Mechanistic Insights and Kinetic Analysis for the Oxidative Hydroxylation of Arylboronic Acids by Visible Light Photoredox Catalysis: A Metal-Free Alternative

Abstract

The photocatalytic hydroxylation of boronic acids with methylene blue as photosensitizer proceeds with high efficiency. Detailed time-resolved studies of the relevant rate constants provide a clear mechanistic understanding of excited-state processes and guided the selection of the photocatalyst and the optimization of experimental conditions.

Photocatalytic hydroxylation of arylboronic acids using continuous flow reactors

“…67 : 1. According to previous studies, 17 the use of pure oxygen is expected to increase kinetics. However, increasing the air to substrate ratio led to lower conversion which might be attributed to a decrease in residence time (entry 4).…”

“…17,18 Rose Bengal (RB) and [Ru(bpy) 3 ] 2+ were used as photocatalysts providing an opportunity to compare organic and transition-metal catalysts. We also investigated the use of both iPr 2 EtN and ascorbic acid (AA) as reductive quenchers for the photocatalytic cycle.…”

“…3,[7][8][9][10][11][12][13][14] Recently, a number of alternative methods have been described, based on the generation of superoxide free radical species (O 2 c À ). 15,16 In this context, photocatalysis has emerged as a promising methodology [17][18][19] which is compatible with a variety of substrates and catalysts, including metal-free dyes.…”

“…Aer hydrolysis, the reaction yields phenol with high selectivity. 18 Scaiano and co-workers 17,20 recently outlined important mechanistic details of the reaction and, in particular, found that organic dyes such as methylene blue exhibited Â40 higher activity. Even so, the batch photosynthesis of phenols requires relatively long irradiation times (>6 h) using O 2 saturated solutions.…”

Porous conjugated polymer via metal-free synthesis for visible light-promoted oxidative hydroxylation of arylboronic acids

“…We also demonstrate that the resulted PCP could serve as a reusable and efficient visible light heterogeneous photocatalyst for light-induced oxidative hydroxylation of arylboronic acids using molecular oxygen as green oxidant. [39,40] …”