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• Because of the unique reactivity of sulfonyl fluoride (SO₂F) group, such compound has found increasing applications in the areas of organic synthesis, new drug development, pesticide, material science and click chemistry, etc. It is of current interest to develop novel strategies and methods for the facile preparation of sulfonyl fluorides in a green and highly efficient manner [1,2]. Although the traditional approach for the synthesis of sulfonyl fluorides relies on the nucleophilic substitution of fluoride atom with sulfonyl chlorides, the recent process has identified the possibility by using the electrophilic fluorination with the corresponding sulfinates or sulfonyl radicals, as well as the direct use of fluorosulfonylation reagents, which dramatically broadens the scope for the efficient preparation of versatile sulfonyl fluriodes (Scheme 1A). Nevertheless, several issues remain unresolved in this territory. First of all, while most known methods can be applied for the facile synthesis of aryl sulfonyl fluorides via the fluorosulfonylation of pre-functionalized substrates, the corresponding protocols for the preparation of alkyl sulfonyl fluorides leg behind, and methods via direct selective fluorosulfonylation of very challengeable C(sp³)–H bonds remains underdeveloped. Additionally, it would be highly significant to explore innovative and effective approaches to enhance the assemble strategy of SO₂F group, as this could provide valuable insights for developing future protocols in this area. To date, such an area has become a testing ground for new synthetic methodology evolution.

  Scheme 1

  

  Scheme 1.  Strategies for the construction of SO₂F group.

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  To meet these challenges, Cao and co-workers have always focused on developing new and efficient methods for streamlining the synthesis of highly functionalized sulfonyl fluorides from readily available feedstocks [4-6]. For example, by using paired electrolysis, they have achieved the first formal arene C—H sulfonylation by borrowing thianthrenium salt chemistry [3]. In addition, the employment of an organomediated paired electrolysis strategy enables to transform aryl triflates into the corresponding sulfonyl fluorides under transition metal-free conditions for the first time [4]. Furthermore, by employing ionic liquid N-methylimidazolium p-toluenesulfonate as an electrolyte and acidic additive, they also realized the denitrative fluorosulfonylation of feedstock nitroarenes for the first time [5]. On the basis of these, very recently, Cao and Zhang further noticed that simple and cheap 4-methoxypyridine N-oxide could serve as an effective ligand to tune the reactivity of copper center, which could promote the selective radical fluorine-atom transfer (FAT) [6,7] to the in situ formed sulfonyl radical, providing a conceptually interesting strategy to assemble SO₂F group (Scheme 1B). Guided by the concept, coupled with the power and unique reactivity of copper catalysis, three types of methods for delivering C(sp³)-rich aliphatic sulfonyl fluorides, which involves site-selective fluorosulfonylation of inert C(sp³)−H bonds in an intra- or intermolecular manner and 1,2-aminofluorosulfonylation of inactivated alkenes, have been established [8].

  As shown in Scheme 2, by using cheap 4-methoxypyridine N-oxide as ligand and Na₂S₂O₅ as SO₂ source, Zhang and Cao noticed that the use of copper catalysis could facilely activate N-F bond of sulfluoroamides to form electrophilic nitrogen radical in situ, furnishing the subsequent δ-C(sp³)−H fluorosulfonylation under mild reaction conditions. The protocol features good functional group tolerance and can be applied for gram-scale synthesis, providing a new way for the preparation of aliphatic sulfonyl fluorides [9].

  Scheme 2

  

  Scheme 2.  Intermolecular C(sp³)−H fluorosulfonylation.

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  Furthermore, such a simple catalytic system can be extended for the more challengable intermolecular C(sp³)−H fluorosulfonylation as well as 1,2-aminofluorosulfonylation of inactivated alkenes (Scheme 3), which dramatically broadens the scope for the preparation of versatile alkyl sulfonyl fluorides.

  Scheme 3

  

  Scheme 3.  Intermolecular fluorosulfonylation and 1,2-aminofluorosulfonylation of alkenes.

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  Meanwhile, Li and Wang also reported the very similar strategy for achieving the intramolecular C(sp³)−H fluorosulfonylation and 1,2-aminofluorosulfonylation of alkenes as well (Scheme 4) [9]. It was disclosed that the use of simple bipyridine/CuCl₂ complex enabled to promotion of the above transformations under mild conditions, too. Different from Zhang and Cao's work, DABSO has been chosen as the optimal SO₂ source for achieving satisfactory results.

  Scheme 4

  

  Scheme 4.  Li and Wang's strategy for intermolecular C(sp³)−H fluorosulfonylation and 1,2-aminofluorosulfonylation of alkenes.

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  To give a deep study of the applications of these protocols, a series of interesting transformations have been conducted by both groups. It has been identified that the prepared alkyl sulfonyl fluorides are useful building blocks for preparing versitale targets [8,9].

  To give a better understanding of the mechanism of intramolecular C(sp³)−H fluorosulfonylation, several experiments which include the characterization of copper complex, radical trapping experiments, kinetic studies, and DFT calculations have been conducted by Zhang, Li, Cao, and wo-workers [9]. As shown in Scheme 5, the reaction starts with the reduction of N−F bond of sulfluoroamides 1 by L_nCu(I) complex A, to deliver nitrogen radical I and L_nCu(II)F species B. I undergoes the intramolecular 1,5-hydrogen atom transfer (HAT) and the subsequent SO₂ insertion could deliver sulfonyl radical III. The outer-sphere FAT process between III and B furnishes the final product 2 along with the regeneration of A.

  Scheme 5

  

  Scheme 5.  Proposed mechanism for intramolecular fluorosulfonylation.

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  In conclusion, the groups of Zhang and Cao, Li and Wang have independently reported a conceptually interesting strategy for the facile assemble of SO₂F group by copper catalysis. Based on this concept, both the C(sp³)−H fluorosulfonylation and 1,2-aminofluorosulfonylation of alkenes have been developed, providing a new avenue for the preparation of highly functionalized alkyl sulfonyl fluorides that cannot be easily achieved by other known methods. Although the results showed the potential of FAT strategy to prepare alkyl sulfonyl fluorides, how to further expand such strategy for other more challengeable transformations such as selective intermolecular C(sp³)−H fluorosulfonylation with broad substrate scope remains elusive. It is anticipated such strategies should have found more applications in the near future.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Yu-Yu Tan: Writing – original draft. Lin-Heng He: Writing – original draft. Wei-Min He: Writing – review & editing.

  
  
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• 

  Scheme 1  Strategies for the construction of SO₂F group.

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  Scheme 2  Intermolecular C(sp³)−H fluorosulfonylation.

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  Scheme 3  Intermolecular fluorosulfonylation and 1,2-aminofluorosulfonylation of alkenes.

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  Scheme 4  Li and Wang's strategy for intermolecular C(sp³)−H fluorosulfonylation and 1,2-aminofluorosulfonylation of alkenes.

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  Scheme 5  Proposed mechanism for intramolecular fluorosulfonylation.

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