Solvent and micellar effects upon the cyclisation of o-3-halopropyloxyphenoxide ions

Abstract

Rates of intramolecular cyclisations of o-(3-halopropyloxy)phenoxide ion [(1a, b), halogen = Br or I, respectively] increase with decreasing solvent polarity in the order H₂O < MeOH < EtOH < PrⁱOH < MeCN, except that reaction of (1a) is faster in H₂O than in MeOH, and in H₂O (1a) is more reactive than (1b). These effects depend upon the solvation of the oxide ion and the leaving halide ion in the transition state, and relative reactivities k^I/k^Br indicate the relative importance of these interactions. Micelles of CTAX [C₁₆H₃₃NMe₃X, X = Br, Cl, NO₃, OH, OTs, or (SO₄)_0.5] increase the rates by factors of ca. 2 for (1a) and by 4 for (1b), with little effect by the counter-ion. Rate enhancements and k^I/k^Br increase sharply in the sequence of surfactant head groups N⁺Me₃ < N⁺ Et₃ < N⁺Pr₃ < N⁺Bu₃. Increasing the size of the n-alkyl group apparently decreases hydration of the aryloxide without destabilising the transition state. These micellar medium effects can be related to medium effects in intermolecular reactions in micelles.