Chiral cyclopentadienyl (Cpx) ligands have widespread applications in asymmetric transition-metal catalysis. Yet, further ligand development is crucial to unlock different reactivity outcomes and break through current selectivity boundaries. Past ligand design efforts have primarily focused on incorporating new chiral backbones into disubstituted Cpx entities. In contrast, modification of the substitution degree of the Cpx ring and diversification of the nature of these substituents remain largely underexplored. In this respect, a concise synthetic approach toward highly substituted Cpx ligands with profound substituent flexibility is most desirable. Herein, we report a modular strategy for the rapid assembly of structurally diverse pentasubstituted Cpx ligands (CpV). Readily accessible 1,2,3-trifunctionalized cyclopentadiene building blocks are leveraged in a robust one-step dialkylation procedure, integrating a wide range of chiral bis-electrophiles. This combinatorial approach reduces the synthetic upfront investment of catalyst screenings and enables fast ligand diversification with extensive steric and electronic substituent tunability. Subsequent complexation with group 9 metals (Co, Rh, Ir) was amply demonstrated (50+ examples), and electronic parametrization of the ligands via their respective CpVRh phosphite species was performed. In selected exemplary asymmetric C-H functionalizations, the pentasubstituted CpV cobalt and rhodium catalysts acted as true chiral Cp* clones, delivering excellent reactivity under identical conditions. Easily introduced modifications of the CpV substituents prompted strong responses in stereoselectivity, including several instances of enantio-inversion. For each catalytic benchmark assessment, multiple CpV ligands directly outperformed their di- or trisubstituted Cpx counterparts with simultaneously improved yields, diastereo-, and enantioselectivities. As such, the CpV platform addresses catalyst performance issues in challenging transformations, as well as substantially expands the reactivity and selectivity optimization options for future methodology development.
Bossche et al. (2026) studied this question.