Intramolecular Ni-mediated cyclizations with alpha,omega-dienals

Fifteen alpha,omega -dienals and one alpha -en-omega -ynal were prepared. T hese substrates were treated with Ni(COD)(2)/TMSCl under carbonylative and noncarbonylative conditions, with subsequent methanol quenching of the resu lting metal intermediates to afford different types of cycloadducts. From t he compounds thus obtained, it is concluded that the reaction proceeds thro ugh an intermediate (pi -allyl)Ni complex, which readily cyclises in a "met allo-ene"-type reaction. The different pathways were characterised by furth er carbonyl insertion or by protonolysis, resulting either in cyclopentanon e formation (insertion followed by beta -elimination) when under carbon mon oxide 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 h eteroatoms (N,S), excessive tether length and particular substituents hampe ring satisfactory double bond chelation all disrupt the process, with the k etals of the original aldehyde (occasionally as Michael adducts) largely be ing recovered in such cases.