Equilibrium acidities in Me2SO are reported for six ketimines of the type Ph2C=NCH(R)CO2Et and five aldimines, ArCH=NCH(R)CO2Et. Changing R in the ketimine from H to Ph increased the pKa 2.2 units. This surprising acidity decrease for Ph substitution points to a substantial increase in steric effect, as do the increases in pKa of 3.8 and 4.2 units observed for the replacement of hydrogen by Me and PhCH2, respectively. Phase-transfer alkylation of the Ph2C=NCH2CO2Et ketimine gave over 90% of monoalkylate whereas, under similar conditions, the aldimine 4-ClC6H4CH=NCH2CO2Et gave a mixture of mono- and dialkylate. The difference is that the pKa of the monoalkylated aldimine is essentially the same as that of the parent, which leads to rapid equilibration with the parent anion and consequent dialkylation. The rates of alkylation in Me2SO of these parent and monoalkylated anions did not differ greatly, showing that the relative pKHAs of the parent acid and its monoalkyl derivative, rather than the relative rates of the mono- and dialkylation reactions, is the principal factor that determines the extent of the competition between monoalkylation and dialkylation.
ASJC Scopus subject areas
- Colloid and Surface Chemistry