Deacylative Thiolation by Redox‐Neutral Aromatization‐Driven C−C Fragmentation of Ketones

Abstract

Herein we report the development of deacylative thiolation of diverse methyl ketones. The reaction is redox-neutral, and heavy-metal-free, which provides a new way to introduce thioether groups site-specifically to unactivated aliphatic positions. It also features excellent functional group tolerance and broad substrate scope, thus allowing late-stage derivatization. The process benefits from efficient condensation between the activation reagent and ketone substrates, which triggers aromatization-driven CÀ C fragmentation and rapid radical coupling with thiosulfonates. Experimental and computational mechanistic studies suggest the involvement of a radical chain pathway.Ketones commonly exist in bioactive natural products, pharmaceuticals, feedstock chemicals, and synthetic intermediates. [1] Among various transformations of ketones, those involving fragmentation of α CÀ C bonds, such as Baeyer-Villiger oxidation, Beckmann rearrangement, and Schmidt reaction, have been strategically significant in organic synthesis, [2] as they can convert a common alkylÀ acyl bond into a useful alkylÀ O or alkylÀ N bond (Scheme 1A). However, to the best of our knowledge, there has been no direct method to transform an acyl group in a regular alkyl ketone into a thio group, which, if successful, could offer a new bond-disconnecting strategy for preparing alkyl thioethers. On the other hand, alkyl thioethers have been important pharmacophores (Figure 1) and can be used to regulate lipophilicity, solubility, and metabolism of drug molecules. [3] In particular, trifluoromethyl thioethers exhibit high Hansch lipophilicity (π = 1.44), [4] suggesting that SCF 3containing molecules could be easily transported through a lipid membrane. In addition, sulfoxides and sulfones, readily accessible from thioethers, are also frequently found in bioactive compounds and can serve as bioisosteres of ketones and carboxylic acids. [5] Recently, transition metal-catalyzed CÀ C bond activation emerges as a unique approach to functionalize unstrained ketones; [6] however, the corresponding deacylative thiolation reaction remains elusive, likely owing to the strong coordination of sulfur moieties to transition-metal catalysts. As a complementary CÀ C cleavage approach, we have been engaged in developing deacylative transformations capitalized on aromatization as a driving force since 2019 (Scheme 1B). [7] More recently, an effective activation reagent, N'-methylpicolinohydrazonamide (MPHA), [7c] was discovered, which can efficiently condense with various alkyl ketones to form a pre-aromatic intermediate (PAI). The subsequent NÀ H bond cleavage triggers CÀ C homolytic scission to afford a carbon-center radical species (R2), [8]