Reactivity of a Bis(alkynyl)-Vinylidene Osmium Complex: Access to Carbynes, Vinyl Ester Derivatives, and C–C and C–N Bond-Forming Reactions

Hexahydride OsH6(PiPr3)2 (1) reacts with tert-butylacetylene to yield bis(alkynyl)-vinylidene Os(C≡CtBu)2(=C=CHtBu)(PiPr3)2 (2). Subsequent protonation of 2 with HOTf or Et3N·3HF affords the bis(alkynyl)-carbyne derivatives [Os(C≡CtBu)2(≡CCH2tBu)(PiPr3)2]OTf (3) and OsF(C≡CtBu)2(≡CCH2tBu)(PiPr3)2 (4), respectively. In contrast, treatment of 2 with acetic acid leads to the alkynyl-vinylidene-vinyl ester derivative Os(C≡CtBu){κ2-O,C-OC(CH3)OC=CHtBu}(=C=CHtBu)(PiPr3)2 (5). Density functional theory (DFT) calculations, together with deuterium labeling experiments, demonstrate that the protonation may occur either at the Cβ carbon of the vinylidene ligand or at the Cβ carbon atom of one of the alkynyl groups, and that the formation of 5 is governed by thermodynamic control. The reaction of 2 with 2-aminopyridine provides a new route for the selective synthesis of a 2,3-disubstituted imidazo[1,2-a]pyridine derivative starting from tert-butylacetylene and 2-aminopyridine.