Generation and reactivity of sterically hindered iridium carbenes. Competitive α- vs. β-hydrogen elimination in iridium(iii) alkyls

Abstract

Some tris(pyrazolyl)borate complexes of iridium are able to cleave regioselectively the two α-CH bonds of ethers (cyclic and non-cyclic) and aliphatic amines, RCH₂X (X = OR′, NR₂′), with formation of Fischer-type carbene complexes, [Ir]C(X)R. The last step of these rearrangements, namely an α-H elimination from an alkyl intermediate, [Ir]–CH(X)R, takes place even when β-H atoms are present. Migratory insertion reactions of hydride or alkyl ligands onto highly electrophilic iridium alkylidenes have also been investigated. It has been found that an in situ generated [Ir]–C₂H₅⁺ species yields the corresponding [Ir](H)(CHCH₃)⁺ derivative, that is, the α-H elimination product, at a rate faster than that of formation of the isomeric hydride ethene complex derived from β-H elimination.