Iridium and rhodium complex-catalyzed cycloadditions

Transition metal-catalyzed cycloaddition is an atom-economical and powerful synthetic tool for the construction of cyclic carbon skeletons. Various types of cycloadditions, including [2 + 2 + 1], [2 + 2 + 2], [4 + 2] cycloaddition, etc., have been reported. Their asymmetric versions using chiral transition metal catalysts have also been reported to give enantiomerically-enriched multi-cyclic compounds. First, an iridium-catalyzed enantioselective Pauson-Khand-type reaction is summarized. Pauson-Khand(-type) reaction is a [2 + 2 + 1] cycloaddition of an alkyne, alkene and carbon monoxide, and gives synthetically useful cyclopentenones. Rhodium- and iridium-catalyzed Pauson-Khand-type reactions using an aldehyde as a CO source were also mentioned. Second, two types of enantioselective [2 + 2 + 2] cycloadditions are described: One is an iridium-catalyzed [2 + 2 + 2] cycloaddition of diynes and monoalkynes for the synthesis of chiral teraryls with two axial chiralities. Another is a rhodium-catalyzed [2 + 2 + 2] cycloaddition of enynes and monoalkynes for the synthesis of bicyclic cyclohexa-1, 3-dienes with a chiral quaternary carbon center. Third, a rhodium-catalyzed enantioselective [2 + 2] cycloaddition of alkynes and alkenes for the synthesis of chiral cyclobutenes is mentioned.