Organoiridium compounds with bidentate phosphines as highly regioselective catalysts for alkynes cyclotrimerization

The iridium derivatives HIr(cod)(L-L) (cod=1,5-cyclooctadiene; L-L=Ph2PCH2PPh2 (dppm); Ph2P(CH2)2PPh2 (dppe); Ph2P(CH2)3PPh2 (dppp); Ph2P(CH2)4PPh2 (dppb); o-C6H4(PPh2)2; Cy2P(CH2)2PCy2 (dcpe); o-Me2NC6H4PPh2 (P-NMe2)) and Ir(OMe)(cod)(dppe-F) (dppe-F=(C6F5)2P(CH2)2P(C6F5)2) are active catalysts for the cyclotrimerization of phenylacetylene and substituted derivatives. The nature of the phosphine ligand has a pronounced effect on catalytic activity and selectivity of the reactions: in some cases (L-L=dcpe, dppe-F) mixtures of oligomerization and polymerization products are obtained, in others only cyclomerization is observed, with regioselectivities as high as 100% of the 1,2,4-trisubstituted benzenes in the reactions catalyzed by HIr(cod)(dppm). The observed regioselectivity is discussed in terms of preferential formation of one of the possible metallacyclic intermediates of the catalytic reaction.