawa T. Kobayashi S. Green Chem. 2010; 12: 776
• 3 Kiyooka S. Ueno M. Ishii E. Tetrahedron Lett. 2005; 46: 4639
• 4 Guha NR. Sharma S. Bhattacherjee D. Thakur V. Bharti R. Reddy CB. Das P. Green Chem. 2016; 18: 1206
• 5 Yoo WJ. Li CJ. Tetrahedron Lett. 2007; 48: 1033
• 6 Oda Y. Hirano K. Satoh T. Kuwabata S. Miura M. Tetrahedron Lett. 2011; 52: 5392
• 7 Rajbongshi KK. Sarma MJ. Phukan P. Tetrahedron Lett. 2014; 55: 5358
• 8 Tan L. Chen C. Liu W. Beilstein J. Org. Chem. 2017; 13: 1079
• 9 Li P. Zhao J. Lang R. Xia C. Li F. Tetrahedron Lett. 2014; 55: 390
• 10 Liu L. Yun L. Wang Z. Fu X. Yan C. Tetrahedron Lett. 2013; 54: 5383
• 11 Hackbusch S. Franz AH. Tetrahedron Lett. 2016; 57: 2873
• 12 Guo S. Yu JT. Dai Q. Yang H. Cheng J. Chem. Commun. 2014; 50: 6240
• 13 Wang H. Chen C. Liu W. Zhu Z. Beilstein J. Org. Chem. 2017; 13: 2023
• 14a Ghatak A. Khan S. Roy R. Bhar S. Tetrahedron Lett. 2014; 55: 7082
• 14b Khan S. Ghatak A. Bhar S. Tetrahedron Lett. 2015; 56: 2480 
• 15 Ghatak A. Khan S. Bhar S. Adv. Synth. Catal. 2016; 358: 435
• 16a Rout SK. Guin S. Ghara KK. Banerjee A. Patel BK. Org. Lett. 2012; 14: 3982
• 16b Majji G. Guin S. Gogoi A. Rout SK. Patel BK. Chem. Commun. 2013; 49: 3031
• 16c Majji G. Rout SK. Rajamanickam S. Guin S. Patel BK. Org. Biomol. Chem. 2016; 14: 8178
• 17 Representative Procedure for the Reaction To a mixture of benzyl alcohol (108 mg, 1.0 mmol) and TBHP (180 mg, 2.0 mmol) in water (5 ml), the catalyst TBAI (73.8 mg, 0.2 mmol) and imidazole (136 mg, 2.0 mmol) were added, and the mixture was stirred at 80 °C for 8 h. The progress of the reaction was monitored by thin layer chromatography (TLC). After completion of reaction, the reaction mixture was cooled to room temperature. Then the organic product was extracted with ethyl acetate (3 × 10 ml), repeatedly washed with distilled water (4 × 5 ml) to remove the unreacted TBHP, dried with anhydrous sodium sulfate, and the solvent was evaporated under reduced pressure to afford benzyl benzoate (182 mg, yield 86%). To a mixture of benzyl alcohol (108 mg, 1.0 mmol) and TBHP (180 mg, 2.0 mmol) in water (3 ml), the catalyst TBAI (73.8 mg, 0.2 mmol), imidazole (136 mg, 2.0 mmol), and MeOH (2 ml) were added, and the mixture was stirred at 80 °C for 8 h. The progress of the reaction was monitored by TLC. After completion of reaction, the reaction mixture was cooled to room temperature. Then MeOH was distilled out, and the organic product was extracted with ethyl acetate (3 × 10 ml), repeatedly washed with distilled water (4 × 5 ml) to remove the unreacted TBHP, dried with anhydrous sodium sulfate, and the solvent was evaporated under reduced pressure to afford methyl benzoate (112 mg, yield 82%).
• 18 Spectral and Analytical Data of some Representative Compounds (Thiophen-2-yl) Methyl Thiophene-2-carboxylate (2c) Yellow oil (yield 85%).¹H NMR (300 MHz, CDCl₃): δ = 5.47 (2 H, s), 7.00–7.02 (1 H, m), 7.07–7.10 (1 H, m), 7.17–7.18 (1 H, m), 7.33–7.35 (1 H, m) 7.55–7.57 (1 H, m), 7.82–7.83 (1 H, m). ¹³C NMR (75 MHz, CDCl₃): δ = 61.1, 126.8, 126.9, 127.7, 128.3, 132.7, 133.4, 133.7, 137.7, 161.9. HRMS (ESI-TOF): m/z calcd for C₁₀H₈O₂S₂ [M + Na⁺]: 246.9866; found: 246.9868. Methyl Benzo[1,3]dioxole-5-carboxylate (6n) Colorless viscous oil (yield 80%). ¹H NMR (300 MHz, CDCl₃): δ = 3.89 (3 H, s), 6.05 (2 H, s), 6.82 (1 H, d, J = 8.8 Hz), 7.45 (1 H, d, J = 1.4 Hz), 7.64 (1 H, dd, J₁ = 9.0 Hz, J₂ = 1.5Hz). ¹³C NMR (75 MHz, CDCl₃): δ = 51.9, 101.7, 107.9, 109.5, 124.2, 125.3, 147.7, 151.6, 166.4. Methyl 4-(2-Acetoxyethoxy)benzoate (9) White solid (mp 76 °C; yield 82%). ¹H NMR (300 MHz, CDCl₃): δ = 2.10 (3 H, s), 3.87 (3 H, s), 4.22–4.23 (2 H, m), 4.44 (2 H, t, J = 4.3 Hz), 6.92 (2 H, d, J = 8.7 Hz), 7.99 (2 H, d, J = 8.7 Hz). ¹³C NMR (75 MHz, CDCl₃): δ = 20.8, 51.9, 62.5, 65.9, 114.1, 123.1, 131.6, 162.2, 166.7, 170.9. HRMS (ESI-TOF): m/z calcd for C₁₂H₁₄O₅ [M + Na⁺]: 261.0741; found: 261.0736 1-[4-(2-Hydroxyethoxy)phenyl]ethanone (11) Yellowish oil (yield 88%). ¹H NMR (300 MHz, CDCl₃): δ = 2.54 (3 H, s), 3.98 (2 H, t, J = 4.3 Hz), 4.13 (2 H, t, J = 4.4 Hz), 6.93 (2 H, d, J = 8.7 Hz), 7.91 (2 H, d, J = 8.7 Hz). ¹³C NMR (75 MHz, CDCl₃): δ = 29.6, 61.7, 72.2, 114.7, 129.1, 130.1, 162.3, 200.4. HRMS (ESI-TOF): m/z calcd for C₁₀H₁₂O₃ [M + Na⁺]: 203.0687; found: 203.0684 2-(4-Acetyl-phenoxy) Ethyl Acetate (12) Colorless semisolid (yield 85%). ¹H NMR (300 MHz, CDCl₃): δ = 2.08 (3 H, s), 2.54 (3 H, s), 4.13–4.23 (2 H, m), 4.43 (2 H, t, J = 4.4 Hz), 6.93 (2 H, d, J = 8.6 Hz), 7.92 (2 H, d, J = 8.6 Hz). ¹³C NMR (75 MHz, CDCl₃): δ = 20.8, 26.3, 62.5, 66.0, 114.2, 130.6, 130.7, 162.3, 170.9, 196.7. HRMS (ESI-TOF): m/z calcd for C₁₂H₁₄O₄ [M + K⁺]: 261.1872; found: 261.1876.