Nanoconfinement Effects of Micellar Media in Asymmetric Catalysis

Surface Involvement in the Boosting of Chiral Organocatalysts for Efficient Asymmetric Catalysis

Nanostructures with curved surfaces and chiral-directing residues are highly desirable in the synthesis of asymmetric chemicals, but they remain challenging to synthesize without using unique templates due to the disfavored torsion energy of twisted architectures toward chiral centers. Here, a strategy for the facile fabrication of highly cured capsule-shaped catalysts with chiral interiors by the amplification of molecular chirality via the irreversible cross-linking of 2D asymmetric laminates is presented. The key to the success of these irregular 2D layers is the use of hierarchical assembly of chiral macrocycles, which can exactly regulate the cured nanostructures as well as asymmetric catalysis. The cross-linking of 2D laminates from the assembly of hexameric macrocycles with one proline edge gave rise to rarely curled capsules with a diameter of 200-400 nm and excellent enantioselectivities as well as diastereoselectivities for asymmetric aldol reactions (94% ee and 1:13 dr). The tetrameric macrocycles decorated with the chiral block produced further curled porous structures, giving an outstanding enantioselectivities (up to 98% ee and 1:17 dr). The strategy of mechanical surface folding will provide a new insight related to increasing the enantioselectivity of chiral organocatalysts.

A double-chain based metallomicellar catalyst for aerobic oxidative synthesis of benzimidazoles in water

The oxomolybdenum complexes Mo1, Mo2 and Mo3, which share a common ONO donor ligand backbone but differ in their peripheral substituents, were explored to study their reactivity in organic transformations in water. The ligand backbones of Mo1 and Mo2 were covalently linked to a methyl group and a single hydrophobic n-hexadecyl chain via an ether linkage, respectively. The complex Mo3 was found to possess two n-hexadecyl chains attached to the ligand backbone via a common amine-N. Complexes Mo2 and Mo3 formed metallomicelle when dispersed in water due to the surfactant presence in their structures, enabling them to uptake organic substrates. The catalytic potential of the complexes was evaluated for the oxidative coupling of benzylamine with 1,2-diaminobenzene to synthesize benzimidazole in neat water using open air as the sole oxidant. The double-chain surfactant-type catalyst Mo3 displayed superior activity compared to the single-chain surfactant-type complex, Mo2. A wide variety of benzimidazoles were synthesized in good to excellent yields under environmentally benign conditions using Mo3 as the catalyst. The practical utility of the process was validated through multi-gram scale-up reactions and recyclability experiments. A plausible mechanism was proposed based on several controlled experiments and literature support.

Determining the Absolute Configuration of Small Molecules by Diffusion NMR Experiments

Enantiomers are ubiquitous in many areas of science, such as pharmaceuticals, agriculture, and food. Nuclear magnetic resonance (NMR) alone is not able to differentiate enantiomers as their spectra are identical. However, these can be distinguished using chiral auxiliaries (such as chiral complexing agents) that form diastereomeric complexes, but absolute identification is still troublesome, usually requiring a chemical reaction with a chiral derivatizing agent. Here, we propose a new method that uses a hybrid mixture of solvating agents in a simple comparison of diffusion NMR experiments, which can discriminate enantiomers in both frequency and diffusion domains, dubbed CHIMERA (CHIral Micelle Enantiomer Resolving Agent). The new method was assessed for twenty-three small chiral molecules using a combination of BINOL and (-)-DMEB, a chiral surfactant, and initial results indicate that absolute configuration can be obtained from a simple experiment.

Rapid Aminations of Functionalized Aryl Fluorosulfates in Water

Aryl fluorosulfates of varying complexities have been used in amination reactions in water using a new Pd oxidative addition complex (OAC-1) developed specifically to match the needs of the fine chemicals industry, not only in terms of functional group tolerance, but also reflecting time considerations associated with these important C-N couplings. Also especially noteworthy is that they replace both PFAS-related triflates and nonaflates, which are today out of favor due to recent government regulations. The new complex based on the BippyPhos ligand is used at low loadings and under aqueous micellar conditions. Moreover, it is easily prepared and stable to long term storage. DFT calculations on the OAC precatalyst compare well with the X-ray structure of the crystals with π-complexation to the aromatic system of the ligand and also confirm the NMR data showing a mixture of conformers in solution that differ from the X-ray structure in rotation of the phenyl and t-butyl ligand substituents. An extensive variety of coupling partners, including pharmaceutically relevant APIs, readily participate under mild and environmentally responsible reaction conditions.

Micelle-mediated synthesis of quinoxaline, 1,4-benzoxazine and 1,4-benzothiazine scaffolds from styrenes

A range of heterocycles based on quinoxalines, 1,4-benzoxazines and 1,4-benzothiazines have been accessed from styrenes by reacting them with benzene-1,2-diamine, 2-aminophenol and 2-aminothiophenol respectively in micellar medium. This reaction occurring in a less explored cetylpyridinium bromide (CPB) micellar medium operates in the presence of NBS through a tandem hydrobromination-oxidation cascade, converting styrenes to phenacyl bromides. Its subsequent nucleophilic addition with aromatic 1,2-dinucleophiles and further transformations led to the formation of heterocyclic constructs. The locus of the reaction site was confirmed through NMR studies and the types of interactions between the CPB and solubilizates were established by DFT calculations.

The impact of earth-abundant metals as a replacement for Pd in cross coupling reactions

Substitution of one metal catalyst for another is not as straightforward as simply justifying this change based on the availability and/or cost of the metals. Methodologies to properly assess options for reaction design, including multiple factors like a metal's availability, cost, or environmental indicators have not advanced at the pace needed, leaving decisions to be made along these lines more challenging. Isolated indicators can lead to conclusions being made in too hasty a fashion. Therefore, an extensive life cycle-like assessment was performed documenting that the commonly held view that methods using earth-abundant metals (and in this case study, Ni) are inherently green replacements for methods using palladium in cross-coupling reactions, and Suzuki-Miyaura couplings, in particular, is an incomplete analysis of the entire picture. This notion can be misleading, and unfortunately derives mainly from the standpoint of price, and to some degree, relative natural abundance associated with the impact of mining of each metal. A more accurate picture emerges when several additional reaction parameters involved in the compared couplings are considered. The analysis points to the major impact that use of organic solvents has in these couplings, while the metals themselves actually play subordinate roles in terms of CO2-release into the environment and hence, the overall carbon footprint (i.e., climate change). The conclusion is that a far more detailed analysis is required than that typically being utilized.

Nanoparticles as Heterogeneous Catalysts for ppm Pd-Catalyzed Aminations in Water

A general protocol employing heterogeneous catalysis has been developed that enables ppm of Pd-catalyzed C-N cross-coupling reactions under aqueous micellar catalysis. A new nanoparticle catalyst containing specifically ligated Pd, in combination with nanoreactors composed of the designer surfactant Savie, a biodegradable amphiphile, catalyzes C-N bond formations in recyclable water. A variety of coupling partners, ranging from highly functionalized pharmaceutically relevant APIs to educts from the Merck Informer Library, readily participate under these environmentally responsible, sustainable reaction conditions. Other key features associated with this report include the low levels of residual Pd found in the products, the recyclability of the aqueous reaction medium, the use of ocean water as an alternative source of reaction medium, options for the use of pseudohalides as alternative reaction partners, and associated low E factors. In addition, an unprecedented 5-step, one-pot sequence is presented, featuring several of the most widely used transformations in the pharmaceutical industry, suggesting potential industrial applications.

Benchmarking Cationic Monolayer Protected Nanoparticles and Micelles for Phosphate-Mediated and Nucleotide-Selective Proton Transfer Catalysis

Both micelles and self-assembled monolayer (SAM)-protected nanoparticles are capable of efficiently hosting water-immiscible substrates to carry out organic reactions in aqueous media. Herein, we have analyzed the different catalytic effect of SAM-protected cationic nanoparticles and cationic surfactants of varying chain length towards base-catalyzed proton transfer mediated ring-opening reaction of 5-nitrobenzisoxazole (NBI) (also known as Kemp Elimination (KE) reaction). We use inorganic phosphate ion or different nucleotide (phosphate-ligated different nucleoside) as base to promote the reaction on micellar or nanoparticle interface. We find almost 2-3 orders of magnitude higher concentration of surfactants of comparable hydrophobicity required to reach the similar activity which attained by low cationic head group concentration bound on nanoparticle. Additionally, at low concentration of nanoparticle-bound surfactant or with high surfactant in micellar form, nucleotide-selectivity has been observed in activating KE reaction unlike free surfactant at low concentration. Finally, we showed enzyme-mediated nucleotide hydrolysis to generate phosphate ion which in situ upregulate the KE activity much more in GNP-based system compared to CTAB. Notably, we show a reasonable superiority of SAM-protected nanoparticles in activating chemical reaction in micromolar concentration of headgroup which certainly boost up application of SAM-based nanoparticles not only for selective recognition but also as eco-friendly catalyst.

Observation of Nanoconfinement Effect on the Kinetics of Hydrated Electron in the Nanoscale Water Pools of Water-AOT-Cyclohexane Microemulsions by Picosecond Pulse Radiolysis

The decay kinetics of the hydrated electron (eaq-) in aerosol OT (AOT)-based ternary microemulsions with pool sizes ranging from 0.34 to 4.85 nm were studied using picosecond pulse radiolysis coupled with transient absorption UV-vis spectroscopy. Electron transfer from oil to water and the subsequent solvation occurred within a time resolution of 7 ps. The decay kinetics of eaq- were accurately modeled using a double-exponential decay model, revealing the occurrence of two types of reactions, i.e., the recombination reaction at the water-oil interface and the radical-radical reactions in the water pools. The apparent lifetimes of both types of decays decreased significantly as the size of water pools decreased, indicating the influence of nanoconfinement effects. Moreover, the importance of the water-oil interface increased with decreasing water content, regardless of the presence or absence of NO3- as an electron scavenger in the water pools. Our findings provide a comprehensive understanding on the kinetics of the radiation reaction in AOT-based microemulsions.

A micelle-mediated approach enables facile access to bridged oxabicyclo[n.3.1]alkene scaffolds

Oxabicyclo[n.3.1]alkene scaffolds present in a diverse range of complex natural products have been accessed by reacting 2-cycloalkenones with 1,3-cycloalkadiones in a micellar medium. This reaction occurring in a micellar confinement environment operates through a Michael addition/enolization/oxygen addition cascade to furnish highly functionalized constructs using a sustainable organic synthesis protocol. NMR analysis confirms that the locus of the solubilizates is within the palisade and stern regions of the micellar cavity.

Is Micellar Catalysis Green Chemistry?

Many years ago, twelve principles were defined for carrying out chemical reactions and processes from a green chemistry perspective. It is everyone's endeavor to take these points into account as far as possible when developing new processes or improving existing ones. Especially in the field of organic synthesis, a new area of research has thus been established: micellar catalysis. This review article addresses the question of whether micellar catalysis is green chemistry by applying the twelve principles to micellar reaction media. The review shows that many reactions can be transferred from an organic solvent to a micellar medium, but that the surfactant also has a crucial role as a solubilizer. Thus, the reactions can be carried out in a much more environmentally friendly manner and with less risk. Moreover, surfactants are being reformulated in their design, synthesis, and degradation to add extra advantages to micellar catalysis to match all the twelve principles of green chemistry.

Surfactant Micellar and Vesicle Microenvironments and Structures under Synthetic Organic Conditions

Fluorescence lifetime imaging microscopy (FLIM) reveals vesicle sizes, structures, microenvironments, reagent partitioning, and system evolution with two chemical reactions for widely used surfactant-water systems under conditions relevant to organic synthesis, including during steps of Negishi cross-coupling reactions. In contrast to previous investigations, the present experiments characterize surfactant systems with representative organohalide substrates at high concentrations (0.5 M) that are reflective of the preparative-scale organic reactions performed and reported in water. In the presence of representative organic substrates, 2-iodoethylbenzene and 2-bromo-6-methoxypyridine, micelles swell into emulsion droplets that are up to 20 μm in diameter, which is 3-4 orders of magnitude larger than previously measured in the absence of an organic substrate (5-200 nm). The partitioning of reagents in these systems is imaged through FLIM─demonstrated here with nonpolar, amphiphilic, organic, basic, and oxidative-addition reactive compounds, a reactive zinc metal powder, and a palladium catalyst. FLIM characterizes the chemical species and/or provides microenvironment information inside micelles and vesicles. These data show that surfactants cause surfactant-dictated microenvironments inside smaller micelles (<200 nm) but that addition of a representative organic substrate produces internal microenvironments dictated primarily by the substrate rather than by the surfactant, concurrent with swelling. Addition of a palladium catalyst causes the internal environments to differ between vesicles─information that is not available through nor predicted from prior analytical techniques. Together, these data provide immediately actionable information for revising reaction models of surfactant-water systems that underpin the development of sustainable organic chemistry in water.

Aqueous micellar technology: an alternative beyond organic solvents

Solvents are the major source of chemical waste from synthetic chemistry labs. Growing attention to more environmentally friendly sustainable processes demands novel technologies to substitute toxic or hazardous solvents. If not always, sometimes, water can be a suitable substitute for organic solvents, if used appropriately. However, the sole use of water as a solvent remains non-practical due to its incompatibility with organic reagents. Nonetheless, over the past few years, new additives have been disclosed to achieve chemistry in water that also include aqueous micelles as nanoreactors. Although one cannot claim micellar catalysis to be a greener technology for every single transformation, it remains the sustainable or greener alternative for many reactions. Literature precedents support that micellar technology has much more potential than just as a reaction medium, i.e., the role of the amphiphile as a ligand obviating phosphine ligands in catalysis, the shielding effect of micelles to protect water-sensitive reaction intermediates in catalysis, and the compartmentalization effect. While compiling the powerful impact of micellar catalysis, this article highlights two diverse recent technologies: (i) the design and employment of the surfactant PS-750-M in selective catalysis; (ii) the use of the semisynthetic HPMC polymer to enable ultrafast reactions in water.

Recent Developments and Trends in Asymmetric Organocatalysis

Asymmetric organocatalysis has experienced a long and spectacular way since the early reports over a century ago by von Liebig, Knoevenagel and Bredig, showing that small (chiral) organic molecules can catalyze (asymmetric) reactions. This was followed by impressive first highly enantioselective reports in the second half of the last century, until the hype initiated in 2000 by the milestone publications of MacMillan and List, which finally culminated in the 2021 Nobel Prize in Chemistry. This short Perspective aims at providing a brief introduction to the field by first looking on the historical development and the more classical methods and concepts, followed by discussing selected advanced recent examples that opened new directions and diversity within this still growing field.