Electronic Effects in a Green Protocol for (Hetero)Aryl-S Coupling

Abstract

Aryl and heteroaryl iodides have been efficiently converted into the corresponding thioacetates in cyclopentyl methyl ether (CPME), a green solvent, under Cu catalysis. The chemoselectivity of the reaction is mainly controlled by electronic factors, enabling the conversion of both electron-rich and electron-deficient substrates into the corresponding thioacetates in good to excellent yields. The products can be easily deprotected to the corresponding thiolates to carry out additional synthetic transformations in situ. Surprisingly, despite CPME's relatively low dielectric constant, the reaction rate significantly increased when conducted under microwave irradiation conditions. This synthetic methodology exhibits a remarkable tolerance to functional groups, mild reaction conditions, and a wide substrate scope, utilizing a safe and inexpensive CuI pre-catalyst in the green solvent CPME. A non-aqueous workup allowing for the complete recovery of both catalyst and solvent makes this approach an environmentally sustainable protocol for C(sp²) sulfur functionalization. Additionally, the reaction shows selective cross-coupling with iodides in competition with chlorides and bromides, allowing its use in multistep syntheses. To demonstrate the potential of this methodology, it was applied to the high-yield synthesis of a photochromic dithienylethene, where a selective synthesis had not been reported before.

Keywords: Ar-S coupling; CPME; Cu catalysis; electronic effects; green chemistry.

Figure 1

Ar-S-R bioactive molecules.

Scheme 1

Plausible reaction mechanism for the Cu^I−phen-catalyzed Ar−SAc coupling (a); plausible reaction mechanism for the Cu^I−phen-catalyzed Ar−S−Ar coupling (b).

Scheme 2

Acyl transfer on the heteroaryl iodides 1p.

Scheme 3

Synthesis of the protected photochromic dithienylethene 13 [37].