Metal- and additive-free photoinduced borylation of haloarenes

Abstract

Boronic acids and esters have crucial roles in the areas of synthetic organic chemistry, molecular sensors, materials science, drug discovery, and catalysis. Many of the current applications of boronic acids and esters require materials with very low levels of transition metal contamination. Most of the current methods for the synthesis of boronic acids require, however, transition metal catalysts and ligands that have to be removed via additional purification procedures. This protocol describes a simple, metal- and additive-free method of conversion of haloarenes directly to boronic acids and esters. This photoinduced borylation protocol does not require expensive and toxic metal catalysts and ligands and produces innocuous and easy-to-remove by-products. Furthermore, the reaction can be carried out on multigram scales in common grade solvents without the need for reaction mixtures to be deoxygenated. The setup and purification steps are typically accomplished within 1–3 h. The reactions can be run overnight, and the protocol can be completed within 13–16 h. Two representative procedures that are described in this protocol provide details for preparation of a boronic acid (3-cyanopheylboronic acid) and a boronic ester (1,-benzenediboronic acid bis(pinacol)ester). We also discuss additional details of the method that will be helpful in the application of the protocol to other haloarene substrates.

Flow Photochemistry as a Tool in Organic Synthesis

“…[86] The incorporation of more than one boronate group into an aromatic system leads to highly interesting aryl buildings blocksw ith multiple sites for coordination [87] or further synthetic manipulation. [88] An ovel methodology for ar egioselective CÀH/CÀXd iborylation of iodo-, bromo-, and chloroaryls with B 2 pin 2 (pin = pinacolato) was developed by Larionov et al [89] The additive-and metal-free photoinduced process allowst he straightforward synthesis of 1,2-or 1,3-diborylated aryls depending on the choice of the solvent and the position and electronic properties of other substituents in the aryl halogen substrate (Figure 29). In the case of am edium polar solvent like 2-propanol, the 1,2-regioisomer is favored, whereas highly polar hexafluoro-2-propanol directst he reaction pathway towards the 1,3-isomer.F urthermore, the authors point out that, if as trong electron-withdrawing substituent is present in the para-position or as trong electron-donatings ubstituent is present in the meta-position, 1,2-diborylation also becomes dominanti nh exafluoro-2-propanol.…”

Photoinduced Borylation Reactions: An Overview

“…Usually, preparation of aryl chlorides into its corresponding borylated product was very difficult but some reports were available by using several transition metal catalysts (e. g., Pd, Cu, Ni, Co, [71] and Zn [72–75] ). Various research groups across the world have developed the new methodologies particularly on photochemical borylation reactions of haloarenes [21,49,67,68,70] . However, most of the reported methods have shown limitations compare to transition metal mediated reactions and by the use of most reactive aryl iodides and bromides as substrates.…”

“…Various research groups across the world have developed the new methodologies particularly on photochemical borylation reactions of haloarenes. [21,49,67,68,70] However, most of the reported methods have shown limitations compare to transition metal mediated reactions and by the use of most reactive aryl iodides and bromides as substrates.…”

Organoboron chemistry comes to light: Recent advances in photoinduced synthetic approaches to organoboron compounds

“…A detailed experimental procedure was subsequently reported to facilitate the use of the photoinduced borylation method. 29 Mechanistically, initial excitation is followed by intersystem crossing to triplet 4 ( Figure 4). Three divergent pathways were proposed for the subsequent borylation.…”