Macrocyclic polyethers as enolate activators in base-catalyzed phase-transfer reactions

A kinetic study of the alkylation reaction of deoxybenzoin (1) with alkylating agents 2-4, catalyzed by PHDB18C6 7 or [2.2.2,C-10]cryptand 8, has been performed in chlorobenzene-aqueous (or solid) base (NaOH, KOH) two-phase systems, under liquid-liquid (LL) and solid-liquid (SL) phase-transfer catalysis (PTC) conditions. The results obtained seem to indicate a multistep mechanism where the Ligand transfers the enolate from the interface to the bulk organic phase and the alkylation reaction is rate determining for all the process. The enolate reactivity strongly depends on the ligand and, to a lesser extent, on the base. The highest rate constant values are obtained under SL-PTC conditions and with [2.2.2,C-10]cryptand 8,which realizes the best cation-anion separation and hence anion activation. In line with these data, UV-vis spectroscopic determinations of the complexed enolate (M(+)subset of Lig)R- exhibit bathochromic shifts of the absorption maximum wavelength (lambda(max)) on changing from crown ether to cryptand. The distribution of the reaction products (O/C alkylation ratio) is also related to the different ability of the ligand to separate cation from anion in the ion pair.