9-Ethynylfluoroenyl Radicals: Regioselective Dimerization and Post Ring-Cyclization Reactions

Synthesis, properties and chemical modification of a persistent triisopropylsilylethynyl substituted tri(9-anthryl)methyl radical

In studies aimed at developing new organic spin materials, we prepared a triisopropylsilylethynyl substituted tri(9-anthryl)methyl (TAntM) radical. The TIPS-ethynyl group in this radical effectively suppresses its reactivity, resulting in extremely high stability in air for at least one month. Chemical modification of the radical using [4+2] Diels-Alder reaction proceeds even at room temperature. Because harsh conditions and metal-catalyzed reactions are not required, this post-modification strategy should be highly versatile for use in constructing unique spin-labelled molecules.

Metalloradical Activation of In Situ-Generated α-Alkynyldiazomethanes for Asymmetric Radical Cyclopropanation of Alkenes

α-Alkynyldiazomethanes, generated in situ from the corresponding sulfonyl hydrazones in the presence of a base, can serve as effective metalloradicophiles in Co(II)-based metalloradical catalysis (MRC) for asymmetric cyclopropanation of alkenes. With D₂-symmetric chiral amidoporphyrin 2,6-DiMeO-QingPhyrin as the optimal supporting ligand, the Co(II)-based metalloradical system can efficiently activate different α-alkynyldiazomethanes at room temperature for highly asymmetric cyclopropanation of a broad range of alkenes. This catalytic radical process provides a general synthetic tool for stereoselective construction of alkynyl cyclopropanes in high yields with high both diastereoselectivity and enantioselectivity. Combined computational and experimental studies offer several lines of evidence in support of the underlying stepwise radical mechanism for the Co(II)-catalyzed olefin cyclopropanation involving a unique α-metalloradical intermediate that is associated with two resonance forms of α-Co(III)-propargyl radical and γ-Co(III)-allenyl radical. The resulting enantioenriched alkynyl cyclopropanes, as showcased with several stereospecific transformations, may serve as valuable chiral building blocks for stereoselective organic synthesis.

Singlet Biradical Versus Triplet Biradical/Zwitterion Characteristics in Isomers of C6 -C5 -C6 -C7 -C6 -Fused Pentacyclic Aromatic Hydrocarbons Revealed through Reactivity Patterns

Among various polycyclic aromatic hydrocarbons, C₆ -C₅ -C₆ -C₇ -C₆ fused pentacyclic aromatic hydrocarbons have the unique potential to adopt quinonoid, zwitterion, singlet, or triplet biradical electronic configurations. Two such hybrid structures between pentacene and azulene were synthesized and their ground state electronic configurations were deduced from the reactivity patterns they exhibit respectively. Compound 6, where the radicaloid carbons are linked through a para-phenylene, forms a head-to-head dimer like a singlet biradical. In contrast, isomer 7, where the para-linkage was switched to meta, reacts readily with oxygen which resembles the reactivity of a triplet state. The oxidized intermediate(s) then undergoes rearrangement to furnish the C₆ -C₅ -C₆ -C₆ -C₆ ring contraction product 13. Cation 14, the protonated form of 7, was synthesized, which implies 7 also reacts like a zwitterion. It was revealed the oxidative rearrangement takes place even with mesityl dibenzotropylium cation despite its perceived aromaticity. DFT calculations confirm the most stable forms of 6 and 7 are singlet and triplet diradical, which is consistent with the observed reactivity of respective molecules.

Photochemical Synthesis of 1,4-Dicarbonyl Bifluorene Compounds via Oxidative Radical Coupling Using TEMPO as the Oxygen Atom Donor

A visible-light-induced metal-free synthesis of 1,4-dicarbonyl compounds from alkyne-containing aryl iodides via photochemical C-I bond cleavage, intramolecular cyclization, oxidation, and intermolecular radical coupling sequence is reported. TEMPO was employed as the oxygen atom donor in this transformation. This protocol provided a new strategy for the synthesis of 1,4-dicarbonyl bifluorene compounds.

Modulating the ground state, stability and charge transport in OFETs of biradicaloid hexahydro-diindenopyrene derivatives and a proposed method to estimate the biradical character

Biradicaloid compounds with an open-shell ground state have been the subject of intense research in the past decade. Although diindenoacenes are one of the most developed families, only a few examples have been reported as active layers in organic field-effect transistors (OFETs) with a charge mobility of around 10-3 cm2 V-1 s-1 due to a steric disadvantage of the mesityl group to kinetically stabilize compounds. Herein, we disclose our efforts to improve the charge transport of the diindenoacene family based on hexahydro-diindenopyrene (HDIP) derivatives with different annelation modes for which the most reactive position has been functionalized with (triisopropylsilyl)ethynyl (TIPS) groups. All the HDIP derivatives show remarkably higher stability than that of TIPS-pentacene, enduring for 2 days to more than 30 days, which depends on the oxidation potential, the contribution of the singlet biradical form in the ground state and the annelation mode. The annelation mode affects not only the band gap and the biradical character (y 0) but also the value of the singlet-triplet energy gap (ΔE S-T) that does not follow the reverse trend of y 0. A method based on comparison between experimental and theoretical bond lengths has been disclosed to estimate y 0 and shows that y 0 computed at the projected unrestricted Hartree-Fock (PUHF) level is the most relevant among those reported by all other methods. Thanks to their high stability, thin-film OFETs were successfully fabricated. Well balanced ambipolar transport was obtained in the order of 10-3 cm2 V-1 s-1 in the bottom-gate/top-contact configuration, and unipolar transport in the top-gate/bottom-contact configuration was obtained in the order of 10-1 cm2 V-1 s-1 which is the highest value obtained for biradical compounds with a diindenoacene skeleton.

Ring-expansion approach towards extended asymmetric benzopentafulvalenes: overcrowded olefinic structure and chain length-dependent properties

A strain energy-induced ring-expansion is described to construct novel asymmetric benzopentafulvalenes precisely, which show chain length-dependent physical properties and air-stable ambipolar carrier transport.

Selectfluor-Mediated Stereoselective [1 + 1 + 4 + 4] Dimerization of Styrylnaphthols

Stereoselective [1 + 1 + 4 + 4] dimerization of 1-styrylnaphthols has been developed by using Selectfluor as the oxidant for the first time. The reaction was compatible with various functional groups, giving a class of ethanodinaphtho[b,f][1,5]dioxocines with novel 3D skeletons. DFT calculations indicate that this method merges an intriguing stereoselective intermolecular 1 + 1 radical coupling to construct a bridged C-C bond and then an intramolecular [4 + 4] formal cycloaddition of the in situ generated o-quinone methide intermediate.

MoS2 Quantum Dot Growth Induced by S Vacancies in a ZnIn2S4 Monolayer: Atomic-Level Heterostructure for Photocatalytic Hydrogen Production

It is highly demanded to steer the charge flow in photocatalysts for efficient photocatalytic hydrogen reactions (PHRs). In this study, we developed a smart strategy to position MoS₂ quantum dots (QDs) at the S vacancies on a Zn facet in monolayered ZnIn₂S₄ (Vs-M-ZnIn₂S₄) to craft a two-dimensional (2D) atomic-level heterostructure (MoS₂QDs@Vs-M-ZnIn₂S₄). The electronic structure calculations indicated that the positive charge density of the Zn atom around the sulfur vacancy (Vs) was more intensive than other Zn atoms. The Vs confined in monolayered ZnIn₂S₄ established an important link between the electronic manipulation and activities of ZnIn₂S₄. The Vs acted as electron traps, prevented vertical transmission of electrons, and enriched electrons onto the Zn facet. The Vs-induced atomic-level heterostructure sewed up vacancy structures of Vs-M-ZnIn₂S₄, resulting in a highly efficient interface with low edge contact resistance. Photogenerated electrons could quickly migrate to MoS₂QDs through the intimate Zn-S bond interfaces. As a result, MoS₂QDs@Vs-M-ZnIn₂S₄ showed a high PHR activity of 6.884 mmol g^-1 h^-1, which was 11 times higher than 0.623 mmol g^-1 h^-1 for bulk ZnIn₂S₄, and the apparent quantum efficiency reached as high as 63.87% (420 nm). This work provides a prototype material for looking into the role of vacancies between electronic structures and activities in 2D photocatalytic materials and gives insights into PHR systems at the atomic level.

Toward helical-shaped diradicaloids: cyclobutenyl o-quinodimethane-bridged indeno[1,2-b]fluorenes

A new class of helical-shaped singlet biradicals disclosed featured properties and unusual half-field ΔM_s = 2 transitions.

Bay- and Ortho-Octasubstituted Perylenes

A key intermediate compound, 2,5,8,11-tetrabromo-1,6,7,12-tetrabutoxyperylene (Per-4Br), was synthesized from 3,6-dibromo-2,7-dioxylnaphthalene via simple regioselective oxidative radical-radical coupling, followed by reduction and nucleophilic substitution. Various bay- and ortho-octasubstituted perylenes containing cyano, methoxy and aryl groups were then obtained by nucleophilic substitution or Pd-catalyzed coupling reactions. X-ray crystallographic analyses reveal that these new perylene molecules process a twisted structure due to steric congestion at the bay-regions and there is no obvious intermolecular π-π interaction. As a result, they exhibit moderate fluorescence quantum yields even in solid state. Therefore, Per-4Br can serve as a versatile building block for various functional perylene dyes with tunable optoelectronic property.

A facile approach toward 1,2-diazabenzo[ghi]perylene derivatives: structures and electronic properties

Novel pyridazine-fused perylenes are synthesized via simple intermolecular oxidative coupling followed by a condensation strategy, in which attractive structural and electronic features are observed.