Azolium/Hydroquinone Organo‐Radical Co‐Catalysis: Aerobic C−C‐Bond Cleavage in Ketones

Abstract

Organo‐radical catalysts have recently attracted great interest, and the development of this field can be expected to broaden the applications of organocatalysis. Herein, the first example of a radical‐generating system is reported that does not require any photoirradiation, radical initiators, or preactivated substrates. The oxidative C−C‐bond cleavage of 2‐substituted cyclohexanones was achieved using an azolium salt and a hydroquinone as co‐catalysts. A catalytic mechanism was proposed based on the results of diffusion‐ordered spectroscopy and cyclic voltammetry measurements, as well as computational studies.

Visible Light-Driven, Copper-Catalyzed Aerobic Oxidative Cleavage of Cycloalkanones

“…49−50 °C; R f = 0.15 (EtOAc/ petroleum ether = 1:1); 1 H NMR (400 MHz, CDCl 3 ): δ = 7.74 −7.72 (m, 1H), 7.64−7.61 (m, 1H), 7.47−7.42 (m, 1H), 7.28−7.24 (m, 1H), 2.99 (t, J = 6.8 Hz, 2H), 2.43 (t, J = 6.8 Hz, 2H), 1.84−1.71 (m, 4H); 13 C{ 1 H} NMR (100 MHz, CDCl 3 ): δ = 198. 4,178.8,162.9 (d,J = 246.4 Hz),138.9 (d,J = 5.9 Hz),130.3 (d,J = 7.6 Hz),123.7 (d,J = 3.0 Hz),120.1 (d,J = 21.4 Hz),114.8 (d,J = 22.1 Hz),38.2,33.7,24.2,23.3 ppm;19 F NMR (376 MHz,CDCl 3 ): δ = −111.84 (s, 1F); IR (neat): υ max 3078,2944,2872,1698,1681,1589,1485,784, 681 cm −1 ; HRMS (ESI) m/z: [M + Na] + calcd for C 12 H 13 FO 3 Na,247.0741;found,hexanoic Acid (2j). Reaction conditions A.…”

“…3 In this field, the catalytic oxidative cleavage of C−C bonds with O 2 has drawn a lot of attention for a long time because it provides straightforward and sustainable routes to high-value fine chemicals. 4 On the other hand, visible light is another natural and infinite energy. Since 2008, visible light photoredox catalysis has displayed great potential in organic synthesis.…”

Selective C–C Bond Cleavage of Cycloalkanones by NaNO2/HCl

“…The oxidative C–C cleavage of α-substituted cycloalkanones usually requires transition metals or a high temperature and suffers from a low yield and selectivity . Recently, Kobayashi and Takemoto developed an azolium/hydroquinone cocatalytic system that worked only for two-substituted cyclohexanones . During the preparation of our manuscript, Guo and coworkers disclosed a copper-catalyzed C–C bond cleavage of cycloalkanones under visible-light irradiation …”

“…6 Recently, Kobayashi and Takemoto developed an azolium/hydroquinone cocatalytic system that worked only for two-substituted cyclohexanones. 7 During the preparation of our manuscript, Guo and coworkers disclosed a copper-catalyzed C−C bond cleavage of cycloalkanones under visible-light irradiation. 8 On the contrary, sodium nitrite and other nitroso reagents be could also utilized for C−C bond cleavage reactions.…”