Formation of hemiaminals by N-protonation of ketenimines(etheneimines) sterically hindered at carbon
1997
The bis(pentamethylphenyl)-N-isopropyl-ketenimine
[N-isopropyl-2,2-bis(pentamethylphenyl)ethenimine] 11 undergoes
pre-equilibrium N-protonation followed by water attack in
1:1 acetonitrile–water at 25 °C. This
is confirmed by the inverse solvent isotope effect for this
acid-catalysed reaction
(kH2O/kD2O
= 0.48) and the observation of the
2,2-bis(pentamethylphenyl)ethene-1,1-diol 23 as an intermediate. This is
formed from the ketene 22 on fragmentation of the intermediate
hemiaminal 26. At pH 1.15, proton transfer to the hemiaminal 26 (which
is formally an enol of an amide) and pH independent fragmentation of 26
proceed at equal rates. This change in protonation site from the normal
carbon [observed with the phenyl and diphenyl ketenimines (8, 9)] is
ascribed to the steric crowding about carbon in the pentamethylphenyl-
and mesityl-ketenimines (11, 10).
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