311
High yield of DPA has been reported using Bronsted acidic ionic liquids (BAILs)
as catalyst [90]. The combination of ionic liquid and propanethiol showed 91%
yield of DPA including high selectivity of p,p′-DPA isomer (ratio of p,p′/o,p′ is
>100) at 60 °C for 48 h (Scheme 9). The catalytic system is also applied for the
preparation of diphenolic esters by reacting alkyl levulinates with phenol. The alkyl
levulinates, methyl, ethyl and n-butyl, gave 57%, 58% and 75% yield of corresponding diphenolic esters, and diphenolic acid was observed as the side product.
Interestingly, high yield of diphenolic esters was observed using levulinic acid as
the reactant in a suitable alcoholic medium. Methanol, ethanol and n-butanol
resulted in 87%, 83% and 86% yield of the corresponding diphenolic esters, and a
minor yield of diphenolic acid (2–6%) was reported. The catalytic system had good
stability and was recyclable for four reaction cycles, with a minor decrease in the
product yield.
Recently, Shen et al. reported different acid catalysts for this conversion such as
[BSMim]OAc, as [BSMim]CF 3 SO 3 , [BSMim]HSO 4 , [Bpy]HSO 4 , [AMim]Br,
[BMin]Cl, NH 3 SO 3 H, p-TSA and HCl at 60 °C, 24 h, LA to phenol (1:4) ratio,
50 mol% of catalyst and 1 mol% ethanethiol w.r.t LA. Among them, [BSMim]CF 3 SO 3
and [BSMim]HSO 4 resulted in 100% selectivity for p,p′-DPA with 81 and 75%
conversion of LA. At optimized condition, [BSMim]HSO 4 showed 93% conversion
with desired p,p′-DPA as the only product [91].
Preparation of 4,4-Bis(5-Methylfuran-2-yl) Pentanoic Acid and Esters
Zhang research group disclosed the acid-catalysed preparation of γ,γ-di-2methylfuran pentanoic acid (4,4-bis(5-methylfuran-2-yl) pentanoic acid) and its
esters from levulinic acid and its derivatives (angelica lactone and ethyl levulinate)
with 2-methylfuran by hydroxyalkylation-alkylation (HAA) (Schemes 10 and 11)
[92, 93]. Nafion-212 resin showed good catalytic activity towards the desired product. At reaction conditions (50 °C for 1 h), angelica lactone showed high yield of
HAA product with 81.3%, wherein 12.0% and 4.8% yield of HAA product was
reported from ethyl levulinate and levulinic acid. The reusability of Nafion-212
resin was investigated for six reaction cycles with marginal decrease after the third
run, attributed to handling loss of the catalyst.
Scheme 9 Ionic liquid-catalysed diphenolic acid formation using propanethiol
Levulinic Acid- and Furan-Based Multifunctional Materials: Opportunities…
High yield of DPA has been reported using Bronsted acidic ionic liquids (BAILs)
as catalyst [90]. The combination of ionic liquid and propanethiol showed 91%
yield of DPA including high selectivity of p,p′-DPA isomer (ratio of p,p′/o,p′ is
>100) at 60 °C for 48 h (Scheme 9). The catalytic system is also applied for the
preparation of diphenolic esters by reacting alkyl levulinates with phenol. The alkyl
levulinates, methyl, ethyl and n-butyl, gave 57%, 58% and 75% yield of corresponding diphenolic esters, and diphenolic acid was observed as the side product.
Interestingly, high yield of diphenolic esters was observed using levulinic acid as
the reactant in a suitable alcoholic medium. Methanol, ethanol and n-butanol
resulted in 87%, 83% and 86% yield of the corresponding diphenolic esters, and a
minor yield of diphenolic acid (2–6%) was reported. The catalytic system had good
stability and was recyclable for four reaction cycles, with a minor decrease in the
product yield.
Recently, Shen et al. reported different acid catalysts for this conversion such as
[BSMim]OAc, as [BSMim]CF 3 SO 3 , [BSMim]HSO 4 , [Bpy]HSO 4 , [AMim]Br,
[BMin]Cl, NH 3 SO 3 H, p-TSA and HCl at 60 °C, 24 h, LA to phenol (1:4) ratio,
50 mol% of catalyst and 1 mol% ethanethiol w.r.t LA. Among them, [BSMim]CF 3 SO 3
and [BSMim]HSO 4 resulted in 100% selectivity for p,p′-DPA with 81 and 75%
conversion of LA. At optimized condition, [BSMim]HSO 4 showed 93% conversion
with desired p,p′-DPA as the only product [91].
Preparation of 4,4-Bis(5-Methylfuran-2-yl) Pentanoic Acid and Esters
Zhang research group disclosed the acid-catalysed preparation of γ,γ-di-2methylfuran pentanoic acid (4,4-bis(5-methylfuran-2-yl) pentanoic acid) and its
esters from levulinic acid and its derivatives (angelica lactone and ethyl levulinate)
with 2-methylfuran by hydroxyalkylation-alkylation (HAA) (Schemes 10 and 11)
[92, 93]. Nafion-212 resin showed good catalytic activity towards the desired product. At reaction conditions (50 °C for 1 h), angelica lactone showed high yield of
HAA product with 81.3%, wherein 12.0% and 4.8% yield of HAA product was
reported from ethyl levulinate and levulinic acid. The reusability of Nafion-212
resin was investigated for six reaction cycles with marginal decrease after the third
run, attributed to handling loss of the catalyst.
Scheme 9 Ionic liquid-catalysed diphenolic acid formation using propanethiol
Levulinic Acid- and Furan-Based Multifunctional Materials: Opportunities…