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The iridium derivatives HIr(cod)(L-L) (cod=1,5-cyclooctadiene; L-L=Ph 2 PCH 2 PPh 2 (dppm); Ph 2 P(CH 2 ) 2 PPh 2 (dppe); Ph 2 P(CH 2 ) 3 PPh 2 (dppp); Ph 2 P(CH 2 ) 4 PPh 2 (dppb); o -C 6 H 4 (PPh 2 ) 2 ; Cy 2 P(CH 2 ) 2 PCy 2 (dcpe); o -Me 2 NC 6 H 4 PPh 2 (P-NMe 2 )) and Ir(OMe)(cod)(dppe-F) (dppe-F=(C 6 F 5 ) 2 P(CH 2 ) 2 P(C 6 F 5 ) 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. |
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