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Transformation of alcohols to esters promoted by hydrogen bonds using oxygen as the oxidant under metal-free conditions

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Science Advances  05 Oct 2018:
Vol. 4, no. 10, eaas9319
DOI: 10.1126/sciadv.aas9319
  • Fig. 1 Plausible pathway for the oxidative self- or cross-esterification reaction.
  • Fig. 2 The synergistic and/or cooperative effect of the cation and anion of [EMIM] OAc detected by 1H NMR spectra.

    (A) 1H nuclear magnetic resonance (NMR) spectra of benzyl alcohol and its mixtures with [EMIM] OAc with various molar ratios (molar ratio of benzyl alcohol to [EMIM] OAc = 10:1, 5:1, 1:1, and 1:5). (B) 1H NMR spectra of benzyl alcohol and its mixtures with [EMIM] TFA. (C) 1H NMR spectra of benzyl alcohol and its mixtures with [EMIM] BF4. For recording the spectra, DMSO was used as internal standard. The resonance band of hydroxyl protons is labeled by asterisk. Details are in figs. S4 to S7 and table S2 to S4.

  • Fig. 3 1D-selective 1H NOE and 1H NMR spectra of the mixture of benzyl alcohol and [EMIM] OAc.

    (1) the hydroxyl resonance at 5.57 ppm was selectively irradiated. The integral of the irradiated (negative, peak 10) peak was assigned a value of 100%. The absolute values of other NOE intensities were measured relative to peak 10. (2) The methyl resonance of acetate at 1.20 ppm was selectively irradiated. (3) Normal 1H NMR spectrum. The molar ratio of benzyl alcohol to [EMIM] OAc in the mixture was 10:1. s-Trioxane was used as an internal standard at 5.11 ppm.

  • Table 1 Self-esterification of benzyl alcohol to benzyl benzoate in various ILs.

    Reaction conditions: Benzyl alcohol (2 mmol), 1 MPa O2, 2 g of ILs (12 hours, 80°C). Yields were determined using gas chromatography (GC).


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    EntryILsConversion (%)Yield (%)
    1[EMIM] OAc>9994
    2[EMIM] TFA100
    3[EMIM] HSO4160
    4[EMIM] BF430
    5[EMIM] N(CN)2<10
    6[OMIM] OAc<10
    7*[N4,4,4,4] OAc260
    8NH4Ac/DMSO50

    *Benzyl alcohol (2 mmol), 1 MPa O2, 2 g of ILs, 4 hours, 110°C.

    †Benzyl alcohol (2 mmol), 2 MPa O2, 0.4 g of NH4Ac, 2 g of DMSO, 12 hours, 80°C.

    • Table 2 Self-esterification of aryl- and alkyl- Alcohols in [EMIM] OAc.

      Reaction conditions: benzyl alcohol (2 mmol), 1 MPa O2, 2 g of [EMIM] OAc, 12 hours, 80°C. Yields were determined using GC.


      Embedded Image

      *2 MPa O2.

      †100°C, 36 hours.

      ‡2 MPa O2, 110°C, 48 hours.

      • Table 3 Cross-esterification of benzylic alcohols and aliphatic alcohols in [EMIM] OAc.

        Reaction conditions: Benzyl alcohol (2 mmol), aliphatic alcohol (8 mmol), 2 MPa O2, 2 g of [EMIM] OAc, 12 hours, 80°C.


        Embedded Image

        *90°C, 24 hours.

        †100°C, 24 hours.

        Supplementary Materials

        • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/10/eaas9319/DC1

          Fig. S1. The structures of ILs used in this study.

          Fig. S2. Product distributions for the oxidative transformation of propanol to propyl propionate in [EMIM] OAc.

          Fig. S3. NMR spectra of benzaldehyde and [EMIM] OAc.

          Fig. S4. NMR spectra of the mixture of benzaldehyde and [EMIM] OAc.

          Fig. S5. 1H NMR spectra of the reaction media of the oxidative esterification reaction of benzyl alcohol (that is, the reaction in Table 1, entry 1) after 3- (yellow) and 12-hour reaction (green), together with that of the mixture of benzaldehyde and [EMIM] OAc (red).

          Fig. S6. Recycling of [EMIM] OAc for oxidative esterification of benzyl alcohol and ethanol.

          Fig. S7. Possible acid-base equilibrium in [EMIM] OAc.

          Fig. S8. Effects of free carbene (products of the reaction between azolium salts A or B and DBU) on the oxidative reactions of benzyl alcohol and benzyl aldehyde.

          Fig. S9. Isotope labeling experiments with 18O-enriched benzyl alcohol.

          Fig. S10. Mass spectra of substrates and products for the experiments with 18O-enriched benzyl alcohol.

          Fig. S11. 1H NMR spectrum of mixture of benzyl alcohol and [EMIM] OAc (molar ratio of benzyl alcohol to [EMIM] OAc = 1:1).

          Fig. S12. 1H NMR spectrum of mixture of benzyl alcohol and [EMIM] OAc (molar ratio of benzyl alcohol to [EMIM] OAc = 1:5).

          Fig. S13. 1H NMR spectra of mixture of benzyl alcohol and [EMIM] TFA (molar ratio of benzyl alcohol to [EMIM] TFA = 1:1).

          Fig. S14. 1H NMR spectra of mixture of benzyl alcohol and [EMIM] TFA (molar ratio of benzyl alcohol to [EMIM] TFA = 1:5).

          Fig. S15. FTIR spectra of benzyl alcohol, [EMIM] OAc, and their mixtures with various molar ratios.

          Fig. S16. DOSY NMR spectra.

          Fig. S17. 1H NMR spectrum of [EMIM] OAc before (above) and after (below) drying in vacuum.

          Fig. S18. Variation of moisture content and pH of the reaction media during the oxidative esterification reaction.

          Fig. S19. Effects of water content for the oxidative esterification of benzyl alcohol.

          Table S1. Self-esterification of benzyl alcohol to benzyl benzoate in various ILs with or without additives.

          Table S2. Chemical shifts and FWHM of the hydroxyl group resonance band in 1H NMR spectra of the mixtures of benzyl alcohol and [EMIM] OAc with various molar ratios.

          Table S3. Chemical shifts and FWHM of the hydroxyl group resonance band in 1H NMR spectra of mixture of benzyl alcohol and [BMIM] TFA with various molar ratios.

          Table S4. Chemical shifts and FWHM of the hydroxyl group resonance band in 1H NMR spectra of mixture of benzyl alcohol and [EMIM] BF4 with various molar ratios.

          Table S5. Summary of assignments and shifts of frequencies (cm−1) of absorption bands in the FTIR spectra in fig. S15. (ν, stretch; s, symmetric; as, antisymmetric).

          Table S6. Summary of diffusion coefficients.

        • Supplementary Materials

          This PDF file includes:

          • Fig. S1. The structures of ILs used in this study.
          • Fig. S2. Product distributions for the oxidative transformation of propanol to propyl propionate in EMIM OAc.
          • Fig. S3. NMR spectra of benzaldehyde and EMIM OAc.
          • Fig. S4. NMR spectra of the mixture of benzaldehyde and EMIM OAc.
          • Fig. S5. 1H NMR spectra of the reaction media of the oxidative esterification reaction of benzyl alcohol (that is, the reaction in Table 1, entry 1) after 3- (yellow) and 12-hour reaction (green), together with that of the mixture of benzaldehyde and EMIM OAc (red).
          • Fig. S6. Recycling of EMIM OAc for oxidative esterification of benzyl alcohol and ethanol.
          • Fig. S7. Possible acid-base equilibrium in EMIM OAc.
          • Fig. S8. Effects of free carbene (products of the reaction between azolium salts A or B and DBU) on the oxidative reactions of benzyl alcohol and benzyl aldehyde.
          • Fig. S9. Isotope labeling experiments with 18O-enriched benzyl alcohol.
          • Fig. S10. Mass spectra of substrates and products for the experiments with 18O-enriched benzyl alcohol.
          • Fig. S11. 1H NMR spectrum of mixture of benzyl alcohol and EMIM OAc (molar ratio of benzyl alcohol to EMIM OAc = 1:1).
          • Fig. S12. 1H NMR spectrum of mixture of benzyl alcohol and EMIM OAc (molar ratio of benzyl alcohol to EMIM OAc = 1:5).
          • Fig. S13. 1H NMR spectra of mixture of benzyl alcohol and EMIM TFA (molar ratio of benzyl alcohol to EMIM TFA = 1:1).
          • Fig. S14. 1H NMR spectra of mixture of benzyl alcohol and EMIM TFA (molar ratio of benzyl alcohol to EMIM TFA = 1:5).
          • Fig. S15. FTIR spectra of benzyl alcohol, EMIM OAc, and their mixtures with various molar ratios.
          • Fig. S16. DOSY NMR spectra.
          • Fig. S17. 1H NMR spectrum of EMIM OAc before (above) and after (below) drying in vacuum.
          • Fig. S18. Variation of moisture content and pH of the reaction media during the oxidative esterification reaction.
          • Fig. S19. Effects of water content for the oxidative esterification of benzyl alcohol.
          • Table S1. Self-esterification of benzyl alcohol to benzyl benzoate in various ILs with or without additives.
          • Table S2. Chemical shifts and FWHM of the hydroxyl group resonance band in 1H NMR spectra of the mixtures of benzyl alcohol and EMIM OAc with various molar ratios.
          • Table S3. Chemical shifts and FWHM of the hydroxyl group resonance band in 1H NMR spectra of mixture of benzyl alcohol and BMIM TFA with various molar ratios.
          • Table S4. Chemical shifts and FWHM of the hydroxyl group resonance band in 1H NMR spectra of mixture of benzyl alcohol and EMIM BF4 with various molar ratios.
          • Table S5. Summary of assignments and shifts of frequencies (cm−1) of absorption bands in the FTIR spectra in fig. S15. (ν, stretch; s, symmetric; as, antisymmetric).
          • Table S6. Summary of diffusion coefficients.

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