Abstract
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 + cyclic polyether). Results have been rationalized on the basis of a transition state where the complexed cation (M + ⊂ 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 - .
Highlights
Macrocyclic and open-chain polyethers are known for their unique ability to bind inorganic salts, giving rise to very stable inclusion complexes
The ligand topology plays a leading role in reactions where cations are involved in the activation process ("electrophilic catalysis").1b,5 We revealed metal ion electrophilic catalysis in nucleophilic substitution reactions of alkyl sulfonates6 and, very recently, in demethylation reactions of phosphinic7 and phosphate8 esters promoted by complexes of polyether ligands with alkali and alkaline-earth metal iodides in solvents of low polarity
The present results clearly indicate the solid-liquid phase-transfer system catalysed by PEG400Me2 7 as the best one for obtaining the ring opening of 1,2-epoxides under mild conditions and in high yields
Summary
Macrocyclic and open-chain polyethers (polyethylene glycols, crown ethers, cryptands) are known for their unique ability to bind inorganic salts, giving rise to very stable inclusion complexes. Like solid-liquid (SL) or liquid-liquid (LL) phase transfer catalysis (PTC) In these complexes the anion is remarkably activated due to the reduced interaction with the bulky complexed cation and little stabilization by the weakly polar medium .1.2-4 Reactivity is mainly determined by the ability of the polyether to induce cation-anion separation, increasing in the order: open-chain
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