Solvolysis of secondary alkyl azoxytosylates. A new reaction related to solvolytic deamination and arenesulphonate solvolysis

Abstract

The rates of solvolysis of 2-adamantyl azoxytosylate have been measured in hexafluoropropan-2-ol(HFIP), trifluoroethanol (TFE), aqueous hexafluoropropan-2-ol (97HFIP), aqueous trifluoroethanol (97TFE), aqueous ethanol (50E and 80E), and aqueous methanol (80M) at 3–5 temperatures between ca. 23 and 70 °C, and activation parameters have been determined. The O-²H solvent kinetic isotope effect has been measured for HFIP (k^H/k^D 1.02₅± 0.02 at 50.8 °C) and secondary α-deuterium kinetic isotope effects have been determined in 97HFIP (k^H/k^D 1.090 ± 0.005; 40.2 °C),50E (1.091 ∓ 0.006; 51.0 °C), and 80E (1.133 ∓ 0.009; 60.9 °C). Neither methanesulphonic acid in 80M nor pyridine in TFE gives rise to a significant rate enhancement, and there is hardly any salt effect due to low concentrations of tetrabutylammonium perchlorate in 97TFE. 2-Adamantyl tosylate is formed in parallel with solvolysis product in ethanol and weakly aqueous ethanol, and is the sole organic product from reactions in chloroform and toluene. No relatively long lived intermediate such as 2-adamantyl tosylate is involved in the reaction in 97TFE, 80M, and 90M. The Kinetics and product results are satisfactorily accommodated by a mechanism involving initial unimolecular rate-determining synchronous fragmentation of the substrate to give nitrous oxide and an ion-pair. This ion-pair is then either captured by solvent to give solvolysis product or, in the less polar media, undergoes ion-pair combination to yield covalent 2-adamantyl tosylate. Bicyclo[2.2.2]octan-2-yl azoxytosylate has also been prepared, and rates and activation parameters have been determined in 97HFIP and 80M. There is no evidence from this limited study of a mechanism different from that of the 2-adamantyl analogue. Rates and activation parameters for the solvolysis of cyclohexyl azoxytosylate have also been determined in an even more limited study using only 97HFIP. Rates are slower than those for the other two compounds, but the mechanism appears to be the same.