FABI: A New, Solid State Fluorosulfonyl Radical Reagent

Radical flurosulfonylation has recently emerged as a promising approach for introducing the sulfonyl fluoride functional groups. In this work, we introduce a solid-state, bench-stable type of new reagents, FABI, which can serve as effective FSO2 radical precursors under photoredox conditions.

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Sulfonyl fluorides have attracted considerable and growing research interests from various disciplines, which raises a high demand for novel and effective methods to access this class of compounds. So far most of the fluorosulfonylating reagents are synthetic equivalents of the “FSO2+” synthons, fluorosulfonylation with the corresponding fluorosulfonyl radical (FSO2•) is rarely explored. Recently, by employing sulfuryl chlorofluoride (FSO2Cl) as a radical precursor, we could be able to developed the radical fluorosulfonylation reaction of alkenes and alkynes with success, representing a new method for the synthesis of important alkenyl sulfonyl fluorides (under photoinduced conditions: Angew. Chem. Int. Ed. 2021, 60, 3956; Angew. Chem. Int. Ed. 2021, 60, 22035; also achieved under electrochemical conditions: Angew. Chem. Int. Ed. 2021, 60, 27271, etc.). However, when we applied this reagent to the development of other transformations, e.g. the alkoxy-fluorosulfonylation reaction of styrene, we failed to obtained any desired product even after extensive optimization. In addition, inconvenience in storage and handling due to the gaseous (b.p. 7 ºC) and moisture-sensitive nature of FSO2Cl, the development of a new and convenient FSO2 radical precursor is highly desirable.

Figure 1. Examples of FSO2 cationic and radical reagents

In this work, we introduced a solid-state, bench-stable type of reagents, 1-fluorosulfonyl 2-aryl benzoimidazolium triflate (FABI) salts (patent filed, CN 113248444, 2021), which can serve as effective FSO2 radical precursors and enable the development of radical fluorosulfonylation of olefins via a photoredox pathway. In our previous study (Angew. Chem. Int. Ed. 2021, 60, 3956, SI), we tried imidazolium 2a (FDIT) in the reaction, which was introduced by Dong and Sharpless for modification of alcohols and amines (Angew. Chem. Int. Ed. 2018, 57, 2605), but it gave no product under the photoredox conditions. In our subsequent further investigation, we found that 2a can be reduced via single electron transfer from excited Ir photocatalyst, but it enabled to produce the FSO2 radicals. We thus conceived that there may need a higher driving force of re-aromatization to favor the homolytic cleavage of the N-S bond (please see the mechanistic discussion in the article). We thus tried to replace the methyl group in 2a with phenyl (2b) and also introduce a benzo moiety (2c), and finally, the structure optimization led us to FABI 2d & 2e. To our delight, excellent yield can be achieved in the end with the new reagent 2e.

Figure 2. Reactivity of different fluorosulfonyl radical reagents

FABI could well accommodate many substrates that were not compatible or low yielding before with FSO2Cl, such as electron-rich alkenes and triaryl ethylenes. Moreover, a cascade alkoxy-fluorosulfonyl difunctionalization of olefins with FABI has been successfully developed for the first time, by trapping the postulated cationic intermediate with alcohols via a photoredox pathway.

Figure 3. Our new radical reagent for photoredox catalytic radical fluorosulfonylation of olefins

For more details, especially on reagent development, substrate scope and mechanistic studies for the new fluorosulfonyl radical reagents, please have a look at our article.

Article Link: https://www.nature.com/articles/s41467-022-31089-7

Saihu Liao

Professor, Fuzhou University