Shape-Matching and Halogen Bonding in Chiral Pyrazine-Allene Hosts: Confining an Unstable Guest Conformation

A novel chiral host with axially chiral allenes and pyrazine units was synthesized in high yield via a three-step sequence. In the solid-state, it adopts a twist-boat conformation, forming discrete double chains in an extended network. Host-guest studies with halogen bond donors, analyzed by NMR and X-ray diffraction, revealed size-selective inclusion within the macrocyclic cavity. Remarkably, a syn-gauche conformer of diiodoperfluorobutane was crystallized inside the host, highlighting the role of guest structure, host flexibility, and crystallization conditions.