Scope and Mechanism of the Iridium-Catalyzed Cleavage of Alkyl Ethers with Triethylsilane

The cationic iridium pincer complex [(POCOP)Ir(H)(acetone)](+)[B(C6F5)(4)](-) (1, POCOP = 2,6-[OP(tBu)(2)](2)C6H3) was found to be a highly active catalyst for the room-temperature cleavage and reduction of a wide variety of unactivated alkyl ethers including primary, secondary, and tertiary alkyl ethers as well as aryl alkyl ethers by triethylsilane. Mechanistic studies have revealed the full details of the catalytic cycle with the catalyst resting state(s) depending on the basicity of the alkyl ether. During the catalytic reduction of diethyl ether, cationic iridium silane complex, [(POCOP)Ir(H)(eta(1)-Et3SiH)](+)[B(C6F5)(4)] (3), and Et2O are in rapid equilibrium with neutral dihydride, (POCOP)Ir(H)(2) (5) and diethyl(triethylsilyl)oxonium ion, [Et3SiOEt2](+)[B(C6F5)(4)] (7), with 5 + 7 strongly favored. Species 7 has been isolated from the reaction mixture and fully characterized. The turnover-limiting step in this cycle is the reduction of 7 by the neutral dihydride 5. The relative rates of reduction of 7 by dihydride 5 and Et3SiH were determined to be similar to 30,000: 1. In the cleavage of the less basic ethers anisole and EtOSiEt3, the cationic iridium silane complex, 3, was found to be the catalyst resting state. The hydride reduction of the intermediate oxonium ion EtO(SiEt3)(2)(+), 9, occurs via attack by Et3SiH. In the case of anisole, the intermediate PhMeOSiEt3, 10, is reduced by 5 and/or Et3SiH.