Aerobic Oxidation of Alkynes to 1,2‐Diketones by Organic Photoredox Catalysis

Abstract

Am ethod for the synthesis of 1,2-diketones through the mild and metal-free catalytic photooxygenation of alkynes is described. This reaction, using 9,10-dicyanoanthracene as ac atalytic sensitizer with/without biphenyl as ac o-sensitizer, readily furnished av ariety of desired products upon visible-light irradiation. Mechanistic studies indicated that ar adicalc ation process might have been involved. This report provides the first examples of the transformation of alkynes via acetenyl radical cation intermediates,w hichh ave thus far not been explored in organic synthesis.

ortho‐Alkylation of Pyridine N‐Oxides with Alkynes by Photocatalysis: Pyridine N‐Oxide as a Redox Auxiliary

“…Alternatively, alkyne cation radicals, which can be directly generated from a carbon–carbon triple bond by single‐ electron oxidation, present appealing intermediates for the construction of carbon–carbon and carbon–heteroatom bonds (Scheme a) . However, the achievement of alkyne single‐electron oxidation and its application in synthetically meaningful organic transformations has yet to be accomplished in a catalytic manner . This dearth of methods is a result of the high oxidation potential of carbon–carbon triple bonds that requires a potent oxidant to generate alkyne cation radicals .…”

“…This dearth of methods is a result of the high oxidation potential of carbon–carbon triple bonds that requires a potent oxidant to generate alkyne cation radicals . In addition, the exceptionally high reactivity of alkyne cation radicals makes them challenging intermediates for the development of selective and meaningful organic syntheses . Reported approaches for the direct single‐electron oxidation of alkynes are limited to γ irradiation or the use of strongly oxidizing conditions .…”

“…[4] However,the achievement of alkyne single-electron oxidation and its application in synthetically meaningful organic transformations has yett ob eaccomplished in ac atalytic manner. [5] This dearth of methods is ar esult of the high oxidation potential of carbon-carbon triple bondsthat requires ap otent oxidantt og enerate alkyne cation radicals. [6] In addition, the exceptionally high reactivity of alkyne cation radicals makes them challenging intermediates for the developmento f selectivea nd meaningfulo rganic syntheses.…”

KI‐I2‐DMSO: An Improved Microwave‐Assisted Selective Oxidation of Alkenes into 1,2‐Diketones

“…Recently, X. Liu et al . and H. T. Qin et al . reported that the aerobic photo‐oxidation of alkynes to 1,2‐diketones use blue light irradiation and organic photo‐redox catalysis respectively.…”

Visible‐Light‐Promoted Generation of α‐Ketoradicals from Vinyl‐bromides and Molecular Oxygen: Synthesis of Indenones and Dihydroindeno[1,2‐c]chromenes

“…Fluorescence quenching studies (see the Supporting Information) revealed that the starting material 1a strongly interacts with the photocatalysts.Adirect photoelectron transfer from the iridium(III) catalyst to 1 is thermodynamically unfavorable due to the large difference in reduction potentials of substrates 1 and the employed photocatalyst (see above). Also,nooxidation of the triple bond in 6 to the diketone occurs under the reaction conditions, [18] thus,the reaction of I* through attack of oxygen on the triple bond appears to be unlikely.Instead, I* loses bromine BrC to form the critical vinyl radical II. Also,nooxidation of the triple bond in 6 to the diketone occurs under the reaction conditions, [18] thus,the reaction of I* through attack of oxygen on the triple bond appears to be unlikely.Instead, I* loses bromine BrC to form the critical vinyl radical II.…”