Unexpected Solvent Effects in the Isomerization of iPr PCPIr(η 2 ‐PhC≡CPh) to a 1‐Iridaindene

The reaction of iPr PCPIr with diphenylacetylene ( iPr PCP=κ 3 ‐2,6‐C 6 H 3 (CH 2 P( i Pr) 2 ) 2 ), generated by dehydrogenation of iPr PCPIrH 4 with tert ‐butylethylene, readily provides the π‐adduct iPr PCPIr(η 2 ‐PhC≡CPh) in quantitative yield at room temperature. Heating solutions of iPr PCPIr(η 2 ‐PhC≡CPh) in aromatic hydrocarbon solvents leads to isomerization forming the 1‐iridaindene iPr PCPIr(C 8 H 5 (2‐Ph)). Surprisingly, this seemingly intramolecular reaction presents a kinetic isotope effect of 4.6 in C 6 H 6 versus C 6 D 6 solvent. The rate of this isomerization is effected by the availability of easily activated C−H bonds (i. e. aromatic C( sp 2 )‐H without ortho ‐substituents), and is strongly dependent upon the ratio of C−H to C−D bonds. Experiments indicate that many, or all, of the steps in this process may be reversible and that a highly fluxional C−H/D addition product is present during isomerization.