Elucidating Mechanism and Selectivity in Pyridine Functionalization through Silylium Catalysis.

The functionalization of aromatic N-heterocycles through silylium activation demonstrates exceptional selectivity and efficiency. Density functional theory (DFT) calculations unveil the detailed silylium catalysis mechanism and elucidate the origins of selectivity in this reaction. The phosphoramidimidate sulfonamide (PADI) precatalyst orchestrates of the catalytic cycle via three elementary steps. The Brønsted acidity of precatalyst significantly influences both the formation of silylium-based Lewis acid active species and the silylium activation of pyridine. Unlike disulfonimide (DSI)-type precatalysts, both Tf2NH and PADI precatalysts with strong acidities can easily promote the generation of activated silylium pyridine species. A semi-enclosed 'rigid' electronegative cavity in PADI-type anions constructs a well-defined recognition site, facilitating engagement with the positively charged silylium pyridine species. Due to the high electrophilicity and less steric demand at the C4-position of the pyridine substrate, the product with C4-regioselectivity was predominantly generated.

Photoswitchable and long-lived seven-membered cyclic singlet diradicals for the bioorthogonal photoclick reaction.

Annularly 1,3-localized singlet diradicals are energetic and homolytic intermediates, but commonly too short-lived for widespread utilization. Herein, we describe a direct observation of a long-lived and seven-membered singlet diradical, oxepine-3,6-dione-2,7-diyl (OXPID), via spectroscopic experiments and also theoretical evidence from computational studies, which is generated via photo-induced ring-expansion of 2,3-diaryl-1,4-naphthoquinone epoxide (DNQO). The photo-generated OXPID reverts to the thermally stable σ-bonded DNQO with t1/2 in the µs level, thus constituting a novel class of T-type molecular photoswitches with high light-energy conversion efficiency (η = 7.8-33%). Meanwhile, the OXPID is equilibrated to a seven-membered cyclic 1,3-dipole as an electronic tautomer that can be captured by ring-strained dipolarophiles with an ultrafast cycloaddition rate (k2CA up to 109 M-1 s-1). The T-type photoswitchable DNQO is then exploited to be a highly selective and recyclable photoclick reagent, enabling spatiotemporal-resolved bioorthogonal ligation on living cell membranes via a tailored DNQO-Cy3 probe.

Asymmetric [3+2]-cyclization of α-imino amide surrogates to construct 3,4-diaminopyrrolidine-2,5-diones.

Using newly designed α-imino amide surrogates and azlactones as amphiphilic reactants, catalyzed by a chiral bifunctional guanidine, the construction of chiral 3,4-diaminopyrrolidine-2,5-diones and their derivatives was realized via formal [3+2]-cyclization. The role of guanidine as a multiple hydrogen bond donor was demonstrated by DFT calculations.

Photo-induced defluorination acyl fluoride exchange as a fluorogenic photo-click reaction for photo-affinity labeling.

Photo-click chemistry has emerged as a powerful tool for revolutionizing bioconjugation technologies in pharmacological and various biomimetic applications. However, enriching the photo-click reactions to expand the bioconjugation toolkit remains challenging, especially when focusing on spatiotemporal control endowed by light activation. Herein, we describe a photo-induced defluorination acyl fluoride exchange (photo-DAFEx) as a novel type of photo-click reaction that is mediated through acyl fluorides produced by the photo-defluorination of m-trifluoromethylaniline to covalently conjugate with primary/secondary amines and thiols in an aqueous environment. (TD)-DFT calculations, together with experimental discovery, indicate that the m-NH2PhF2C(sp3)-F bond in the excited triplet state is cleaved by water molecules, which is key to inducing defluorination. Intriguingly, the benzoyl amide linkages built by this photo-click reaction exhibited a satisfactory fluorogenic performance, which allowed visualization of its formation in situ. Accordingly, this photo-controlled covalent strategy was exploited not only for the decoration of small molecules, peptide cyclization and functionalization of proteins in vitro, but also for designing photo-affinity probes targeting endogenous carbonic anhydrase II (hCA-II) in living cells.

Dibenzo[b,f][1,4,5]chalcogenadiazepine Photoswitches: Conversion of Excitation Energy into Ring Strain.

Tetra-ortho-substituted, heteroaryl and cyclic azobenzenes have emerged as three key strategies on morphology design of photoswitch to diversify controllability. Cyclic azobenzene is of particular utilization in photo-energy conversion due to rigid and ring-strain structure. Despite the well-recognized diazocine, the photo-switching properties of seven-membered cyclic azobenzenes (diazepines) have yet been exploited. Herein, we report a family of dibenzo[b,f][1,4,5]chalcogenadiazepines (DBChDs) and their T-type photo-switching nature with tunable relaxation rate. Based on experiments together with DFT calculations, we found that an unsymmetric 2-bithiophenyl-dibenzo[b,f][1,4,5]thiadiazepine exhibited an efficient response to 445 nm laser stimulation (quantum efficiency, ΦZ→E =0.71) with millisecond relaxation half-life (t1/2 =40 ms). Photo-energy transduction efficiency was also exceptionally high with 29.1 % converted into ring-strain energy mainly loaded on azo π-bond.

A low-polarity small organic molecule with a stable keto form for photocatalytic H2 evolution.

Five small organic molecules (SOMs) with different degrees of enol to keto tautomerism were synthesized for photocatalytic H2 evolution. The SOM possessing the highest activity features a stable keto form that greatly facilitates the flowing of the excited electrons toward the carbonyl O site where the reduction reaction occurs.