Mechanism Investigations of the Endo Cycloisomerization of Alkynols through Isolation and Characterization of Ruthenium Complexes from the Reactions of Alkynes with a Ruthenium Complex

In our efforts to probe the reaction mechanism of the endo cycloisomerization of alkynols catalyzed by the complex [Ru(N3P)(OAc)](BPh4) (1; NP3 = N,N-bis[(pyridin-2-yl)methyl][2-(diphenylphosphino)phenyl]methanamine), the reactions of 1 with alkynes were investigated. Several complexes related to intermediates in the catalytic reactions were isolated and characterized. In the presence of DIPEA (N,N-diisopropylethanamine), complex 1 reacted with 3-butyn-1-ol to afford the Ru−oxocyclocarbene complex {Ru(N3P)[C(CH2)3O](OAc)}(BPh4) (4). Under anhydrous conditions, the reaction of complex 1 with tert-butylacetylene afforded [Ru(N3P)(η3-tBuCCCCHBut)](BPh4) (8). In the presence of water, the reaction of complex 1 with tert-butylacetylene afforded [Ru(N3P)(CO)(η1-CH2But)](BPh4) (13). All the reactions are likely to proceed through ruthenium vinylidene intermediates. These results support that ruthenium vinylidene complexes are involved as the key intermediates in the cycloisomerization of alkynols catalyzed by complex 1.