Gold-Catalyzed Cyclization of 2-Alkynylaldehyde Cyclic Acetals via Hydride Shift for the Synthesis of Indenone Derivatives

An Exploration of Substituent Effects on the Photophysical Properties of Monobenzopentalenes

Monobenzopentalenes have received moderate attention compared to dibenzopentalenes, yet their accessibility as stable, non-symmetric structures with diverse substituents could be interesting for materials applications, including molecular photonics. Recently, monobenzopentalene was considered computationally as a potential chromophore for singlet fission (SF) photovoltaics. To advance this compound class towards photonics applications, the excited state energetics must be characterized, computationally and experimentally. In this report we synthesized a series of stable substituted monobenzopentalenes and provided the first experimental exploration of their photophysical properties. Structural and opto-electronic characterization revealed that all derivatives showed 1H NMR shifts in the olefinic region, bond length alternation in the pentalene unit, low-intensity absorptions reflecting the ground-state antiaromatic character and in turn the symmetry forbidden HOMO-to-LUMO transitions of ~2 eV and redox amphotericity. This was also supported by computed aromaticity indices (NICS, ACID, HOMA). Accordingly, substituents did not affect the fulfilment of the energetic criterion of SF, as the computed excited-state energy levels satisfied the required E(S1)/E(T1)>2 relationship. Further spectroscopic measurements revealed a concentration dependent quenching of the excited state and population of the S2 state on the nanosecond timescale, providing initial evidence for unusual photophysics and an alternative entry point for singlet fission with monobenzopentalenes.

Intramolecular Carboxyamidation of Alkyne-Tethered O-Acylhydroxamates through Formation of Fe(III)-Nitrenoids

We developed intramolecular carboxyamidations of alkyne-tethered O-acylhydroxamates followed by either thermally induced spontaneous or 4-(dimethylamino)pyridine-catalyzed O→O or O→N acyl group migration. Under iron-catalyzed conditions, the carboxyamidation products were generated in high yield from both Z-alkene and arene-tethered substrates. DFT calculations indicate that the iron-catalyzed carboxyamidation proceeds via a stepwise mechanism involving iron-imidyl radical cyclization followed by intramolecular acyloxy transfer from the iron center to the alkenyl radical center to furnish the cis-carboxyamidation product. Upon treatment with 4-(dimethylamino)pyridine, the Z-alkene-tethered carboxyamidation products underwent selective O→O acyl migration to generate 2-acyloxy-5-acyl pyrroles. Thermal O→N acyl migration occurs during carboxyamidation if the Z-alkene linker contains an alkyl or an aryl substituent at the β-position of the carbonyl group. On the other hand, the arene linker-containing compounds selectively undergo O→N acyl migration to generate N-acyl-3-acylisoindolinones, and the corresponding O→O acyl migration forming isoindole derivatives was not observed.

Gold(I)-Mediated Cyclizations of Triene-Yne Systems: An Easy Access to Cyclopenta-Fused Anthracenes

The direct synthesis of cyclopenta-fused anthracenes (CP-anthracenes) through a gold(I)-catalyzed cyclization reaction of easily available triene-yne systems bearing a benzofulvene substructure, is reported. The targets are of great interest due to the potential use as organic materials and the preparation of these compounds is gaining huge importance. The applied starting materials are readily available through a three-step synthesis, which further contributes to the advantages of this route. In addition, UV-Vis and fluorescent spectra of the obtained CP-anthracenes were recorded.

Gold(I)-Catalyzed Benzylic C(sp3 )-H Functionalizations: Divergent Synthesis of Indole[a]- and [b]-Fused Polycycles

Phenyl azides substituted by an (alkylphenyl)ethynyl group facilitate benzylic sp³ (C-H) functionalization in the presence of a JohnPhosAu catalyst, resulting in indole-fused tetra- and pentacycles via divergent N- or C-cyclization. The chemoselectivity is influenced depending on the counter-anion, the electron density of the α-imino gold(I) carbene, and the alkyl groups stabilizing the benzylic carbocation originating from a 1,5-hydride shift. An isotopic labeling experiment demonstrates the involvement of an indolylgold(I) species resulting from a tautomerization that is much faster than the deauration. The formation of a benzylic sp³ (C-H) functionalization leading to an indole-fused seven-membered ring is also demonstrated.

Recent Advances in Gold(I)-Catalyzed Approaches to Three-Type Small-Molecule Scaffolds via Arylalkyne Activation

Gold catalysts possess the advantages of water and oxygen resistance, with the possibility of catalyzing many novel chemical transformations, especially in the syntheses of small-molecule skeletons, in addition to achieving the rapid construction of multiple chemical bonds and ring systems in one step. In this feature paper, we summarize recent advances in the construction of small-molecule scaffolds, such as benzene, cyclopentene, furan, and pyran, based on gold-catalyzed cyclization of arylalkyne derivatives within the last decade. We hope that this review will serve as a useful reference for chemists to apply gold-catalyzed strategies to the syntheses of related natural products and active molecules, hopefully providing useful guidance for the exploration of additional novel gold-catalyzed approaches.

Addition of benzyl ethers to alkynes: a metal-free synthesis of 1H-isochromenes

Bromotrimethylsilane (TMSBr)-promoted intramolecular cyclization of (o-arylethynyl)benzyl ethers to form 1H-isochromenes at room temperature is reported. Further studies indicated that vinyl carbocations are the reaction intermediates which are stabilized by the conjugated aryl groups. Thus, O-addition of benzyl ethers/tetrahydropyrans to alkynes was achieved under metal-free, acidic conditions. These reaction conditions were compatible with an alkynyl Prins reaction; therefore, 1H-isochromenes were produced directly from alkynyl benzaldehydes and alkynyl alcohols using a one-pot procedure.

Cleavage and Reassembly of the C═O Bond of 2-Alkynylbenzaldehydes: A Metal-Free Access to Inden-1-ones

A metal-free approach to inden-1-ones from 2-alkynylbenzaldehydes mediated by pyrrolidine has been developed. The reaction proceeds under mild conditions in a step- and atom-economy process by cleaving the C═O bond and constructing new C-C as well as C═O bonds. Oxygen-18 and deuterium labeling experiments revealed an aza-Petasis-Ferrier rearrangement of an intermediate 1-amino-3-methylene-dihydroisobenzofuran.

Transition-metal-free oxidative cyclization reaction of enynals to access pyrane-2-one derivatives

A novel and efficient metal-free C-H functionalization of enynals is developed to synthesize α-pyrone derivatives via the formation of two C-O bonds. In this project, K₂S₂O₈ has been introduced as an efficient oxygen source and C-H functionalization agent in regioselective oxidative cyclization reaction with a relatively broad substrate scope.

Revealing the Mechanism and Origin of Reactivity of Au(I)-Catalyzed Functionalized Indenone Formation of Cyclic and Acyclic Acetals of Alkynylaldehydes

A density functional theory study is presented here to offer mechanistic insights and explications of experimentally intriguing observations in the Au(I)-catalyzed cyclization of cyclic and acyclic acetals of alkynylaldehydes that leads to indenone formation. The reactivity of catalytic cycles with and without methoxy migration is clearly defined when the alkyne terminus is phenylated. The reaction mechanism of indenone formation proceeds first with the coordination of Au(I) to alkyne to initiate the reaction with 1,5-H shift as a rate-determining step (RDS), and the fastest 1,5-H shift is achieved when one phenyl ring carries an electron-donating group and the other one is substituted with an electron-withdrawing group. Following the 1,5-H shift, the reaction undergoes feasible steps that are cyclization and 1,2-H shift before elimination to persist the iterative cycle, but the reactivity of both steps is highly affected by the existence of the phenyl group on the alkyne terminus. The unreactivity of the alkyne terminus not bearing a phenyl ring is because the cyclization is thermodynamically disfavorable, subsequently deactivating the 1,2-H shift kinetically and thermodynamically. The absence of a tether in the acetal unit considerably outpaces any 1,5-H shift and instead activates 1,5-methoxy migration, giving methoxy-migrated indenone, with the 1,2-OMe shift being an RDS.