Solid-State Generation of Diarylisonaphthofuran and Its Mechanochemical Diels-Alder Reaction with Epoxynaphthalene.

Invited for the cover of this issue are Toshiyuki Hamura and co-workers at Kwansei Gakuin University. The image depicts solid-state generation of diarylisonaphthofuran from 1,3-diaryl-1,3-dihydronaphthofuranol and its mechanochemical Diels-Alder reaction with epoxynaphthalene. Read the full text of the article at 10.1002/chem.202302660.

Solid-State Generation of Diarylisonaphthofuran and Its Mechanochemical Diels-Alder Reaction with Epoxynaphthalene.

A solid-state method was developed for generating diarylisonaphthofurans from 1,3-diaryl-1,3-dihydronaphthofuranols. The generated reactive molecules were stable in the solid state and could be stored without any extra precautions. X-ray diffraction analysis revealed a typical quinoidal structure. Furthermore, the mechanochemical Diels-Alder reaction of 1,3-diarylisonaphthofurans with epoxynaphthalenes afforded synthetically attractive diepoxypentacenes.

Intramolecular Singlet Fission in Pentacene Oligomers via an Intermediate State.

Intramolecular singlet fission (iSF) is an efficient strategy of multiexciton generation via a singlet exciton splitting into a correlated triplet pair in one organic molecule with more than two chromophores. Propeller-shaped iptycene-linked triisopropylsilyl(TIPS)-ethynyl functionalized pentacene oligomers (pent-monomer, pent-dimer, and pent-trimer) were synthesized, and the iSF dynamics of pent-dimer and -trimer were monitored by a visible-near-IR transient absorption (TA) spectroscopy. Quantum yields of the triplet pair, ∼80%, of both estimated by near-IR TA spectral analysis are in good agreement with the results of global analysis and triplet sensitization experiments. The iSF rate of pent-trimer is slightly faster than that of pent-dimer even with one more chromophore site. The unexpectedly weak difference indicates the existence of an intermediate process to realize iSF. The intermediate process might be determined by through-bond electronic coupling of the homoconjugation bridge in the pentacene oligomers. Our results suggest the importance of the rigid bridge to the fast iSF rate and the elongated lifetime of the correlated triplet pair in pentacene oligomers.

Thermodynamically Stable o-Quinodimethane: Synthesis, Structure, and Reactivity.

Thermal isomerization of cyclobutaphenanthrene to o-quinodimethane was investigated. Sterically congested substituents or electron-donating substituents on the four-membered ring promoted the ring-opening, affording o-quinodimethane in a relatively stable form. Isolation of the newly prepared o-quinodimethane allowed its structural elucidation and investigation of its potential reactivities. Dual [4+2] cycloaddition of an aryne and o-quinodimethane afforded tetrabenzopentacene, demonstrating the synthetic application of the isolated compound.

Isoacenofuran: a novel quinoidal building block for efficient access to high-ordered polyacene derivatives.

Herein, a simple and practical method for generating isoacenofuran, a new π-extended quinoidal building block, was developed. A three-step protocol involving double nucleophilic additions of alkynyllithiums to acene-2,3-dicarbaldehyde, mono-oxidation, and acid-promoted cyclization enables the generation of the target molecule, which is trapped by a dienophile to produce highly condensed acenequinones. Further transformations by double nucleophilic additions of alkynyllithium to hexacenequinone, followed by reductive aromatization, produce tetraalkynylhexacenes with a remarkably higher stability than that of the previously reported substituted hexacenes.

Intramolecular benzoallene-alkyne cycloaddition initiated by site-selective SN2' reaction of epoxytetracene en route to π-extended pyracylene.

A hydrogen halide promoted cascade reaction of epoxytetracene to afford halo-benzoindenotetracene including a benzoallene intermediate was developed. The remaining two alkynyl groups in benzoindenotetracene were further reacted with norbornadiene or arylamine through transition metal-catalyzed cyclization to give π-extended pyracylene derivatives.

A new synthetic route to 5,6,11,12-tetraarylethynyltetracenes.

A new synthetic route to 5,6,11,12-tetrakis(arylethynyl)tetracenes, π-extended rubrenes, was developed via [4 + 2] cycloadditions of dialkynylisobenzofuran and 1,4-naphthoquinone. Introduction of arylethynyl groups by double nucleophilic additions to tetracenequinone gave sterically congested (arylethynyl)tetracenes after reductive aromatization. The photophysical properties of the newly prepared π-conjugated molecules are also evaluated.

Synthesis and Evaluation of a 1,3a,6a-Triazapentalene (TAP)-Bonded System.

A method of synthesizing a directly connected 1,3a,6a-triazapentalene (TAP) ring system as a linearly bonded aromatic system with a planar form was established. Various TAP-dimers and a 2-alkyl-TAP-trimer were synthesized and their fluorescence properties were evaluated. Although the direct connection of the TAP ring with other TAP rings did not affect the fluorescence properties in diluted solvent, TAP-dimers showed unique fluorescence properties derived from the aggregation state under highly concentrated conditions. In particular, TAP-dimer 5 f showed aggregation-induced emission in highly concentrated solution, and 5 b showed typical mechanochromic fluorescence in the solid state despite their compact molecular size.

Didehydroisobenzofuran: A New Reactive Intermediate for Construction of Isoacenofuran.

An efficient generation method of didehydroisobenzofuran, a new heteroaryne species, was developed by bromine/lithium exchange of the dibromoisobenzofuran. The reactive intermediate, thus generated, was trapped by appropriate arynophile to give the [2+2], [2+3], and [2+4] cycloadducts, respectively. Moreover, the reaction could be applied to the syntheses of isoanthracenofurans (anthra[2,3-c]furans), a new class of heteroacenes, with isoelectoronic structure to the corresponding acenoheteroles (anthra[2,3-b]furans).

Tetrakis(phenylethynyl)tetracene: A New π-Extended Rubrene Derivative.

An efficient synthetic route to 5,6,11,12-tetrakis(arylethynyl)tetracenes, new π-extended rubrene derivatives, was developed by means of [2+4] cycloaddition of dialkynylnaphthalyne and dialkynylisobenzofuran. Importantly, two alkynyl groups introduced into the aryne exerts a significant effect in lowering the LUMO energy, allowing practical access to sterically overcrowded polycyclic structures through an efficient HOMO-LUMO interaction. Study on the potential reactivity inherent in the peri-ethynyl-substituted tetracenes revealed several interesting reactivities. X-ray analysis of these new π-extended derivatives showed distorted structures to reduce steric repulsion due to the existence of the substituents at the peri-positions.

Star-Shaped Polycyclic Aromatic Ketones via 3-Fold Cycloadditions of Isobenzofuran Trimer Equivalent.

Three-directional annulations of isobenzofuran trimer equivalent are developed. Importantly, the successive cycloadditions could be controlled under suitable conditions, selectively affording the dual or triple cycloadduct, which leads to the alternative preparation of the symmetrical and unsymmetrical star-shaped polycyclic aromatic ketones. 1H NMR analysis of the star-shaped aromatic ketone indicated the π-π interactions through the aggregation in solution.

Selective Halogen-Lithium Exchange of 1,2-Dihaloarenes for Successive [2+4] Cycloadditions of Arynes and Isobenzofurans.

Successive [2+4] cycloadditions of arynes and isobenzofurans by site-selective halogen-lithium exchange of 1,2-dihaloarenes were developed, allowing the rapid construction of polycyclic compounds which serve as a useful synthetic intermediates for the preparation of various polyacene derivatives.

Ring Selective Generation of Isobenzofuran for Divergent Access to Polycyclic Aromatic Compounds.

Ring selective generation of isobenzofuran, a formal equivalent to bis-isobenzofuran, was developed. Importantly, selective introduction of functionalities and/or fused rings in the isobenzofuran core by iterative cycloadditions can achieve the divergent construction of polycyclic compounds. This selective approach enables us to prepare a regioisomeric pair of pentacenes.

A new synthetic route to substituted tetracenes and pentacenes via stereoselective [4+2] cycloadditions of 1,4-dihydro-1,4-epoxynaphthalene and isobenzofuran.

Stereoselective [4+2] cycloadditions of 1,4-dihydro-1,4-epoxynaphthalene and isobenzofuran were described. Among several possibilities, syn-exo and/or anti-endo isomers were selectively produced depending on the substitution pattern of the reactants. Importantly, the syn-exo isomer underwent acid promoted aromatization, affording the corresponding tetracene. These findings enabled us to prepare a substituted pentacene with electron withdrawing groups.

An efficient synthetic route to 1,3-bis(arylethynyl)isobenzofuran using alkoxybenzocyclobutenone as a reactive platform.

An efficient synthetic method of 1,3-bis(arylethynyl)isobenzofurans is developed. Nucleophilic addition of alkynyllithium to benzocyclobutenone and subsequent oxidative ring cleavage of the four-membered ring gave a keto-aldehyde, which, in turn, accepted the second nucleophile to produce isobenzofurans after acid treatment.

Dibromoisobenzofuran as a formal equivalent of didehydroisobenzofuran: reactive platform for expeditious assembly of polycycles.

Two-directional annulation of dibromoisobenzofuran, a formal equivalent to didehydroisobenzofuran, was developed. Importantly, selective bromine-lithium exchange allows the tandem generation of benzynes and dual cycloadditions with two different arynophiles. Also described is the application to the synthesis of a substituted pentacene.

Catalytic generation of arynes and trapping by nucleophilic addition and iodination.

A fair exchange: in the title reaction, alkynyllithium serves as an initiator for benzyne generation through an iodine-lithium exchange. When performed in the presence of stoichiometric amounts of a nucleophile, the generated benzyne undergoes attack by lithio nucleophiles to generate aryllithium, which is then iodinated by iodoalkyne to give the iodoarenes 1.

A domino pericyclic route to polysubstituted salicylic acid derivatives: four sequential processes from enynones and ketene silyl acetals.

Alkenyl alkynyl ketones and ketene silyl acetals (KSAs) undergo regioselective [2+2]-cycloaddition under thermal conditions, triggering domino pericyclic reactions en route to various poly-substituted salicylic acid derivatives.

Siloxy(trialkoxy)ethene undergoes regioselective [2+2] cycloaddition to ynones and ynoates en route to functionalized cyclobutenediones.

Regioselective [2+2] cycloaddition of ynones or ynoates to siloxy(trialkoxy)ethene (KSA) is described. A siloxy group on the KSA directs the perfect regioselectivity, allowing rapid construction of various functionalized cyclobutenedione derivatives.