Mechanisms of the PtCl2-catalyzed intramolecular cyclization of o-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 sp(3) 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 a and c for the cyclization reaction. Pathway a involves [1,2]-R migration, [1,5]-H shift, and 4π-electrocyclization, giving the indenes with the R group at the C3 position. Pathway c 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 a is favored. When R = alkyl group, the [1,2]-R migration is difficult and pathway c is preferred. When R = Ar, DFT calculations predicted and experiments verified that both pathways a and c occur to give two indene products. Comparison of different models of sp(3) C-H activations has been presented to guide further understanding and prediction of new C-H bond activations.