Comparative Reactivities of Phosphate Ester Cleavages by Metallomicelles

The hydrolytic efficiency of the hydrophilic Cu(II) complex of tetramethylethylenediamine (TMED C-1 . Cu(II)) and the amphiphilic N-n-hexadecyl-N,N',N'-trimethylethylenediamine complex (TMED C-16. Cu(II)) in comicelles with CTANO(3) was studied toward a series of phosphate triesters and diesters that were either neutral (1-7) or cationic and amphiphilic (8-10). While the pseudo-first-order rate constants at [catalyst] = 1 x 10(-3) M are larger (ca. one order of magnitude) for the micellar catalyst, the second-order rate constants are higher for the hydrophilic catalyst. With phosphate triesters, regardless of the structure of the substrate, TMED C1(.) Cu(II)-bound OH- is a better nucleophile than free OH-, while TMED C-16. Cu(II)-bound OH- is poorer. This is explained by electrophilic assistance of the metal center which greatly diminishes in micelles. With phosphate diesters the OH- bound to both TMED C-1 . Cu(II) and TMED C-16. Cu(II) is a better nucleophile than free OH-. Partial neutralization of the negative charge of the substrate may explain this behavior. In all cases the amphiphilic substrates are intrinsically more reactive than the neutral ones. Comparison with an iodosobenzoate-based catalyst shows that this is ineffective with phosphate diesters while it is a better reagent than the metallomicellar one with phosphate triesters. Its higher effectiveness, however, seems to vanish as the substrate becomes less reactive.