Halogen bonding in functional chiral systems

The halogen bond (XB) is a directional noncovalent interaction formed between a halogen atom acting as an electrophilic site and a Lewis base. In recent years, it has shown great potential in the design of functional chiral systems. Due to its tunable interaction strength and pronounced directionality, the XB is becoming a powerful tool for constructing and controlling chirality at both molecular and supramolecular levels. In functional chiral systems, XBs are widely employed to induce, amplify, and transfer chirality, primarily arising from the directionally anisotropic electronic distribution on the surfaces of iodine, bromine, or chlorine atoms. This strategy enables precise construction and stereocontrol of chiral molecular assemblies, asymmetric catalysts, and chiroptical materials. In this review, we first discuss the fundamental properties of XBs and introduce several unconventional types of XBs to stimulate readers' interest. Subsequently, we summarize the latest research progress in XB-based crystal engineering, chiral recognition and separation, chiroptical materials, and supramolecular assemblies. Various XB-mediated asymmetric catalytic systems are also examined, with particular attention paid to their reaction mechanisms and catalyst design strategies. Undoubtedly, living organisms are multilevel chiral systems-from individual protein molecules to large and complex biological structures-thus, this review also explores the potential roles of XBs in biological and biomimetic systems. Finally, we provide a summary and outlook on the current research status, existing challenges, and future opportunities of XBs in functional chiral systems. Through these discussions, this review aims to inspire further exploration of XBs in chiral systems, opening new avenues for supramolecular chemistry, asymmetric synthesis, catalysis, and the development of advanced chiroptical materials.