Photocatalytic Hydrodecarboxylation of Fatty Acids for Drop‐in Biofuels Production

Abstract

A mild, practical, and environmentally friendly method for the hydrodecarboxylation of fatty acids using an acridine‐based photoredox catalyst and thiophenol was developed. Cn‐1 alkanes were synthesized in good to excellent yields (up to 99 %) from C10−C18 saturated fatty acids under visible light irradiation (405 nm). The developed protocol was employed for a mixture of fatty acids obtained from the hydrolysis of Licuri oil, affording a mixture of C9−C17 hydrocarbons in quantitative yield, which demonstrates the potential application of the method to produce drop‐in biofuels.

Photocatalyzed hydrodecarboxylation of fatty acids: a prospective method to produce drop-in biofuels

“…The solvents were chosen based on the existing literature. 26–28,40 Reactions were carried out on a 0.2 mmol scale in N 2 -sparged solvents, considering a concentration of about 0.1 mol L −1 , at ambient temperature for 24 h, and the yield was determined from GC-MS data by using hexadecane as an internal standard. According to the data the mixture toluene/H 2 O (9 : 1) was considered the ideal solvent system providing 2a in 44% (Table 1, entry 2).…”

“…Among the approaches that use less severe operating conditions, photocatalytic processes have emerged as an alternative to traditional protocols. 24–28 In this sense, photocatalysis has been demonstrating significant advancements in the synthesis of value-added chemical products. 29–32 Specifically, it has increased the interest in investigating these processes aiming at deoxygenation reactions of carboxylic acids to generate alkanes, including instances involving bioderived fatty acids that produce hydrocarbons chemically identical to drop-in biofuels.…”

“…To steer clear of using hydrogen gas and simultaneously prevent secondary reactions arising from the instability of the primary radicals formed, several protocols use more easy-to-handle hydrogen atom transfer (HAT) co-catalyst, like thiols and disulfides. 26,35 In this context, Nicewicz and colleagues reported a direct photocatalytic system using an acridinium salt as a photocatalyst (Mes-Acr-Ph), diphenyl disulfide as HAT co-catalyst, and N , N -diisopropylethylamine (DIPEA) for the hydrodecarboxylation of various aliphatic carboxylic acids into alkanes 27 in high yields. However, the reaction scope was limited to only one fatty acid (tridecanoic acid).…”

Multimodal Acridine Photocatalysis Enables Direct Access to Thiols from Carboxylic Acids and Elemental Sulfur

“…The structural diversity of carboxylic acids that is evident from the broad span of their chemical space across the domains of molecular complexity, fraction of sp 3 carbon atoms (Fsp 3 ), and geometric diversity , renders them some of the most effective alkyl radical precursors for the purpose of accessing the chemical space of diverse functionalities that is not achievable by currently available transformations. − …”

“…Acridine photocatalysis is an emergent catalytic platform that has enabled direct decarboxylative functionalization of carboxylic acids, providing previously unavailable synthetic shortcuts to bioisosteric functional groups, synthetic intermediates, and new advanced materials. ,, The photocatalytic radical generation is facilitated by photoinduced proton-coupled electron transfer (PCET) within the singlet excited state of the acridine–carboxylic acid hydrogen bond complex, obviating stepwise preactivation of the carboxylic group that is typically required to bypass the challenging oxidative decarboxylation. However, the scope of the direct decarboxylative functionalization remains narrow, and the primary catalytic mode of 9-arylacridines is confined to PCET with carboxylic acids, while the possibility of catalyzing other types of reactions, e.g., hydrogen atom transfer (HAT) from other substrates is understudied …”