Conformational analysis. Part XI. Chemical equilibration of diastereoisomeric alkyl-1,3-dithians. Conformational preferences of alkyl substituents and the chair–boat energy difference. A revision of the chair–boat energy difference of cyclohexane
Acid-catalysed equilibration of diastereoisomeric 2-alkyl-4-methyl-1,3-dithians led to the following equatorial–axial free energy differences (in kJ mol–1): 2-Me, 8·02; 4- or 6-Me, 6·49; 2-Pri, 9·70. For 5-alkyl groups the conformational energies were also re-estimated. The enthalpy and entropy differences, 12·21 ± 0·04 kJ mol–1 and –20·10 ± 0·11 J mol–1 K–1, for the equilibrium between the chair form of the r-2-t-butyl-2,c-4-dimethyl derivative and the twist form of the r-2-t-butyl-2,t-4-dimethyl derivative were also determined by a study of the epimer equilibrium at several temperatures. These data together with some earlier findings were used to derive the following thermodynamic parameters for the chair–twist interconversion of the 1,3-dithian ring: ΔH0CT 16·7 ± 1·0 kJ mol–1, ΔS0CT 19·0 ± 2·0 J mol–1 K–1, ΔG0CT(298 K) 11·0 ± 1·6 kJ mol–1. Known equilibrium data for the diastereoisomeric 1,3-di-t-butylcyclohexanes, together with the observation that the trans-chair form is thermochemically more stable by 1·55 ± 0·84 kJ mol–1 than the trans-twist form, were used to re-estimate the conformational enthalpy of an axial t-butyl group, –23·7 ± 0·9 kJ mol–1, and the chair–twist energy parameters, ΔH0CT 25·2 ± 0·9 kJ mol–1 and ΔS0CT 13·4 ± 1·5 J mol–1 K–1 for cyclohexane.