Entropy reduction from strong localization - an explanation for enhanced reaction rates of organic synthesis in aqueous micelles

Aqueous Micellar Environment Impacts the Co-Catalyzed Phototransformation: A Case Study

In recent years, methodologies that rely on water as the reaction medium have gained considerable attention. The unique properties of micellar solutions were shown to improve the regio-, stereo-, and chemoselectivity of different transformations. Herein, we demonstrate that the aqueous environment is a suitable medium for a visible light driven cobalt-catalyzed reaction involving radical species. In this system, reduced vitamin B12 reacts with alkyl halides, generating radicals that are trapped by the lipophilic olefin present in the Stern layer. A series of NMR measurements and theoretical studies revealed the location of reaction components in the micellar system.

Molecular Dynamic Simulations of Aqueous Micellar Organometallic Catalysis: Methane Functionalization as a Case Study

Molecular Dynamics (MD) simulations constitute a powerful tool that provides a 3D perspective of the dynamical behavior of chemical systems. Herein the first MD study of the dynamics of a catalytic organometallic system, in micellar media, is presented. The challenging methane catalytic functionalization into ethyl propionate through a silver-catalyzed process has been targeted as the case study. The intimate nature of the micelles formed with the surfactants sodium dodecylsulfate (SDS) and potassium perfluorooctane sulfonate (PFOS) has been ascertained, as well as the relative distribution of the main actors in this transformation, namely methane, the diazo reagent and the silver catalyst, the latter in two different forms: the initial compound and a silver-carbene intermediate. Catalyst deactivation occurs with halide containing surfactants dodecyltrimethylammonium chloride (DTAC) and Triton X-100. Computed simulations allow explaining the experimental results, indicating that micelles behave differently regarding the degree of accumulation and the local distribution of the reactants and their effect in the molecular collisions leading to net reaction.

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.

Where Chemocatalysis Meets Biocatalysis: In Water

Chemoenzymatic catalysis, by definition, involves the merging of sequential reactions using both chemocatalysis and biocatalysis, typically in a single reaction vessel. A major challenge, the solution to which, however, is associated with numerous advantages, is to run such one-pot processes in water: the majority of enzyme-catalyzed processes take place in water as Nature's reaction medium, thus enabling a broad synthetic diversity when using water due to the option to use virtually all types of enzymes. Furthermore, water is cheap, abundantly available, and environmentally friendly, thus making it, in principle, an ideal reaction medium. On the other hand, most chemocatalysis is routinely performed today in organic solvents (which might deactivate enzymes), thus appearing to make it difficult to combine such reactions with biocatalysis toward one-pot cascades in water. Several creative approaches and solutions that enable such combinations of chemo- and biocatalysis in water to be realized and applied to synthetic problems are presented herein, reflecting the state-of-the-art in this blossoming field. Coverage has been sectioned into three parts, after introductory remarks: (1) Chapter 2 focuses on historical developments that initiated this area of research; (2) Chapter 3 describes key developments post-initial discoveries that have advanced this field; and (3) Chapter 4 highlights the latest achievements that provide attractive solutions to the main question of compatibility between biocatalysis (used predominantly in aqueous media) and chemocatalysis (that remains predominantly performed in organic solvents), both Chapters covering mainly literature from ca. 2018 to the present. Chapters 5 and 6 provide a brief overview as to where the field stands, the challenges that lie ahead, and ultimately, the prognosis looking toward the future of chemoenzymatic catalysis in organic synthesis.