Equilibration of alkene regioisomers in trans-and cis-octalins

Abstract

As models for studying the energetics of double-bond regiochemistry in the octalin system, the enol acetates of trans-and cis-2-decalone (1, 4), trans-and cis-10-methyl-2-decalone (7, 10) and trans-and cis-9-methyl-2-decalone (13, 16) have been synthesised. Acid-catalysed equilibrations of double-bond position were conducted in acetic anhydride at ca.60, 100 and 140°C and assessed by integration of the vinyl-proton NMR signals, which were unambiguously assigned to each enol acetate either by the observed splitting pattern or synthesis. Values of ΔH and ΔS(Δ²:Δ¹) are derived and compared with experimental and theoretical literature values. Values of ΔH for the enol acetates of 1 and 4 are –0.69 and 0.0 kcal mol^–1, respectively, and addition of an angular methyl decreases the relative stability (ΔH)of the Δ¹ isomer, by 1.25–2.25 kcal mol^–1 for the trans skeleton and by 1.0–1.4 kcal mol^–1 for the cis skeleton. For a given angular substituent, changing cis stereochemistry to trans also decreases the relative Δ¹-stability. by 0.7 kcal mol^–1 when R = H and 0.95–1.55 kcal mol^–1 when R = Me. Values of ΔS are all small, between +0.1 and –3.2 cal mol^–1K^–1. Trends in the data and features of the ¹H NMR spectra related to conformation are discussed, and an approach is suggested for calculating approximate ΔH, ΔS and ΔG values for Δ¹-9-methyl-vs.Δ¹-10-methyloctalin in the trans and cis series.