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Effect of Internal Hydrogen Bonding on Base‐Catalyzed NH Proton‐Exchange Reactions of Isomeric Enamines

Proton NMR spectra of the E and Z isomers of enamines 2–4, R′CH2NHC(CO2R) CHCO2R [2, R′ = C6H5, R = C(CH3)3; 3, R′ = 2‐furyl, R = CH3; 4, R′ = 2‐furyl, R = C(CH3)3], were obtained in neutral and basic solutions at 25°C. Base‐catalyzed exchange of the E isomer was found to be faster than that for the Z isomer. These differences in exchange rate are discussed in terms of steric, electronic and hydrogen bonding effects. The results of AM1 semiempirical quantum‐mechanical calculations on various geometries of 3 show that the NH proton is involved in simultaneous three‐center hydrogen bonds.