Micellar induced regioselectivity in the two-step consecutive reaction of SO2−3 with Br(CH2CH2)nBr (n=2–5n=2–5)

High field (800 MHz) 1H NMR was used to monitor the two-step consecutive reaction of excess SO2−3 with symmetrical bifunctional α,ωα,ω-dibromoalkanes with butane (DBB), hexane (DBH), octane (DBO), and decane (DBD) chains in CTAB micelles at 25 °C. The first-order rate constant for the first substitution step for DBB and DBH is about 5 times faster than for the second, but the kinetics for DBO and DBD were not cleanly first-order. After 40 min, the solution contained about 80% of the intermediate bromoalkanesulfonate from DBB and DBH and the remainder is alkanedisulfonate and unreacted starting material. The same reactions were carried out in homogeneous MeOH/D2O solutions at 50 °C. The rate constants for all four α,ωα,ω-dibromoalkanes were first-order throughout the time course of the reaction and the same within ±10%. However, because micellar solutions are organized on the nanoscale and bring together lipophilic and hydrophilic reactants into a small reaction volume at the micellar interface, they speed this substitution reaction considerably compared to reaction in MeOH/D2O. The CTAB micelles also induce a significant regioselectivity in product formation by speeding the first step of the consecutive reaction more than the second. The results are consistent with the bromoalkanesulfonate intermediates having a radial orientation within the micelles with the CH2SO−3 group in the interfacial region and the CH2Br group directed into the micellar core such that the concentration of CH2Br groups in the reactive zone, i.e., the micellar interface, is significantly reduced. These results provide the first example of self-assembled surfactant system altering the relative rates of the reaction steps of a consecutive reaction and, in doing so, enhancing monosubstitution of a symmetrically disubstituted species.

Figure optionsDownload as PowerPoint slide