Journal of the Chemical Society. Perkin transactions II p. 967 - 972 (1993)
Update date:2022-08-11
Topics:
Thompson, Hugh W.
Gaglani, Kamlesh D.
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 synthesized.Acid-catalyzed equilibrations of double-bond position were conducted in acetic anhydride at ca. 60, 100 and 140 deg 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-1 K-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.
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