Synthesis of Hybrid Ferrocene-Proline Amides as Active Catalysts for Asymmetric Aldol Reactions in Water

Resonant transport in a highly conducting single molecular junction via metal-metal covalent bond

Achieving highly transmitting molecular junctions through resonant transport at low bias is key to the next-generation low-power molecular devices. Although resonant transport in molecular junctions was observed by connecting a molecule between the metal electrodes via chemical anchors by applying a high source-drain bias (>1 V), the conductance was limited to <0.1G0, G0 being the quantum of conductance. Herein, we report electronic transport measurements by directly connecting a ferrocene molecule between Au electrodes under ambient conditions in a mechanically controllable break junction setup (MCBJ), revealing a conductance peak at ∼0.2G0 in the conductance histogram. A similar experiment was repeated for ferrocene terminated with amine (-NH2) and cyano (-CN) anchors, where conductance histograms exhibit an extended low conductance feature, including the sharp high conductance peak, similar to pristine ferrocene. The statistical analysis of the data and density functional theory-based transport calculation suggest a possible molecular conformation with a strong hybridization between the Au electrodes, and that the Fe atom of ferrocene is responsible for a near-perfect transmission in the vicinity of the Fermi energy, leading to the resonant transport at a small applied bias (<0.5 V). Moreover, calculations including van der Waals/dispersion corrections reveal a covalent-like organometallic bonding between Au and the central Fe atom of ferrocene, having bond energies of ∼660 meV. Overall, our study not only demonstrates the realization of an air-stable highly transmitting molecular junction, but also provides important insights about the nature of chemical bonding at the metal/organo-metallic interface.

Thermal and thermo-oxidative stability of a series of palladium and platinum ferrocenylamine sulfides and selenides

Ferrocenylamine complexes have found increasing applications in the fields of catalysis in various organic reactions, industry, medical treatments and enzyme–activity determinations. Therefore, information related to the thermal and thermo-oxidative stability of these compounds is important for such applications; however, this information is currently limited. Twenty previously prepared palladium and platinum ferrocenylamine complexes with systematic structural variations are examined for their thermal (under nitrogen) and thermo-oxidation stability (under atmospheric air) using thermogravimetry (TG), differential thermal analysis (DTG), and differential scanning calorimetry (DSC) techniques. Degradation products are identified by comparing thermogravimetric analysis and theoretical calculations. Structure–stability studies are also discussed. The results show that all the compounds have high thermal and thermo-oxidative stabilities of up to 265 and 173 °C, respectively. Electron–donating substituents enhance the thermal and thermo-oxidative stabilities of the palladium complexes ( t-Bu, selenide electrophiles and dielectrophiles), while those with destabilizing effects are aromatic substituents (Ph and tolyl). Most platinum ferrocenylamine sulfides and selenides show higher thermal and thermo-oxidative stabilities than their corresponding palladium analogs. All the prepared compounds show high thermal and thermo-oxidative stability which reinforces their catalytic and industrial applications. However, their thermal stability is higher than their thermo-oxidative stability.

Dissipative self-assembly of a proline catalyst for temporal regulation of the aldol reaction

The spatiotemporal regulation of chemical reactivity in biological systems permits a network of metabolic reactions to take place within the same cellular environment. The exquisite control of reactivity is often mediated by out-of-equilibrium structures that remain functional only as long as fuel is present to maintain the higher energy, active state. An important goal in supramolecular chemistry aims to develop functional, energy dissipating systems that approach the sophistication of biological machinery. The challenge is to create strategies that couple the energy consumption needed to promote a molecule to a higher energy, assembled state to a functional property such as catalytic activity. In this work, we demonstrated that the assembly of a spiropyran (SP) dipeptide (1) transiently promoted the proline-catalyzed aldol reaction in water when visible light was present as fuel. The transient catalytic activity emerged from 1 under light illumination due to the photoisomerization of the monomeric, O-protonated (1-MCH⁺) merocyanine form to the spiropyran (1-SP) state, which rapidly assembled into nanosheets capable of catalyzing the aldol reaction in water. When the light source was removed, thermal isomerization to the more stable MCH⁺ form caused the nanosheets to dissociate into a catalytically inactive, monomeric state. Under these conditions, the aldol reaction could be repeatedly activated and deactivated by switching the light source on and off.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

A self-assembled nanotube for the direct aldol reaction in water

Nanotubes formed by the aqueous assembly of a proline-lysine dipeptide (1) were used to create the hydrophobic microenvironments required to catalyze the aldol reaction in water. The self-assembly process occurred most efficiently in the presence of the substrates, producing an array of homogeneous nanotubes under the reaction conditions. The nanotubes formed by dipeptide 1 served as an efficient catalyst for the aldol reaction that functioned at low loading levels and provided good to excellent conversions. The catalytic activity of 1 was minimal under conditions that dissociated the nanotube into soluble monomers.

Binaphthyl-based chiral bifunctional organocatalysts for water mediated asymmetric List–Lerner–Barbas aldol reactions

Binaphthyl-based organocatalysts were synthesized and successfully applied to the asymmetric List–Lerner–Barbas aldol reaction in water medium. These organocatalysts were found to be effective catalysts for the reactions of ketones with different aldehydes to give aldol products with higher yield and ee's.

Novel ferrocene-based bifunctional amine-thioureas for asymmetric Michael addition of acetylacetone to nitroolefins

An efficient method was developed to synthesize ferrocene-based bifunctional amine-thioureas bearing multiple hydrogen-bonding donors. Asymmetric Michael addition of acetylacetone to nitroolefins catalyzed by these novel bifunctional catalysts affords the Michael adducts in high yield and moderate to excellent enantioselectivities. Multiple hydrogen-bonds play an important role in accelerating the reaction.

Ferrocene as a scaffold for effective bifunctional amine–thiourea organocatalysts

This work demonstrates that ferrocene could be an excellent scaffold for chiral organocatalysts.

The smallest organocatalyst in highly enantioselective direct aldol reaction in wet solvent-free conditions

The catalytic efficacy of the smallest organocatalyst, l-proline hydrazide, prepared from a cheaply available natural amino acid, such as l-proline, was studied for the direct asymmetric aldol reaction of various ketones with aromatic aldehydes at room temperature in the presence of several acid additives.

Bifunctional ferrocene-based squaramide-phosphine as an organocatalyst for highly enantioselective intramolecular Morita–Baylis–Hillman reaction

Ferrocene is an excellent scaffold for organocatalysts. Bifunctional ferrocene-based phosphine (R_C,S_Fc)-1 shows high enantioselectivity in the intramolecular Morita–Baylis–Hillman reaction of 7-aryl-7-oxo-5-heptenals.