The aqua complex (S-Ir,R-C)-[(eta(5)-C5Me5)Ir(Prophos)(H2O)][SbF6](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,R-C)-[(eta(5)-C5Me5)Ir(Prophos)(ethyl acrolein)][SbF6](2) has been isolated and completely characterized, including the X-ray crystal structure determination. Structural parameters indicate that the disposition of the coordinated dienophile is controlled by CH/pi attractive interactions established between a phenyl group of the Prophos ligand and the aldehyde proton of the coordinated enal. Proton NMR data indicate that these interactions are maintained in solution. From diffractometric and spectroscopic data, the origin of the enantioselectivity is discussed. (C) 2014 Elsevier B.V. All rights reserved.