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. Copyright © 2016 John Wiley & Sons, Ltd.