Since the discovery of catalytic homogeneous hydrogenation in 1938, CC bond formation has only been observed under hydrogenation conditions in alkene hydroformylation and Fischer–Tropsch-type processes, that is, in catalytic processes involving migratory insertion of carbon monoxide. As revealed in the present study, organometallic intermediates obtained upon diene hydrogenation are subject to electrophilic trapping by glyoxal partners, thus demonstrating catalytic CC bond formation under hydrogenation conditions in the absence of carbon monoxide (see scheme). DCE: 1,2-dichloroethane. R=aryl, heteroaryl, alkyl. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2003/z51986_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.