Azulenesulfonium Salts: Accessible, Stable, and Versatile Reagents for Cross‐Coupling

Abstract

Azulenesulfonium salts may be readily prepared from the corresponding azulenes by an S E Ar reaction. These azulene sulfonium salts are bench-stable species that may be employed as pseudohalides for cross-coupling.S pecifically, their application in Suzuki-Miyaura reactions has been demonstrated with ad iverse selection of coupling partners. These azulenesulfonium salts possess significant advantages in comparison with the corresponding azulenyl halides,whichare knownt ob eu nstable and difficult to prepare in pure form.Azulene (1)isanon-alternant aromatic hydrocarbon which has fascinated chemists for many years owing to its blue color and high dipole moment. [1] Substituted azulenes have been employed in diverse contexts,i ncluding medicinal chemistry (as antiulcer, [2] antidiabetic, [3] anticancer, [4] antiarrhythmic, [5] and anti-erectile-dysfunction [6] agents,a nd as TXA 2 t receptor antagonists [7] ), solar cells, [8] metal-organic frameworks for hydrogen storage, [9] and organic electronics, [10] among others. Uses of azulenes in stimuli-responsive systems have also been disclosed, most commonly in halochromic materials, [11] but also in probes for soft metal cations, [12] fluoride, [13] other anions, [14] and biomolecule analytes. [15] Furthermore,t he ability to tune the absorption and emission maxima of azulenes by attaching conjugated substituents [16] has led to applications in bioimaging and fluorescence. [17] In all of the above instances,t he ability to introduce substituents onto the azulene skeleton in acontrolled manner is crucial. Substitution at the azulene 1-and 3-positions has been most extensively explored, since these positions are the most reactive in S E Ar reactions.I nc ertain specific cases the desired substituent may be installed directly in one step by such an S E Ar process. [18] Alternatively,c ross-coupling methodologies should allow access to am uch wider range of substituted azulenes.T he reactivity described above suggests

Radical C−C Bond Formation using Sulfonium Salts and Light

“…More recently, several groups have contributed to establishing intermolecular C−H sulfenylation using sulfoxides as a facile means for accessing sulfonium salts . Within these reports, sulfonium salts have proved effective electrophilic partners in transition metal‐catalysed C−C cross‐couplings . In particular, formal C−H cross‐couplings can occur when C−H sulfenylation and C−C bond formation are carried out in one pot …”

High Site Selectivity in Electrophilic Aromatic Substitutions: Mechanism of C–H Thianthrenation

“… 64 − 66 However, although trifluoroacetylated sulfoxides are frequently operating in Pummerer-type rearrangements, their reactivity toward arenes has rarely been described 67 and is generally restricted to highly electron rich substrates. 68 − 73 Recently, Procter and Alcarazo independently reported the formation of aryl sulfonium salts from a broad set of arenes by using dibenzothiophene S -oxide ( DBTO ) in combination with triflic anhydride (Tf 2 O). 42 , 43 It was proposed that electrophilic intermediates of the type R 2 S + –OTf are responsible for the extended reactivity with aromatic substrates.…”

Recent Advances in the Functionalization of Azulene Through Pd‐Catalyzed Cross‐Coupling Reactions

“…In the presence of a palladium catalyst, Suzuki‐Miyaura cross‐coupling reactions of the 1‐azulenylsulfonium salt 68 with boronic acid 24 or boronic acid esters 69 of arenes or heteroarenes yielded the corresponding 1‐(aryl) or (heteroaryl)azulenes 70 (Scheme 26, Table 2). In comparison to the equivalent azulenyl halides, which are known to be unstable and difficult to manufacture in pure form, these azulenesulfonium salts have considerable benefits [76] …”