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Selectivity and Mechanism of Iridium-Catalyzed Cyclohexyl Methyl Ether Cleavage

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posted on 05.06.2020, 07:14 by Caleb D. Fast, Caleb A. H. Jones, Nathan D. Schley
Cationic bis­(phosphine)iridium complexes are found to catalyze the cleavage of cyclohexyl methyl ethers by triethylsilane. Selectivity for C–O cleavage is determined by the relative rates of SN2 demethylation versus SN1 demethoxylation, with the axial or equatorial disposition of the silyloxonium ion intermediate acting as an important contributing factor. Modulation of the electron-donor power of the supporting phosphine ligands enables a switch in selectivity from demethylation of equatorial methyl ethers to 2° demethoxylation. Applications of these accessible catalysts to the selective demethoxylation of the 3α-methoxy group of cholic acid derivatives is demonstrated, including a switch in observed selectivity controlled by 7α-substitution. The resting state of the catalyst has been characterized for two phosphine derivatives, demonstrating that the observed switch in C–O cleavage selectivity likely results from electronic factors rather than from a major perturbation of the catalyst structure.

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