Intramolecular [2+2+2] Cyclotrimerization of a Model Triyne to [7]Helical Indeno[2,1-c]Fluorene with Air-Stable Ni(0) and Other Precatalysts

In this work, we demonstrated that air-stable Ni(0) complexes with phosphine ligands can effectively catalyze intramolecular cyclotrimerization of a triyne to a compound with a [7]helical indeno[2,1-c]fluorene skeleton. The obtained results are comparable to those achieved by using Rh-based catalytic systems. Screening of the reaction conditions showed that bidentate phosphine ligands with small bite angles (70–80°) gave the best results in terms of yields. The highest asymmetric induction with the investigated air-stable Ni(0) precatalyst was obtained using the PROPHOS ligand in HFIP (62% ee). Other catalytic systems, like [Rh(CH2=CH2)2Cl]2 and [CpCo(P{OEt}3)(trans-MeO2CHC=CHCO2Me)], have also been investigated, showing promising results.