Kinetic and Thermodynamic Selectivity in Subcomponent Substitution

Fe(II)-Iminopyridine Catalyst for the Regioselective Synthesis of Oxazolidinones Using Carbon Dioxide

This study presents the application of a novel Fe-iminopyridine catalyst for the regioselective synthesis of oxazolidinones from carbon dioxide and aziridines. Our findings demonstrate that the Fe-iminopyridine catalyst containing imidazole functional group offers promising efficiency and facilitates a sustainable approach to green chemical synthesis at 50 °C and 10 bar CO2 pressure in a single-component Fe catalyst system. Various aziridines with carboxylic acid-derived substituents were transformed into 5-carbonyl substituted oxazolidinone products. The regioselective synthesis of oxazolidinones followed by the reduction enhances their utility for the pharmaceutically valuable compounds.

Kinetic Control of Complexity in Multiple Dynamic Libraries

Multiple dynamic libraries of compounds are generated when more than one reversible reaction comes into play. Commonly, two or more orthogonal reversible reactions are used, leading to non-communicating dynamic libraries which share no building blocks. Only a few examples of communicating libraries have been reported, and in all those cases, building blocks are reversibly exchanged from one library to the other, constituting an antiparallel dynamic covalent system. Herein we report that communication between two different dynamic libraries through an irreversible process is also possible. Indeed, alkyl amines cancel the dynamic regime on the nucleophilic substitution of tetrazines, generating kinetically inert compounds. Interestingly, such amine can be part of another dynamic library, an imine-amine exchange. Thus, both libraries are interconnected with each other by an irreversible process which leads to kinetically inert structures that contain parts from both libraries, causing a collapse of the complexity. Additionally, a latent irreversible intercommunication could be developed. In such a way, a stable molecular system with specific host-guest and fluorescence properties, could be irreversibly transformed when the right stimulus was applied, triggering the cancellation of the original supramolecular and luminescent properties and the emergence of new ones.

Supramolecular polymerization provides non-equilibrium product distributions of imine-linked macrocycles

Supramolecular polymerization of imine-linked macrocycles has been coupled to dynamic imine bond exchange within a series of macrocycles and oligomers. In this way, macrocycle synthesis is driven by supramolecular assembly, either into small aggregates supported by π–π interactions, or high-aspect ratio nanotubes stabilized primarily by electrostatic and solvophobic interactions. For the latter, supramolecular polymerization into nanotubes restricts imine exchange, thereby conferring chemical stability to the assemblies and their constituent macrocycles. Competition in the formation and component exchange among macrocycles favored pyridine-2,6-diimine-linked species due to their rapid synthesis, thermodynamic stability, and assembly into high-aspect ratio nanotubes under the reaction conditions. In addition, the pyridine-containing nanotubes inhibit the formation of similar macrocycles containing benzene-1,3-diimine-linkages, presumably by disrupting their assembly and templation. Finally, we exploit rapid imine exchange within weak, low-aspect ratio macrocycle aggregates to carry out monomer exchange reactions to macrocycles bearing pyridine moieties. Once a pyridine-containing dialdehyde has exchanged into a macrocycle, the macrocycle becomes capable of nanotube formation, which dramatically slows further imine exchange. This kinetic trap provides chemically diverse macrocycles that are not attainable by direct synthetic methods. Together these findings provide new insights into coupling supramolecular polymerization and dynamic covalent bond-forming processes and leverages this insight to target asymmetric nanotubes. We envision these findings spurring further research efforts in the synthesis of nanostructures with designed and emergent properties.

Supramolecular polymerization of imine-linked macrocycles has been coupled to dynamic imine bond exchange within a series of macrocycles and oligomers.

Diastereoselective Control of Tetraphenylethene Reactivity by Metal Template Self-Assembly

The reaction of 4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)tetraaniline with 2-pyridinecarboxaldehyde and iron(II) chloride resulted, after aqueous workup, in the diastereoselective formation of an [Fe₂ L₃ ]⁴⁺ triple-stranded helicate structure, irrespective of the stoichiometry employed. The helicate structure was characterized in solution by multinuclear NMR spectroscopy, and in the solid state by single-crystal X-ray crystallography. The reaction of iron(II) tetrafluoroborate or iron(II) bistriflimide with the tetraaniline and 2-pyridinecarboxaldehyde allowed the formation of an [Fe₈ L₆ ]¹⁶⁺ cube when the appropriate stoichiometry was used, but these structures were unstable with respect to hydrolysis. The pendant amine groups on the helicate can be functionalized by reaction with acid chlorides or anhydrides, and the resulting functionalized tetraphenylethene (TPE) units were isolated by the reaction of the helicate with tris(2-aminoethyl)amine. The emission properties of the TPE units were studied in THF/water mixtures, and they were found by dynamic light scattering to self-assemble into large (av. diameter 250 nm) structures.

Multifunctional and Sustainable Fe-Iminopyridine Complexes for the Synthesis of Cyclic Carbonates

The use of multifunctional and sustainable Fe catalysts for the formation of cyclic carbonates from epoxides and carbon dioxide at 80 °C and 3 bar pressure is presented. The optimal catalyst possesses a halide counteranion and a hydrogen bond donor to activate the epoxide for ring opening, affording a single-component, cocatalyst-free catalytic system.

mer and fac isomerism in tris chelate diimine metal complexes

In this perspective, we highlight the issue of meridional (mer) and facial (fac) orientation of asymmetrical diimines in tris-chelate transition metal complexes. Diimine ligands have long been the workhorse of coordination chemistry, and whilst there are now good strategies to isolate materials where the inherent metal centered chirality is under almost complete control, and systematic methodologies to isolate heteroleptic complexes, the conceptually simple geometrical isomerism has not been widely investigated. In systems where the two donor atoms are significantly different in terms of the σ-donor and π-accepting ability, the fac isomer is likely to be the thermodynamic product. For the diimine complexes with two trigonal planar nitrogen atoms there is much more subtlety to the system, and external factors such as the solvent, lattice packing and the various steric considerations play a delicate role in determining the observed and isolable product. In this article we discuss the possibilities to control the isomeric ratio in labile systems, consider the opportunities to separate inert complexes and discuss the observed differences in their spectroscopic properties. Finally we report on the ligand orientation in supramolecular systems where facial coordination leads to simple regular structures such as helicates and tetrahedra, but the ability of the ligand system to adopt a mer orientation enables self-assembled structures of considerable beauty and complexity.

Chiral amine enantiomeric excess determination using self-assembled octahedral Fe(II)-imine complexes

A receptor assembly composed of iron(II) triflate and pyridine-2,6-dicarbaldehyde was used to determine the enantiomeric excess (ee) of alpha-chiral primary amines using circular dichroism spectroscopy. The alpha chiral amines condense with the dialdehyde to form a diimine, which forms a 2:1 octahedral complex with iron(II). The ee values of unknown concentrations of alpha-chiral amines were determined by constructing calibration curves for each amine and then measuring the ellipticity at 600 nm. This improves our previously reported assay for ee determination of chiral primary amines by further increasing the wavelength at which CD is measured and reducing the absolute error of the assay.

Template-directed synthesis employing reversible imine bond formation

The imine bond--formed by the reversible condensation of an amine and an aldehyde--and its applications as a dynamic covalent bond in the template-directed synthesis of molecular compounds, will be the focus of this tutorial review. Template-directed synthesis--or expressed another way, supramolecular assistance to covalent synthesis--relies on the use of reversible noncovalent bonding interactions between molecular building blocks in order to preorganise them into a certain relative geometry as a prelude to covalent bond formation to afford the thermodynamically preferred product. The use of this so-called dynamic covalent chemistry (DCC) in templated reactions allows for an additional amount of reversibility, further eliminating potential kinetic products by allowing the covalent bonds that are formed during the template-directed reaction to be 'proofread for errors', thus making it possible for the reaction to search out its thermodynamic minimum. The marriage of template-directed synthesis with DCC has allowed chemists to construct an increasingly complex collection of compounds from relatively simple precursors. This new paradigm in organic synthesis requires that each individual piece in the molecular self-assembly process is preprogrammed so that the multiple recognition events expressed between the pieces are optimised in a highly cooperative manner in the desired product. It offers an extremely simple way of making complex mechanically interlocked compounds--e.g., catenanes, rotaxanes, suitanes, Borromean rings and Solomon knots--from relatively simple precursors.

Disulfides, Imines, and Metal Coordination within a Single System: Interplay between Three Dynamic Equilibria

We report a system in which three distinct dynamic linkages, disulfide (S-S), imine (C=N), and coordinative (N-->metal) bonds were shown to be capable of simultaneous reversible exchange. The "disulfide layer" of the system under study consists of two homo-disulfides, bis(4-aminophenyl) disulfide 1 and bis(4-methoxyphenyl) disulfide 2 that equilibrate in the presence of catalytic amount of triethylamine to favor the formation of a hetero-disulfide product, 4-aminophenyl-4'-methoxyphenyl disulfide 3. The addition of 2-formylpyridine and a metal salt strongly perturbed this 1+2<-->3 equilibrium through the formation of metal complexes incorporating disulfide 1 as a subcomponent. CuI perturbed the equilibrium by a factor of 3.3, and FeII by a factor of 179, in both cases in favor of the homo-disulfides. The disulfide equilibrium could be further modified, following metal-complex formation, by coordinative (transmetallation: substitution of FeII for CuI) or covalent (imine exchange: the substitution of one amine residue for another) exchange. Thus, although the three kinds of dynamic linkages were demonstrated to be mutually compatible, changes at one kind of linkage could be used to predictably perturb an equilibrium involving another.