Catalysis by 18 + δ Compounds. Cyclooligomerization of Acetylenes Catalyzed by Co(CO)3L2

The ability of 18 + δ complexes to act as homogeneous catalysts was tested by studying the cyclooligomerization reactions of alkynes using the Co(CO)3L2 complex as the catalyst (L2 = 2,3-bis(diphenylphosphino)maleic anhydride). Cyclotrimerization occurred for RC⋮CR, where R = Ph, CH2OSiMe3, and CO2Me. No reaction was observed for R = H, CH3, SiMe3, or CH2CH2CH3. It is proposed that the alkynes need to be activated with electron-withdrawing groups to prevent the formation of high-energy 19-electron reaction intermediates. Although the mechanistic data are not conclusive, it is proposed that the reactions follow an odd-electron pathway involving the interconversion of 17- and 19-electron species. The 18 + δ complexes are labile to ligand dissociation, and it is this feature that makes them potentially good catalysts. The importance of ligand lability was demonstrated by comparing the activity of the Co(CO)3L2 complex to that of its inert 18-electron counterpart, Co(CO)3L2+; the 18-electron molecule was not ...