Formation of hemiaminals by N-protonation of ketenimines (etheneimines) sterically hindered at carbon

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 degrees C. This is confirmed by the inverse solvent isotope effect for this acid-catalysed reaction (k(H2O)/k(D2O) = 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).