Homolytic Aromatic Sulfonation with K₂S₂O₅ Promoted by a Combination of Mn(OAc)₃·2H₂O and HFIP

Abstract

Herein, we reported a so far unprecedented Mn­(OAc)3·2H2O-promoted homolytic aromatic sulfonation. The reaction was performed under mild conditions with K2S2O5 employed as a green sulfonating reagent. Various arenes were successfully converted into desired sulfonic acids or sulfonates in high efficiency. Preliminary mechanistic studies demonstrated that the present reaction proceeds via a homolytic aromatic substitution-type mechanism involving an SO3 – radical. The combination of Mn­(OAc)3·2H2O and HFIP plays a crucial role.

Mn(OAc)3-Promoted Sulfonation-Cyclization Cascade via the SO3– Radical: The Synthesis of Heterocyclic Sulfonates

“…Intriguingly, we found that the reaction of 1aa could proceed smoothly in several other solvents besides HFIP, such as DCM, MeOH, EtOH, DMSO, CH 3 COOH, and toluene (entries 5 and 6). The success of nonfluorinated solvents is in contrast with our previously reported radical sulfonation of arenes, probably attributed to a higher radical trapping ability of alkenes. Considering the solvent cost and the ease of workup, we chose DCM as the optimized solvent for the synthesis of oxindole-based sulfonates (entry 7).…”

“…Inspired by previous works, − we envisioned that the SO 3 – radical could also trigger the sulfonation-cyclization cascade via a radical cyclization mechanism (Scheme D). The challenge may come from the competition between direct hyrdrosulfonation of alkenes and cyclization, which requires an ingenious substrate design.…”

“…8 Recently, we identified a mild radical sulfonation of (hetero)arenes promoted by the Mn(OAc) 3 -HFIP oxidative system. 9 Mechanism studies showed that the reaction proceeded via an SO 3 − radical involved homolytic aromatic substitution (Scheme 1C). Heterocyclic sulfonates containing oxindole, quinolinone, isoquinoline-1,3-dione, and benzoxazine structural fragments are valuable skeletons.…”

“…With efforts toward the effective construction of C–S bonds, we have developed a series of oxidative couplings promoted by earth-abundant first-row transition metals and fluorinated solvents . Recently, we identified a mild radical sulfonation of (hetero)­arenes promoted by the Mn­(OAc) 3 -HFIP oxidative system . Mechanism studies showed that the reaction proceeded via an SO 3 – radical involved homolytic aromatic substitution (Scheme C).…”

Mn(OAc)3-Promoted Sulfonation–ipso-Cyclization Cascade via the SO3– Radical: The Synthesis of Spirocyclic Sulfonates

“…Our methodology not only broadens the scope of synthetic applications of the SO 3 – radical as a versatile sulfonic synthon but also offers a promising strategy for the modular synthesis of sulfonyl derivatives, thereby contributing new insights and methods to the field of sulfonyl chemistry. In conjunction with our prior research efforts, , we have established a sulfonate library comprising over 100 new compounds, and further optimization and scope-expanding studies are underway in our lab.…”