Direct Anodic Conversion of 4-Hydroxybenzaldehydes into Benzoquinones

Abstract

A novel electrosynthetic protocol for the direct conversion of easily accessible 4-hydroxybenzaldehydes into valuable benzoquinones is reported. The transformation is enabled through anodic oxidation and is performed in a reagent-free manner devoid of terminal oxidants and redox mediators to effectively minimize waste. Environmentally benign and inexpensive graphite serves as a metal-free anode material. A broad range of substituents, including halogens, are tolerated in this electrochemical protocol, and the quinones are selectively accessible in yields up to 99%. Furthermore, the compatibility with electrochemical flow synthesis is demonstrated. Moreover, the flow protocol is successfully applied to the biopolymer lignosulfonate and provides quinones in yields up to 7.6 wt%. A mechanistic proposal for this Dakin-type reaction is offered. The sustainability and synthetic utility are evaluated quantitatively and compared to established protocols by application of different green chemistry metrics.

E-Dakin reaction: oxidation of hydroxybenzaldehydes to phenols with electrochemically generated peroxodicarbonate as sustainable ex-cell oxidizer

“…In contrast to that the direct electrochemical conversion inevitably yields the benzoquinones due to the galvanostatic conditions. 52,72 Therefore, this novel protocol complements our previous efforts by expanding the space of obtainable products. Ultimately, we tested the scalability of our established protocol.…”

“…50,51 Recently, we reported an electrosynthetic method to transform 4-hydroxybenzaldehydes into para -benzoquinones via direct anodic oxidation. 52 Although hydroquinones are formed as intermediates in this process, a galvanostatic electrolysis will always proceed all the way to benzoquinones. After all, the hydroquinone oxidation has a lower oxidation potential than the first step.…”