N-Alkylated TsDPEN derivatives bearing a small alkyl group act as highly efficient ligands in Ru(II) complexes for the asymmetric transfer hydrogenation of imines and ketones. A larger alkyl group serves to significantly reduce the activity of the catalyst; however, high enantiomeric excesses are still obtained. An X-ray crystal structure of the N-benzyl derivative reveals a conformation that permits hydrogen transfer through a six-membered transition state. A transition state structure for the imine reduction process is proposed.

. (2011) The importance of the N-H bond in Ru/TsDPEN complexes for asymmetric transfer hydrogenation of ketones and imines. Organic & Biomolecular Chemistry,9 (9). pp. 3290-3294. Permanent WRAP url:http://wrap.warwick.ac.uk/40377 Copyright and reuse:The Warwick Research Archive Portal (WRAP) makes this work by researchers of the University of Warwick available open access under the following conditions. Copyright © and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available.Copies of full items can be used for personal research or study, educational, or not-for profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. Publisher's statement:Final published version : http://dx.doi.org/10.1039/C1OB05208J A note on versions:The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP url' above for details on accessing the published version and note that access may require a subscription. For more information, please contact the WRAP Team at: publications@warwick.ac.uk Ru(II) complexes of TsDPEN containing two alkyl groups on the non-tosylated nitrogen atom are poor catalysts for asymmetric transfer hydrogenation of ketones and imines; this observation provides direct evidence for the importance of the N-H interaction in the transition state for ketone reduction. CREATED USING THE RSC COMMUNICATION TEMPLATE (VER. 3.0) -SEE WWW.RSC.ORG/ELECTRONICFILES FOR DETAILS

The preparation of a range of asymmetric iron and ruthenium-cyclone complexes, and their application to the asymmetric reduction of a ketone, are described. The enantioselectivity of of ketones reduction is influenced by a single chiral centre in the catalyst, as well as by the planar chirality in the catalyst. This represents the first example of asymmetric ketone reduction using an iron cyclone catalyst. Introduction 10The ruthenium complex 1 (the Shvo catalyst)1-4 reversibly splits to give hydride 2 and the unsaturated species 3.15 By 'shuttling' between 2 and 3, the Shvo catalyst transfers pairs of hydrogen atoms between secondary alcohols and ketones and has been used to good effect in dynamic kinetic resolution (DKR) reactions of alcohols and amines.2,3 There is evidence, 4 largely based on kinetic isotope effects, that the 20 hydrogen transfer to ketones and aldehydes, by the Shvo catlyst, takes place via a concerted 'outer sphere' mechanism ( Figure 1a). This is analogous to that of of ketone reduction by the Noyori catalyst 4 (Figure 1b). The Shvo catalyst is also an efficient ketone reducing agent when using an excess of an alcohol (usually iPrOH) or formic acid as hydrogen source, 1d and can also catalyse hydrogenation reactions. 1b,c,4d,6 The closely related iron complex 5 has recently been prepared from the tricarbonyl 35 precursor 6 7 and employed in catalytic reduction reactions of ketones by Casey and Guan.8 The mechanism appears to be analogous to that of the Shvo catalyst 2 (Figure 1c). In recent studies, complex 5 has been applied to the oxidation of alcohols using acetone as an acceptor, and a number of its 40 derivatives have been reported and evaluated in this role. 9 In our own studies, 10 we reported the synthesis and applications of racemic complexes 7a-7g in alcohol oxidations. The complexes were formed by an intramolecular cyclisation from a linear dialkyne precursor 8, followed by diastereoisomer 45 separation. 7,1150

Rh(III) catalysts containing a tetramethylcyclopentadienyl group linked by a 'tether' to a tosylated diamine ligand have previously been reported by our group for the asymmetric transfer hydrogenation (ATH) of ketones. The extension of these catalysts to the asymmetric reduction of imines, as well as to more highly functionalized substrates is reported. In some cases, the catalysts give better ee values than other methods for these transformations at lower catalyst loadings. The introduction of a methoxy group into the tethering aryl ring does not negate the performance of the catalyst, thus opening up a route to supported derivatives.

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