Nature of Proline-induced Enantiodifferentiation in Asymmetric Pd Catalyzed Hydrogenations: Is the Catalyst Really Indif
- PDF / 485,209 Bytes
- 8 Pages / 595.276 x 790.866 pts Page_size
- 98 Downloads / 131 Views
Nature of Proline-induced Enantiodifferentiation in Asymmetric Pd Catalyzed Hydrogenations: Is the Catalyst Really Indifferent? Shilpa C. Mhadgut Æ Marianna To¨ro¨k Æ Sujaya Dasgupta Æ Be´la To¨ro¨k
Received: 12 December 2007 / Accepted: 19 January 2008 / Published online: 8 February 2008 Ó Springer Science+Business Media, LLC 2008
Abstract The mode of enantioselection in the proline modified asymmetric hydrogenation of isophorone (3,5,5trimethyl-2-cyclohexenone) on supported Pd catalysts has been studied. It is shown that several experimental factors, such as modifier structure and chemical nature of the catalyst support, strongly affect the outcome of the hydrogenations. Secondary kinetic resolution was found to be the major reason for obtaining high enantioselectivities on most catalysts. Extensive studies have been carried out to clarify the importance of the interaction of the proline– dihydroisophorone complex with the catalyst. The secondary kinetic resolution of dihydroisophorone was investigated under different conditions. First, racemicdihydroisophorone was studied using several (S)-proline modified supported Pd catalysts, then the individual enantiomers were subjected to a similar reaction on Pd/ BaCO3 catalyst in the presence of (S)-proline. Our results provide convincing support for the heterogeneous enantioselection model under the current experimental conditions. Keywords Hydrogenation Enantiodifferentiation Pd catalysts Proline Isophorone Kinetic resolution
S. C. Mhadgut M. To¨ro¨k S. Dasgupta B. To¨ro¨k (&) Department of Chemistry, University of Massachusetts Boston, 100 Morrissey Blvd, Boston, MA 02125, USA e-mail: [email protected] S. C. Mhadgut M. To¨ro¨k B. To¨ro¨k Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA
123
1 Introduction The high demand for chiral pharmaceuticals, agrochemicals, flavors and fragrances provides extraordinary inspiration to develop new synthetic methods to prepare enantiomerically pure compounds [1]. Asymmetric saturation of alkenes, ketones and imines provides ideal access to chiral alkanes, alcohols and amines. Thus asymmetric hydrogenation has become increasingly important in organic syntheses. Heterogeneous methods possess many potential advantages over homogeneous approaches, such as the ease of storage, use, separation and recycling [2]. Heterogeneous chiral hydrogenation is clearly dominating the asymmetric catalysis on surfaces. The most prominent systems are the cinchona modified Pt [3], and the tartaric acid modified Raney Nickel [4]. The number of practical applications increases continuously and heterogeneous catalytic asymmetric hydrogenations are in the forefront of contemporary catalysis research [5, 6]. Following the extensive number of applications with carbonyl compounds, C=C double bond hydrogenation has also attracted significant attention [7]. After many years of struggling, the hydrogenation of 4-methoxypyrone and 4-hydroxypyrone on cinchona-modified Pd/TiO2 catalyst (up to 94% ee) opened up t
Data Loading...
