Energetic Preferences for α,β versus β,γ Unsaturation

Density functional theory has been applied at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level to examine the energetics of α,β- versus β,γ-unsaturation for some common organic functional groups. Specifically, the relative stabilities of allyl-X (H2CCHCH2X) and 1-propenyl-X (H3CCHCHX) isomers have been computed for X = methyl, vinyl, phenyl, formyl, acetyl, methoxy, methylthio, methylsulfinyl, methylsulfonyl, sulfamoyl, and methoxysulfonyl, and the results are compared to available experimental data. The intrinsic preference of 3 kcal/mol for the 1-propenyl isomer when X = CH3 is exceeded by 2−4 kcal/mol for first-row conjugating groups, but it is not met for the sulfur-containing groups. In particular, α,β-unsaturation is favored by less than 1 kcal/mol for the sulfone and sulfonamide analogues, while it is preferred by 8 kcal/mol for the vinyl-substituted case. Detailed structural results and torsional energy profiles are also reported.