Preparation and solvolysis of trans,trans,trans-tricyclo[7.3.1.0]tridecan-3-yl toluene-p-sulphonates in acetic acid and aqueous hexafluoropropan-2-ol. The role of non-chair conformers and the question of internal return in the limiting SN1 solvolytic mechanism

Abstract

Details are reported of a general synthesis of the epimeric trans,trans,trans-tricyclo[7.3.1.0]tridecan-3-ols and the corresponding toluene-p-sulphonates, (5a and b). Both compounds react in buffered acetic acid slightly more slowly than simpler unbridged axial and equatorial cyclohexyl toluene-p-sulphonates, but the axial: equatorial epimeric rate ratio is very similar at 3.4 : 1 (50 °C) and is quite different from the divergent values obtained for bridged systems. The epimeric rate ratio is even smaller in 97 : 3 (w/w) hexafluoropropan-2-ol–water (97HFIP), 1 : 1 at 25 °C, and the equatorial compound is estimated to be the more reactive in this medium below ca. 6 °C. Remarkably, the standard enthalpy of activation for solvolysis in 97HFIP is lower for the equatorial than the axial isomer. These results, the role of non-chair conformers for the equatorially substituted isomer, and the nature of the rate-determining step in the limiting S_N1 solvolytic mechanism generally are discussed.