Unexpected Transformation of Aldehydes into Benzoins with Copper(I)/Samarium

Abstract

The reductive coupling of aldehydes to afford pinacolic alcohols using all kinds of reducing agents involving samarium is well known. In this report, however, treatment of aromatic aldehydes with Cu(I)/Sm system produces benoins and/or benzils in good yields. A possible mechanism is proposed where Cu(I) not only activates the Sm metal but also promotes the dehydrogenation of the intermediates, during which a Cu(III) species may be involved.

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GRAPHICAL ABSTRACT

Keywords:

• Aldehydes
• benzoins
• copper
• iodides
• samarium

ACKNOWLEDGMENTS

The project was supported by the Promotive Research Fund for Excellent Young and Middle-Aged Scientists of Shandong Province (BS2011CL019) and the Natural Science Foundation of Shandong Province (Project No. 2009ZRA02039).

Notes

^a The ratio of aldehyde–samarium–Cu(I) is 2:1:1.

^b No reaction.

^c The ratio of aldehyde–samarium–Cu(II) is 2:2:1.

^a Entries 1′, 5′, 10′, 13′, 19′, 20′, and 21′ are performed in the presence of CuI alone without potassium iodide.

^b A complex mixture was obtained.

^c 4-Nitrobenzyl alcohol was afforded.

^a Based on aryl aldehydes.

^b Not detected.