Flash photolytic generation of primary, secondary, and tertiary ynamines in aqueous solution and study of their carbon-protonation reactions in that medium

Article abstract of DOI:10.1021/ja9601797

A group of nine phenylynamines (PhC≡CNH₂, PhC≡CNHCH(CH₃)₂, PhC≡CNHC₆H₁₁, PhC≡CNHC₆H₅, PhC≡CNHC₆F₅, PhC≡CN(CH₂)₅, PhC≡CN(CH₂CH₂)₂O, PhC≡CN(CH₂CH₂CN)₂, and PhC≡CN(CH₃)C₆F₅) were generated in aqueous solution by flash photolytic decarbonylation of the corresponding phenylaminocyclopropenones, and the kinetics of their facile decay in that medium were studied. This decay is catalyzed by acids for all ynamines-primary, secondary, and tertiary-and also by bases for primary and secondary ynamines. Solvent isotope effects and the form of acid-base catalysis show that the acid-catalyzed path involves formation of keteniminium ions by rate-determining proton transfer to the β-carbon atoms of the ynamines. The ions generated from primary and secondary ynamines then lose nitrogen-bound protons to give ketenimines, and the ketenimines obtained from secondary ynamines are hydrated to phenylacetamides, whereas that from the primary ynamine tautomerizes to phenylacetonitrile. Keteniminium ions formed from tertiary ynamines have no nitrogen-bound protons that can be lost, and they are therefore captured by water instead, and the amide enols thus produced then ketonize to phenylacetamides. The base-catalyzed decay of primary and secondary ynamines also generates ketenimines, but protonation on the β-carbon is now preceeded by proton removal from nitrogen. Rate constants for β-carbon protonation of PhC≡CNHCH(CH₃)₂ and PhC≡CN(CH₂)₅ by a series of carboxylic acids give linear Bronsted relations with exponents α = 0.29 and 0.28, respectively, whereas inclusion of literature data for protonation of PhC≡CN-(CH₂)₅ by a group of weaker acids gives a curved Bronsted relation whose exponent varies from 0.25 to 0.97. Application of Marcus rate theory to this curved Bronsted relation produces the intrinsic barrier ΔG((+))(o) = 3.26 ± 0.19 kcal mol^-1 and the work term w(r) = 8.11 ± 0.15 kcal mol^-1.