| Popis: |
Equilibrium acidities in Me2S0 are reported for six ketimines of the type Ph2C=NCH(R)C02Et and five aldimines, ArCH=NCH(R)C02Et. Changing R in the ketimine from H to Ph increased the pKa by 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 PhCH,, respectively. Phase-transfer alkylation of the Ph2C=NCH2C02Et ketimine gave over 90% of monoalkylate whereas, under similar conditions, the aldimine 4-ClC6H4CH=NCH2C02Et gave a mixture of monoand 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 Me2S0 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 monoand dialkylation reactions, is the principal factor that determines the extent of the competition between monoalkylation and dialkylation. |
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