Short Communication Asymmetric transfer hydrogenation of 1-phenyl dihydroisoquinolines using Ru(II) diamine catalysts Jan Přech a , Jiří Václavík a , Petr Šot a , Jan Pecháček a , Beáta Vilhanová a , Jakub Januščák a , Kamila Syslová a , Richard Pažout b , Jaroslav Maixner b , Jakub Zápal c , Marek Kuzma c , Petr Kačer a, ⁎ a Department of Organic technology, Institute of Chemical Technology, Technická 5, 166 28 Prague 6, Czech Republic b Central Laboratories, Institute of Chemical Technology, Technická 5, 166 28 Prague 6, Czech Republic c Laboratory of Molecular Structure Characterization, Institute of Microbiology, v.v.i., Academy of Sciences of the Czech Republic, Vídeňská 1083, 142 20 Prague 4, Czech Republic abstract article info Article history: Received 2 January 2013 Received in revised form 5 March 2013 Accepted 6 March 2013 Available online 15 March 2013 Keywords: Asymmetric transfer hydrogenation N-alkylsulfonyl Dihydroisoquinoline Ruthenium A new [Ru(II)(η 6 -p-cymene)(1R,2R)-N-((1S,2S)-borneol-10-sulfonyl)-1,2-diphenylethylenediamine] catalyst for the asymmetric transfer hydrogenation of both 1-alkyl and 1-aryl dihydroisoquinolines has been isolated. For the first time in this type of reaction, the catalyst employs an N-alkylsulfonyl group instead of N-arylsulfonyl. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Chiral amines are important building blocks in the fine-chemical, agrochemical and pharmaceutical industries. As the natural resources of optically pure substances are limited, their synthesis from achiral precursors is strongly in demand. The asymmetric reduction of corre- sponding imines is nowadays a prevalent procedure of their preparation [1]. Chiral ruthenium(II)-diamine catalysts (for instance 1a and 1b in Fig. 1) together with the formic acid-triethylamine azeotropic mixture (HCOOH-TEA) as a hydrogen donor represents an efficient and popular system for the asymmetric transfer hydrogenation (ATH) of various imines and ketones [2–4]. Regarding imine substrates, this catalytic system is capable of reducing the C_N double bonds of aryl-N-benzyl imines, β-carbolines and 1-alkyl- or 1-benzyl-3,4-dihydroisoquinolines (DHIQs). On the other hand, the ATH of 1-phenyl-DHIQs represents a con- siderable challenge since catalysts bearing the N-p-toluenesulfonyl-1, 2-diphenylethylenediamine (TsDPEN) ligand (like 1a) fail to catalyze this reduction under standard reaction conditions. Some homogeneous catalysts have been described for the asymmetric hydrogenation of 1-phenyl substituted DHIQs, such as Rh(III)-diamine catalysts structurally similar to 1a [5] and Ir(III)-phosphine complexes [6,7]. Catalytic activity was also reached in aqueous environment with the substrate in the form of iminium over modified 1a [8], and with 1a upon activation with AgSbF 6 /La(OTf) 3 [9]. Among ruthenium-based catalysts, there is only one example of a complex which is able to catalyze the ATH of 6,7-dimethoxy-1-phenyl-DHIQ (2a) to corresponding tetrahydroisoquinoline under the original conditions. Catalyst 1a, bearing the N-naphthalene-1-sulfonyl-DPEN ligand (NpsDPEN), was first pro- posed and tested by Noyori [10] and subsequently used by Vedejs and Boros [11–13]. Although the original Noyori-type ATH catalysts always bear an N-arylsulfonyl substituent, Ohkuma et al. [14], Ikariya et al. [15] and others [16–18] showed that Ir and Ru catalysts containing methanesulfonyl-DPEN (MsDPEN) hydrogenate various aryl-N-benzyl imines, N-sulfonylimines and aryl ketones with molecular hydrogen. Carreira and co-workers followed with their work on the ATH of β-keto esters in water catalyzed by an Ir(III)-MsDPEN complex [19]. The MsDPEN ligand belongs to a very small group of N-alkylsulfonyl diamines that proved utilizable in asymmetric (transfer) hydrogenation. Another such ligand is alicyclic N-(camphor-10-sulfonyl)-DPEN (CsDPEN) that has been employed in the ATH of various ketones [20–23]. However, to the best of our knowledge, no N-alkylsulfonyl diamines have applied in the ATH of imines such as DHIQs. We report here a CsDPEN-based Ru(II) catalyst that is suitable for the reduction of both 1-alkyl- and 1-phenyl-DHIQ substrates. 2. Experimental 2.1. General information [RuCl(η 6 -p-cymene)(1R,2R)-N-((1S,2S)-borneol-10-sulfonyl)-1,2- diphenylethylenediamine] (5) and [RuCl(η 6 -benzene) (1R,2R)-N- Catalysis Communications 36 (2013) 67–70 ⁎ Corresponding author. Tel.: +420 220 444 158. E-mail addresses: petr.kacer@vscht.cz (P. Kačer), richard.pazout@vscht.cz (R. Pažout), kuzma@biomed.cas.cz (M. Kuzma). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.03.004 Contents lists available at SciVerse ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom