Scheme 1. Proposed catalytic cycle. Cp = cyclopentadienyl, M = metal.

The electrochemical properties of titanocene(III) complexes and their stability in THF in the presence and absence of chloride additives were studied by cyclic voltammetry (CV) and computational methods. The anodic peak potentials of the titanocenes can be decreased by as much as 0.47 V through the addition of an electron-withdrawing substituent (CO2Me or CN) to the cyclopentadienyl ring when compared with Cp2TiCl. For the first time, it is demonstrated that under the conditions of catalytic applications low-valent titanocenes can decompose by loss of the substituted ligand. The recently discovered effect of stabilizing titanocene(III) catalysts by chloride additives was analyzed by CV, kinetic, and computational studies. An unprecedented supramolecular interaction between [(C5H4R)2TiCl2](-) and hydrochloride cations through reversible hydrogen bonding is proposed as a mechanism for the action of the additives. This study provides the critical information required for the rational design of titanocene-catalyzed reactions in single electron steps.

Mechanistic studies show the importance of iodide displacement by additives that accelerate reactions of samarium diiodide. The key feature important for acceleration of reaction rate is the use of proton donors and other additives that have a high enough affinity for Sm(II) to displace iodide yet do not saturate the coordination sphere inhibiting substrate reduction.

An atom-economical and catalytic arylation of epoxide-derived radicals is described. The key step of the catalytic system is a sequential electron and proton transfer for the rearomatization of the radical σ-complex and catalyst regeneration. Kinetic, computational, spectroscopic, and cyclovoltammetric investigations highlight the key issues of the reaction mechanism and catalyst stabilization by collidine hydrochloride. Studies employing radicophiles rule out the participation of cations as reactive intermediates.