Dissociative Nucleophilic Substitution of η²-Olefin Complexes via a Novel η²-Vinyl Cation Intermediate

A series of η 2 -[Os(NH 3 ) 5 (vinyl ether)] 2+ complexes have been prepared by three independent methods that involve direct coordination of a vinyl ether, alcohol addition to an η 2 -alkyne complex, or nucleophilic substitution of an η 2 -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 η 2 -vinyl cation intermediate. In cases where the α-carbon bears an alkyl group, an η 2 -vinyl cation species can be isolated and characterized. For example, protonation of [Os(NH 3 ) 5 (η 2 -2-methoxypropene)] 2+ ( 3 ) in neat HOTf allows the characterization of the substitution reaction intermediate η 2 -[Os(NH 3 ) 5 (C 3 H 5 )] 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(NH 3 ) 5 (η 2 -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 [(NH 3 ) 5 Os⋮CCH 3 ] 3+ ( 33 ). Examples of nucleophilic substitution reactions for other types of η 2 -[Os(NH 3 ) 5 (olefin)] n + complexes are also demonstrated.