Mechanistic Study of Ir(H)z-Assisted Transformations of Ethyne: Cyclotrimerization, Cooligomerization with Ethene, and Reductive Coupling

The (etheneldihydride complex [(triphos)Ir(H)z(CzH4)1BPh4( 1) is capable of promoting a variety of transformations of ethyne, including cyclotrimerization to benzene, cooligomerization with ethene to hexa-1,3,5-triene, reductive coupling to buta-1,3-diene, and hydrogenation to ethene (triphos = MeC(CH2PPh&). A detailed study under various experimental conditions, the detection of several intermediates along the various reaction paths, and the use of isolated complexes in independent reactions, taken together, permit mechanistic conclusions that account for the varied products. In particular, the cyclotrimerization and cooligomerization reactions are mediated by an iridacyclopentadiene species which is trapped by either ethyne or ethene. Consumption of the hydride ligands of 1 by C2H2 or C2H4 is an ingredient for both cyclotrimerization and cooligomerization reactions but is not necessary to accomplish the reductive dimerization of ethyne to buta-1,3-diene for which, conversely, the two hydride ligands are mandatory.