Azobenzene is one of the most extensively studied molecular photoswitches because of its reversible trans–cis photoisomerization. Incorporation of azobenzene into cyclic architectures profoundly modifies its photochemical and thermal isomerization behavior through geometric constraint, ring strain, and conformational restriction. This review summarizes recent advances in cyclic azobenzenes with particular emphasis on the relationship between molecular architecture and photoresponsive properties. Covalently linked macrocycles, diazocines, azobenzenophanes, mechanically interlocked systems, coordination-driven macrocycles, supramolecular assemblies, and metal complexes are discussed from the viewpoint of isomerization control. The synthetic approaches to these cyclic systems are first classified according to whether macrocyclization is achieved using preformed azobenzene building blocks or through formation of the azo bond itself, and the consequences of these strategies for the accessible trans/cis configurations are discussed. Particular attention is paid to systems exhibiting enhanced thermal stability, selective stabilization of either trans or cis isomers, inhibition of photoisomerization, and visible- or near-infrared-light responsiveness. Recent studies have also demonstrated that metal coordination provides an effective strategy for locking or unlocking azobenzene isomerization and for switching additional functions such as fluorescence, host–guest recognition, and spin states. These findings illustrate how cyclic structural design enables precise control over the photophysical behavior of azobenzene. The concepts summarized in this review provide useful guidelines for the rational design of next-generation photoresponsive molecular systems and functional materials based on cyclic azobenzene architectures.
Primary amines represent the most fundamental and widely employed building blocks in organic synthesis. For a long time, the functionalization of primary amines has relied predominantly on the strong nucleophilicity of the nitrogen atom to achieve N–H bond modification. In contrast, direct deaminative functionalization that converts the amino group into other functional groups via selective C–N bond cleavage has remained a central challenge in this field. This difficulty stems from the high bond dissociation energy of the C–N bond and the extremely poor thermodynamic leaving ability of the amide anion. Traditional deaminative coupling strategies rely on preactivation methods such as diazonium or pyridinium salts. Although these approaches enable the desired transformations, they suffer from drawbacks, including poor thermal stability of intermediates, high safety risks, and poor atom and step economy. In recent years, driven by the principles of green and sustainable chemistry, direct deaminative transformation of primary amines without the isolation of preactivated intermediates has emerged as a cutting-edge research direction. This review provides a comprehensive summary of direct deaminative functionalization strategies, with a detailed discussion of reaction conditions, substrate scope, mechanisms, and late-stage applications.
Hypervalent iodine reagents have emerged as environmentally benign, metal-free oxidants and versatile reaction mediators in modern synthetic organic chemistry, featuring mild reaction conditions, excellent functional group tolerance, and facile post-reaction separation. This review systematically summarizes the advances in intermolecular cycloaddition reactions mediated/catalyzed by trivalent hypervalent iodine species over the past two decades. The discussed cycloaddition manifolds cover [2 + 1], [2 + 2], [2 + 2+1], [3 + 2], [4 + 2] and other cascade domino cyclization modes. The construction of diverse molecular skeletons is highlighted, including epoxides, cyclobutanes, 1-pyrrolines, isoxazolines/isoxazoles, fused polycyclic heterocycles, quinoxalines, isoquinolones, bridged bicyclic frameworks and fullerene-fused heterocycles. This review provides a comprehensive reference for the development of novel hypervalent iodine-based cycloaddition methodologies and inspires further exploration of metal-free oxidative heterocyclization for complex molecule assembly.
The effects of the formation of a hydrogen or halogen bond on the aromaticity of a substituted benzene are assessed via DFT calculations. The ability of the mobile π-electrons to respond to an external magnetic field is evaluated using several variants of the NICS scheme as is the drive toward uniformity of C-C bond lengths and strengths. Formation of a hydrogen/halogen bond between the Lewis base lone pair and either the proton donor group of a substituent or the halogen substituent σ-hole reduces the aromaticity, whereas an external proton donor yields a rise in this property when attached to a nucleophilic substituent. The effects are mixed when a proton donor is poised directly above the phenyl ring. Replacement of the aromaticity in the ring by a CC-CC conjugated system has a mixed effect on the interactions. Aromaticity favors interactions in which the arene accepts density and disfavors those in which it donates.
