Stereoselective Haliranium, Thiiranium and Seleniranium Ion-Triggered Friedel-Crafts-Type Alkylations for Polyene Cyclizations

The review is aimed at describing the advances in electrophilic iranium ion-triggered Friedel-Crafts-type alkylation for polyene cyclizations. Owing to the great potential of halo-, seleno- and sulfeno-functionalized Friedel-Crafts alkylated compounds, numerous catalytic methods have been developed in either racemic or enantiopure forms. More importantly, these arene-alkylated or polyene-cyclized products containing halo, seleno and sulfeno functionality could be useful in further synthetic planning to access biologically active molecules and natural products with highly complex architectures. Starting from a wide range of classical Lewis acids, BrOnsted acids and Lewis bases through to the very recently developed halogen bond catalysts (XB-cat.), selenonium cations as Lewis acids etc. are utilized to access these types of compounds. More recently, some of the co-operative catalytic systems such as phosphite-thiourea, sulfonic acid-phosphoramidite, sulfonic acid-selenophosphoramide, sulfonic acid-anion binding thiourea, etc. catalyzing highly diastereo- and enantioselective polyene cyclizations have also been described. The configurational stability, reactivity and selectivity of three-membered cationic iranium ions generated in situ from alkenes and their subsequent trapping by electron-rich aromatic compounds as terminating nucleophiles is systematically discussed. This review directs attention to both the developments that have been accomplished in this research field and to suggestions for future work most warranted in this important area.