Optimal Regioselectivity Control in Electrocatalytic Oxidation of Terminal Olefins

Abstract

Wacker-type reaction has been considered as an effective industrially viable route for oxidation of various organic compounds since 1970s [1]. Products of these processes, such as aldehydes and ketones are extremely valuable in fine chemicals industry [2]. Despite the historical success of these reactions, there hasn’t been many studies in terms improving their processes throughput, and particularly the attributed Palladium catalyst efficiency. During the past decade, oxidation of organic compounds via electrocatalytic oxidation has provided a valuable opportunity to increase the reaction efficiency as well as tuning the metallic catalyst specifically designed for these processes [3]. Electrochemistry is a well-known method used for a multitude of purposes ranging from renewable energy production [4], corrosion protection [5], and organic syntheses [3]. This method has been used in large scale production of different metal oxides, commodity chemicals and active pharmaceutical ingredients (e.g. acetone, and cephalosporin, respectively) [6]. However, the true potential of this configuration for organic synthesis is yet to be discovered. The recent reports on electrochemical Wacker oxidation of olefins as model organic substrates has shown that through this process modification, a higher yield compared to traditional batch configuration can be achieved [3]. Nonetheless, the effect of catalyst tuning and property optimization on electrochemical Wacker oxidation is a critical component towards process intensification. Solid-supported catalysis has been found to be one of the most successful methods to improve upon traditional homogeneously catalyzed reactions [7]. During this catalyst-preparation method, metallic particles of catalyst are immobilized on the surface of a support capable of high charge transfer (e.g. graphene [8]). Hence, the redox reactions occurring during the catalytic cycle of the metal particles are improved by faster and more efficient electrical charge transfer through the support. One of the major drawbacks of the electrochemical approach for Wacker-type reactions is the lack of control over regioselectivity of olefin oxidation. Therefore, a mixture of two different products (aldehyde and ketone) (Figure 1) is often obtained. It has been shown that by promoting an anodic oxidation or cathodic reduction of catalyst, a certain regioselectivity can be promoted [9]. Thus, the type of product (aldehyde or ketone) can be controlled. Figure 1 Regioselective Wacker-type electrocatalytic oxidation of terminal olefins to ketone (top) and aldehyde (bottom) In this study, a palladium on graphene nanoplatelet solid-supported catalyst was prepared via an all-aqueous ascorbic acid-mediated chemical reduction process for Wacker-type electrochemical oxidation of 1-octene as a model substrate. During the process optimization turn over frequency (TOF) of the prepared electrocatalyst compared to conventional homogeneous PdCl­2 [9] and solid-supported Pd on carbon nanotube [3] catalysts was improved by approximately 110 and 50 time fold, respectively. Nevertheless, through controlling the promoted half-cell reaction, oxidation or reduction, the regioselectivity of the reaction was controlled where aldehyde and ketone products were successfully prepared with optimal selectivity. Higher activity of prepared catalysts due to improved charge transfer via graphene as well as improved control over anodic oxidation through cell-type alternation are thought to be the governing factors in such improved yield and regioselectivity. References Michel, B.W., L.D. Steffens, and M.S. Sigman, The Wacker Oxidation. 2014, Wiley.Calter, M., Palladium Reagents and Catalysts: Innovations in Organic Synthesis By Jiro Tsuji (Okayama University of Science). 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