N-Heterocyclic Carbene Adducts of Alkynyl Functionalized 1,3,2-Dithioborolanes
Alkynyl functionalized boron compounds are versatile intermediates in the areas of medicinal chemistry, materials science, and optical materials. In particular, alkynyl boronate esters [R1-C≡C-B(OR2)2] 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 [R1-C≡C-B(S2X)]. Access to this poorly investigated class of compounds starts form chloro dithioborolane cyclo-Cl-B(S2C2H4) as a representative example. Whereas syntheses of three coordinate alkynyl boronate thioesters [R1-C≡C-B(S2X)] proved to be ineffective, the reactions of NHC-adducts (NHC = N-heterocyclic carbene) of cyclo-Cl-B(S2C2H4) afforded the alkyne substituted thioboronate esters in good yield. The products NHC-B(S2C2H4)(C≡C-R1) are remarkably stable towards water and air, which suggests their use as boron-based building blocks for applications akin to oxygen-based boronate esters.