A Guide to Low-Valent Titanocene Complexes as Tunable Single-Electron Transfer Catalysts for Applications in Organic Chemistry

Abstract

Low-valent titanocene catalysts are a versatile tool for organic synthesis. They promote inter- and intramolecular reactions ranging from homolytic bond cleavages to reductive umpolung reactions to additions and cyclizations in single electron steps. These reactions heavily depend on the redox potential of an in situ formed titanium(III) center, which can be adjusted by the choice of appropriate ligands. We herein review various chiral and achiral ligand-modified titanocene catalysts and their reduction potentials Ep/2 obtained via cyclic voltammetry. The latter are found to correlate with the Hammett parameters σp of the cyclopentadienyl substituents and to the pKa values of the corresponding acids of the Ti−X ligands. For selected examples, we further discuss how the adjustment of the redox properties through modifications of the titanocene ligands can lead to greatly improved reaction outcomes in titanium(III) catalyzed single-electron transfer reactions.