Mechanisms of the PtCl₂‑Catalyzed Intramolecular Cyclization of <i>o</i>‑Isopropyl-Substituted Aryl Alkynes for the Synthesis of Indenes and Comparison of Three sp³ C–H Bond Activation Modes

Chatani and He respectively reported an efficient way to synthesize indenes through PtCl₂ catalyzed sp³ C–H bond activation. Interestingly, the R group (R = H or Br) in the alkyne moiety of the substrates in Chatani’s experiments migrates to the C3 position in indenes, whereas the R group (R = Ar) stays in the original C2 position of final indenes in He’s experiments. DFT calculations indicated that there are two competing pathways <i>a</i> and <i>c</i> for the cyclization reaction. Pathway <i>a</i> involves [1,2]-R migration, [1,5]-H shift, and 4π-electrocyclization, giving the indenes with the R group at the C3 position. Pathway <i>c</i> takes place through irreversible [1,5]-H shift/cyclization and [1,2]-H shift, generating indenes with the R group at the C2 position. DFT calculations found that, when R = H or Br, pathway <i>a</i> is favored. When R = alkyl group, the [1,2]-R migration is difficult and pathway <i>c</i> is preferred. When R = Ar, DFT calculations predicted and experiments verified that both pathways <i>a</i> and <i>c</i> occur to give two indene products. Comparison of different models of sp³ C–H activations has been presented to guide further understanding and prediction of new C–H bond activations