Ring opening reactions of 1,2-epoxides catalyzed by complexes of polyether ligands with metal halides in low polarity solvents. Comparison with heterogeneous systems

Metal ion electrophilic catalysis has been revealed in ring opening reactions of 1,2-epoxides 1-5 promoted by complexes of polyethers 6-9 with alkali metal halides MHal (M+ = Li+, Na+, K+; Hal(-) = I-, Br-) in solvents of low polarity (chlorobenzene, 1,2-dichlorobenzene). The catalytic effect depends on the Lewis acid character of the cation (K+ <<Na+ < Li+) as well as on the ligand topology (open-chain > cyclic polyether). Results have been rationalized on the basis of a transition state where the complexed cation (M+ subset of Lig) stabilizes the negative charge developing on the oxygen atom of the oxirane ring while favors the nucleophilic attack at the adjacent carbon by the halide anion Hal(-).