Catalyst-free decarboxylation of 4-hydroxycinnamic acids: efficient synthesis of 4-vinylphenols

Abstract

We report herein an efficient protocol for the synthesis of 4-vinylphenols by a catalyst-free decarboxylation of trans -4-hydroxycinnamic acids. A variety of 4-vinylphenols has been synthesized in moderate to excellent yields. This protocol also features no polymerization.

Characterization of Colorants Formed by Non-Enzymatic Browning Reactions of Hydroxycinnamic Acid Derivatives

“…In a more recent investigation, it was reported that heat treatment in polar and protic solvents enabling proton transfer resulted in a yield twice as high compared to reactions performed in apolar media. In contrast to the data discussed herein, longer reaction times (30 min at 200 °C) were needed for a comparable conversion of caffeic acid, and no polymerization of 4-vinylcatechol was reported under these conditions [ 13 ]. In the present study, a revised mechanism that does not require any medium for proton transfer and makes the rapid conversion of caffeic acid more plausible is proposed in Figure 4 : Under roasting conditions, caffeic acid 1 performs an intramolecular proton transfer and decarboxylation, yielding 4-vinylcatechol 2 .…”

“…Nevertheless, consecutive reactions leading to further enlargement of the conjugated electron system must occur to explain the intense color formation observed in this study. In a recent investigation, it was reported that the electron-donating and -withdrawing groups of 4-hydroxycinnamic acids increase the decarboxylation rate during thermal treatment [ 13 ]. In combination with the data discussed herein, the hypothesis could be formulated that the substitution of the structurally related phenols significantly impacts their color formation under roasting conditions, whereby a higher electron density of the aromatic system correlates with higher extinction coefficients of the resulting reaction products.…”

“…The conversion of the reactants during the reaction was analyzed by HPLC-DAD ( Figure 2 A). Earlier studies reported that the thermal treatment of these compounds induced decarboxylation [ 11 , 13 , 32 ]. In the present study, pH values were used as an indication of the prevalence of decarboxylation reactions ( Figure 2 B).…”

“…Phenolic compounds have been proven to mitigate toxic effects by trapping reactive carbonyl intermediates [ 8 , 9 ] and, thereby, inhibiting the formation of heterocyclic aromatic amines and advanced glycation end-products (AGEs) [ 10 ]. Previous studies have revealed the high reactivity of phenolic compounds during thermic treatment, but little is known about their part in the Maillard reaction, especially regarding the formation of colorants [ 11 , 12 , 13 ]. α-Dicarbonyl compounds formed from the Maillard reaction such as 3-deoxyglucosone, methylglyoxal, and glyoxal were characterized as important reactants in browning reactions [ 14 , 15 , 16 , 17 ] and are prevalent in a large number of plant-based foods and ingredients [ 18 ].…”

Highly efficient synthesis of sustainable bisphenols from hydroxycinnamic acids

“…However, recently, Yang and coworkers synthesized various functionalized 4-vinylphenols from hydroxycinnamic acids by heating dilute solutions of the acids in N , N -dimethylformamide (DMF), above the boiling point, in sealed reactors. 57 This approach eliminated the need for catalysts or other additives, but may be difficult to scale-up due to the use of low concentrations and elevated pressures.…”