Showing papers on "Reactivity (chemistry) published in 2021"

Gold-Catalyzed Reactions of Specially Activated Alkynes, Allenes, and Alkenes

Abstract: This review describes the gold-catalyzed reactions of specially activated alkynes, allenes, and alkenes. Such species are characterized by the presence of either electron-donating or electron-withdrawing groups as substituents of the carbon π-system. They are intrinsically polarized, and when compared to their nonspecially activated counterparts can therefore be involved in gold-catalyzed transformations featuring increased regio-, stereo-, and chemoselectivities. The chemistry of specially activated carbon π-systems under homogeneous gold catalysis is extremely rich and varied. The reactivity observed with nonspecially activated unsaturated systems can often be transposed to specially activated ones without loss of efficiency. However, specially activated carbon π-systems also exhibit specific reactivities that cannot be attained with regular substrates. In this family of carbon π-systems, ynamides and their analogs, along with alkynyl carbonyl derivatives, are the classes of substrates that have retained the most attention. This review provides an overview of the chemistry developed with all classes of specially activated carbon π-systems by discussing their general and specific reactivities, presenting and commenting on their gold-catalyzed transformations as well as their applications.

The Aluminyl Anion: A New Generation of Aluminium Nucleophile

Abstract: Trivalent aluminium compounds are well known for their reactivity as Lewis acids/electrophiles, a feature that is exploited in many pharmaceutical, industrial and laboratory-based reactions Recently, a series of isolable aluminium(I) anions ("aluminyls") have been reported, which offer an alternative to this textbook description: these reagents behave as aluminium nucleophiles This minireview covers the synthesis, structure and reactivity of aluminyl species reported to date, together with their associated metal complexes The frontier orbitals of each of these species have been investigated using a common methodology to allow for a like-for-like comparison of their electronic structure and a means of rationalising (sometimes unprecedented) patterns of reactivity

Photocatalytic C–H activation and the subtle role of chlorine radical complexation in reactivity

Abstract: The functionalization of methane, ethane, and other alkanes derived from fossil fuels is a central goal in the chemical enterprise. Recently, a photocatalytic system comprising [CeIVCl5(OR)]2− [CeIV, cerium(IV); OR, –OCH3 or –OCCl2CH3] was disclosed. The system was reportedly capable of alkane activation by alkoxy radicals (RO•) formed by CeIV–OR bond photolysis. In this work, we present evidence that the reported carbon-hydrogen (C–H) activation of alkanes is instead mediated by the photocatalyst [NEt4]2[CeCl6] (NEt4+, tetraethylammonium), and RO• are not intermediates. Spectroscopic analyses and kinetics were investigated for C–H activation to identify chlorine radical (Cl•) generation as the rate-limiting step. Density functional theory calculations support the formation of [Cl•][alcohol] adducts when alcohols are present, which can manifest a masked RO• character. This result serves as an important cautionary note for interpretation of radical trapping experiments.

Electrophotocatalytic diamination of vicinal C–H bonds

Abstract: The conversion of unactivated carbon-hydrogen (C-H) bonds to carbon-nitrogen (C-N) bonds is a highly valued transformation. Existing strategies typically accomplish such reactions at only a single C-H site because the first derivatization diminishes the reactivity of surrounding C-H bonds. Here, we show that alkylated arenes can undergo vicinal C-H diamination reactions to form 1,2-diamine derivatives through an electrophotocatalytic strategy, using acetonitrile as both solvent and nitrogen source. The reaction is catalyzed by a trisaminocyclopropenium (TAC) ion, which undergoes anodic oxidation to furnish a stable radical dication while the cathodic reaction reduces protons to molecular hydrogen. Irradiation of the TAC radical dication (wavelength of maximum absorption of 450 to 550 nanometers) with a white-light compact fluorescent light generates a strongly oxidizing photoexcited intermediate. Depending on the electrolyte used, either 3,4-dihydroimidazole or aziridine products are obtained.

Catalytic three-component dicarbofunctionalization reactions involving radical capture by nickel

Abstract: The catalytic dicarbofunctionalization of unsaturated π bonds represents a powerful platform for the rapid construction of complex motifs. Despite remarkable progress, novel and efficient methods for achieving such transformations under milder conditions with chemo-, regio-, and stereoselectivity still remain a significant challenge; thus, their development is highly desirable. Recently, the merging of nickel catalysis with radical chemistry offers a new and benign platform for the catalytic dicarbofunctionalization of unsaturated π bonds with unprecedented reactivity and selectivity. In this review, we summarize the recent advances in this area by underpinning the catalytic domino transformations involving radical capture by nickel to provide a clear overview of reaction designs and mechanistic scenarios.

Recognition of Surface Oxygen Intermediates on NiFe Oxyhydroxide Oxygen-Evolving Catalysts by Homogeneous Oxidation Reactivity.

Abstract: NiFe oxyhydroxide is one of the most promising oxygen evolution reaction (OER) catalysts for renewable hydrogen production, and deciphering the identity and reactivity of the oxygen intermediates on its surface is a key challenge but is critical to the catalyst design for improving the energy efficiency. Here, we screened and utilized in situ reactive probes that can selectively target specific oxygen intermediates with high rates to investigate the OER intermediates and pathway on NiFe oxyhydroxide. Most importantly, the oxygen atom transfer (OAT) probes (e.g., 4-(diphenylphosphino) benzoic acid) could efficiently inhibit the OER kinetics by scavenging the OER intermediates, exhibiting lower OER currents, larger Tafel slopes, and larger kinetic isotope effect (KIE) values, while probes with other reactivities demonstrated much smaller effects. Combining the OAT reactivity with electrochemical kinetic and operando Raman spectroscopic techniques, we identified a resting Fe═O intermediate in the Ni-O scaffold and a rate-limiting O-O chemical coupling step between a Fe═O moiety and a vicinal bridging O. DFT calculation further revealed a longer Fe═O bond formed on the surface and a large kinetic energy barrier of the O-O chemical coupling step, corroborating the experimental results. These results point to a new direction of liberating lattice O and expediting O-O coupling for optimizing NiFe-based OER electrocatalyst.

Radical Chain Reduction via Carbon Dioxide Radical Anion (CO 2

Abstract: We developed an effective method for reductive radical formation that utilizes the radical anion of carbon dioxide (CO2•-) as a powerful single electron reductant. Through a polarity matched hydrogen atom transfer (HAT) between an electrophilic radical and a formate salt, CO2•- formation occurs as a key element in a new radical chain reaction. Here, radical chain initiation can be performed through photochemical or thermal means, and we illustrate the ability of this approach to accomplish reductive activation of a range of substrate classes. Specifically, we employed this strategy in the intermolecular hydroarylation of unactivated alkenes with (hetero)aryl chlorides/bromides, radical deamination of arylammonium salts, aliphatic ketyl radical formation, and sulfonamide cleavage. We show that the reactivity of CO2•- with electron-poor olefins results in either single electron reduction or alkene hydrocarboxylation, where substrate reduction potentials can be utilized to predict reaction outcome.

C(sp 3 )-H methylation enabled by peroxide photosensitization and Ni-mediated radical coupling.

Abstract: The "magic methyl" effect describes the change in potency, selectivity, and/or metabolic stability of a drug candidate associated with addition of a single methyl group. We report a synthetic method that enables direct methylation of C(sp3)-H bonds in diverse drug-like molecules and pharmaceutical building blocks. Visible light-initiated triplet energy transfer promotes homolysis of the O-O bond in di-tert-butyl or dicumyl peroxide under mild conditions. The resulting alkoxyl radicals undergo divergent reactivity, either hydrogen-atom transfer from a substrate C-H bond or generation of a methyl radical via β-methyl scission. The relative rates of these steps may be tuned by varying the reaction conditions or peroxide substituents to optimize the yield of methylated product arising from nickel-mediated cross-coupling of substrate and methyl radicals.

Photochemical C-H Activation Enables Nickel-Catalyzed Olefin Dicarbofunctionalization.

Abstract: Alkenes, ethers, and alcohols account for a significant percentage of bulk reagents available to the chemistry community. The petrochemical, pharmaceutical, and agrochemical industries each consume gigagrams of these materials as fuels and solvents each year. However, the utilization of such materials as building blocks for the construction of complex small molecules is limited by the necessity of prefunctionalization to achieve chemoselective reactivity. Herein, we report the implementation of efficient, sustainable, diaryl ketone hydrogen-atom transfer (HAT) catalysis to activate native C-H bonds for multicomponent dicarbofunctionalization of alkenes. The ability to forge new carbon-carbon bonds between reagents typically viewed as commodity solvents provides a new, more atom-economic outlook for organic synthesis. Through detailed experimental and computational investigation, the critical effect of hydrogen bonding on the reactivity of this transformation was uncovered.

Cobalt(III)‐Catalyzed Enantioselective Intermolecular Carboamination by C−H Functionalization

Abstract: High-valent cyclopentadienyl cobalt catalysis is a versatile tool for sustainable C-H bond functionalizations. To harness the full potential of this strategy, control of the stereoselectivity of these processes is necessary. Herein, we report highly enantioselective intermolecular carboaminations of alkenes through C-H activation of N-phenoxyamides catalyzed by CoIII -complexes equipped with chiral cyclopentadienyl (Cpx ) ligands. The method converts widely available acrylates as well as bicyclic olefins into attractive enantioenriched isotyrosine derivatives as well as elaborated amino-substituted bicyclic scaffolds under very mild conditions. The outlined reactivity is unique to the Cpx CoIII complexes and is complementary to the reactivity of 4d- and 5d- precious-metal catalysts.

Steam-created grain boundaries for methane C-H activation in palladium catalysts.

Abstract: Defects may display high reactivity because the specific arrangement of atoms differs from crystalline surfaces. We demonstrate that high-temperature steam pretreatment of palladium catalysts provi...

A radical approach for the selective C–H borylation of azines

Abstract: Boron functional groups are often introduced in place of aromatic carbon–hydrogen bonds to expedite small-molecule diversification through coupling of molecular fragments1–3. Current approaches based on transition-metal-catalysed activation of carbon–hydrogen bonds are effective for the borylation of many (hetero)aromatic derivatives4,5 but show narrow applicability to azines (nitrogen-containing aromatic heterocycles), which are key components of many pharmaceutical and agrochemical products6. Here we report an azine borylation strategy using stable and inexpensive amine-borane7 reagents. Photocatalysis converts these low-molecular-weight materials into highly reactive boryl radicals8 that undergo efficient addition to azine building blocks. This reactivity provides a mechanistically alternative tactic for sp2 carbon–boron bond assembly, where the elementary steps of transition-metal-mediated carbon–hydrogen bond activation and reductive elimination from azine-organometallic intermediates are replaced by a direct, Minisci9-style, radical addition. The strongly nucleophilic character of the amine-boryl radicals enables predictable and site-selective carbon–boron bond formation by targeting the azine’s most activated position, including the challenging sites adjacent to the basic nitrogen atom. This approach enables access to aromatic sites that elude current strategies based on carbon–hydrogen bond activation, and has led to borylated materials that would otherwise be difficult to prepare. We have applied this process to the introduction of amine-borane functionalities to complex and industrially relevant products. The diversification of the borylated azine products by mainstream cross-coupling technologies establishes aromatic amino-boranes as a powerful class of building blocks for chemical synthesis. Selective borylation of azines—nitrogen-containing aromatic heterocycles used in the synthesis of many pharmaceuticals—is made possible by forming a radical from an aminoborane using a photocatalyst.

Efficient Photoelectrochemical Conversion of Methane into Ethylene Glycol by WO3 Nanobar Arrays.

Abstract: Photoelectrochemical (PEC) conversion of methane (CH4 ) has been extensively explored for the production of value-added chemicals, yet remains a great challenge in high selectivity toward C2+ products Herein, we report the optimization of the reactivity of hydroxyl radicals ( OH) on WO3 via facet tuning to achieve efficient ethylene glycol production from PEC CH4 conversion A combination of materials simulation and radicals trapping test provides insight into the reactivity of OH on different facets of WO3 , showing the highest reactivity of surface-bound OH on {010} facets As such, the WO3 with the highest {010} facet ratio exhibits a superior PEC CH4 conversion efficiency, reaching an ethylene glycol production rate of 047 μmol cm-2 h-1 Based on in situ characterization, the methanol, which could be attacked by reactive OH to form hydroxymethyl radicals, is confirmed to be the main intermediate for the production of ethylene glycol Our finding is expected to provide new insight for the design of active and selective catalysts toward PEC CH4 conversion

Constructing Electron lever in Perovskite Nanocrystal to Regulate Local Electron Density for Intensive Chemodynamic Therapy.

Abstract: The local electron density of an atom is one key factor that determines its chemical properties Regulating electron density can promote the atom's reactivity and so reduce the reaction activation energy, which is highly desired in many chemical applications Herein, we report an intra-crystalline electron lever strategy, which can regulate the electron density of reaction centre atoms via manipulating ambient lattice states, for Fenton activity improvement Typically, with the assistance of ultrasound, the Mn4+ -O-Fe3+ bond in BiFe097 Mn003 O3 perovskite nanocrystals can drive valence electrons and free electrons to accumulate on Fe atoms by a polarization electric field originated from the designed lattice strain The increase of electron density significantly improves the catalytic activity of Fe, decreasing the activation energy of BiFe097 Mn003 O3 -mediated Fenton reaction by 5255 %, and increasing the OH yield by 921-fold This study provides a new way to understand the sono-Fenton chemistry, and the increased OH production enables a highly effective chemodynamic therapy

Click Nucleophilic Conjugate Additions to Activated Alkynes: Exploring Thiol-yne, Amino-yne, and Hydroxyl-yne Reactions from (Bio)Organic to Polymer Chemistry.

Abstract: The 1,4-conjugate addition reaction between activated alkynes or acetylenic Michael acceptors and nucleophiles (i.e., the nucleophilic Michael reaction) is a historically useful organic transformation. Despite its general utility, the efficiency and outcomes can vary widely and are often closely dependent upon specific reaction conditions. Nevertheless, with improvements in reaction design, including catalyst development and an expansion of the substrate scope to feature more electrophilic alkynes, many examples now present with features that are congruent with Click chemistry. Although several nucleophilic species can participate in these conjugate additions, ubiquitous nucleophiles such as thiols, amines, and alcohols are commonly employed and, consequently, among the most well developed. For many years, these conjugate additions were largely relegated to organic chemistry, but in the last few decades their use has expanded into other spheres such as bioorganic chemistry and polymer chemistry. Within these fields, they have been particularly useful for bioconjugation reactions and step-growth polymerizations, respectively, due to their excellent efficiency, orthogonality, and ambient reactivity. The reaction is expected to feature in increasingly divergent application settings as it continues to emerge as a Click reaction.

Role of the ionic environment in enhancing the activity of reacting molecules in zeolite pores.

Abstract: Tailoring the molecular environment around catalytically active sites allows for the enhancement of catalytic reactivity through a hitherto unexplored pathway. In zeolites, the presence of water creates an ionic environment via the formation of hydrated hydronium ions and the negatively charged framework aluminum tetrahedra. The high density of cation-anion pairs determined by the aluminum concentration of a zeolite induces a high local ionic strength that increases the excess chemical potential of sorbed and uncharged organic reactants. Charged transition states (carbocations for example) are stabilized, which reduces the energy barrier and leads to higher reaction rates. Using the intramolecular dehydration of cyclohexanol on H-MFI zeolites in water, we quantitatively show an enhancement of the reaction rate by the presence of high ionic strength as well as show potential limitations of this strategy.

In-Depth Characterization of Lithium-Metal Surfaces with XPS and ToF-SIMS: Toward Better Understanding of the Passivation Layer

Abstract: To significantly increase the energy density of lithium-based batteries, the use of lithium metal as an anode is an option despite all of the associated challenges. Due to its high reactivity, lith...

Origin of the improved reactivity of MoS2 single crystal by confining lattice Fe atom in peroxymonosulfate-based Fenton-like reaction

Abstract: In this study, we describe a confinement of trace amount of Fe atoms (0.66 at. ‰) in Mo lattice, via a chemical vapor transport growth of MoS2 single crystal. In the Fenton-like reaction for the degradation of atrazine, the Fe@MoS2 as catalyst to activate PMS could produce more reactive oxygen species and exhibit a rate constant (1.30 min−1) three times higher than that of pristine MoS2 (0.43 min−1). Theoretical simulation suggests that the diluted confinement of Fe atoms in Mo sites activates the inert basal plane of MoS2, creating new active sites of Mo both nearby the Fe site and afar, for the adsorption and decomposition of PMS. Our work provides a clear atomic mechanism of the improvement of the chemical reactivity of MoS2 single crystal in Fenton-like reaction via heteroatom confinement.

Regiospecific and Enantioselective Arylvinylcarbene Insertion of a C-H Bond of Aniline Derivatives Enabled by a Rh(I)-Diene Catalyst.

Abstract: Asymmetric insertion of an arylvinylcarbenoid into the C-H bond for direct enantioselective C(sp2)-H functionalization of aniline derivatives catalyzed by a rhodium(I)-diene complex was developed for the first time The reaction occurred exclusively at the uncommon vinyl terminus site with excellent E selectivity and enantioselectivities, providing various chiral γ,γ-gem-diarylsubstituted α,β-unsaturated esters with broad functional group compatibility under simple and mild conditions It provides a rare example of the asymmetric C-H insertion of arenes with selective vinylogous reactivity Synthesis applications of this protocol were featured by several versatile product transformations Systematic DFT calculations were also performed to elucidate the reaction mechanism and origin of the uncommon enantio- and regioselectivity of the Rh(I)-catalyzed C(sp2)-H functionalization reaction The measured and computed inverse deuterium kinetic isotope effect supports the C-C bond-formation step as the rate-determining step Attractive interactions between the chiral ligand and substrates were also proposed to control the enantioselectivity