Monthly Archives: October 2023

Synthesis of Partially Fluorinated Alkyl Triflates by Electrochemical Fluorination (Simons Process)

Posted on 11 October 2023 by

The electrosynthesis of partially fluorinated alkyl triflates and nonaflates by using electrochemical fluorination (ECF) has been developed; it provides access to CF₃SO₂OCH₂F and CF₃SO₂OCHF₂ as well as their deuterated analogs. Selected ethyl triflates and nonaflates are also accessible through ECF.

Abstract

A scalable straightforward synthesis of monofluoro- and difluoromethyl triflate CF₃SO₂OCH₂F (M^H2F ) and CF₃SO₂OCHF₂ (M^HF2 ) through electrochemical fluorination (ECF, Simons process) of methyl triflate M^H3 in anhydrous hydrogen fluoride at nickel anodes is presented. The ECF method is also feasible for the preparation of the deuterated analogues CF₃SO₂OCD₂F (M^D2F ) and CF₃SO₂OCDF₂ (M^D2F ). Surprisingly, no H/D exchange occurs during ECF of CF₃SO₂OCD₃ (M^D3 ); this provides further evidence for a NiF₃/NiF₄-mediated ECF mechanism. The ECF of selected partially fluorinated ethyl triflates is described, and electrochemical fluorination of CF₃SO₂OCH₂CF₃ (E^H2F3 ) leads to the until now unknown chiral CF₃SO₂OCHFCF₃ (E^HFF3 ). The analogous fluoromethyl and fluoroethyl nonaflates are also accessible by ECF. This study contains detailed spectroscopic, structural, and thermal data on (fluoro)methyl and fluoro(ethyl) triflates.

Synthesis of Photochromic Phosphines by Pd‐Catalyzed Annulation Reaction of Alkynes Bearing Phosphinyl Substituent with a Silacyclopropene

Posted on 11 October 2023 by

A methodological approach for the preparation of unconventional DTE-based photochromic phosphines is reported. It relies on the Pd-catalyzed annulation reaction of alkynyl phosphines in the presence of a silirene.

Abstract

The synthesis of phosphines with light controlled basicity is presented in this study. A methodological approach for the preparation of these unconventional photochromic phosphines based on a dithienylethene organic moiety is reported. It relies on the palladium-catalyzed annulation of alkynyl phosphines in the presence of a 2,3-Dithienylsilacyclopropene. Accordingly, a diphenyphosphino moiety is connected to the organic photochrome thanks to different linkers. Their influence on the photochromism and on the phosphinyl group basicity is studied and evaluated based on experimental an NMR descriptor as well as DFT calculations.

Tailoring the CO2 Hydrogenation Performance of Fe‐Based Catalyst via Unique Confinement Effect of the Carbon Shell

Posted on 11 October 2023 by

Core-shell structured Fe-based catalyst Na−Fe₃O₄@C with confinement effect delivered higher productivity of light olefins compared with the naked Na−Fe₂O₃ counterpart. Multiple characterization and DFT calculation verified that the timely carbon chain-growth termination, as well as unique CO₂/H₂ ratio and electronic property on the catalytic interface played vital roles in boosting light olefins synthesis.

Abstract

Even though Fe-based catalysts have been widely employed for CO₂ hydrogenation into hydrocarbons, oxygenates, liquid fuels, etc., the precise regulation of their physicochemical properties is needed to enhance the catalytic performance. Herein, under the guidance of the traditional concept in heterogeneous catalysis-confinement effect, a core-shell structured catalyst Na−Fe₃O₄@C is constructed to boost the CO₂ hydrogenation performance. Benefiting from the carbon-chain growth limitation, tailorable H₂/CO₂ ratio on the catalytic interface, and unique electronic property that all endowed by the confinement effect, the selectivity and space-time yield of light olefins (C₂ ⁼−C₄ ⁼) are as high as 47.4 % and 15.9 g mol_Fe ⁻¹ h⁻¹, respectively, which are all notably higher than that from the shell-less counterpart. The function mechanism of the confinement effect in Fe-based catalysts are clarified in detail by multiple characterization and density functional theory (DFT). This work may offer a new prospect for the rational design of CO₂ hydrogenation catalyst.

Metal‐Free Electrochemical Reduction of Disulfides in an Undivided Cell under Mass Transfer Control

Posted on 11 October 2023 by

Electrochemical reductive cleavage of disulfides has been accomplished without the need for sacrificial metal anodes or a divided cell. Excellent conversion and selectivity toward the corresponding thiol can be achieved by tuning the current density at the electrodes. Very high current density at the anode creates a mass transfer barrier that impedes re-oxidation of the thiol generated at the cathode.

Abstract

Electroorganic synthesis is generally considered to be a green alternative to conventional redox reactions. Electrochemical reductions, however, are less advantageous in terms of sustainability, as sacrificial metal anodes are often employed. Divided cell operation avoids contact of the reduction products with the anode and allows for convenient solvent oxidation, enabling metal free greener electrochemical reductions. However, the ion exchange membranes required for divided cell operation on a commercial scale are not amenable to organic solvents, which hinders their applicability. Herein, we demonstrate that electrochemical reduction of oxidatively sensitive compounds can be carried out in an undivided cell without sacrificial metal anodes by controlling the mass transport to a small surface area electrode. The concept is showcased by an electrochemical method for the reductive cleavage of aryl disulfides. Fine tuning of the electrode surface area and current density has enabled the preparation of a wide variety of thiols without formation of any oxidation side products. This strategy is anticipated to encourage further research on greener, metal free electrochemical reductions.

Reductive Activation of Aryl Chlorides by Tuning the Radical Cation Properties of N‐Phenylphenothiazines as Organophotoredox Catalysts

Posted on 11 October 2023 by

Two photons are better than one: Intramolecular charge transfer in the intermediate radical cation of the N-phenylphenothiazines as photocatalysts improves the yields for borylation and phosphonylation of aryl chlorides.

Abstract

Aryl chlorides as substrates for arylations present a particular challenge for photoredox catalytic activation due to their strong C(sp²)−Cl bond and their strong reduction potential. Electron-rich N-phenylphenothiazines, as organophotoredox catalysts, are capable of cleaving aryl chlorides simply by photoinduced electron transfer without the need for an additional electrochemical activation setup or any other advanced photocatalysis technique. Due to the extremely strong reduction potential in the excited state of the N-phenylphenothiazines the substrate scope is high and includes aryl chlorides both with electron-withdrawing and electron-donating substituents. We evidence this reactivity for photocatalytic borylations and phosphonylations. Advanced time-resolved transient absorption spectroscopy in combination with electrochemistry was the key to elucidating and comparing the unusual photophysical properties not only of the N-phenylphenothiazines, but also of their cation radicals as the central intermediates in the photocatalytic cycle. The revealed photophysics allowed the excited-state and radical-cation properties to be fine-tuned by the molecular design of the N-phenylphenothiazines; this improved the photocatalytic activity.

Water‐Soluble Arylazoisoxazole Photoswitches

Posted on 11 October 2023 by

Water-soluble arylazoisoxazoles are introduced as an additional scaffold to the existing arylazoheteroarenes and open the door for tailor-made properties in molecular and materials chemistry. They were found to combine excellent photochromic behavior with long thermal life times. Supramolecular aggregates were formed by host–guest interaction and gelation.

Abstract

Azoheteroarenes are emerging as powerful alternatives to azobenzene molecular photoswitches. In this study, water-soluble arylazoisoxazole photoswitches are introduced. UV/vis and NMR spectroscopy revealed moderate to very good photostationary states and reversible photoisomerization between the E- and Z-isomers over multiple cycles with minimal photobleaching. Several arylazoisoxazoles form host–guest complexes with β- and γ-cyclodextrin with significant differences in binding constants for each photoisomer as shown by isothermal titration calorimetry and NMR experiments, indicating their potential for photoresponsive host–guest chemistry in water. One carboxylic acid functionalized arylazoisoxazole can act as a hydrogelator, allowing gel properties to be manipulated reversibly with light. The hydrogel was characterized by rheological experiments, atom force microscopy and transmission electron microscopy. These results demonstrate that arylazoisoxazoles can find applications as molecular photoswitches in aqueous media.

Improved Catalytic Performance toward Selective Oxidation of Benzyl Alcohols Originated from New Open‐Framework Copper Molybdovanadate with a Unique V/Mo Ratio

Posted on 11 October 2023 by

A copper molybdovanadate with mixed-valent vanadium (V⁴⁺/V⁵⁺=4/3) and molybdenum (Mo⁵⁺/Mo⁶⁺=8/2) cations exhibits improved catalytic activity (conv.: 96.8 %) compared with the complex (Cpyr)₅PV₂Mo₅W₅O₄₀ [conv.: 88.51 %, Cpyr=(C₁₆H₃₂C₅H₄N)⁺)] and could serve as a highly efficient heterogeneous catalyst in the selective oxidation of benzyl alcohols to benzaldehydes.

Abstract

A new organic-inorganic hybrid open-framework molybdovanadate with mixed-valences of vanadium (V⁴⁺/V⁵⁺=4/3) and molybdenum (Mo⁵⁺/Mo⁶⁺=8/2) cations has been synthesized. The complex possesses the unique V/Mo ratio (7/10), fascinating 8-C topological network and 1D 4-MR channels (7.793 Å×6.699 Å). Importantly, its catalytic activities for the selective oxidation of benzyl alcohol to benzaldehyde (oxidant: H₂O₂, 30 wt %) have been well evaluated. The results indicated that it exhibited improved catalytic activities (conv.: 96.8 %) compared with the catalyst (Cpyr)₅PV₂Mo₅W₅O₄₀ [conv.: 88.51 %, Cpyr=(C₁₆H₃₂C₅H₄N)⁺)], high recyclability and structural stability. Moreover, the conversions and selectivities (conv.: 82.4–92.5 %; sele.: 91.5–95.7 %) of the substrates containing electron donating groups (−OH, −CH₃, −OCH₃ and −Cl) were significantly higher than those of the substrate containing electron withdrawing group (−NO₂) (conv. 67.4 %; sele.: 80.8 %). This is due to the fact that the −NO₂ with a large Hammett substituent constant is not conducive to the generation of transition state products. The studies revealed the complex could act as a highly efficient heterogeneous catalyst in selective oxidation of benzyl alcohols.

Sulfone/Carbonyl‐Based Donor‐Acceptor Fluorescent Dyes: Synthesis, Structures, Photophysical Properties and Cell Imaging

Posted on 11 October 2023 by

Sulfone or carbonyl-based acceptor units were used to construct donor-acceptor conjugate organic fluorophores. Both TPA-SO and TPA-CO have solvent polarity-dependent photophysical properties for strong intramolecular charge transfer. As sulfone has weak intersystem crossing ability, TPA-SO has high fluorescence efficiency in solution and nanoparticles, and is a promising fluorescence dye.

Abstract

Electron-accepting units play vital roles in constructing donor-acceptor (D-A) conjugated organic optoelectronic materials; the electronic structures and functions of the acceptors need to be carefully unveiled to controllably tailor the optoelectronic properties. We have synthesized two D-A conjugated organic fluorophores, TPA-SO and TPA-CO, with similar molecular skeletons based on sulfone- or carbonyl-containing polycyclic aromatic acceptors. Both TPA-SO and TPA-CO display obvious solvent polarity-dependent photophysical properties and large Stokes shift of over 100 nm for strong intramolecular charge transfer processes. Experimental evidence indicates that the sulfone group in TPA-SO merely serves as a strong electron-withdrawing unit. TPA-SO shows yellowish-green emission with a peak at 542 nm and an absolute photoluminescence quantum yield (PLQY) of 98 % in solution, whereas the carbonyl group in TPA-CO can act as both an electron-withdrawing unit and spin transition convertor, so TPA-CO displays red emission with a low absolute PLQY of 0.32 % in solution. Impressively, upon going from solution to aggregate state, TPA-SO nanoparticles keep a high PLQY of 9.5 % and moderate biocompatibility, thus they are good nano-agents for cellular fluorescence imaging. The results reveal that the inherent acceptor characteristic acts as a crucial effect in the photophysical properties and applications of the organic fluorophores.

Two‐Dimensional Living Supramolecular Polymerization: Improvement in Edge Roughness of Supramolecular Nanosheets by Using a Dummy Monomer

Posted on 11 October 2023 by

Controlled/living supramolecular polymerization has enabled the synthesis of well-defined nanostructures which are otherwise inaccessible under thermodynamically controlled, spontaneous self-assembly process. Yet, there is still room for improvement; here we show two-dimensional living supramolecular polymerization improved using an additive, which permits the synthesis of supramolecular nanosheets of better quality.

Abstract

Supramolecular polymers are formed through nucleation (i. e., initiation) and polymerization processes, and kinetic control over the nucleation process has recently led to the realization of living supramolecular polymerization. Changing the viewpoint, herein we focus on controlling the polymerization process, which we expect to pave the way to further developments in controlled supramolecular polymerization. In our previous study, two-dimensional living supramolecular polymerization was used to produce supramolecular nanosheets with a controlled area; however, these had rough edges. In this study, the growth of the nanosheets was controlled by using a ‘dummy’ monomer to produce supramolecular nanosheets with smoothed edges.

Germanium(II) Dithiolene Complexes

Posted on 11 October 2023 by

The chemistry at the germylene and dithiolene interface has rarely been explored. Here, we report a series of Lewis base-coordinated germanium(II) dithiolene complexes. Controlled hydrolysis of the carbene-coordinated germanium(II) dithiolene complex affords a dianionic bis-dithiolene-based germanium(II) species.

Abstract

The 1 : 2 reaction of the imidazole-based dithiolate (2) with GeCl₂ • dioxane in THF/TMEDA gives 3, a TMEDA-complexed dithiolene-based germylene. Compound 3 is converted to monothiolate-complexed (5) and N-heterocyclic carbene-complexed (7) germanium(II) dithiolene complexes via Lewis base ligand exchange. A bis-dithiolene-based germylene (8), involving a 3c–4e S-Ge-S bond, has also been synthesized through controlled hydrolysis of 7. The bonding nature of 3, 5, and 8 was investigated by both experimental and theoretical methods.