| Summary: | Alkynyl functionalized boron compounds are versatile intermediates in the areas of medicinal chemistry, materials science, and optical materials. In particular, alkynyl boronate esters [R<sup>1</sup>−C≡C−B(OR<sup>2</sup>)<sub>2</sub>] are of interest since they provide reactivity at both the alkyne entity, with retention of the B−C bond or alkyne transfer to electrophilic substrates with scission of the latter. The boron atom is commonly well stabilized due to (i) the extraordinary strength of two B−O bonds, and (ii) the chelate effect exerted by a bifunctional alcohol. We reasoned that the replacement of a B−O for a B−S bond would lead to higher reactivity and post-functionalization in the resulting alkynyl boronate thioesters [R<sup>1</sup>−C≡C−B(S<sub>2</sub>X)]. Access to this poorly investigated class of compounds starts form chloro dithioborolane <i>cyclo</i>-Cl−B(S<sub>2</sub>C<sub>2</sub>H<sub>4</sub>) as a representative example. Whereas syntheses of three coordinate alkynyl boronate thioesters [R<sup>1</sup>−C≡C−B(S<sub>2</sub>X)] proved to be ineffective, the reactions of NHC-adducts (NHC = <i>N</i>-heterocyclic carbene) of <i>cyclo</i>-Cl-B(S<sub>2</sub>C<sub>2</sub>H<sub>4</sub>) afforded the alkyne substituted thioboronate esters in good yield. The products NHC−B(S<sub>2</sub>C<sub>2</sub>H<sub>4</sub>)(C≡C-R<sup>1</sup>) are remarkably stable towards water and air, which suggests their use as boron-based building blocks for applications akin to oxygen-based boronate esters.
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