Self-Thickening Materials Derived from Phenylpropanoid Ene Reactions

Abstract

In this work, we report the observation of uncatalyzed ene reactions between several phenylpropanoid compounds and diethyl azodicarboxylate (DEAD). For allylbenzene, the reaction produces the ene product at molar ratios of up to 1:2 of allylbenzene to DEAD. At higher levels of DEAD, more complex reactions are observed. For the reaction between methyl eugenol and DEAD, similar ene reaction products have been found. However, the reaction of eugenol with DEAD is more complex; in addition to the ene reaction, other reactions happen at the same time. Most of the structures of the resulting products have been elucidated using NMR spectroscopy (¹H, ¹³C, and 2D), and the findings have been further corroborated by FTIR analysis. Interestingly, these products appear to undergo molecular aggregation, which results in self-thickening in their neat form. However, the viscosity significantly decreases upon dilution with a solvent. This self-thickening property suggests their potential use as thickening agents in organic solvent formulations.

Keywords: allylbenzene; diethyl azodicarboxylate; ene reaction; eugenol; methyl eugenol; phenylpropanoids; self-thickening.

Scheme 1

Natural products with allylbenzene functionality: eugenol (I), estragole (II), anethole (III), safrole (IV), and methyl eugenol (V).

Scheme 2

Reaction using heat between allylbenzene and DEAD.

Figure 1

Photographs of (a) samples A-2, M-2, and E-2 (at ca. 1:2 molar ratio of alkene/DEAD); (b) the same three samples after overnight heating at 90 °C.

Figure 2

NMR spectra of the 1:2 reaction products between allylbenzene and DEAD (sample A-2): (a) ¹³C spectrum and (b) ¹H spectrum. U and A denote unreacted and ene products, respectively; the subscripts correspond to the numbering shown in Scheme 3. The letter X indicates the CDCl₃ peaks, and the letter s denotes ethyl acetate peaks.

Scheme 3

Structures for unreacted allylbenzene (U) and the AB-DEAD ene reaction derivative (A) and three other structures (C,D,E). Structures (U) and (A) are numbered to facilitate NMR assignments.

Scheme 4

Structures for unreacted methyl eugenol (V) and the methyl eugenol/DEAD ene reaction derivative (G), numbered to facilitate NMR assignments.

Scheme 5

Structures for the eugenol/DEAD ene derivative (H), numbered to facilitate NMR assignments, and two other possible structures produced in the eugenol/DEAD reaction.

Figure 3

NMR spectra of the 1:3 reaction products between AB and DEAD (sample A-5): (a) ¹³C spectrum, (b) ¹H spectrum. A and C denote ene products and ethoxyformaldehyde, respectively; the subscripts correspond to the numbering shown in Scheme 3. R denotes the peaks from residual DEAD moieties after the reaction and X the CDCl₃ peaks.

Figure 4

NMR spectra of the reaction products between methyl eugenol and DEAD (sample M-1): (a) ¹³C spectrum and (b) ¹H spectrum. V and G denote unreacted methyl eugenol and ene products; the subscripts correspond to the numbering shown in Scheme 4. X denotes the CDCl₃ peak, and s denotes ethyl acetate peaks.

Figure 5

NMR spectra of the reaction products between eugenol and DEAD (sample E-1). (a) ¹³C spectrum, (b) ¹H spectrum. I and H denote unreacted eugenol and ene products, respectively; the subscripts correspond to the numbering shown in Scheme 5. R denotes the peaks from residual DEAD moieties after the reaction, and X represents the CDCl₃ peaks.

Figure 6

FTIR spectra of starting alkenes and ene reaction products (at 1:2 molar ratio of alkene/DEAD). From bottom to top: AB and AB-DEAD (sample A-2), ME and ME-DEAD (sample M-2), and eugenol and eugenol/DEAD (sample E-2).

Figure 7

Viscosity buildup (in Pa-s) during the reaction of AB and DEAD as a function of reaction time (in seconds) at 87 °C for 19.4 h.

Figure 8

Viscosity of the AB-DEAD reaction products at different wt % DEAD measured as a function of increasing temperature; the samples are (from bottom to top) A-R1, A-R2, A-R3, and A-R4.

Figure 9

Viscosity of the ME-DEAD reaction products at different wt % DEAD measured as a function of temperature; the samples are (from bottom to top) M-R1, M-R2, M-R3, and M-R4.

Figure 10

Viscosity of the eugenol/DEAD reaction products at different wt % DEAD measured as a function of temperature; the samples are (from bottom to top) E-R1, E-R2, and E-R3.