Reversible Double C-H Bond Activation of Linear and Cyclic Ethers To Form Iridium Carbenes

The double C?H bond activation of a series of linear and cyclic ethers by the iridium complex [Tptol'Ir(C6H5)(N2)] (2 center dot N2), which features a cyclometalated hydrotris(3-p-tolylpyrazol-1-yl)borate ligand (Tptol') coordinated in a ?4-N,N',N'',C manner, has been studied. Two methyl ethers, namely, Me2O and MeOtBu, along with diethyl ether and the cyclic ethers tetrahydrofuran, tetrahydropyran (THP), and 1,4-dioxane have been investigated with formation in every case of the corresponding hydride carbene complexes 38, which are stabilized by ?4-coordination of the ancillary Tptol' ligand. Five of the compounds have been structurally authenticated by X-ray crystallography. A remarkable feature of these rearrangements is the reversibility of the double C?H bond activation of Me2O, MeOtBu, Et2O, and THP. This has permitted catalytic deuterium incorporation into the methyl groups of the two methyl ethers, although in a rather inefficient manner (for synthetic purposes). Although possible in all cases, C?C coupling by migratory insertion of the carbene into the Ir?C s bond of the metalated linkage has only been observed for complex 8 that contains a cyclic carbene that results from a,a-C?H activation of 1,4-dioxane. Computational studies on the formation of iridium carbenes are also reported, which show a role for metalated Tp ligands in the double C?H activation and account for the reversibility of the reaction in terms of the relative stability of the reagents and the products of the reaction.