The α-Effect in Reactions of sp-Hybridized Carbon Atom: Michael-Type Reactions of 1-Aryl-2-propyn-1-ones with Primary Amines

[reaction: see text] Second-order rate constants (kN) have been measured for the Michael-type reaction of 1-(X-substituted phenyl)-2-propyn-1-ones (2a-f) with a series of primary amines in H2O at 25.0 +/- 0.1 degree C. A linear Brønsted-type plot with a small beta(nuc) value (beta(nuc) = 0.30) has been obtained for the reactions of 1-phenyl-2-propyn-1-one (2c) with non-alpha-nucleophile amines. Hydrazine is more reactive than other primary amines of similar basicity (e.g., glycylglycine and glycine ethyl ester) and results in a positive deviation from the linear Brønsted-type plot. The reactions of 2a-f with hydrazine exhibit a linear Hammett plot, while those with non-alpha-nucleophile amines give linear Yukawa-Tsuno plots, indicating that the electronic nature of the substituent X does not affect the reaction mechanism. The alpha-effect increases as the substituent X in the phenyl ring of 2a-f becomes a stronger electron-donating group. However, the magnitude of the alpha-effect for the reactions of 2a-f is small (e.g., kN(hydrazine)/kN(glycylglycine) = 4.6-13) regardless of the electronic nature of the substituent X. The small beta(nuc) has been suggested to be responsible for the small alpha-effect. A solvent kinetic isotope effect (e.g., kN(H2O)/kN(D2O) = 1.86) was observed for the reaction with hydrazine but absent for the reactions with non-alpha-nucleophile amines. The reactions with hydrazine and other primary amines have been suggested to proceed through a five-membered intramolecular H-bonding structure VI and a six-membered intermolecular H-bonding structure VII, respectively. The transition state modeled on VI can account for the substituent dependent alpha-effect and the difference in the solvent kinetic isotope effect exhibited by the reactions with hydrazine and other primary amines. It has been proposed that the beta(nuc) value is more important than the hybridization type of the reaction site to determine the magnitude of the alpha-effect.