Hikaru Fujita

Neutral π-electron donors stronger than dialkylamino substituents enable control of the frontier-orbital energies and optical properties of π-conjugated molecules without altering their net charge. Here we report sp2-type (ammonio)amidyl (AA) groups in which the cationic moiety is extended to pyridinium or guanidinium frameworks. These groups were installed onto a 7-(N,N-dimethylsulfamoyl)benzofurazan scaffold by nucleophilic aromatic substitution using N-aminated salts. The resulting AA-substituted benzofurazan derivatives exhibited large bathochromic shifts, with the absorption maximum reaching 518 nm for the derivative bearing the acyclic guanidinium-derived donor. By cross-calibrating benzofurazan absorption against the p-nitrobenzene-based σ+DCM substituent scale, apparent σ+DCM values of –2.44, –2.57, and –2.72 were estimated for the pyridinium-, cyclic guanidinium-, and acyclic guanidinium-derived donors, respectively. These extrapolated donor-strength assignments were independently supported by empirical oxidation onset potentials and HOMO energies calculated by density functional theory. Charge analysis and X-ray crystallography indicate that cation-charge delocalization attenuates local electrostatic stabilization of the amidyl nitrogen, thereby enhancing π donation. The enhanced donation is accompanied by increased Brønsted basicity, revealing an intrinsic coupling between donor strength and protonation equilibrium. To the best of our knowledge, these cation-delocalized AA groups are the strongest sufficiently stable, neutral nitrogen-centered π-electron donors quantified to date.

A traceless silicon-tether strategy enables the synthesis of tetraphenylammonium (Ph₄N⁺) via intramolecular arylative quaternization. The key cyclization step proceeded in 1.2% yield, a 12-fold improvement over the intermolecular approach (0.1%). Subsequent two-step desilylation furnished the target Ph₄N⁺. DFT calculations suggest β-silicon stabilization in the cyclization transition state, although competing pathways are kinetically favored. This work highlights the potential of intramolecular designs for accessing sterically hindered organic cations.

The (ammonio)amidyl (AA) groups, with the general structure of R3N+N−–, represent a new class of π-electron-donating groups for the benzene-based π-conjugated molecules. This study investigated AA-substituted p-nitrobenzenes to assess the effects of acyclic, monocyclic, and bicyclic ammonium structures in the AA groups on the π-electron-donating ability and thermal stability of the structure. Quantum chemical calculations were performed that supported the experimentally observed favorable properties of the (quinuclidinio)amidyl (QA) group.

A triazinyluronium-based dehydrative condensing reagent, 2-(4,6-dimethoxy-1,3,5-triazin-2-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (DMT-TU), has been developed. Unlike commonly used guanidinium- and uronium-based reagents, DMT-TU does not contain high-energy N-N and N-O bonds, reducing its explosivity, as suggested by differential scanning calorimetry. Using DMT-TU in the presence of ⁱPr₂EtN at room temperature, carboxylic acids and amines were effectively converted to their corresponding amides. Additionally, peptide bond formation with DMT-TU exhibited suppressed racemization ratios.

N,N '-Dimethylated (alkoxy)triazinediones (DMATs-R) with eight different O-alkyl groups (R), such as benzyl, allyl, and tert-butyl groups, were synthesized in 61-94% yield through the alkoxy exchange reaction of DMAT-Me with the corresponding alcohol. Molecular sieves were added to the reaction mixture to shift the alkoxy exchange equilibrium toward the product via the absorptive removal of the methanol formed in situ. All the DMATs-R were obtained as stable solids, in contrast to previously reported N,N '-diallyl analogs that included oil compounds. The treatment of alcohols with DMATs-R in the presence of an acid catalyst afforded the corresponding alkyl ethers in up to 92% yield. A base-labile bromoalkyl group is compatible with these reactions. The O-tert-butylation of a sterically hindered tert-alcohol with DMAT-Bu-t yielded the di-tert-butyl ether product. The DMATs-R are suitable reagents for the O-alkylative protection of alcohols under nonbasic conditions.

Ylidic nitrogen-based π-electron-donating groups, (quinuclidinio)amidyl (QA) and (1-azanorbornanio)amidyl (ANA) groups, were developed to shift the absorption of push-pull benzenes toward longer wavelengths. Changing the pyrrolidinyl group to the QA or ANA group achieved a bathochromic shift of 45-100 nm in the maximum absorption wavelength, depending on the push-pull π-conjugated system investigated (p-nitrobenzene, 1,8-naphthalimide, and an azo dye).

A triazine-based reagent, 2-hydroperoxy-4,6-dimorpholino-1,3,5-triazine (Triazox-II), was developed for alkene epoxidation. This reagent can be prepared from inexpensive starting materials (cyanuric chloride and morpholine) on a 15 mmol scale in two steps with 54% overall yield and isolated as a pure, bench-stable solid with low sensitivity to impact and friction. Triazox-II exhibited higher solubility in chlorinated solvents than the previously reported reagent Triazox. Epoxidation using Triazox-II was conducted in various solvents, with a preference for CH₂Cl₂ at 0.5 M concentration, resulting in epoxides in 83–94% yield. The reaction was conducted under mild conditions owing to the low acidity of the reaction coproduct. Received: 26 May 2024 Accepted after revision: 27 June 2024 Accepted Manuscript online: 28 June 2024 © . Thieme. All rights reserved. Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Dialkylamino groups are electrically neutral π-electron-donating groups (πEDGs) typically used in push–pull benzene π-conjugated systems to shift their absorption toward longer wavelengths. To push the upper limit of the π-electron donation imposed by the dialkylamino groups while maintaining the neutral net electric charge, we developed novel ylidic nitrogen-based πEDGs, (quinuclidinio)amidyl (QA) and (1-azanorbornio)amidyl (ANA) groups, which were introduced to push–pull benzenes through nucleophilic aromatic substitution. The bicyclic ammonium structures incorporated in these groups enhanced their thermal stability and π-electron-donating ability. The QA and ANA groups donate more π-electrons than the pyrrolidinyl group, as demonstrated by comparing their σ+DCM values and conducting a competitive electrophilic bromination experiment. Changing the pyrrolidinyl group to the QA or ANA group achieved a bathochromic shift of 45–100 nm in the maximum absorption wavelength, depending on the push–pull π-conjugated system investigated (p-nitrobenzene, 1,8-naphthalimide, and an azo dye).

Carbocations, which are positively charged highly electrophilic intermediates, are efficacious for the direct alkylation of low-reactive nucleophiles. The utilization of carbocations in S_N1 reactions relies on the activation of their precursors in the presence of a nucleophile. However, undesirable interactions between the nucleophile and the leaving group activator limit the scope of acceptable nucleophiles. Here we report a strategy to conduct S_N1 reactions involving unstable carbocations in an alternative stepwise procedure, which was demonstrated by the benzylation of various neutral nucleophiles. In the first step, carbocations were accumulated in a nucleophile-free solution in the form of carbocationoids utilizing the coordinative stabilization of triazinediones. Subsequently, the addition of these solutions in the second step enabled room-temperature alkylation without the need for acidic additives. This methodology overcomes the inherent challenges of carbocations in S_N1 reactions.