Abstract The aqua complex (S Ir, RC)–[(η 5-C 5 Me 5) Ir (Prophos)(H 2 O)][SbF 6] 2 [Prophos=(R)-propane-1, 2-diyl-bis (diphenylphosphane)] is an active precursor for the asymmetric Diels–Alder reaction of acyclic enals with cyclopentadiene, 2, 3- dimethylbutadiene and isoprene. Enantioselectivities up to 78% ee are achieved. The intermediate Lewis acid-dienophile complex (S Ir, RC)–[(η 5-C 5 Me 5) Ir (Prophos)(ethyl ...