Ethyl (2-cyano-3-ethoxyacryloyl)carbamate: Irreversible thermal isomerization of a push-pull olefin

A push-pull olefin, E-2, was prepared highly stereoselectively by reaction of ethyl (2-cyanoacetyl)carbamate, 1, with ethyl orthoformate in the presence of acetic anhydride. Forty percent of E-2 was isomerized to Z-2 after two hours of irradiation at 254 nm. All the Z-2 was isomerized back to E-2 spontaneously and irreversibly with Ea = 19.6 kcal mol^-1, ΔH‡= 19.0 kcal mol^-1, ΔS‡ = -17.5 cal K^-1 mol^-1, and ΔG‡ = 24.4 kcal mol^-1. The negative entropy of activation for this isomerization indicates that the transition state has much more charge separation than the ground state. The low rotational barrier of this isomerization and very big chemical shift difference (Δδ _C=C = 85.83 ppm) of two sp²-hybridized carbons of C=C partial double bond confirm 2 is a push-pull olefin.