Asymmetric enamine catalysis.

Hydrogen bonding is responsible for the structure of much of the world around us. The unusual and complex properties of bulk water, the ability of proteins to fold into stable three-dimensional structures, the fidelity of DNA base pairing, and the binding of ligands to receptors are among the manifestations of this ubiquitous noncovalent interaction. In addition to its primacy as a structural determinant, hydrogen bonding plays a crucial functional role in catalysis. Hydrogen bonding to an electrophile serves to decrease the electron density of this species, activating it toward nucleophilic attack. This principle is employed frequently by Nature's catalysts, enzymes, for the acceleration of a wide range of chemical processes. Recently, organic chemists have begun to appreciate the tremendous potential offered by hydrogen bonding as a mechanism for electrophile activation in small-molecule, synthetic catalyst systems. In particular, chiral hydrogen-bond donors have emerged as a broadly applicable class of catalysts for enantioselective synthesis. This review documents these advances, emphasizing the structural and mechanistic features that contribute to high enantioselectivity in hydrogen-bond-mediated catalytic processes.

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Asymmetric catalysis by chiral hydrogen-bond donors.

Selective non-covalent interactions have been widely exploited in solution-based chemistry to direct the assembly of molecules into nanometre-sized functional structures such as capsules, switches and prototype machines1,2,3,4,5. More recently, the concepts of supramolecular organization have also been applied to two-dimensional assemblies on surfaces6,7 stabilized by hydrogen bonding8,9,10,11,12,13,14, dipolar coupling15,16,17 or metal co-ordination18. Structures realized to date include isolated rows8,13,14,15, clusters9,10,18 and extended networks10,11,12,17, as well as more complex multi-component arrangements16. Another approach to controlling surface structures uses adsorbed molecular monolayers to create preferential binding sites that accommodate individual target molecules19,20. Here we combine these approaches, by using hydrogen bonding to guide the assembly of two types of molecules into a two-dimensional open honeycomb network that then controls and templates new surface phases formed by subsequently deposited fullerene molecules. We find that the open network acts as a two-dimensional array of large pores of sufficient capacity to accommodate several large guest molecules, with the network itself also serving as a template for the formation of a fullerene layer.

Controlling molecular deposition and layer structure with supramolecular surface assemblies

The engineering of highly organized systems from instructed molecular building blocks opens up new vistas for the control of matter and the exploration of nanodevice concepts. Recent investigations demonstrate that well-defined surfaces provide versatile platforms for steering and monitoring the assembly of molecular nanoarchitectures in exquisite detail. This review delineates the principles of noncovalent synthesis on metal substrates under ultrahigh vacuum conditions and briefly assesses the pertaining terminology-self-assembly, self-organization, and self-organized growth. It presents exemplary scanning-tunneling-microscopy observations, providing atomistic insight into the self-assembly of organic clusters, chains, and superlattices, and the metal-directed assembly of low-dimensional coordination architectures. This review also describes hierarchic-assembly protocols leading to intricate multilevel order. Molecular architectonic on metal surfaces represents a versatile rationale to realize structurally complex nanosystems with specific shape, composition, and functional properties, which bear promise for technological applications.

Molecular architectonic on metal surfaces.

Phthalocyanines: old dyes, new materials. Putting color in nanotechnology

Increased structural complexity leads to higher activity: peptides as efficient and versatile catalysts for asymmetric aldol reactions.

A novel route to molecular self-assembly: self-intermixed monolayer phases.

The adsorption and the two-dimensional (2D) ordering of chloro[subphthalocyaninato]boron(III) (SubPc) on Ag(111) has been studied in detail by combined scanning tunneling microscopy and photoelectron spectroscopy at room temperature. SubPc is a polar, highly symmetric molecule, consisting of an extended aromatic system and a central B-Cl bond. When growing on Ag(111) an interesting phase behavior is observed for the first molecular layer of SubPc. At low coverage, below $\ensuremath{\approx}0.2$ monolayer (ML), a 2D lattice gas is present, whereas at medium coverage (on the order of 0.2--0.5 ML), 2D condensed molecular islands are observed in coexistence with the 2D lattice gas. In these condensed islands, the molecules assemble into a well-ordered honeycomb pattern. At higher coverage (approximately 0.5--0.9 ML) the molecules organize into a 2D hexagonal close-packed (hcp) pattern, in equilibrium with a dense 2D gas phase. In the honeycomb and in the hcp pattern, individual molecules are imaged with submolecular resolution, giving information on their orientation. For both the honeycomb and hcp patterns, islands with two different orientations of the superstructures with respect to the Ag(111) substrate are observed. In case of the honeycomb pattern, the two superstructures are enantiomorphic. The chirality of these layers originates in the loss of the symmetry of the metal surface upon adsorption of SubPc, while the molecules alone are intrinsically achiral. Based on different photoelectron spectroscopy experiments we conclude that the SubPc molecule is adsorbed on Ag(111) with its Cl atom towards the substrate and that the molecule remains intact. Finally, several aspects of the observed 2D condensed phases and the thermodynamic phase behavior are discussed with respect to the charge distribution and the adsorption physics and chemistry of the SubPc molecules.

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Adsorption and two-dimensional phases of a large polar molecule: Sub-phthalocyanine on Ag(111)

Peptides containing a secondary amine and a carboxylic acid in a specific orientation to each other are presented as highly efficient catalysts for asymmetric aldol reactions:  (1) their activity is considerably higher compared to that of proline, and (2) the enantioselectivity of the peptidic catalysts can be changed from (R)- to (S)-selectivity by simple modifications of the secondary structure.

Stanislav Ivan

Papers

Increased structural complexity leads to higher activity: peptides as efficient and versatile catalysts for asymmetric aldol reactions.

A novel route to molecular self-assembly: self-intermixed monolayer phases.

Adsorption and two-dimensional phases of a large polar molecule: Sub-phthalocyanine on Ag(111)

Increased Structural Complexity Leads to Higher Activity: Peptides as Efficient and Versatile Catalysts for Asymmetric Aldol Reactions.

Cover Picture: ChemPhysChem 10/2002