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 E p/2 obtained via cyclic voltammetry. The latter are found to correlate with the Hammett parameters σ p of the cyclopentadienyl substituents and to the pK a values of the corresponding acids of the TiÀ 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.

Coordination-induced bond weakening of water at the surface of an oxygen-deficient polyoxovanadate cluster

“…For example, the titanocene(III) complex reported by Please do not adjust margins Please do not adjust margins Cuevra binds to water, resulting in the formation of a highly reactive O-H bond due to the thermodynamic driving force for oxidation of the Ti(III) centre. 70,71 The change in BDFE(O-H) of water reported here, following coordination to a V(III) center embedded within a multimetallic MO x assembly, is smaller than that described for Cp 2 TiCl(OH 2 ) (∆BDFE(O-H) H2O,avg = 46.5 kcal mol -1 for V(III) vs. ∆BDFE(O-H) H2O,avg = 58.7 kcal mol -1 for Ti(III)), consistent with the modest oxidation potential of the parent cluster (E 1/2 = -0.477 V vs Fc +/0 in THF, Figure S7). The BDFE(O-H) avg obtained for the reduced cluster, (calix)V 6 O 5 (OH 2 ), closely resembles values previously reported by our group for reduced polyoxovanadate cluster containing a similar Lindqvist structure.…”

“…For example, the titanocene(III) complex reported by Cuevra binds to water, resulting in the formation of a highly reactive O-H bond due to the thermodynamic driving force for oxidation of the Ti(III) centre. 70,71 The change in BDFE(O-H) of water reported here, following coordination to a V(III) center embedded within a multimetallic MO x assembly, is smaller than that described for Cp 2 TiCl(OH 2 ) (DBDFE(O-H) = 46.5 kcal mol −1 for V(III) vs. DBDFE(O-H) = 58.7 kcal mol −1 for Ti(III)), consistent with the modest oxidation potential of the parent cluster (E 1/2 = −0.477 V vs. Fc +/0 in THF, Fig. S7 †).…”

Coordination-induced bond weakening and small molecule activation by low-valent titanium complexes

“…The facile transition of Ti-based complexes between the Ti(II), Ti(III) and Ti(IV) oxidation states has enabled accessibility to various mechanistic paradigms in redox chemistry. [3][4][5][6][7] Among the reactions that employ the Ti(III)-Ti(IV) redox couple are the reduction and cyclization of epoxides, 8 Barbiertype reactions, 9 intermolecular couplings, 10 and synthesis of stereo-controlled compounds. 11 A notable and recent example of a Ti(III)-mediated redox transformation is the photocatalytic reduction of epoxides.…”

Reductive Umpolung and Defunctionalization Reactions through Higher-Order Titanium(III) Catalysis

“…Moreover, the reduction potential can be adjusted over a broad range through the choice of matching Cp ligands and counter ions. 4…”

“…Moreover, the reduction potential can be adjusted over a broad range through the choice of matching Cp ligands and counter ions. 4 Scheme 1 Concept for controlling radical reactions with a single-electron transfer catalyst that remains coordinated by the substrate (top). Cp 2 TiCl as an ideal catalyst for accessing controlled reactions of ketyl and homoketyl radicals (bottom).…”