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New Method for Photocatalytic Production of Alkynes Using Visible Light

Recently, a team of researchers from Kanazawa University used visible light responsive photocatalyst to developed a method to produce an extremely reactive alkyne, an organic molecule with a C≡C triple bond, from a cyclopropenone, an organic molecule with a strained three-membered ring.

The first photocatalytic active alkyne generation from a cyclopropenone under visible light conditions has been achieved. The generated active alkynes such as ynamine and cyclooctyne can be directly used, without isolation, for the chemical reactions such as a dehydration condensation and alkyne-azide click chemistry. (Image credit: Kanazawa University)

The associated research has been published in Organic Letters.

Alkynes are organic compounds used in the production of polymers and industrial reagents. Photolysis of a cyclopropenone using UV light is a potent method to produce alkynes that are extremely reactive. But if there are related UV-sensitive compounds in the reaction mixture, then they will deteriorate.

Hence, this reaction can instead be carried out in the presence of visible light to keep such compounds unscathed. Scientists from Kanazawa University have found catalysts that could enable the photolysis of visible-light-stable cyclopropenones under visible light.

They initially screened a group of six potential catalysts to detect those producing the major proportion of aminoalkynes from aminocyclopropenones under visible light. They evaluated this by carrying out a phototriggered dehydration condensation reaction. In this reaction, the aminoalkyne prepared from the aminocyclopropenone directly undergoes several intermediate steps to produce chemical compounds called amides.

Irradiation of this reaction mixture using a household fluorescence lamp showed two catalysts produced the highest yield of the amide. These two catalysts can deplete over 90% of the cyclopropenone, indicating their efficiency under visible light. Such observations were not made in the dark conditions, additionally indicating these two catalysts require light within the visible spectrum.

Fascinatingly, the scientists also observed that these two catalysts appeared to have low reduction potential at their ground states and high reduction potential at their excited states. This redox potential likely causes the excited catalysts to grab an electron from cyclopropenone to initiate the reaction chain.

The oxidized cyclopropenone becomes unstable and decomposes to the ring-opened radical cation, which, in turn, receives an electron from the photocatalyst and produces the active alkyne and carbon monoxide. Thus, the process of visible-light-activated catalysts is believed to be different from the catalysts that function in the presence of UV light.

To find the useful application of these catalysts, the reaction was then carried out using more UV-sensitive tetrazole. As anticipated, tetrazole decomposed under UV light and produced only 46% of the amide. However, it remained intact under visible light and produced almost 75% of the amide.

This study has put forward a new method for preparing alkyne in the absence of UV light. Finally, the scientists stated, “[We] have developed a novel phototriggered active alkyne generation reaction with a visible-light-responsive-photocatalyst. The method would be especially useful when UV light cannot be used for alkyne generation due to the UV light-sensitivity of other coexisting substrates.”


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