Spiro- and Tricyclic Phosphoranes with Six- and Higher-Membered Rings

This article presents developments in synthetic and structural aspects of tetra-, penta-, and hexacoordinate spirocyclic phosphoranes that involve primarily a 1,3,2-dioxaphosphorus heterocyclic skeleton and lie at the border between traditional organic and inorganic chemistry. Studies on these systems have made us rethink the traditional tenets on structural preferences in pentacoordinate phosphorus chemistry. Some compounds have a bearing on the mechanism of the Mitsunobu reaction while others are useful as chiral resolving agents or strong phosphazenic bases.