Aromatization as an impetus to harness ketones for metallaphotoredox-catalyzed benzoylation/benzylation of (hetero) arenes Shao-Chi Lee, †,1,2 Li-Yun Li, †,1,2 Zong-Nan Tsai, †,1,2 Yi-Hsin Lee, 1,2 Yong-Ting Tsao, 1,2 Pin-Gong Huang, 1,2 Cheng-Ku Cheng, 1,2 Heng-Bo Lin, 1,2 Ting-Wei Chen, 1,2 Chung-Hsin Yang, 1,2 Cheng-Chau Chiu, 1,2 and Hsuan-Hung Liao* 1,2 1 Department of Chemistry, National Sun Yat-sen University, R.O.C., 2 Chinese Taipei † S.-C.L.,L.-Y.L.,and Z.-N.T. contributed equally.
We used ketones as our feedstock materials to prepare pre-aromatic radical precursors and employed Ni/ photoredox dual catalysis for reductive cross-coupling reactions, releasing benzyl/benzoyl radicals through the aromatic stabilization energy (ASE) 1 similar to the enzymatic transformation pathway of NADH/NAD+ redox pair in nature. 2 Inspired by these aromatization-promoted processes, researchers have utilized several pre-aromatics such as Hantzsch esters (HE), 3 cyclohexadiene (CHD), 4 and N-alkyl-pyridinium salts (Katritzky salt), 5 in which the aromatic stabilization effect serves as a driving force for the release of radicals. In contrast to the cumbersome preparation of pre-aromatic compounds in the past, our developed method not only features simple preparation process with mild reaction conditions, but it can also be activated by visible light. This method exhibits high functional group tolerance, meaning that it can tolerate a wide range of functional groups in the starting material, and ultimately creates a novel method for benzylation/benzoylation.
References 1. von Ragué Schleyer, P.;Puhlhofer, F.Recommendations for the Evaluation of Aromatic Stabilization Energies. Org. Lett. 2002 , 4 ,2873–2876. 2. Pollak, N.;Dölle, C.;Ziegler, M.The Power to Reduce: Pyridine Nucleotides - Small Molecules with a Multitude of Functions. J. 2007 , 402 ,205–218. 3. Wang, P. Z.;Chen, J. R.;Xiao, W. J.Hantzsch Esters: An Emerging Versatile Class of Reagents in Photoredox Catalyzed Organic Synthesis. Biomol. Chem. 2019 , 17 ,6936–6951. 4. Walton, J. C.; Studer, A. Evolution of Functional Cyclohexadiene-Based Synthetic Reagents: The Importance of Becoming Aromatic. Acc. Chem. Res. 2005 , 38 , 794– 802. 5. He, F. S.;Ye, S.;Wu, J.Recent Advances in Pyridinium Salts as Radical Reservoirs in Organic Synthesis. ACS Catal. 2019 , 9 ,8943–8960.
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