Solvent‐Enabled Radical Selectivities: Controlled Syntheses of Sulfoxides and Sulfides

Abstract

Controlling selectivity is of central importance to radical chemistry. However, the highly reactive and unstable radical intermediates make this task especially challenging. Herein, a strategy for taming radical redox reactions has been developed, in which solvent-bonding can alter the reactivity of the generated radical intermediates and thereby drastically alter the reaction selectivity at room temperature. Various β-oxy sulfoxides and β-hydroxy sulfides can be facilely obtained, some of which are difficult to synthesize by existing methods. Notably, neither a metal catalyst nor any further additives are necessary in these processes.

Organophotocatalytic Generation of N‐ and O‐Centred Radicals Enables Aerobic Oxyamination and Dioxygenation of Alkenes

“…The photocatalyst PC-I* (E ox 1/2 = + +2.06 V) [13] then serves as a1 e-oxidant to oxidizeT EMPO (E 1/2 =+ +0.62 V) into its highly oxidizing state, TEMPO + + ,t hrough an SET process, [24] with the releaseo ft he reduced photocatalyst PC-I À .S ubsequently,a nother SET oxidation of the nitrogen anion A (generatedf rom 1a by deprotonation) by the highly oxidizingT EMPO + + gives rise to the key N-centred radicali ntermediate B,t ogether with the regeneration of TEMPOt oc lose the organocatalytic cycle. A5 -exo radicalc yclization of N-radical B gives rise to C-centred radical C (Scheme 3a,P ath A) that could be easily trapped by O 2 to afford alkylhydroperoxy radi-cal D. [25] Then, the intermediate D undergoes as equential SET reduction and protonation to give 8.F inally,r eduction of 8 by PPh 3 furnishest he target product 2a.T he reduced photocatalyst PC-I À À speciesc an be oxidized by O 2 or hydroperoxy radical D to regeneratet he ground state photocatalyst PC-I and complete the photocatalytic cycle. It should be noted that a6 -endo cyclization of N-centred radical B' would be ap redominant pathway,w hen R 3 is an aryl group, due to the stability of the newly generated benzylr adical intermediate C' (Scheme 3b, Path B).…”

Catalyst‐Free Difunctionalization of Activated Alkenes in Water: Efficient Synthesis of β‐Keto Sulfides and Sulfones

“…Based on previous reports and the above results, [10] we have proposed am echanism for this reaction( Scheme6). Benzenethiol is first oxidized by O 2 ,g enerating the S-centered radical I.…”

“…With al ow SÀHb onddissociation energy, p-toluenethiol could be oxidized by O 2 , forming an S-centered radical. [10] However,w hen the reaction was performed with styrene as the substrate, undesired side products,i ncluding the b-hydroxys ulfide and the H-abstraction product, were formed, and low producty ields and complicated product mixtures were obtained.T oo vercome these obstacles and obtain b-keto sulfides exclusively,a ctivateda lkenes incorporating ag ood leaving group, such as ap hosphate ester,a tt he a-position were investigated for use as ar adical linker (Scheme 1).…”

“…However,o wing to oxygen exchange between the product and water,w ec ould not confirm whether the primary oxygen atom of the b-keto sulfides had come from O 2 or from water. [10] To furtherd emonstrate the syntheticu tility of this catalystfree protocol, we next tested the synthesis of b-keto sulfides on al arger scale, carrying out the reaction on 7mmol scale withouta ny catalyst in water under O 2 atmosphere,w hich afforded the corresponding product 3a in 70 %y ield (Scheme 4).…”

Chloroacetate Promotes and Participates in the Oxidative Annulation of Pyridines/Isoquinoline by Using Oxygen as the Oxidant

“…However,t he direct oxidative annulation of simplea nd readily availables ubstrates like chalcones, pyridines/isoquinoline and ethyl chloroacetate to indolizines by using molecular oxygen ast he oxidantr emains ac hallenging task.Molecular oxygen is an ideal and green oxidanti no rganic chemistry.T he directu tilization of molecularo xygen for the construction of valuable nitrogen-containing compounds is of great interest in both academic and industrial communities due to its economic and environmentally benign features. [11][12][13] The salts of pyridines andi soquinolines have already exhibited highera ctivity than the corresponding pyridines and isoquinolines in the oxidation and hydrogenation. [14] Inspired by these previousp ioneering works, we assumed that the activation of simple pyridines and isoquinolines as the corresponding iminium salts would not only effectively avoid the coordinationa bility between the substrate and metal catalystb ut also improve the activation of CÀHbonds of the pyridines and isoquinolines.…”