Flash Photolytic Generation of Primary, Secondary, and Tertiary Ynamines in Aqueous Solution and Study of Their Carbon-Protonation Reactions in That Medium

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 photolyic 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
wr = 8.11 ± 0.15 kcal
mol-1.