In an effort to augment or displace petroleum as a source of liquid fuels and chemicals, researchers are seeking lower cost technologies that convert natural gas (largely methane) to products such as ...methanol. Current methane to methanol technologies based on highly optimized, indirect, high-temperature chemistry (>800 °C) are prohibitively expensive. A new generation of catalysts is needed to rapidly convert methane and O2 (ideally as air) directly to methanol (or other liquid hydrocarbons) at lower temperatures (∼250 °C) and with high selectivity. Our approach is based on the reaction between CH bonds of hydrocarbons (RH) and transition metal complexes, L n M–X, to generate activated L n M–R intermediates while avoiding the formation of free radicals or carbocations. We have focused on the incorporation of this reaction into catalytic cycles by integrating the activation of the CH bond with the functionalization of L n M–R to generate the desired product and regenerate the L n M–X complex. To avoid free-radical reactions possible with the direct use of O2, our approach is based on the use of air-recyclable oxidants. In addition, the solvent serves several roles including protection of the product, generation of highly active catalysts, and in some cases, as the air-regenerable oxidant. We postulate that there could be three distinct classes of catalyst/oxidant/solvent systems. The established electrophilic class combines electron-poor catalysts in acidic solvents that conceptually react by net removal of electrons from the bonding orbitals of the CH bond. The solvent protects the CH3OH by conversion to more electron-poor CH3OH2+ or the ester and also increases the electrophilicity of the catalyst by ligand protonation. The nucleophilic class matches electron-rich catalysts with basic solvents and conceptually reacts by net donation of electrons to the antibonding orbitals of the CH bond. In this case, the solvent could protect the CH3OH by deprotonation to the more electron-rich CH3O− and increases the nucleophilicity of the catalysts by ligand deprotonation. The third grouping involves ambiphilic catalysts that can conceptually react with both the HOMO and LUMO of the CH bond and would typically involve neutral reaction solvents. We call this continuum base- or acid-modulated (BAM) catalysis. In this Account, we describe our efforts to design catalysts following these general principles. We have had the most success with designing electrophilic systems, but unfortunately, the essential role of the acidic solvent also led to catalyst inhibition by CH3OH above ∼1 M. The ambiphilic catalysts reduced this product inhibition but were too slow and inefficient. To date, we have designed new base-assisted CH activation and L n M–R fuctionalization reactions and are working to integrate these into a complete, working catalytic cycle. Although we have yet to design a system that could supplant commercial processes, continued exploration of the BAM catalysis continuum may lead to new systems that will succeed in addressing this valuable goal.
Cooperative enamine‐metal Lewis acid catalysis has emerged as a powerful tool to construct carbon‐carbon and carbon‐heteroatom bond forming reactions. A concise synthetic method for asymmetric ...synthesis of chromans from cyclohexanones and salicylaldehydes has been developed to afford tricyclic chromans containing three consecutive stereogenic centers in good yields (up to 87 %) and stereoselectivity (up to 99 % ee and 11 : 1 : 1 dr). This difficult organic transformation was achieved through bifunctional enamine‐metal Lewis acid catalysis. It is believed that the strong activation of the salicylaldehydes through chelating to the metal Lewis acid and the bifunctional nature of the catalyst accounts for the high yields and enantioselectivity of the reaction. The absolute configurations of the chroman products were established through X‐ray crystallography. DFT calculations were conducted to understand the mechanism and stereoselectivity of this reaction.
A concise method for the asymmetric synthesis of tricyclic chromans from salicylaldehydes and cyclic ketones was developed through bifunctional enamine‐metal Lewis acid catalysis. Mechanism of the chroman formation reaction was proposed and investigated with deuterium experiments and DFT calculations. Tricyclic chromans containing three consecutive stereogenic centers were prepared in good yields (up to 87 %) and stereoselectivity (up to 99 % ee and 11 : 1 : 1 dr).
Single‐atom catalysts (SACs) bridge homo‐ and heterogeneous catalysis because the active site is a metal atom coordinated to surface ligands. The local binding environment of the atom should thus ...strongly influence how reactants adsorb. Now, atomically resolved scanning‐probe microscopy, X‐ray photoelectron spectroscopy, temperature‐programmed desorption, and DFT are used to study how CO binds at different Ir1 sites on a precisely defined Fe3O4(001) support. The two‐ and five‐fold‐coordinated Ir adatoms bind CO more strongly than metallic Ir, and adopt structures consistent with square‐planar IrI and octahedral IrIII complexes, respectively. Ir incorporates into the subsurface already at 450 K, becoming inactive for adsorption. Above 900 K, the Ir adatoms agglomerate to form nanoparticles encapsulated by iron oxide. These results demonstrate the link between SAC systems and coordination complexes, and that incorporation into the support is an important deactivation mechanism.
The coordination of a single atom to the oxide support has dramatic consequences on its ability to adsorb carbon monoxide. The observed motifs can be rationalized using simple arguments from coordination chemistry, confirming the hypothesis that single‐atom catalysts have much in common with organometallic complexes used in homogeneous catalysis.
An inexpensive and facile one-step method to develop a superhydrophobic coating on the copper surface is reported. Superhydrophobic CuO/Cu(OH)2 surfaces were prepared by a simple solution-immersion ...process at room temperature, without using a low surface energy material. The structure and composition of as-prepared CuO/Cu(OH)2 hierarchical structure were confirmed by X-ray diffraction, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The growth stage was carefully examined by field emission scanning electron microscopy (FESEM), and it was observed that initially Cu(OH)2 nanoneedle arrays were formed on the copper surface and subsequently the CuO microflowers formed on the nanoneedle arrays. The contact angle as a function of immersion time was studied using a contact angle goniometer. The correlation between the microstructure of the immersed copper surface and the contact angle was examined carefully using FESEM and atomic force microscopy (AFM). Our results based on FESEM and AFM studies show that the CuO/Cu(OH)2 coatings demonstrate superhydrophobicity only for an optimal combination of the solid region (i.e., microflowers and nanoneedles) and air pockets (i.e., voids). The maximum static water contact angle on the prepared surface was 159°. The wettability transition of the CuO/Cu(OH)2 surface from superhydrophobicity to superhydrophilicity was studied by the alteration of oxygen plasma treatment and dark storage. The FESEM, AFM, and XPS studies showed that this transformation was mainly due to the morphological changes that occur in addition to the chemical changes taking place on the CuO/Cu(OH)2 surface under the influence of oxygen plasma. XPS analysis demonstrated that the incorporation of oxygen species by oxygen plasma activation accounted for the highly hydrophilic character of the surface.
Rh(III)-Catalyzed 1 : 2 coupling of 3-aryl-2
-benzo
1,4oxazines with α-diazo-β-ketoesters has been realized for the mild synthesis of spiropyrans. The reaction proceeded
twofold C-H activation ...followed by unusual 3+3 and 4+2 annulation with decent functional group tolerance. Moreover, a pyranoid-skeleton intermediate was isolated as a key intermediate as a result of monoalkylation and enol oxygen annulation, which offers direct mechanistic insight.
As practitioners of organic chemistry strive to deliver efficient syntheses of the most complex natural products and drug candidates, further innovations in synthetic strategies are required to ...facilitate their efficient construction. These aspirational breakthroughs often go hand-in-hand with considerable reductions in cost and environmental impact. Enzyme-catalyzed reactions have become an impressive and necessary tool that offers benefits such as increased selectivity and waste limitation. These benefits are amplified when enzymatic processes are conducted in a cascade in combination with novel bond-forming strategies. In this article, we report a highly diastereoselective synthesis of MK-1454, a potent agonist of the stimulator of interferon gene (STING) signaling pathway. The synthesis begins with the asymmetric construction of two fluoride-bearing deoxynucleotides. The routes were designed for maximum convergency and selectivity, relying on the same benign electrophilic fluorinating reagent. From these complex subunits, four enzymes are used to construct the two bridging thiophosphates in a highly selective, high yielding cascade process. Critical to the success of this reaction was a thorough understanding of the role transition metals play in bond formation.
A photoelectrochemical device with a novel hierarchical heterostructure coupled with narrow bandgap semiconductors is demonstrated for efficient hydrogen generation via water splitting. The ...heterostructures consist of ZnO nanowire branches grown on WO x nanowhisker stems, which offer a large surface area and efficient charge transport path. The assembly of CdSe/CdS narrow bandgap cosensitizers on hierarchical ZnO/WO x nanostructures is shown to enhance light harvesting in the visible light region. The cosensitized ZnO/WO x heterostructures demonstrate efficient light absorption up to a wavelength of 800 nm as well as enhanced photoelectrochemical properties when used as photoanodes. Furthermore, CdSe/CdS cosensitized ZnO/WO x has a type II cascade band structure, resulting in efficient charge transport, which was confirmed by open circuit voltage decay measurements. Our photoelectrochemical system produced a high photocurrent density of 11 mA/cm2 at −0.5 V (vs SCE) under 1.5 AM irradiation for hydrogen generation.
Sulfonyl fluorides have attracted considerable and growing research interests from various disciplines, which raises a high demand for novel and effective methods to access this class of compounds. ...Radical flurosulfonylation is recently emerging as a promising approach for the synthesis of sulfonyl fluorides. However, the scope of applicable substrate and reaction types are severely restricted by limited known radical reagents. Here, we introduce a solid state, redox-active type of fluorosulfonyl radical reagents, 1-fluorosulfonyl 2-aryl benzoimidazolium triflate (FABI) salts, which enable the radical fluorosulfonylation of olefins under photoredox conditions. In comparison with the known radical precursor, gaseous FSO
Cl, FABI salts are bench-stable, easy to handle, affording high yields in the radical fluorosulfonylation of olefins with before challenging substrates. The advantage of FABIs is further demonstrated in the development of an alkoxyl-fluorosulfonyl difunctionalization reaction of olefins, which forges a facile access to useful β-alkoxyl sulfonyl fluorides and related compounds, and would thus benefit the related study in the context of chemical biology and drug discovery in the future.
Radical aryl migrations are powerful techniques to forge new bonds in aromatic compounds. The growing popularity of photoredox catalysis has led to an influx of novel strategies to initiate and ...control aryl migration starting from widely available radical precursors. This review encapsulates progress in radical aryl migration enabled by photochemical methodsparticularly photoredox catalysissince 2015. Special attention is paid to descriptions of scope, mechanism, and synthetic applications of each method.
The growth mechanism of monolayer (ML) graphene on Ru(0001) via pyrolysis of C2H4 was studied by scanning tunneling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and ...ultraviolet photoelectron spectroscopy (UPS). On the basis of the mechanistic understanding, graphene overlayers ranging from nanographene clusters to graphene film with 1 ML coverage were prepared in a well-controlled way. O2 adsorption on the graphene/Ru(0001) surface was investigated by STM, UPS, and X-ray photoelectron spectroscopy (XPS). It is revealed that the Ru(0001) surface fully covered by graphene becomes passivated to O2 adsorption at room temperature and only activated again at elevated temperatures (>500 K). The adsorbed oxygen intercalates between the topmost graphene overlayer and the Ru(0001) substrate surface. These intercalated oxygen atoms decouple the graphene layer from the Ru(0001) substrate, forming quasi-freestanding monolayer graphene atomic crystals floating on the O−Ru(0001) surface.
PDF
