photoredox catalysis

유기분자를 전자전달(SET)이나 에너지전달(EnT)을 통해 활성화 상태로 유도하는 광촉매 반응법은 최근 강력한 유기합성 방법으로 주목받고 떠오르고 있다. 가시광 영역(52 kcal/mol)의 광자에너지는 지속가능한 에너지원이며, 기존 반응과 상보적인 선택성을 가진 C-C, C-X 결합 반응을 유도할 수 있고, 또한 다른 전이금속 촉매반응과  결합되면 더욱 새로운 반응 플랫폼으로 개발될 수 있다.

Dual Gold-Photoredox Catalysis for the Cross-Coupling of Vinyl Golds                               Adv. Synth. Catal. 2015, 357, 2622; Org. Lett. 2016, 18, 484

Considering that gold catalysis enables rapid increase in the molecular complexity from simple pi-bolds, such as alkenes, allenes, combining the complexity generating power of gold catalysis with cross-coupling chemistry will add to the practical utility of gold catalysis. Initial challenges lies in the difficulty in the redox cycling between Au(I)-Au(III), due to the high oxidation potential (Eo = 1.4 V). By way of photoredox catalysis, we successfully achieved oxidation of Au(I) by aryldiazonoum salts into the key Au(III) intermediate, enabling cross-coupling of vinyl gold intermediates that was previously believed to be easily undergo side-reaction pathway, proto-deauration.     

N-Carboxyindoles as N-Radical & α-Carbonyl Precursors via Energy Transfer Catalysis                                                                 Org. Lett. 2022, 24, 1774

Photoredox catalysis has two modes of activation, i.e. electron transfer (ET) and energy transfer (EnT), in the activation of radical precursors. The latter mode is gaining increasing popularity because of the atom economy: while electron transfer (ET) sacrifice organic fragments from the substrates as side products, energy transfer (EnT) uses both of radical fragments.

The renewed interest in the radical chemistry lies in the atom-economical development of new radical precursors. Based on our long-standing interest in the chemistry of N-O bond reagents, we developed photochemistry of N-carboxxyindoles in collaboration with Prof. Cheol-Min Park (UNIST) and Prof. E. J. Cho (Chungang Univ).     

N-Enoxybenzotriazoles as N-Radical & α-Carbonyl Precursors via Energy Transfer Catalysis    Org. Lett. 2022, 24, 8337; ACS Catal. 2022, 12, 8833 

In advancing energy transfer photocatalysis, a key component is developing new precursos for the EnT pathway. We developed easily available enoxybenzotriazoles as new radical precursors via EnT. They are prepared from the Au(I)-catalyzed reaction of terminal alkynes and N-hydroxybenzotriazoles. 

The enoxybenzotriazole undergo triplet-sensitized EnT upon irradiation with blue LEDs. In the former paper (Org. lett.), we demonstrated α-carbonyl radicals can be generated and used in the intramolecular radical addition, leading to a general synthesis of 9-phenanthrols and related PAHs. In doing so, benzotriazolyl (Bt) radical should be quenched by HAT reagents. In the second paper (ACS Cat.), both of these radicals were deployed in the synthesis via formal 1,3-shift as well as group-transfer radical addition (GTRA). We found that Bt radical is high electrophilic N-radical and mediates an long-lived radical chain (Ф = 210).