Chemical and Redox Noninnocence of Pentane-2,4-dione Bis(S-methylisothiosemicarbazone) in Cobalt Complexes and Their Application in Wacker-Type Oxidation

Abstract

Cobalt complexes with multiproton- and multielectron-responsive ligands are of interest for challenging catalytic transformations. The chemical and redox noninnocence of pentane-2,4-dione bis­(S-methylisothiosemicarbazone) (PBIT) in a series of cobalt complexes has been studied by a range of methods, including spectroscopy [UV–vis, NMR, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS)], cyclic voltammetry, X-ray diffraction, and density functional theory (DFT) calculations. Two complexes [CoIII(H2LSMe)­I]I and [CoIII(LSMe )­I2] were found to act as precatalysts in a Wacker-type oxidation of olefins using phenylsilane, the role of which was elucidated through isotopic labeling. Insights into the mechanism of the catalytic transformation as well as the substrate scope of this selective reaction are described, and the essential role of phenylsilane and the noninnocence of PBIT are disclosed. Among the several relevant species characterized was an unprecedented Co­(III) complex with a dianionic diradical PBIT ligand ([CoIII(LSMe••)­I]).

Wacker-type oxidation of styrenes with Markovnikov selectivity using copper as a site-selective sequential photooxidant–photoreductant

“…18,24–26 Methodologies using first-row transition metals relied on using hydrosilanes as a hydrogen source leading to the oxidation of olefins with a marked Markovnikov selectivity. 18,27–34 On the other hand, the Lei group demonstrated that the photochemical single-electron oxidation of olefins, in the presence of a cobalt catalyst, leads to the correspondent carbonyl derivate with anti-Markovnikov selectivity (Scheme 1A). 24 With this in mind, we designed a mechanistically distinct Wacker-type oxidation of styrene derivates, capitalizing on phenacyl intermediates' absorbance properties.…”