Electrocatalytic Diels-Alder Reactions Assisted By Aromatic Redox Tag

Abstract

Although the Diels-Alder reaction is one of the most classic organic transformations, it is still in the front line of carbon-carbon bond formation toolbox and has been producing modern research topics in synthetic chemistry. Especially, the reaction mechanism has been studied intensively and also expanding to a wide variety of the combination of diene and dienophile. The reaction partners must be electronically matched, namely, electron-rich and electron-deficient substrates are required. In order to overcome this limitation, electron transfer strategy can be used. Typically, the reaction between both an electron-rich diene and dienophile are electronically mismatched, however, one electron oxidation by chemical oxidants or photosensitizers can produce a radical cation of one component, which is then trapped by the other component. We have been developing electrocatalytic intermolecular carbon-carbon bond formations in lithium perchlorate/nitromethane electrolyte solution. We have designed the reaction based on an aromatic “redox tag” concept, that is, the aromatic ring which provides both as an oxidant and a reductant in an overall electrochemical transformation. Since radicals, ions, and radical ions are all significantly stabilized by an aromatic ring and the redox potentials can be fine-tuned by the number and/or position of substituents, programmed electron transfer events could be achieved. These results in hand, we herein demonstrate electrocatalytic Diels-Alder reactions based on an aromatic redox tag strategy. Initially, we tried to optimize the reaction conditions by using trans-anethole and isoprene as a model reaction partner. These substrates are both relatively electron-rich, therefore, no reaction occurred without an electron transfer. In this context, we found that only a catalytic amount of electrical input was able to complete the reaction, even in high concentration conditions. We also found that the reaction partners were able to be designed by an expected manner by using an aromatic redox tag concept. In the presentation, the synthesis and electrochemical details will be discussed.