The solvolysis mechanism of simple secondary tosylates in 50% aqueous TFE

Abstract

The solvolysis of simple secondary tosylates in 50% trifluoethanol has been investigated using stereochemical and isotopic labels. 2-butyl, 2-pentyl and 2-octyl tosylates all solvolyse at very similar rates (~1 × 10−5 s−1) at 30 °C. Slow racemisation of S-2-butyl tosylate (~4.6 × 10−7 s−1) was observed during solvolysis, but R-2-octyl tosylate did not show any significant racemisation. Competing rearrangement of 3-pentyl tosylate to 2-pentyl tosylate was observed during solvolysis and is attributed to 1,2-hydride transfer, which occurs at a rate sufficient to account for the difference in the rates of racemisation of 2-butyl and 2-octyl tosylates. The stereochemistry of the alcohol product was studied for the reaction of R-2-octyl tosylate by derivatising the corresponding alcohol to 4-nitrobenzoate and showed high but not complete stereoselectivity (92:8 inversion : retention of configuration). 18O isotope exchange at the leaving group tosylate showed that both 2-butyl (5.4 × 10−7 s−1) and 2-octyl (4.2 × 10−7 s−1) tosylates exchange at similar rates. Partitioning of a common carbenium ion intermediate accounts for all these data only if solvolysis proceeds through pre-organisation of the solvent with 1,2-hydride transfer and isotope exchange competing inefficiently with ion return and solvolysis. Alternatively, a series of parallel concerted reactions (solvolysis, 1,2-hydride transfer and isotope exchange) would also account for these data