Journal of Molecular Catalysis A: Chemical 385 (2014) 119–124 Contents lists available at ScienceDirect Journal of Molecular Catalysis A: Chemical jou rn al hom epage: www.elsevier.com/locate/molcata Enantioselective catalysts based on the chiral fragment ( 5 -C 5 Me 5 )Ir(Prophos) for Diels–Alder reactions Daniel Carmona ∗ , Fernando Viguri ∗ , Ainara Asenjo, Fernando J. Lahoz, Pilar García-Ordu ˜ na, Luis A. Oro Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC – Universidad de Zaragoza, Departamento de Química Inorgánica, Pedro Cerbuna 12, 50009 Zaragoza, Spain a r t i c l e i n f o Article history: Received 4 November 2013 Received in revised form 20 January 2014 Accepted 26 January 2014 Available online 3 February 2014 Keywords: Asymmetric catalysis Diels–Alder reaction ˛,ˇ-Unsaturated aldehydes (R)-Prophos Iridium a b s t r a c t The aqua complex (S Ir ,R C )–[( 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 ,R C )–[( 5 -C 5 Me 5 )Ir(Prophos)(ethyl acrolein)][SbF 6 ] 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/ 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Asymmetric catalysis is an indispensable and highly valuable tool for the synthesis of enantioenriched intermediates and prod- ucts [1–3]. Among the wide variety of metal-catalyzed asymmetric processes, the Diels–Alder (DA) reaction is a powerful and versa- tile synthetic transformation that plays an important role in the construction of cyclohexene derivatives with up to four contigu- ous stereocenters [4–6]. In this methodology, coordination of the alkene to the metal lowers the barrier energy of the reaction and an enantiopure chiral ligand, also coordinated to the metal, is the source of chirality. In some instances, the metal becomes a chi- ral center after the coordination of the ligand as, for example, in half-sandwich three-legged piano stool complexes containing chi- ral bidentate ligands of C 1 symmetry (Scheme 1) [7]. During the last years, examples of one point binding cata- lysts for the asymmetric DA have been developed by the groups of Kündig [8–16], Faller [17–24] Davies [25–34] and ourselves [35–50]. When the alkene was an enal, the CHO group, apart from activating the double C C bond, provides an anchoring atom able for the coordination to the metal. By far, the most ∗ Corresponding authors. Tel.: +34 976 762027; fax: +34 976 761187. E-mail addresses: dcarmona@unizar.es (D. Carmona), fviguri@unizar.es (F. Viguri). studied system was the reaction between methacrolein and cyclopentadiene and, in particular, we have shown that the aquo complexes [( 5 -C 5 Me 5 )M(Prophos)(H 2 O)][SbF 6 ] 2 [Prophos = (R)- propane-1,2-diyl-bis(diphenylphosphane); M = Ir (1), Rh (2)] are active catalyst precursors for this reaction [35–45]. How- ever, the catalytic activity of half-sandwich complexes with acyclic ˛,ˇ-unsaturated enals other than methacrolein has been much less studied. At this respect, we have investi- gated the reaction of acrolein with cyclopentadiene catalyzed by cations of formula [( 5 -C 5 Me 5 )Rh(N–N*)(H 2 O)] 2+ or [( 6 - p-cymene)Ru(N–N*)(H 2 O)] 2+ , where N–N* is an enantiopure chiral pyridilimino ligand, obtaining good exo:endo-selectivity and poor enantioselectivity [38], Kündig’s group has reported that the ruthenium complex [( 5 -C 5 H 5 )Ru(R,R-BIPHOP-F) (acetone)][SbF 6 ] 2 [(R,R-BIPHOP-F) = 1,2-bis[(pentafluorophenyl) phosphanyloxy]-1,2-diphenyl)] catalyzes efficiently the reaction between ethyl acrolein and cyclopentadiene [8] and Faller has achieved good yield and enantioselectivity for the latter reaction by using half-sandwich chiral Ru(II) and Os(II) complexes with (S)-BINPO [2,2 ′ -(diphenyl phosphino-1,1 ′ -binaphthyl)] ligand as catalyst [19,21]. Herein we report on: (i) the catalytic asymmetric DA reac- tion of enals 3–7 (Scheme 2) with cyclopentadiene using the aqua complex (S Ir ,R C )–[( 5 -C 5 Me 5 )Ir(Prophos)(H 2 O)][SbF 6 ] 2 (1) as cat- alyst precursor; (ii) the DA reaction of other acyclic dienes with methacrolein (see Table 2) catalyzed by 1 and by the rhodium 1381-1169/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2014.01.021