Asymmetric Heterogeneous Catalysis Enhancing the Enantioselectivity of Novel Homogeneous Organometallic Hydrogenation Catalysts** Matthew D.Jones,RobertRaja,*John MeurigThomas,* Brian F. G.Johnson,*Dewi W.Lewis,JacquesRouzaud, and Kenneth D. M. Harris Theexigentneedtodevelopnewasymmetrichydrogenation catalystsisuniversallyacknowledged,andmanynewfeasible strategies for the design and synthesis of such catalysts continuetobeproposed. [1–5] Herein,inadditiontodescribing a new set of efficient, diamino-type ligands for a central rhodiumorpalladiumionthatleadstogoodenantioselective (ee) performance, homogeneously, we also show that signifi- cant improvement in the stereoselectivity of the organo- metalliccatalystmaybeachievedbyheterogenizingitatthe innerwallsofamesoporoussilicasothatadvantageistaken of the spatial restrictions imposed by the concave surface at whichwehavelocatedtheactivecenter. Comparatively few reports have hitherto been published in which Rh I or Pd II complexes without phosphane ligands have been used to activate hydrogen, but a rapidly growing numberemployingnitrogencontainingligandshasappeared of late for the purpose of asymmetric catalysis. [6] Oneofthe incentives for this change is the rather unstable nature and cost of many chiral phosphanes. [6b] It is not, however, the nitrogen-containing ligands that are the key feature of this work.Ratheritisthefactthatadvantageistakenofaconcave surface of a silica support, on which the homogeneous organometallic catalyst is tethered, thereby boosting the enantioselectivity to a value higher than that obtained from the same catalyst and the same tether attached to a convex silica support. The catalysts themselves are pseudo square-planar rho- dium( i ) or palladium( ii ) complexes of the bidentate amines: (S)-()-2-aminomethyl-1-ethyl pyrrolidine 1a and (1R,2R)- (+)-1,2-diphenylethylenediamine 1b, which may readily act asligandstothemetalcenter.TheRh I catalysts are bonded either to 1,5-cyclooctadiene (COD) or norbornadiene (NBD), whereas for the palladium species we used the allyl (C 3 H 5 ) group as the coordinating alkene. The BF 4  salts of these organometallic cations are readily handled, air-stable, crystallinematerialsamenabletoX-rayanalysis. [7] Forthecationsbasedon 1a (pairedineachcasewiththe BF 4  anion; see Scheme1) X-ray structural analysis shows that the square-planar rhodium( i )orpalladium( ii )centerhas high steric hindrance. Access to it by a relatively bulky incomingreactantisspatiallyrestricted(seebelow),andthis featureisessentialforgoodenantioselectivecatalyticbehav- ior. A fragment of the crystal structure of the Rh I complex withligand 1a isshowninFigure1a,fromwhichitisseenthat the BF 4  anion is hydrogen-bonded to the nitrogen of the aminogroups. By using described procedures [8–12] whereby 3-bromo- propyltrichlorosilaneservesasameansofanchoringisolated organometallicspeciestotheinnerwallsofmesoporoussilica, we may functionalize an ion-pair of all ligands 1a and 1b complexed to metal so as to yield their respective single-site catalysts,asdepictedinFigure1b.Wehaveanchoredligand 1b to a mesoporous silica (pore diameter 30) and to a nonporous silica (known as cabosil). This permits the comparison of catalytic performance of the organometallic specieswhenitiseitherataconvexorataconcavesiliceous surface(Figure2).(Itismucheasiertoprepareconcavethan convexsilicasurfaces.Sincethisworkbegan,wehavestudied aseriesofsilicasinwhichtheporediametersrangefromalow of38toahighof250;but,todate,theonlysatisfactory, well-defined convex silica surface that we have been able to investigate is that of the commercial material known as cabosil). Wechoseasatestreactionthesimple,well-known [12] case ofthehydrogenationof E-a-phenylcinnamicacid(Scheme2) Scheme 1. The ligands used in this study. [*] Dr. R. Raja, Prof. Dr. B. F. G. Johnson, M.D. Jones, Dr. J. Rouzaud Department of Chemistry University of Cambridge Lensfield Road, Cambridge CB21EW (UK) Fax: (+ 44)1223-336017 E-mail: robert@ri.ac.uk bfgj1@cam.ac.uk Prof. Sir J. M. Thomas The Royal Institution of Great Britain Davy Faraday Research Laboratory 21 Albemarle Street, London W1S4BS (UK) and Department of Materials Science and Metallurgy New Museums Site Pembroke Street, Cambridge CB23QZ (UK) Fax: (+ 44)1223-334567 E-mail: jmt@ri.ac.uk Dr. D. W. Lewis Department of Chemistry University College London 20 Gordon Street, London WC1H0AJ (UK) Prof. Dr. K. D. M. Harris School of Chemical Sciences University of Birmingham Edgbaston, Birmingham B152TT (UK) [**] We thank the EPSRC (UK) for a rolling grant to J.M.T. and general support to B.F.G.J., KDMH and a studentship to MDJ, and for financial assistance towards the single crystal diffractometers. We also thank Bayer AG, Leverkusen (Germany) for their support to R.R.,theMarieCuriefellowshipSchemeforJ.R.,ICIandtheNewton trust for M.D.J.. We gratefully acknowledge Dr. J. E. Davies for determining the crystal structures. Dr. S. Kitchin is acknowledged for the 19 F MAS measurements and Mr. G. Burns for technical assistance. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Communications 4326 # 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200250861 Angew. Chem. Int. Ed. 2003, 42, 4326 –4331