Association of Halogen Bonding and Hydrogen Bonding in Metal Acetate-Catalyzed Asymmetric Halolactonization

Cooperative activation using halogen bonding and hydrogen bonding works in metal-catalyzed asymmetric halolactonization. The Zn-3(OAc)(4)-3,3'-bis(aminoimino) binaphthoxide (tri-Zn) complex catalyzes both asymmetric iodolactonization and bromolactonization. Carboxylic acid substrates are converted to zinc carboxylates on the tri-Zn complex, and the N-halosuccinimide (N-bromosuccinimide [NBS] or N-iodosuccinimide [NIS]) is activated by hydrogen bonding with the diamine unit of chiral ligand. Halolactonization is significantly enhanced by the addition of catalytic I-2. Density functional theory calculations revealed that a catalytic amount of I-2 mediates the alkene portion of the substrates and NIS to realize highly enantioselective iodolactonization. The tri-Zn catalyst activates both sides of the carboxylic acid and alkene moiety, so that asymmetric five-membered iodolactonization of prochiral diallyl acetic acids proceeded to afford the chiral gamma-butyrolactones. In the total description of the catalytic cycle, iodolactonization using the NIS-I-2 complex proceeds with the regeneration of I-2, which enables the catalytic use of I-2. The actual iodination reagent is I-2 and not NIS.