Neutral vs. cationic rhodium (I) complexes of bulky N-phosphino sulfinamide ligands: Coordination modes and its influence in the asymmetric hydrogenation of Z-MAC Seán Doran, Thierry Achard, Antoni Riera * , Xavier Verdaguer * Unitat de Recerca en Síntesi Asimètrica (URSA-PCB), Institute for Research in Biomedicine (IRB Barcelona) and Departament de Química Orgànica, Universitat de Barcelona, c/Baldiri Reixac 10, E-08028 Barcelona, Spain article info Article history: Received 8 May 2012 Received in revised form 28 June 2012 Accepted 8 July 2012 Keywords: P,S and P,O ligands Rhodium Asymmetric hydrogenation Sulfinamides abstract Here we report the synthesis of a new N-di-tert-butylphosphino-tert-butylsulfinamide (PNSO) ligand and its corresponding p-tolylsulfinamide analog. The coordination of these compounds to rhodium to form a neutral and apolar complex is described, followed by the subsequent protonation of said complexes to quantitively form the more orthodox, cationic rhodium species containing a tetrafluoroboric counterion. The crystallographic structure of the tert-butylsulfinamide-derived cationic species was obtained and is elucidated. It outlines coordination from the sulfinamide group to the rhodium atom and shows no preference between O- and S-coordination as both complexes can be seen in one unit cell of the crystal. The efficacies of the neutral species and the salt species were tested in the asymmetric hydrogenation of methyl (Z)-a-acetamido cinnamate (Z-MAC). The p-tolylsulfinamide-derived complexes gave no hydro- genation while the tert-butylsulfinamide-derived ones produced hydrogenation with complete conver- sion but low enantioselectivities. The stereochemical outcome of the reaction was analyzed by means of the quadrant method. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Chiral bi-dentate ligands have become highly relevant in the field of asymmetric catalysis. These compounds not only increase reactivity but more importantly, they induce the high enantiose- lectivities currently demanded by the pharmaceutical industry [1]. Thus moving the chiral information of the ligand as close as possible to the catalyzing metal center would be the best way to efficiently confer chirality to and increase enantioselectivities of the catalytic transformations that the catalyst is mediating. This could be achieved by conferring chirality to the metal-coordinating atom. P-stereogenic bidentate diphosphine ligands are extremely profi- cient in asymmetric transformations [2]. In 2004 Hoge et al. re- ported the synthesis and efficacy of the three-hindered quadrant chiral ligand trichickenfootphos (TCFP) in the Rh-catalyzed asym- metric hydrogenations of a- and b-acetamido dehydroamino acid substrates [3]. The excellent enantioselectivities obtained sug- gested the C 1 -symmetric, 3-hindered quadrant chiral ligand design was an excellent template to follow. MaxPHOS, an analog of the TCFP ligand, was developed in our laboratory and was first reported in 2010 [4,5]. As with TCFP, it showed excellent enantioselectivity in the asymmetric hydrogenation of a- and b-acetamido dehy- droamino acid substrates. Despite the great efficiency of this type of ligand, its synthesis remains somewhat laborious [6]. In this respect, here we addressed the preparation of other cost-effective 3-hindered quadrant ligands that can be readily assembled from commercially available materials (Fig. 1). We previously showed that the PNSO family of ligands, those containing a sulfinamide moiety bound to a phosphine group through the nitrogen atom where chirality resides on sulfur, can be highly efficient when applied to the intermolecular asymmetric PausoneKhand reaction [7]. These ligands can be obtained in a very straightforward manner, often involving a one-step, one-pot synthesis using chiral sulfinamides that are commercially available in large amounts [8]. Also, we demonstrated that ligands such as 1 and similar analogs coordinate readily to rhodium and other metals to give either P,O or P,S bidentate coordination [9]. The positive results with these ligands in the PausoneKhand reaction led us to test their efficacy in rhodium-catalyzed asymmetric hydrogenation of dehydroamino acids. We were willing to test whether the N-di- tert-butylphosphino-tert-butylsulfinamide ligand (PNSO) (2) could be another example of the 3-hindered quadrant chiral ligand * Corresponding authors. Tel.: þ34 934034813; fax: þ34 934037095. E-mail address: xavier.verdaguer@irbbarcelona.org (X. Verdaguer). Contents lists available at SciVerse ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2012.07.008 Journal of Organometallic Chemistry 717 (2012) 135e140