(475e) Tuning Pathways for the Diversification of Biomass-Derived Coumalic Acid- Insights from First-Principles
Understanding the factors that control the regio-chemistry of Diels-Alder addition is very crucial so that theoretical yields for different regio-isomers can be predicted prior to synthesizing these compounds. Theoretical results find that electron-donating dienophiles highly favor para addition over meta. While electron-withdrawing dienophiles also favor para addition, the ratio of para to meta is much closer to unity. This is explained using frontier orbitals interactions as well as steric hindrance during addition. We further explore the influence of different solvents on the formation and reactivity of the cycloadduct. We find that the Diels-Alder addition reaction as well as decarboxylation is accelerated in polar protic solvents such as water and methanol via stabilization of the charged transition state. However, protic solvents are found to result in a slight degradation of coumalic acid. In addition, weakly acidic protic solvents such as methanol result in ring opening similar to the BrÃ¸nsted acid catalyzed ring opening reaction instead of decarboxylation. Water degrades coumalic acid, but, does not induce ring opening by itself. Dry polar aprotic solvents such as dioxane were found to result in decarboxylation alone. Thus, the choice of solvent is found to critically alter product selectivity and is very likely the result of rapid proton shuttling offered by protic solvents. Therefore, through the choice of solvent and catalyst (Lewis acid or BrÃ¸nsted acid) we can functionalize coumalic acid to yield different aromatics, dihydro-aromatics, and isophthalic intermediates. Our results have direct implications towards high-throughput and optimized manufacture of conventional compounds as well as a plethora of unique compounds inaccessible via petroleum.
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