Synthesis of [bis(hexamethylene)cyclopentadienone]iron tricarbonyl and its application to catalytic reductions of C=O bonds

Herein, we report the synthesis of [bis(hexamethylene)cyclopentadienone]iron tricarbonyl (1b) by reaction of cyclooctyne with Fe(CO)5, and the investigation of its catalytic properties in C=O bond reduction. Owing to the peculiar reactivity of cyclooctyne, complex 1b was formed in good yield (56%) by intermolecular cyclative carbonylation/complexation with Fe(CO)5. Compound 1b was fully characterized, and its crystal structure was determined by X-ray analysis. Catalytic tests revealed that, upon in situ activation with Me3NO, complex 1b promotes the hydrogenation of ketones, aldehydes and activated esters, as well as the transfer hydrogenation of ketones, showing higher activity compared to the classical “Knölker complex” (1a). Studies on the hydrogenation kinetics in the presence of 1a and 1b (respectively) suggest that this difference in terms of activity is probably due to the better stability of the 1b-derived complex compared to the in situ generated Knölker-Casey catalyst.