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

Abstract

The double C-H bond activation of a series of linear and cyclic ethers by the iridium complex [Tp(tol')Ir(C(6)H(5))(N(2))] (2⋅N(2)), which features a cyclometalated hydrotris(3-p-tolylpyrazol-1-yl)borate ligand (Tp(tol')) coordinated in a κ(4)-N,N',N'',C manner, has been studied. Two methyl ethers, namely, Me(2)O 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 3-8, which are stabilized by κ(4)-coordination of the ancillary Tp(tol') 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 Me(2)O, MeOtBu, Et(2)O, 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 σ bond of the metalated linkage has only been observed for complex 8 that contains a cyclic carbene that results from α,α-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.