Generation and reactivity of sterically hindered iridium carbenes. Competitive (alpha)- vs.(beta)-hydrogen elimination in iridium(III) alkyls

Some tris(pyrazolyl)borate complexes of iridium are able to cleave regioselectively the two (alpha)-CH bonds of ethers (cyclic and noncyclic) and aliphatic amines, RCH2X (X = ORʹ, NR2ʹ), with formation of Fischer-type carbene complexes, [Ir]=C(X)R. The last step of these rearrangements, namely an (alpha)-H elimination from an alkyl intermediate, [Ir]-CH(X)R, takes place even when (beta)-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 (alpha)-H elimination product, at a rate faster than that of formation of the isomeric hydride ethene complex derived from (beta) -H elimination.