Methyl ketones from carboxylic acids as valuable target molecules in the biorefinery

Selective Cross-Ketonization of Carboxylic Acids Enabled by Metallaphotoredox Catalysis

Carboxylic acids are attractive building blocks for synthetic chemistry because they are chemically stable, abundant, and commercially available with substantial structural diversity. The process of combining two carboxylic acids to furnish a ketone is termed ketonization. This is a potentially valuable transformation that has been underutilized in organic synthesis due to the harsh reaction conditions generally required and the lack of selectivity obtained when coupling two distinct carboxylic acids. We report herein a metallaphotoredox strategy that selectively generates unsymmetrical ketones via cross-ketonization of two structurally dissimilar carboxylic acids. Cross-selectivity is achieved by exploiting divergent reactivity of differentially substituted acids towards critical one- and two-electron processes in the proposed coupling mechanism. This method is broadly applicable to a variety of functionalized carboxylic acids. It can also be applied to acids of similar steric profile by exploiting differences in their relative rates of decarboxylation.

Catalytic deoxygenation of fatty acids via ketonization and α-carbon scissions over layered alkali titanate catalysts under N2

The ketonization of fatty acid with subsequent McLafferty rearrangement of the fatty ketone allows the deoxygenation to hydrocarbons. Here, we report the cascade reaction of palmitic acid (C₁₆) to hydrocarbons (≤C₁₄) over lepidocrocite-type alkali titanate K_0.8Zn_0.4Ti_1.6O₄, K_0.8Mg_0.4Ti_1.6O₄, and K_0.8Li_0.27Ti_1.73O₄ and the reassembled TiO₂ catalysts at ≤400 °C under atmospheric N₂ in a continuous fixed-bed flow reactor. The C₁₆ acid is coupled to C₃₁ ketone prior to the scissions mostly to a C₁₇ methyl ketone and C₁₄ hydrocarbons (i.e., the McLafferty rearrangement). The hydrocarbons yield increases with temperature and is proportional to partial charge at the O atom, suggesting that basic sites are responsible for C₃₁ ketone scissions. The layered alkali titanate catalysts with two-dimensional (2D) space inhibit diffusion of the ketone primarily formed and promote its scissions to hydrocarbons within the confined space. Otherwise, low hydrocarbons yield (but high ketone yield) is obtained over TiO₂ and the Mg/Al mixed oxide catalysts possessing no interlayer space. Meanwhile, the semi-batch experiment with pre-intercalated palmitic acid favors a direct deoxygenation, demonstrating the essential role of reaction mode toward ketone scission reaction pathway. Over K_0.8Li_0.27Ti_1.73O₄, the complete palmitic acid conversion leads to ∼47% hydrocarbons yield, equivalent to ∼80% reduction of the oxygen content in the feed under N₂.