Dramatic effect of the metal cation in dealkylation reactions of phosphinic esters promoted by complexes of polyether ligands with metal iodides

Metal ion electrophilic catalysis has been revealed in dealkylation reactions of phosphinic esters 1-4 promoted by complexes of polyether ligands 5-7 with metal iodides MIn (Mn+ = Li+, Na+, K+, Rb+, Ca2+, Sr2+, Ba2+) in low polarity solvents (chlorobenzene, l,e-dichlorobenzene, and toluene) at 60 degreesC. The catalytic effect increases with increasing the Lewis acid character of the cation, in the order Rb+ < K+ < Na+ < Li+ and Ba2+ < Sr2+ < Ca2+. The results are interpreted in terms of a transition state where the complexed cation (Mn+ subset of Lig) assists the departure of the leaving group Ph2P(O)O- and, at the same time, favors the attack at carbon of the nucleophile I- ("push-pull" mechanism). The rate sequence found for 1-4 (Me > Et much greater than i-Pr and t-Bu) shows that this reaction can be utilized for the selective dealkylation of these substrates.