Solvent and crown ether/cryptand effects on the oximate-promoted 1,2-elimination from β-phenylmercaptoethyl p-nitrophenolate. Formation and reactivity of a crown ether-complexed potassium oximate ion pair

Abstract

The reactions of β-phenylmercaptoethyl p-nitrophenolate (1) with three potassium oximates, viz. potassium 2,3-butanedione monoximate (BDOK), acetophenone oximate (APOK), and acetone oximate (AOK) have been investigated in two nonhydroxylic dipolar aprotic solvents, tetraglyme and dimethyl sulfoxide (DMSO). The reaction products, as determined by 1H NMR spectroscopy, were p-nitrophenoxide ion and phenyl vinyl sulfide, in accord with an elimination process. A kinetic spectrophotometric study showed that in tetraglyme, the addition of oxime and water in small amounts drastically decreased the rate of reaction of 1 with APOK as a result of hydrogen-bonding interactions with the oximate anion. In tetraglyme as solvent the addition of 2.2.2 cryptand greatly enhanced the reactivity of the oximates but the macrocyclic crown ether DC-18-C6 had much smaller effect on rate. The results are consistent with formation of a crown ether-complexed potassium oximate ion pair, which is much less reactive than the free anion formed in the presence of 2.2.2 cryptand but more reactive than the potassium oximate ion pair. The kinetic data were analyzed to obtain specific rate coefficients for reaction of APOK as the dissociated anion, the ion-paired species, and as the crown ether-complexed oximate; equilibrium constants for potassium oximate ion pair formation and for the crown ether-complexed ion pair were obtained. In DMSO as solvent the rate of reaction remained unaffected on addition of the metal ion complexing agents, indicating that ion pairing is not important in this solvent. The reactivity of the free oximate anions in both solvents increased in the order BDOK < APOK < AOK, which parallels the pKa's of the corresponding oximes in DMSO. The unusually high reactivity of oximates in the low polarity tetraglyme compared to polar DMSO could be explained on the basis of stabilization of the transition state in tetraglyme. Trends in kTG/kDMSO for the oximates follow reactivity–selectivity considerations.