Alkoxide Attack on Coordinated Olefin Can Be Reversible

Alkoxide (MeO^-, EtO^-, ⁱPrO^-) attack occurs at the coordinated olefin of Ir(Tripod)(COD) (Tripod = MeC(CH₂PPh₂)₃, COD = 1,5-cyclooctadiene) in CH₂Cl₂ to give Ir(Tripod)(2-alkoxycyclooct-5-en-1-yl), primarily as the exo isomer. These products slowly eliminate alcohol to give Ir(Tripod)((1,2-η²)-6-σ-cycloocta-1,4-dienyl), which is the only product detected when the alkoxide is ^tBuO^-. Addition of excess methanol to exo-Ir(Tripod)(2-methoxycyclooct-5-en-1-yl) abstracts MeO^- (i.e., heterolytic O−C bond cleavage), and thus a hydrogen-bonding solvent reverses the nucleophilic attack on coordinated olefin. Alkoxide (MeO^-, ⁱPrO^-, ^tBuO^- and 2-BuO^-) addition occurs at an olefinic carbon of Rh(Tripod)(NBD)⁺ (NBD = norbornadiene) to give Rh(Tripod)(2-alkoxynorborn-4-en-1-yl). The crystal structure of the exo/methoxy example has been determined. While this product cannot unimolecularly eliminate alcohol, attempts to protonate the norbornyl ether in the presence or absence of added NBD were not successful. Here again, acid abstracts RO^- from the ligand ether.