Iron-catalyzed alkene 2+2 cycloaddition reactions represent a promising stepwise pathway to effect the kinetically hindered concerted 2+2 cycloaddition. However, the fundamental reactivity paradigm ...of these reactions remains unclear. Based on high level combined CASPT2/DFT modelings, herein we reveal an unprecedented substrate-dependent two-state reactivity scenario for the key CC coupling in this iron catalysis, in which the representative substrates of mono-olefins only and mono-olefin plus 1,3-diene exhibit different reactivity paradigms. The role of the redox-active ligand is found to generate a ferric oxidation state for the metallacyclic intermediate of CC coupling, thereby rendering a thermodynamically more accessible FeIII/FeI reductive elimination process compared with the otherwise FeII/Fe0 one. The enhancement of the spin state transition efficiency between the singlet and triplet states is predicted as an alternative way to increase the CC coupling reactivity in the cross 2+2 cycloaddition reactions between mono-olefins and dienes. This work highlights the ab initio multi-reference method in describing very complicated open-shell iron catalysis.
Using high level ab initio coupled cluster calculations as reference, the performances of 15 commonly used density functionals (DFs) on activation energy calculations for typical Mo/W-mediated ...reactions have been systematically assessed for the first time in this work. The selected representative Mo/W-mediated reactions cover a wide range from enzymatic reactions to organometallic reactions, which include Mo-catalyzed aldehyde oxidation (aldehyde oxidoreductase), Mo-catalyzed dimethyl sulfoxide (DMSO) reduction (DMSO reductase), W-catalyzed acetylene hydration (acetylene hydratase), Mo/W-mediated olefin metathesis, Mo/W-mediated olefin epoxidation, W-mediated alkyne metathesis, and W-mediated C–H bond activation. Covering both Mo- and W-mediated reactions, four DFs of B2GP-PLYP, M06, B2-PLYP, and B3LYP are uniformly recommended with and without DFT empirical dispersion correction. Among these four DFs, B3LYP is notably improved in performance by DFT empirical dispersion correction. In addition to the absolute value of calculation error, if the trend of DFT results is also a consideration, B2GP-PLYP, B2-PLYP, and M06 keep better performance than other functionals tested and constitute our final recommendation of DFs for both Mo- and W-mediated reactions.
C–H bond activation/functionalization promoted by low-valent iron complexes has recently emerged as a promising approach for the utilization of earth-abundant first-row transition metals to carry out ...this difficult transformation. Herein we use extensive density functional theory and high-level ab initio coupled cluster calculations to shed light on the mechanism of these intriguing reactions. Our key mechanistic discovery for C–H arylation reactions reveals a two-state reactivity (TSR) scenario in which the low-spin Fe(II) singlet state, which is initially an excited state, crosses over the high-spin ground state and promotes C–H bond cleavage. Subsequently, aryl transmetalation occurs, followed by oxidation of Fe(II) to Fe(III) in a single-electron transfer (SET) step in which dichloroalkane serves as an oxidant, thus promoting the final C–C coupling and finalizing the C–H functionalization. Regeneration of the Fe(II) catalyst for the next round of C–H activation involves SET oxidation of the Fe(I) species generated after the C–C bond coupling. The ligand sphere of iron is found to play a crucial role in the TSR mechanism by stabilization of the reactive low-spin state that mediates the C–H activation. This is the first time that the successful TSR concept conceived for high-valent iron chemistry is shown to successfully rationalize the reactivity for a reaction promoted by low-valent iron complexes. A comparative study involving other divalent middle and late first-row transition metals implicates iron as the optimum metal in this TSR mechanism for C–H activation. It is predicted that stabilization of low-spin Mn(II) using an appropriate ligand sphere should produce another promising candidate for efficient C–H bond activation. This new TSR scenario therefore emerges as a new strategy for using low-valent first-row transition metals for C–H activation reactions.
The iron-catalyzed hydroarylation of allenes was accomplished by weak phenone assistance. The C–H activation proceeded with excellent efficacy and high ortho-regioselectivity in proximity to the ...weakly coordinating carbonyl group for a range of substituted phenones and allenes. Detailed mechanistic studies, including the isolation of key intermediates, the structural characterization of an iron–metallacycle, and kinetic analysis, allowed the sound elucidation of a plausible catalytic working mode. This mechanistic rationale is supported by detailed computational density functional theory studies, which fully address multi-spin-state reactivity. Furthermore, in operando nuclear magnetic resonance monitoring of the catalytic reaction provided detailed insights into the mode of action of the iron-catalyzed C–H alkylation with allenes.
The stereoselective hydrogenation of alkynes to alkenes is an extremely useful transformation in synthetic chemistry. Despite numerous reports for the synthesis of Z‐alkenes, the hydrogenation of ...alkynes to give E‐alkenes is still not well resolved. In particular, selective preparation of both Z‐ and E‐alkenes by the same catalytic hydrogenation system using molecular H2 has rarely been reported. In this paper, a novel strategy of using simple alkenes as promoters for the HB(C6F5)2‐catalyzed metal‐free hydrogenation of alkynes was adopted. Significantly, both Z‐ and E‐alkenes can be furnished by hydrogenation with molecular H2 in high yields with excellent stereoselectivities. Further experimental and theoretical mechanistic studies suggest that interactions between H and F atoms of the alkene promoter, borane intermediate, and H2 play an essential role in promoting the hydrogenolysis reaction.
With a little help from an alkene! A HB(C6F5)2‐catalyzed metal‐free hydrogenation of alkynes, promoted by an alkene through HF interactions, was successfully achieved. Z‐ and E‐alkenes can be accessed respectively in high yields with excellent stereoselectivities (see scheme; TS=transition state).
Detailed density functional theory calculations provide valuable insight into reactivity-controlling factors in transition metal-catalyzed C–H activation by carboxylate assistance. The ...chelation-assisted activation of a variety of arenes by 3d and 4d transition metal complexes was analyzed by means of bond order analysis through density functional theory (DFT) calculations as well as energy decomposition analysis through DLPNO–CCSD(T) calculations, thereby providing in-depth information on distinct electronic influences on the key C–H activation transition state and demonstrating a preferred activation through a base-assisted internal electrophilic substitution (BIES) rather than a concerted metalation-deprotonation (CMD) pathway.
Abstract
Clusteroluminogens refer to some non-conjugated molecules that show visible light and unique electronic properties with through-space interactions due to the formation of aggregates. ...Although mature and systematic theories of molecular photophysics have been developed to study conventional conjugated chromophores, it is still challenging to endow clusteroluminogens with designed photophysical properties by manipulating through-space interactions. Herein, three clusteroluminogens with non-conjugated donor-acceptor structures and different halide substituents are designed and synthesized. These compounds show multiple emissions and even single-molecule white-light emission in the crystalline state. The intensity ratio of these emissions is easily manipulated by changing the halide atom and excitation wavelength. Experimental and theoretical results successfully disclose the electronic nature of these multiple emissions: through-space conjugation for short-wavelength fluorescence, through-space charge transfer based on secondary through-space interactions for long-wavelength fluorescence, and room-temperature phosphorescence. The introduction of secondary through-space interactions to clusteroluminogens not only enriches their varieties of photophysical properties but also inspires the establishment of novel aggregate photophysics for clusteroluminescence.
Cyanobacterial aldehyde-deformylating oxygenase (cADO) is a nonheme diiron enzyme that catalyzes the conversion of aldehyde to alk(a/e)ne, an important transformation in biofuel research. In this ...work, we report a highly desired computational study for probing the mechanism of cADO. By combining our QM/MM results with the available 57Fe Mössbauer spectroscopic data, the gained detailed structural information suggests construction of asymmetry from the symmetric diiron cofactor in an aldehyde substrate and O2 activation. His160, one of the two iron-coordinate histidine residues in cADO, plays a pivotal role in this asymmetric aldehyde activation process by unprecedented reversible dissociation from the diiron cofactor, a behavior unknown in any other nonheme dinuclear or mononuclear enzymes. The revealed intrinsically asymmetric interactions of the substrate/O2 with the symmetric cofactor in cADO are inspirational for exploring diiron subsite resolution in other nonheme diiron enzymes.
In light of the current energy requirements, the conversion of CO2 and N2 into useful C–N bond-containing products under mild conditions has become an area of intense research. However, the inert ...nature of N2 and CO2 renders their coupling extremely challenging. Herein, nitrogen and carbon atoms originating from N2 and CO2, respectively, are fixed sequentially by NbH2 – anions in the gas phase at room temperature. Isocyanate and NbO2CN– anions were formed under thermal collision conditions, thus achieving the formation of new C–N bonds directly from simple N2 and CO2. The anion structures and reaction details were studied by mass spectrometry, photoelectron spectroscopy, and quantum chemical calculations. A novel N2 activation mode (metal–ligand activation, MLA) and a related mechanism for constructing C–N bonds mediated by a single non-noble metal atom are proposed. In this MLA mode, the C atom originating from CO2 serves as an electron reservoir to accept and donate electrons.
Microfluidics has received extensive attention due to its ability to rapidly prepare a large number of microdroplets with controlled sizes and defined morphologies. In addition to having large ...surface areas and controllable confinement environments, these prepared microdroplets can be used as analytical detection devices to screen and optimize various kinetic parameters. This review summarizes recent advances in the microfluidic control of droplet‐based catalytic reactions and discusses the role of these droplets in both homogeneous and heterogeneous catalyzes and in the catalysis of macromolecular biological enzymes in water‐in‐oil and oil‐in‐oil environments. Additionally, the existing problems and future development directions of droplets in catalysis are highlighted to promote the development of catalytic reactions in droplet media and provide guidance for the high‐throughput screening of catalysts and the directed evolution of biological enzymes.
Micro/nanofluidics provides a powerful method for controlling the size and shape of synthesized droplets. Thus, it opens a pathway for facilitating microscale chemistry. Here, we summarize recent developments in catalytic reactions occurring in droplets generated by micro/nanofluidics, and highlight the application of droplets in catalytic processes.
