Spontaneous Oxidation of Aromatic Sulfones to Sulfonic Acids in Microdroplets

Abstract

Reactions in microdroplets can be accelerated and can present unique chemistry compared to reactions in bulk solution. Here, we report the accelerated oxidation of aromatic sulfones to sulfonic acids in microdroplets under ambient conditions without the addition of acid, base, or catalyst. The experimental data suggest that the water radical cation, (H2O)+•, derived from traces of water in the solvent, is the oxidant. The substrate scope of the reaction indicates the need for a strong electron-donating group (e.g., p-hydroxyl) in the aromatic ring. An analogous oxidation is observed in an aromatic ketone with benzoic acid production. The shared mechanism is suggested to involve field-assisted ionization of water at the droplet/air interface, its reaction with the sulfone (M) to form the radical cation adduct, (M + H2O)+•, followed by 1,2-aryl migration and C–O cleavage. A remarkably high reaction rate acceleration (∼103) and regioselectivity (∼100-fold) characterize the reaction.

Spontaneous Water Radical Cation Oxidation at Double Bonds in Microdroplets

“…Microdroplet chemistry is an emerging field in chemistry due to the remarkable reaction acceleration ( Girod et al, 2011 ; Müller et al, 2012 ; Yan et al, 2016 , 2017 ; Lee et al, 2017 ; Nam et al, 2017 ; Cooks and Yan, 2018 ; Sahota et al, 2019 ; Zhang et al, 2019 , Zhang et al, 2021 Y. ; Wei et al, 2020 , 2022 ; Cao et al, 2020 ; Kuai et al, 2021 ; Vannoy et al, 2021 ; Zheng et al, 2021 ), and the unique properties of the reaction environment ( Li et al, 2014 ; Lee et al, 2019a , 2019b ; Gao et al, 2019 ; Huang et al, 2021 ; Wang et al, 2021 ; Gong et al, 2022 ; Qiu et al, 2022 ). These features give rise to numerous applications ( Wei et al, 2017 ; Yan et al, 2018 ; Zhu et al, 2019 ; Nie et al, 2020 ; Li Z. et al, 2021 ; Gunawardena et al, 2021 ; Yan, 2021 ; Zhang, 2021 ).…”

“…Both effects accelerate reactions ( Kreuzer, 2004 ; Aragonès et al, 2016 ; Fried and Boxer, 2017 ; Welborn and Head-Gordon, 2019 ; Ashton et al, 2020 ; Shaik et al, 2020 ). Moreover, reactive species can reasonably be proposed to be generated by strong electric fields causing unique reactions ( Lee et al, 2019a , 2019b , 2020 ; Gao et al, 2019 ; Gong et al, 2022 ; Qiu et al, 2022 ).…”

“…Spontaneous oxidation and reduction without the addition of reducing/oxidizing agents has been reported in microdroplets ( Lee et al, 2019a ; Gao et al, 2019 ; Gong et al, 2022 ; Qiu et al, 2022 ), although the species responsible for these redox reactions is still under debate. Data from Zare and coworkers led them to conclude that the reactive species are the hydroxyl radical and hydrogen peroxide ( Gao et al, 2019 ; Lee et al, 2019b , 2019a ).…”

“…On the other hand, Zhang and collaborators postulated that electrons at the microdroplet air-water interface account for the reduction of doubly-charged ethyl viologen to the singly-charged radical cation and subsequent degradation reactions ( Gong et al, 2022 ). Recently, our group suggested that water radical cations can be generated in organic-solvent microdroplets containing trace amounts of water and that they play a key role in the oxidation of aromatic sulfones to sulfonic acids in microdroplets ( Qiu et al, 2022 ). Water radical cations are reported to be naturally abundant in pure bulk water ( Ben-Amotz, 2011 ; Lee and Rick, 2011 ) and are generated by electric discharge in water vapor under otherwise mild conditions ( Wang et al, 2021 ).…”

“…Water radical cations are reported to be naturally abundant in pure bulk water ( Ben-Amotz, 2011 ; Lee and Rick, 2011 ) and are generated by electric discharge in water vapor under otherwise mild conditions ( Wang et al, 2021 ). Notably, in a study preceding this one, water radical cations produced in charged microdroplets were shown to trigger spontaneous oxidation ( Qiu et al, 2022 ). The argument that water radical cation is a key reactive species is supported by the fact that the water radical cation has a higher oxidative power than either the hydroxyl radical or hydrogen peroxide in aqueous solution ( Ma et al, 2014 , 2018 ).…”