Markovnikov‐Selective Cobalt‐Catalyzed Wacker‐Type Oxidation of Styrenes into Ketones under Ambient Conditions Enabled by Hydrogen Bonding

Abstract

The replacement of palladium catalysts for Wacker‐type oxidation of olefins into ketones by first‐row transition metals is a relevant approach for searching more sustainable protocols. Besides highly sophisticated iron catalysts, all the other first‐row transition metal complexes have only led to poor activities and selectivities. Herein, we show that the cobalt‐tetraphenylporphyrin complex is a competent catalyst for the aerobic oxidation of styrenes into ketones with silanes as the hydrogen sources. Remarkably, under room temperature and air atmosphere, the reactions were exceedingly fast (up to 10 minutes) with a low catalyst loading (1 mol %) while keeping an excellent chemo‐ and Markovnikov‐selectivity (up to 99 % of ketone). Unprecedently high TOF (864 h−1) and TON (5,800) were reached for the oxidation of aromatic olefins under these benign conditions. Mechanistic studies suggest a reaction mechanism similar to the Mukaiyama‐type hydration of olefins with a change in the last fundamental step, which controls the chemoselectivity, thanks to a unique hydrogen bonding network between the ethanol solvent and the cobalt peroxo intermediate.

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

“…While investigating the potential for the catalysis of cobalt­(III) complexes 2 and 3 , ,,− we found that in the presence of PhSiH 3 and dioxygen, 2-vinylnaphthalene ( 9a ) could be oxidized to the corresponding ketone ( 10a ) in a Wacker oxidation-type fashion (cf. Scheme ).…”

“…Important contributions in this direction are nickel- and iron-based catalysts to promote the Wacker-type oxidation of olefins (Scheme b,c). − A dual catalytic system involving a cobaloxime and an acridinium photocatalyst that yielded the anti-Markovnikov products under milder conditions was also recently reported (Scheme d) . With few exceptions (Scheme e), previous reports on Co-mediated oxidation of olefins to ketones, in a Markovnikov fashion, either suffer from limitations in scope, − require harsh conditions, or lead to unspecific reactivity. , Some works also showed that, in the presence of Co and dioxygen, olefins can undergo oxidative cleavage, rather than oxidative hydration. , …”