Nucleophilic ion pairs. Part II. Micellar catalysis of proton abstraction by hydroxamate anions

Abstract

Hydroxamate anions in a cationic (cetyltrimethylammonium bromide, CTAB) micelle were found to serve as excellent catalysts for proton abstraction from α-ketols. Proton removal from benzoin by N-methylmyristohydroxamate anion (MMHA) in the CTAB micelle was 3 500 times faster than the corresponding reaction in a non-micellar system, and 56 times faster than that by the much more basic hydroxide ion. A similar, dramatic rate enhancement was attainable by addition of small amounts of CTAB or methyltri-n-octylammonium chloride (TMAC) to MMHA solubilized in aqueous non-ionic micelles. On the other hand, the MMHA, NEt₄ ion pair in organic solvents was also effective for proton abstraction. MMHA, NEt₄ in highly dehydrated acetonitrile gave a rate 103 higher than that in aqueous media. Minute amounts of H₂O and Nal efficiently quenched the rearrinn in dry organic solvents, and it was inhibited in protic solvents. It was suggested from these experimental data that the rate enhancement of MMHA in the oresence of a cationic micelle largely stems from formation of a hydrophobic ion pair which is loose and desolvated in the micellar phase.