Photocatalyzed Functionalization of Alkenoic Acids in 3D-Printed Reactors

Abstract

The valorization of alkenoic acids possibly deriving from biomass (fumaric and citraconic acids) was carried out through conversion in important building blocks, such as γ-keto acids and succinic acid derivatives. The functionalization was carried out by addition onto the C=C double bond of radicals generated under photocatalyzed conditions from suitable hydrogen donors (mainly aldehydes) and by adopting a decatungstate salt as the photocatalyst. Syntheses were performed under batch (in a glass vessel) and flow (by using 3D-printed reactors) conditions. The design of the latter reactors allowed for an improved yield and productivity.

Keywords: 3D-printed reactors; carboxylic acids; flow chemistry; photocatalysis; radical reactions.

Scheme 1

a) Biomass‐derived alkenoic acids tested in this work. b) Decatungstate photocatalyzed addition of aldehydes onto alkenoic acids.

Scheme 2

Photocatalyzed functionalization of fumaric acid 1 a. Isolated yields for reactions performed under batch (2.5 mmol scale) or flow conditions (3D‐printed Reactor A–1.0 mmol scale; figures within parentheses on gray background). Typical reaction conditions: A solution of fumaric acid 1 a (0.1 m), hydrogen donors 2 a–v (0.15–2 m) and TBADT (2 mol%) in a MeCN : H₂O 9 : 1 mixture was deaerated by argon bubbling for 10 min and irradiated using a 370 nm LED lamp (see Supporting Information for details). [a] Irradiation performed at 390 nm. [b] Reaction medium: MeCN/CH₂Cl₂ 5 : 1 mixture; reaction not performed under flow conditions due to incompatibility issues with the reactor material.

Scheme 3

Photocatalyzed functionalization of citraconic acid 1 b. Isolated yields for reactions performed under batch (2.5 mmol scale) of flow conditions (3D‐printed Reactor A–1.0 mmol scale; figures among parentheses on a gray background). Typical reaction conditions: A solution of citraconic acid 1 b (0.1 m), hydrogen donors 2 a–u (0.15–1 m) and TBADT (2 mol %) in MeCN was deaerated by argon bubbling for 10 min and irradiated using a 370 nm LED lamp (see Supporting Information for details). [a] Only major diastereoisomer shown; the reaction between 1 b and 2 s delivered product 4 s as a 1.5 : 1 diastereoisomeric mixture arising, respectively, from the anti and syn addition to the double bond. [b] Only major diastereoisomer shown; the reaction between 1 b and 2 t delivered product 4 t as a 4.5 : 1 diastereoisomeric mixture arising, respectively, from the anti and syn addition to the double bond. [c] Reaction medium: MeCN/CH₂Cl₂ 5 : 1 mixture; reaction not performed under flow conditions due to incompatibility issues with the reactor material.

Figure 1

ORTEP view of 4 o with labeling scheme (thermal ellipsoids are drawn at the 30 % probability level).

Scheme 4

Proposed reaction mechanism.

Scheme 5

Diastereoselectivity in the radical addition onto 1 b.