Intramolecular Ni-Mediated Cyclizations with α,ω-Dienals

Abstract

Fifteen α,ω-dienals and one α-en-ω-ynal were prepared. These substrates were treated with Ni(COD)2/TMSCl under carbonylative and noncarbonylative conditions, with subsequent methanol quenching of the resulting metal intermediates to afford different types of cycloadducts. From the compounds thus obtained, it is concluded that the reaction proceeds through an intermediate (π-allyl)Ni complex, which readily cyclises in a “metallo-ene”-type reaction. The different pathways were characterised by further carbonyl insertion or by protonolysis, resulting either in cyclopentanone formation (insertion followed by β-elimination) when under carbon monoxide gas, or in single cyclization when not (or, even better, when in the presence of protic acids). Coordination of the distal olefin has been found essential for either of the two processes to proceed. Thus, coordinating heteroatoms (N,S), excessive tether length and particular substituents hampering satisfactory double bond chelation all disrupt the process, with the ketals of the original aldehyde (occasionally as Michael adducts) largely being recovered in such cases.