METAL-ION CATALYSIS IN NUCLEOPHILIC-SUBSTITUTION REACTIONS PROMOTED BY COMPLEXES OF POLYETHER LIGANDS WITH ALKALI-METAL SALTS

Metal ion effects have been revealed on nucleophilic substitution reac tions of n-octyl sulfonates 1-3 promoted by complexes of polyether lig ands (PEGs, crown ethers, cryptands) with alkali metal salts MY (M = L i, Na, K; Y = I, Br) in low polarity solvents (chlorobenzene, o-dichlo robenzene, toluene) at 60 degrees C. Rate constants increase, in the o rder K+ < Na+ <Li+, with the complexes of crown ethers 5 and 6 and PEG 4, whereas they are independent of the cation in the case of cryptate s of 7. In the series of sulfonic esters 1-3 these effects progressive ly diminish with increasing nucleofugality of the leaving group. These results are interpreted on the basis of a transition state where the complexed cation (Lig superset of M(+)) assists the departure of the l eaving group (''electrophilic catalysis'').