A Review on Generation and Reactivity of the N-Heterocyclic Carbene-Bound Alkynyl Acyl Azolium Intermediates

N-heterocyclic carbene (NHC) has been widely used as an organocatalyst for both umpolung and non-umpolung chemistry. Previous works mainly focus on species including Breslow intermediate, azolium enolate intermediate, homoenolate intermediate, alkenyl acyl azolium intermediate, etc. Notably, the NHC-bound alkynyl acyl azolium has emerged as an effective intermediate to access functionalized cyclic molecular skeleton until very recently. In this review, we summarized the generation and reactivity of the NHC-bound alkynyl acyl azolium intermediates, which covers the efforts and advances in the synthesis of achiral and axially chiral cyclic scaffolds via the NHC-bound alkynyl acyl azolium intermediates. In particular, the mechanism related to this intermediate is discussed in detail.     

Solid-State Photochemical Cascade Process Boosting Smart Ultralong Room-Temperature Phosphorescence in Bismuth Halides

Molecular ultralong room-temperature phosphorescence (RTP), exhibiting multiple stimuli-responsive characteristics, has garnered considerable attention due to its potential applications in light-emitting devices, sensors, and information safety. This work proposes the utilization of photochemical cascade processes (PCCPs) in molecular crystals to design a stepwise smart RTP switch. By harnessing the sequential dynamics of photo-burst movement (induced by [2+2] photocycloaddition) and photochromism (induced by photogenerated radicals) in a bismuth (Bi)-based metal-organic halide (MOH), a continuous and photo-responsive ultralong RTP can be achieved. Furthermore, utilizing the same Bi-based MOH, diverse application demonstrations, such as multi-mode anti-counterfeiting and information encryption, can be easily implemented. This work thus not only serves as a proof-of-concept for the development of solid-state PCCPs that integrate photosalient effect and photochromism with light-chemical-mechanical energy conversion, but also lays the groundwork for designing new Bi-based MOHs with dynamically responsive ultralong RTP.     

Solvent-dependence of KI Mediated Electrosynthesis of Imidazo[1,2-a]pyridines

Iodized salts are widely used as mediators to promote C—H functionalization. Solvents and additives have been described as significant roles in these reactions. However, the further electrochemical investigations have rarely been reported. Herein, a KI mediated electrochemical annulation between acetophenones and 2-amniopyridines was developed. We revealed the effect of acids and solvents by cyclic voltammetry(CV), differential pulse voltammetry(DPV), and square wave voltammetry(SWV). The oxidation of 2-aminopyridine is inhibited at the potential window with the addition of strong acids, and the lowest oxidation potential difference of KI was obtained by utilizing EtOH as solvent. The experimental studies also show that the mixture solvent of EtOH/DMSO(9/1, volume ratio) facilitates the electrochemical cyclization due to the solubility improvement of KI. CF₃SO₃H has been screened as the optimal acid. A range of Imidazo[1,2-a]- pyridines have been synthesized in yields of 42% to 96%. Electrochemical investigations present that the KI mediated electro- chemical reaction is probably solvent-dependence.     

A Dynamic Defect Generation Strategy for Efficient Enzyme Immobilization in Robust Metal-Organic Frameworks for Catalytic Hydrolysis and Chiral Resolution

Enzyme immobilization has been demonstrated to be a favorable protocol for promoting the industrialization of bioactive molecules, but still with formidable challenge. Addressing this challenge, we create a dynamic defect generation strategy for enzyme immobilization by using the dissociation equilibrium of metal-organic frameworks (MOFs) mediated by enzymes. Enzymes can act as “macro ligands” to generate competitive coordination against original ligands, along with the release of metal clusters of MOFs to generate defects, hence promoting the gradual transport of enzymes from the surface to inside. Various enzymes can be efficiently immobilized in MOFs to afford composites with good enzymatic activities, protective performances and exceptional reusabilities. Moreover, multienzyme bioreactors capable of efficient cascade reactions can also be generated. This study provides new opportunities to construct highly efficient biocatalysts incorporating different types of enzymes.     

Easy Access to Diverse Multiblock Copolymers with On-Demand Blocks via Thioester-Relayed In-Chain Cascade Copolymerization

Multiblock copolymers are envisioned as promising materials with enhanced properties and functionality compared with their diblock/triblock counterparts. However, the current approaches can construct multiblock copolymers with a limited number of blocks but tedious procedures. Here, we report a thioester-relayed in-chain cascade copolymerization strategy for the easy preparation of multiblock copolymers with on-demand blocks, in which thioester groups with on-demand numbers are built in the polymer backbone by controlled/living polymerizations. These thioester groups further serve as the in-chain initiating centers to trigger the acyl group transfer ring-opening polymerization of episulfides independently and concurrently to extend the polymer backbone into multiblock structures. The compositions, number of blocks, and block degree of polymerization can be easily regulated. This strategy can offer easy access to a library of multiblock copolymers with ≈100 blocks in only 2 to 4 steps.     

Programming Super DNA-Enzyme Molecules for On-Demand Enzyme Activity Modulation**

Dynamic interactions of enzymes, including programmable configuration and cycling of enzymes, play important roles in the regulation of cellular metabolism. Here, we constructed a super DNA-enzymes molecule (SDEM) that comprises at least two cascade enzymes and multiple linked DNA strands to control and detect metabolism. We found that the programmable SDEM, which comprises glucose oxidase (GOx) and horseradish peroxidase (HRP), has a 20-fold lower detection limit and a 1.6-fold higher reaction rate than free enzymes. An SDEM can be assembled and disassembled using a hairpin structure and a displacement DNA strand to complete multiple cycles. An entropically driven catalytic assembly (catassembly) enables different SDEMs to switch from an SDEM with GOx and HRP cascades to an SDEM with sarcosine oxidase (SOX) and HRP cascades in over six orders of magnitude less time than without the catassembly to detect different metabolisms (GO and sarcosine) on demand.     

Oxidative C(sp3)−N Cleavage Cascade Cyclization of N-Uracil Amidines and Tertiaryamines: Switchable Divergent Synthesis of Functionalized Pyrimidouracils and Dihydropyrimidouracils

The divergent synthesis of dihydropyrimidouracil and pyrimidouracil derivatives from N-uracil amidines and tertiaryamines by an oxidative C(sp³)−N cleavage cascade cyclization reaction is first described. This transformation enables the formation of new C(sp³)−N and C(sp²)−N bonds via the selection of different oxidation conditions. The features of this method include tunable product selectivity, excellent chemoselectivity, readily available starting materials, broad substrate scope, good tolerance of functional groups, and moderate to high yields.     

Synthesis of Modified Poly(vinyl Alcohol)s and Their Degradation Using an Enzymatic Cascade

Poly(vinyl alcohol) (PVA) is a water-soluble synthetic vinyl polymer with remarkable physical properties including thermostability and viscosity. Its biodegradability, however, is low even though a large amount of PVA is released into the environment. Established physical-chemical degradation methods for PVA have several disadvantages such as high price, low efficiency, and secondary pollution. Biodegradation of PVA by microorganisms is slow and frequently involves pyrroloquinoline quinone (PQQ)-dependent enzymes, making it expensive due to the costly cofactor and hence unattractive for industrial applications. In this study, we present a modified PVA film with improved properties as well as a PQQ-independent novel enzymatic cascade for the degradation of modified and unmodified PVA. The cascade consists of four steps catalyzed by three enzymes with in situ cofactor recycling technology making this cascade suitable for industrial applications.     

Silver-Mediated Cascade Synthesis of Functionalized 1,4-Dihydro-2H-benzo-1,3-oxazin-2-ones from Carbon Dioxide

A conceptually novel catalytic domino approach is presented for the synthesis of highly functional 1,4-dihydro-2H-1,3-benzoxazine-2-one derivatives. Key to the chemoselectivity is a proper design of the precursor to override thermodynamically favored parasitic cyclization processes and empower the formation of the desired product through Thorpe–Ingold effects. The synthetic diversity of these CO₂-based heterocycles is further demonstrated, and the isolation of a reaction intermediate supports an unusual ring-expansion sequence from an α-alkylidene, five-membered cyclic carbonate to a six-membered cyclic carbamate by N-induced isomerization.     

Cobalt(III)-catalyzed synthesis of isoquinolines from oximes and alkynes in deep eutectic solvents

A cobalt-catalyzed redox-neutral [4 + 2] annulation of oximes and alkynes in deep eutectic solvents comprising quaternary ammonium compounds and hydrogen bond donors has been developed, the best system being Me₃N+CH₂CO₂- / HOCH(CF₃)₂. No external oxidizing reagent was required, and only water was generated as the secondary product. The catalytic system can be reused in three consecutive cycles.