Visible Light Excitation of BODIPYs Enables Dehydrogenative Enamination at Their α-Positions with Aliphatic Amines

A novel strategy for the functionalization and design of 4-methylene-4H-pyran merocyanines via enamination and 1,8-conjugate addition

4-Methylene-4H-pyrans are popular merocyanine dyes, but their functionalization is limited by the Knoevenagel condensation with aromatic aldehydes. In this work, we developed a novel approach for the construction of a new class of pyran fluorophores based on enamination and subsequent nucleophilic substitution of the dimethylamino group via 1,8-conjugate addition/elimination. This methodology includes selective transformations leading to previously unknown symmetrical and asymmetrical structures. The dimethylaminovinyl-substituted pyrans are reactive intermediates and can be considered as a convenient synthetic tool for the construction of new merocyanines with tunable fluorescence (417-628 nm). The main strategies for the modification of the pyran moiety have been determined for the construction and targeted design of fluorophores. Pyrans bearing two enamine moieties demonstrate significant light extinction coefficients (up to 116 000 M^-1 cm^-1), high quantum yields (up to 69%) and large Stokes shifts (up to 152 nm) because of their strong push-pull nature. Density Functional Theory (DFT) calculations were performed for the explanation of the structural and photophysical features of the prepared merocyanines. The developed approach can be considered as a useful platform for further application of 4-methylene-4H-pyrans as promising fluorophores for sensors and solar cells, and in bioimaging.

2-(2-(Dimethylamino)vinyl)-4H-pyran-4-ones as Novel and Convenient Building-Blocks for the Synthesis of Conjugated 4-Pyrone Derivatives

A straightforward approach for the construction of the new class of conjugated pyrans based on enamination of 2-methyl-4-pyrones with DMF-DMA was developed. 2-(2-(Dimethylamino)vinyl)-4-pyrones are highly reactive substrates that undergo 1,6-conjugate addition/elimination or 1,3-dipolar cycloaddition/elimination followed by substitution of the dimethylamino group without ring opening. This strategy includes selective transformations leading to conjugated and isoxazolyl-substituted 4-pyrone structures. The photophysical properties of the prepared 4-pyrones were determined in view of further design of novel merocyanine fluorophores. A solvatochromism was found for enamino-substituted 4-pyrones accompanied by a strong increase in fluorescence intensity in alcohols. The prepared conjugated structures demonstrated valuable photophysical properties, such as a large Stokes shift (up to 204 nm) and a good quantum yield (up to 28%).

BODIPY Photocatalyzed Beckmann Rearrangement and Hydrolysis of Oximes under Visible Light

A novel, efficient, and mild protocol for rearrangement of oximes to amides or hydrolyzing to ketone/aldehyde using a simple BODIPY dye as a photocatalyst and air as an oxidant via propagation reaction under visible-light irradiation is reported. The triplet excited state of BODIPY played a significant role in the catalytic process. It was found that the various substituted ketoximes, both with electron-withdrawing and electron-donating substituents, afforded the corresponding products with moderate to excellent yields, and the catalytic efficiency was up to 0.01 mol %.

4,7-Diarylbenzo[c][1,2,5]thiadiazoles as fluorophores and visible light organophotocatalysts

A library of 26 electron donor-acceptor organophotocatalysts based on the benzo[c][1,2,5]thiadiazole (BTZ) group has been developed. These visible light organophotocatalysts were then used in a Minisci-type alkylation of heteroarenes under both batch and continuous flow conditions.

Substituent effects on opticalproperties of pyrrolizine-fused BOPYIN

Three new pyrrolizine-fused BOPYINs (DAB-H, DAB-OMe, DAB-ester) have been reported in 26-35% yield. The relationship between structures and optical spectra was investigated, which all the compounds show large Stokes Shift (3146-3884 cm^-1) and high quantum yield (up to 99%) in solvents. Among these dyes, the decoration of electron donating/withdrawing groups on indole, pyrrole and pyrrolizine units has a significant impact on optical properties, especially emission spectra. The results suggested that electron withdrawing group on pyrrole and pyrrolizine units has hypsochromic shift on emission spectra (DAB-H, DAB-OMe, DAB-ester versus DAB-1,4,5). The optimized structure, electron distribution on frontier molecular orbital, energy gap and simulated stick spectra of DABs are discussed by Density Functional Theory (DFT) calculation. We claim the agreement between the experimental and theoretical absorption spectra.

Chiral Near-Infrared Fluorophores by Self-Promoted Oxidative Coupling of Cationic Helicenes with Amines/Enamines

In one pot, tertiary alkyl amines are oxidized to enamines by cationic dioxa[6]helicene, which further reacts as electrophile and oxidant to form mono or bis donor-π-acceptor coupling products. This original and convergent synthetic approach provides a strong extension of conjugation yielding chromophores that absorb intensively in far-red or NIR domains (λ_max up to 791 nm) and fluoresce in the NIR as well (λ_max up to 887 nm). Intense ECD properties around 790 nm with a |Δϵ| value up to 60 M^-1  cm^-1 are observed.

Vinylation of α-Aminoazoles with Triethylamine: A General Strategy to Construct Azolo[1,5-a]pyrimidines with a Nonsubstituted Ethylidene Fragment

A new general synthesis of pharmaceutically important azolo[1,5-a]pyrimidines starting from widely available 3(5)-aminoazoles, aldehydes, and triethylamine is developed. The key is to enable the vinylation reaction that allows the in situ generation of elusive acyclic enamines and the subsequent annulation reaction to occur. This direct and practical strategy is capable of constructing a range of 5,6-unsubstituted pyrazolo[1,5-a]pyrimidines and [1,2,4]triazolo[1,5-a]pyrimidines. More importantly, this protocol provides a concise synthetic route to prepare the clinically used zaleplon.