Photochemical and electrochemical processes both have proven to be powerful tools that enable unique bond formations to construct various carbon skeletons. In this context, we have been developing electrochemical reactions in lithium perchlorate/nitromethane (LiClO₄/MeNO₂) electrolyte solution. This solvent system possibly stabilizes the anodically-generated cation species and acts as a Lewis acid catalyst. The feature of the LiClO₄/MeNO₂ allows effective syntheses of various compounds, such as dihydrobenzofuran derivatives formed via phenoxonium cation intermediates ^[1] and azanucleoside derivatives formed via iminium cation intermediates.^[2] Furthermore, it is also demonstrated that the LiClO₄/MeNO₂ is effective for several [2+2] cycloaddition reactions between enol ethers and unactivated olefins,^[3] affording cyclobutane products, and electrocatalytic Diels-Alder reactions^[4]. In this way, our reaction development is governed by the use of a high concentration LiClO₄/MeNO₂ electrolyte solution. Recent ⁷Li NMR and calorimetric studies showed that Li⁺ is weakly coordinated in the solution to exhibit Lewis acidity, which we termed entropic effect.^[5]

During such reaction developments, we serendipitously found the [2+2] cycloaddition reactions in LiClO₄/MeNO₂ also proceed under UV (365 nm) irradiation without any other reagents such as photo-catalyst. UV-Vis spectra study suggested that MeNO₂ would absorb UV light and drive the reaction. This could be reasonably understood based on energy transfer mechanism, namely, irradiation of the MeNO₂ with UV light produces the excited state, which then generates the excited state of the enol ether that undergoes the reaction. Molecular oxygen must also be involved in the overall process, since Ar bubbling had a negative impact on the reaction. . The reaction was also dramatically suppressed in other solvent systems Furthermore, we found this photo-induced reaction in LiClO₄/MeNO₂ system also can be applied to [4+2] and [3+2] cycloadditions. Details of reaction mechanism and scope & limitation would be discussed in the presentation.

Reference

[1] Kim, S.; Hirose, K.; Uematsu, J.; Mikami, Y.; Chiba., K. Chem. Eur. J., 2012, 18, 6284–6288.

[2] Shoji, T.; Kim, S.; Chiba, K. Angew. Chem. Int. Ed., 2017, 56, 4011-4014.

[3] Okada, Y.; Nishimoto, A.; Akaba, R.; Chiba, K. J. Org. Chem., 2011, 76, 3470-3476.

[4] Okada, Y.; Yamaguchi, Y.; Ozaki, A.; Chiba, K. Chem. Sci., 2016, 7, 6387-6393.

[5] Imada, Y.; Yamaguchi, Y.; Shida, N.; Okada, Y.; Chiba, K. Chem. Comm., 2017, 53, 3960-3963.