Abstract
Here, 22-carbon tricarboxylic acid (C22TA) and its ester (C22TAE) were prepared via the Diels–Alder reaction of polyunsaturated fatty acids (PUFAs) and their esters (PUFAEs) as dienes with fumaric acid (FA) and dimethyl fumarate (DF) as dienophiles, respectively. The role of an iodine catalyst for the synthesis of C22TA and C22TAE in the Diels–Alder type reaction was investigated using a spectroscopic approach. The chemical structures of the products were characterized using proton nuclear magnetic resonance (1H-NMR) and electrospray ionization mass spectrometry (ESI-MS) analysis. Results showed that nonconjugated dienes can react with dienophiles through a Diels–Alder reaction with an iodine catalyst, and that iodine transformed the nonconjugated double bonds of dienes into conjugated double bonds via a radical process. DF was more favorable for the Diels–Alder reaction than FA. This was mainly because the dienophile DF contained an electron-withdrawing substituent, which reduced the highest and lowest occupied molecular orbital (HOMO–LUMO) energy gap and accelerated the Diels–Alder reaction. By transforming nonconjugated double bonds into conjugated double bonds, iodine as a Lewis acid increased the electron-withdrawing effect of the carbonyl group on the carbon–carbon double bond and reduced the energy difference between the HOMO of diene and the LUMO of dienophile, thus facilitating the Diels–Alder reaction.
Highlights
With the dramatic decrease in global fossil fuels and the increasing environmental problems and concerns, the use of renewable resources received great attention
In the work reported here, we focused on answering the following questions: What is the role of iodine in the Diels–Alder reaction? What is the effect of substituents of reactants on the reaction efficiency of dienophiles? What is the difference between conjugated double bonds and nonconjugated double bonds when participating in the Diels–Alder reaction? We took a spectroscopy approach by combining the results of 1 H-NMR, 13 C-NMR, and electrospray ionization mass spectrometry (ESI-MS), and we systematically investigated the reaction processes of different
C22TA and C22TAE were prepared via the Diels–Alder reaction of polyunsaturated fatty acids (PUFAs) and Polyunsaturated fatty acid esters (PUFAEs) as dienes with fumaric acid (FA) and dimethyl fumarate (DF) as dienophiles, respectively, under various experimental conditions (Table 1)
Summary
With the dramatic decrease in global fossil fuels and the increasing environmental problems and concerns, the use of renewable resources received great attention. Social, and economic development is a global consensus [1]. One approach is the development and utilization of vegetable oil-based lubricants to partially or even comprehensively substitute petrochemical resources. Mineral oil cannot be used as the base oil of biodegradable lubricating oil because it causes serious pollution when discharged into the environment. It stays in water, soil, forest, and other ecosystems for a long time. It would be highly desirable to develop biodegradable, environmentally benign lubricating oil and its derivatives from plant oil and vegetable oil as raw materials [2]
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