Asymmetric hydrogenation of heteroarenes and arenes.

The discovery and investigation of novel and efficient pathways for the conversion of biomass into fuels and chemicals are among the big challenges facing heterogeneous catalysis nowadays. However, not all experience gained in the transformation of hydrocarbons over the last 100 years can directly be transferred to biomass conversion. In this article, we will discuss how the specific properties of biomass pose new requirements on the processes and on the solids that are used as catalysts for their conversion. Due to the importance of lignocellulosic materials, which constitute ca. 95% of the total plant biomass, we will focus mostly on the desired properties of solid catalysts for the conversion of these polymeric biomolecules. Research in this field is very intense presently and novel transformations and catalysts are being discovered at a high rate. This paper thus focuses on the concepts that govern biomass transformation instead of giving a complete and comprehensive survey of the literature, which would be outdated within a short time.

Design of solid catalysts for the conversion of biomass

Limited natural resources and an increasing demand for enantiomerically pure compounds render catalysis and especially heterogeneous asymmetric catalysis a key technology. The field has rapidly advanced from the initial use of chiral biopolymers, such as silk, as a support for metal catalysts to the modern research areas. Mesoporous supports, noncovalent immobilization, metal-organic catalysts, chiral modifiers: many areas are rapidly evolving. This Review shows that these catalysts have more to them than facile separation or recycling. Better activities and selectivities can be obtained than with the homogeneous catalyst and novel, efficient reaction mechanisms can be employed. Especially fascinating is the outlook for highly ordered metal-organic catalysts that might allow a rational design, synthesis, and the unequivocal structural characterization to give tailor-made catalysts.

Asymmetric heterogeneous catalysis.

In this review, a brief survey is offered on the main nanotechnology synthetic approaches available to heterogeneous catalysis, and a few examples are provided of their usefulness for such applications. We start by discussing the use of colloidal, reverse micelle, and dendrimer chemistry in the production of active metal and metal oxide nanoparticles with well-defined sizes, shapes, and compositions, as a way to control the surface atomic ensembles available for selective catalysis. Next we introduce the use of sol–gel and atomic layer deposition chemistry for the production and modification of high-surface-area supports and active phases. Reference is then made to the more complex active sites that can be created or carved on such supports by using organic structure-directing agents. We follow with an examination of the ability to achieve multiple functionality in catalysis via the design of dumbbells, core@shell, and other complex nanostructures. Finally, we consider the mixed molecular-nanostructure approach that can be used to develop more demanding catalytic sites, by derivatizing the surface of solids or tethering or immobilizing homogeneous catalysts or other chemical functionalities. We conclude with a personal and critical perspective on the importance of fully exploiting the synergies between nanotechnology and surface science to optimize the search for new catalysts and catalytic processes.

Nanostructured materials for applications in heterogeneous catalysis

Asymmetric catalysis at chiral metal surfaces.

Various derivatives of (R)-1-(1-naphthyl)-ethylamine have been synthesized and tested as chiral modifiers of Pt/alumina in the enantioselective hydrogenation of ketopantolactone. The best modifiers (ee up to 79%) possess an ester function in the α-position to the amino group. The modifiers performed far better in AcOH than in toluene, indicating that protonation of the N atom is important in enantioselection. The striking non-linear behaviour of modifier mixtures with cinchonine indicates that the alkaloid adsorbs much stronger on Pt than the naphthylethyl-amine derivatives. Two mechanistic models are proposed for interpretation of the results, involving an N-H-O or N + -H-O bond between the amine-type modifier and the keto carbonyl O atom of ketopantolactone, in apolar and protic media, respectively. In both cases the H atom originates from the modifier and not from the substrate ("half-hydrogenated-state"). The higher ee achieved in acidic medium is attributed to the better proton donor ability of the protonated amine modifiers. The models are applicable also to the hydrogenation of ethyl pyruvate.

/pdf/synthetic-modifiers-for-platinum-in-the-enantioselective-4daes3slil.pdf

Synthetic Modifiers for Platinum in the Enantioselective Hydrogenation of Ketopantolactone: A Test for the Mechanistic Models of Ketone Hydrogenation

Quaternary ammonium derivatives of cinchonidine as new chiral modifiers for platinum

A new chiral modifier pantoyl-naphthylethylamine (PNEA) was synthesized by reductive alkylation of 1-(1-naphthyl)ethylamine with ketopantolactone. Platinum-on-alumina modified by PNEA afforded 93% ee and 100% chemoselectivity in the hydrogenation of the activated carbonyl group of 1,1,1-trifluoro-2,4-pentanedione. Reductive heat treatment and ultrasonication of the catalyst, and the use of chlorinated solvents under mild conditions (10 bar, 10 °C) enhanced the enantioselectivity. This is the first case in heterogeneous catalysis that a synthetic modifier gives more than 90% ee, better than the commonly used modifier of natural origin (cinchonidine or O-methyl-cinchonidine).

/pdf/an-efficient-synthetic-chiral-modifier-for-platinum-13lsk5ccxb.pdf

An efficient synthetic chiral modifier for platinum

The application range of platinum modified by new synthetic chiral amines was investigated in the enantioselective hydrogenation of α,α,α-trifluoromethyl ketones to the corresponding chiral alcohols. High-throughput screening was used to study the transformation of eight different substrates over a 5 wt.% Pt/Al2O3 catalyst chirally modified by eight derivatives of (R)-1-(1-naphthyl)ethylamine. The chiral modifiers possessed the same anchoring moiety (naphthalene ring) that allows strong adsorption on the Pt surface and the surroundings of the basic N atom was varied systematically. All modifiers improved the chemoselectivity to hydrogenation of the activated carbonyl group. The yield achieved in 2 h varied in a broad range (2–100%) and the modifiers had no clear positive or negative influence on the rate of the unmodified reaction. The best modifier pantoyl-naphthylethylamine (H) afforded better than 99% chemoselectivity and 90% enantiomeric excess (ee) (at 5% conversion in 2 h) in the hydrogenation of 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione (1). Critical parameters of the reactions were the nature of solvent and catalyst pretreatments. This new catalyst system provided far better enantioselectivities than the commonly used cinchonidine-modified Pt.

Elisabeth Orglmeister

Papers

Asymmetric catalysis at chiral metal surfaces.

Synthetic Modifiers for Platinum in the Enantioselective Hydrogenation of Ketopantolactone: A Test for the Mechanistic Models of Ketone Hydrogenation

Quaternary ammonium derivatives of cinchonidine as new chiral modifiers for platinum

An efficient synthetic chiral modifier for platinum

Chemo and enantioselective hydrogenation of fluorinated ketones on platinum modified with (R)-1-(1-naphthyl)ethylamine derivatives