Metal-Free, DTBP-Mediated Methylthiolation of Arylboronic Acids with Dimethyldisulfide

Abstract

An efficient method for the C-S bond formation via the coupling reaction of arylboronic acids with dimethyldisulfide has been developed under the metal-free conditions. This novel protocol provides an attractive route for the synthesis of aryl methyl sulfides, due to its operational simplicity, satisfactory yields, excellent functional-group tolerance, as well as the mild reaction conditions. Aryl methyl sulfides are valuable structural motifs present in many pharmaceuticals and bioactive natural molecules. 1 Besides, they also serve as useful substrates for transition-metal-catalyzed cross-coupling reactions, including C-C 2 and C-N 3 coupling reaction as electrophiles, and the carbothiolation of terminal alkynes. 4 Undoubtedly, palladium 5 and other transition metals, such as nickel, 6 copper, 7 cobalt, 8 and indium 9 catalyzed C(aryl)-S coupling of thiols with aryl halides or pseudohalides to provide aryl sulfides that have emerged as mild and effective protocols compared with the traditional C(aryl)-S bond synthetic approaches via nucleophilic aromatic substitution, which generally required harsh reaction conditions 10 and the coupling reaction of arylmagnesium halides with a suitable electrophilic arylsulfur reagent 11 over the past several decades. However, the formation of aryl methyl sulfides in this fashion has not been reported so far, presumably owing to the low boiling point of methanethiol. In general, an effective strategy for the preparation of aryl methyl sulfides mainly consist of reduction of sulfoxides, 12 coupling reaction of aryl thiols with iodomethane, 13 and aryl halides with dimethyldisulfide. 14 An alternative method is the heteroatom-facilitated lithiation of aromatic C-H bonds followed by electrophilic substitution with dimethyldisulfide, which is more attractive from an atom-economical viewpoint. 15 It is worthwhile to mention that visible-light-mediated synthesis of aryl methyl sulfides from arenediazonium salts and dimethyldisulfide in the presence of eosin Y, a metal-free dye, has also been developed under mild reaction conditions, which exactly conforms to green and sustainable chemistry. 16 Recently, DMSO has been explored as an effective methylthiolation surrogate for this transformation. Apart from aryl halides 17 and aryl carboxylic acids, 18 the direct C-S bond formation via C-H bond activation with DMSO is also successful. 19 Nevertheless, generally, some or one of the items including catalytic or stoichiometric amount of transitionmetal sources, an appropriate ligand, Lewis acid, restricted substrates, or harsh reaction conditions were necessary in the aforementioned protocols. Furthermore, due to the fact that the reactions under metal-free conditions are desired particularly in the pharmaceutical synthesis and environmentally benign synthetic procedures, and arylboronic acids have been widely employed in the formation of carbonheteroatom bonds such as C-N 20 and C-O, 20f,r,21 but seldom in carbon-sulfur bond, we herein describe a transitionmetal-free C-S bond formation...

BF3·SMe2 for Thiomethylation, Nitro Reduction and Tandem Reduction/SMe Insertion of Nitrogen Heterocycles

“…[2][3][4][5][6][7][8][9] Although thiolate salts are good nucleophiles, these types of reagents suffer from a range of drawbacks including volatility, instability, toxicity, and strong, unpleasant odors. [10][11][12][13] Disulfides can also be used for the thiomethylation of aryl halides using nickel catalysts and zinc [14][15][16][17][18][19][20] (Scheme 1a) or aryl diazaonium salts [21] and boronic acids [22] (Scheme 1c) under radical conditions although they are also known to be highly odorous. Recently, DMSO has emerged as an attractive thiomethyl source for the functionalization of a range of substrates [23][24][25][26][27][28][29] (Scheme 1a and Scheme 1b) however, these reactions require high temperatures as well as the use of stoichiometric amounts of copper salts.…”

Transition‐Metal‐Free C−S, C−Se, and C−Te Bond Formation from Organoboron Compounds

“…The intermediate V is formed after the loss of water, leading to delocalization of electrons VI and forming the desired selenide 3 (Scheme 7). [36] Wu and co-workers [37] developed a metal-free method to obtain aryl methyl sulfides 12 from different aryl boronic acids 2 and dimethyl disulfide 14 a (Scheme 8). After optimizing the reaction conditions, the authors determined that the best condition for the coupling reaction was using aryl boronic acids 2 (1 mmol), dimethyl disulfide 14 a (2 equiv), di-t butyl peroxide (DTBP) (3 equiv), and acetonitrile for 12 h at 120 °C.…”

“…This intermediate II reacts with the radical intermediate I, yielding the expected arylmethyl sulfide 12. [37] In 2018, an efficient method for the oxidative cleavage of the SÀ S and SeÀ Se bonds was described by An and coworkers [38] using a metal-free system leading to the formation of chalcogenides 3 and 12 (Scheme 9). The use of an acid/ DEAD (diethyl azodicarboxylate) system was capable of generating chalcogenium ions, that can be captured by nucleophilic aryl reagents, such as aryl boronic acids 2.…”

“…Simultaneously, the homolytic cleavage of the S−S bond of the dimethyl disulfide 14 a occurred to form the MeS radical II . This intermediate II reacts with the radical intermediate I , yielding the expected arylmethyl sulfide 12 [37] …”

Copper-Catalyzed Methylthiolation of Aryl Iodides and Bromides with Dimethyl Disulfide in Water

“…28 At about the same time, we reported a transitionmetal-free C-S bond formation from arylboronic acids and dimethyl disulfide under neutral conditions. 29 Moreover, a…”