Unified Synthesis Platform for 1,2,3-Trisubstituted Cyclopentadienyl Ligands Decouples Sterics from Electronics.

Cyclopentadienyl (Cp) ligands are a cornerstone of coordination chemistry and transition-metal catalysis. Their tuning profoundly influences the chemical and biological reactivity, the induced selectivity, and the stability of the corresponding metal complexes. However, compared to phosphines for instance, the accessible chemical space of Cps is rather narrow, exhibiting major limitations regarding the nature, pattern, and number of Cp substituents. A unified synthetic strategy toward partially substituted Cps bearing diverse functionalities and closing gaps in chemical space is highly desirable. Herein, we report a streamlined general strategy to prepare 1,2,3-trisubstituted cyclopentadienes (1,2,3-Cps) from a central inexpensive precursor. Operationally straightforward reactions and purifications ensure scalable sequences. The robust and versatile synthesis platform opens access to underexplored Cp substitution patterns - including flexible incorporation of diverse alkyls, aryls, and previously elusive or rare functionalities such as halogens, chalcogens, and alkynes - with a profound ligand tunability and decoupling of sterics from electronics. The complexation ability with a selection of catalytically relevant early and late transition metals was demonstrated, and parametrization of the Cps with respect to their stereoelectronic environment was performed via the corresponding Cp rhodium phosphite species. In exemplary selected benchmark catalytic transformations, the cobalt and rhodium complexes directly outperformed classical Cp ligands with respect to individual reactivity, regioselectivity, and catalyst loading. Regarding catalytic turnover, a 1,2,3-Cp cobalt complex achieved an attractive turnover number (TON) of 180 for a benchmark C-H annulation.