TiO₂ Photocatalysis in Aromatic “Redox Tag”-Guided Intermolecular Formal [2 + 2] Cycloadditions

Abstract

Since the pioneering work by Macmillan, Yoon, and Stephenson, homogeneous photoredox catalysis has occupied a central place in new reaction development in the field of organic chemistry. While heterogeneous semiconductor photocatalysis has also been studied extensively, it has generally been recognized as a redox option in inorganic chemistry where such "photocatalysis" is most often used to catalyze carbon-carbon bond cleavage and not in organic chemistry where bond formation is usually the focal point. Herein, we demonstrate that titanium dioxide photocatalysis is a powerful redox option to construct carbon-carbon bonds by using intermolecular formal [2 + 2] cycloadditions as models. Synergy between excited electrons and holes generated upon irradiation is expected to promote the overall net redox neutral process. Key for the successful application is the use of a lithium perchlorate/nitromethane electrolyte solution, which exhibits remarkable Lewis acidity to facilitate the reactions of carbon-centered radical cations with carbon nucleophiles. The reaction mechanism is reasonably understood based on both intermolecular and intramolecular single electron transfer regulated by an aromatic "redox tag". Most of the reactions were completed in less than 30 min even in aqueous and/or aerobic conditions without the need for sacrificial reducing or oxidizing substrates generally required for homogeneous photoredox catalysis.

Interplay of arene radical cations with anions and fluorinated alcohols in hole catalysis

“…In such reactions, radical cations are generated by various methods, including photoredox catalysts 14,17,19,21,23 , semiconducting nanomaterials 30,31 , chemical oxidants 7,20 and electrochemistry 16,24,25 . Due to the high reactivity of the radical cation intermediate, the efficiency in hole catalysis is highly dependent on the reaction media.…”

Synthetic Semiconductor Photoelectrochemistry

“… [31] In particular, the reaction with 2‐ethyl‐1‐butene ( 7 ) is used as a benchmark in this account [32] . Compared to the radical cation Diels‐Alder reaction, which was completed with 0.1 F/mol of electricity, the turnover number for the electron seems to be much smaller in the radical cation [2+2] cycloaddition, since 0.5 F/mol is required for full conversion (Table 3, Entries 1 and 2) [33] . Nevertheless, based on the fact that less than 1 F/mol is sufficient to consume the starting material, it is reasonable to consider that the chain pathway is involved (Figure 13).…”

Photocatalytic Cycloadditions Enabled by a Lithium Perchlorate/Nitromethane Electrolyte Solution

“…[12] The electrolyte solution exhibits remarkable Lewis acidity and facilitates reactions be-form and nitromethane was expected to function as the initial sensitizer for the reactions. [14] Described herein is our serendipitous development of sensitizer-free photocatalytic cycloadditions in this electrolyte solution. tween carbon-centered radical cations and carbon nucleophiles.…”