Science of Synthesis

Landais, Y.; Vincent, J. M., Science of Synthesis, (2005) 26, 656.

α-Oxonitriles react similarly to acyl halides in the presence of reducing agents. Samarium, zinc, and also indium have been used as electron-transfer reagents for the preparation of symmetrical 1,2-diketones. Electrochemical reduction has also been employed. The first report on the one-pot transformation of α-oxonitriles into 1,2-diketones appeared in 1997 and used either samarium(II) iodide or zinc(II) iodide (‌Scheme 12‌).[‌35‌] With 2 equivalents of samarium(II) iodide in tetrahydrofuran, the reaction proceeds at room temperature and provides good yields of the corresponding symmetrical 1,2-diketone ‌48‌. The reaction works well with aromatic and aliphatic α-oxonitriles ‌47‌ but is slower for the latter. α-Hydroxy ketones, which are frequently byproducts in this type of reaction, are not detected. The method is compatible with the presence of esters and ethers and the reaction proceeds without dehalogenation with iodo- and chloroarenes. Interestingly, the method is also efficient with only 1 equivalent of inexpensive zinc(II) iodide. The reaction is slower with zinc as compared to samarium but 1,2-diketones are obtained in comparable yield. The mechanism has not been firmly established but it is likely to involve an electron transfer from samarium(II) iodide to α-oxonitrile ‌47‌ to generate a radical anion that can be cleaved to give an acyl radical and a cyanide anion. The acyl radical may then dimerize or trap another electron to form an acyl anion, which reacts further with a second molecule of α-oxonitrile to afford the 1,2-diketone.

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References