Abstract
A series of η2-[Os(NH3)5(vinyl ether)]2+ complexes have been prepared by three independent methods that involve direct coordination of a vinyl ether, alcohol addition to an η2-alkyne complex, or nucleophilic substitution of an η2-vinyl ether species. In the presence of an acid catalyst, the vinyl ether ligand undergoes a novel acid-catalyzed substitution reaction at the α-carbon with a broad range of nucleophiles that includes alcohols, amines, carboxylates, hydrides, silylated enols, nitriles, phosphines, and dialkyl sulfides. These reactions appear to proceed through an elimination−addition process where the first step is loss of an alcohol to form an η2-vinyl cation intermediate. In cases where the α-carbon bears an alkyl group, an η2-vinyl cation species can be isolated and characterized. For example, protonation of [Os(NH3)5(η2-2-methoxypropene)]2+ (3) in neat HOTf allows the characterization of the substitution reaction intermediate η2-[Os(NH3)5(C3H5)]3+ (32), formally a metallocyclopropene that behaves chemically like a vinyl cation. In contrast, when the α-carbon of the vinyl ether bears a hydrogen such as with [Os(NH3)5(η2-ethoxyethene)]2+ (1), the hypothetical vinyl cation intermediate, in absence of a suitable nucleophile, undergoes an intramolecular 1,2-hydrogen shift to yield the Fischer carbyne [(NH3)5Os⋮CCH3]3+ (33). Examples of nucleophilic substitution reactions for other types of η2-[Os(NH3)5(olefin)]n+ complexes are also demonstrated.
