Electro-Oxidation of Cumene Using Diamond Electrodes in Batch and Flow

Abstract

Selective oxidation of aromatic alkyl side chains is an important molecular transformation process to obtain various materials ranging from rubber and resin to fine chemicals. Generally, molecular oxygen has been utilized in straightforward oxidation of aromatic alkyls. However, since molecular oxygen is highly stable, activation of the molecular oxygen itself is necessary, which requires a specific catalyst and/or harsh condition such as high temperature and high pressure. Recent environmental and sustainable concerns lead to a growing demand for the development of greener oxidation processes.Recently, electro-organic synthesis, referring to an organic synthesis method combined with electrochemistry, has drawn attention as a sustainable synthetic method. Since electro-organic synthesis utilizes electricity itself as a reagent, reactions proceed well even under ambient conditions and reagent wastes are reduced to a minimum. In electro-organic synthesis, electrode materials are one of the most significant parameters because reactions occur at the anode and/or cathode. Boron-doped diamond (BDD) is a relatively new electrode material and shows a wide potential window, which can be applied to the transformation of compounds with high redox potentials. Therefore, BDD electrodes would enable a straightforward oxidation reaction of aromatic alkyls, which is difficult to achieve with other conventional electrode materials.Herein, we report the straightforward electro-oxidation of cumene, one of the most important and extensively investigated aromatic alkyls, by BDD electrodes. The role of electrode materials was investigated with electrochemical measurements. Furthermore, the cell design (i.e. batch vs. flow) was examined, and we realized that the product selectivity of cumene oxidation can be switched by simply changing the cell design.First, we carried out the electrolysis of cumene under the constant current condition in an undivided batch cell. The main product was acetophenone under the following optimum conditions: (solvent) MeCN, (supporting electrolyte) Et4NClO4, (current density) 2.1 mA/cm2, (amount of charge) 5 F referring to mole of cumene. It should be noted that almost no acetophenone was obtained when the combination of anode and cathode was graphite/graphite or Ni/Ni. Cyclic voltammetry using BDD as a working electrode showed a clear oxidation peak of cumene at around 2.40 V (vs. Ag/Ag+). On the other hand, no clear oxidation peak was observed when using graphite or Ni as a working electrode. This is because potential windows of graphite and Ni are too narrow to oxidize cumene directly. Overall, the electrochemical measurements indicate the BDD’s wide potential window enables direct oxidation of cumene to produce a key reaction intermediate to afford acetophenone. A series of electrolysis experiments confirmed that the reaction intermediate is the cumyl cation and the oxygen source is the superoxide generated by reduction of dissolved oxygen on the cathode. A proposed mechanism is as follows. Anodic oxidation of cumene on the BDD electrode affords a cumyl cation. On the other hand, cathodic reduction of dissolved oxygen produces the superoxide and even the hydroperoxide anion. Addition of the hydroperoxide anion to the cumyl cation yields cumene hydroperoxide, which is further converted into acetophenone.Second, we carried out the electrolysis of cumene under the constant current condition in an undivided flow cell. The main product was a-cumyl alcohol under the following optimum conditions: (solvent) MeCN, (supporting electrolyte) Et4NClO4, (current density) 0.25 mA/cm2, (amount of charge) 1 F referring to mole of cumene, (flow rate) 0.375 mL/min. A series of flow electrolysis experiments indicated that cumene is converted into a-cumyl alcohol via cumene hydroperoxide, and then to acetophenone, which is supported by cyclic voltammetry measurements: the potential at which the current value increases was lower in the order of cumene hydroperoxide > a-cumyl alcohol > cumene > acetophenone. A proposed mechanism is as follows. A hydroperoxide anion generated at the cathode added to a cumyl cation generated at the anode to form cumene hydroperoxide. The O-O bond in cumene hydroperoxidewas cleaved anodically to form alkoxy radical, which underwent a hydrogen atom transfer (HAT) between cumene to yield a-cumyl alcohol and cumyl radical. Furthermore, a-cumyl alcoholwas oxidized into the alkoxy radical, followed by b-scission to yield acetophenone. Therefore, it is suggested that the selective conversion of cumene into a-cumyl alcohol in flow electrolysis was due to suppressing overoxidation of a-cumyl alcoholinto acetophenone. Figure 1