SELECTIVITY IN C-ALKYLATION OF DIANIONS OF ACYCLIC BETA-ENAMINO KETONES

The regioselectivity of dianion formation from 4-(N-alkylamino)pent-3-en-2-one has been probed by deuteriation and alkylation techniques. The results obtained indicate that the regioselectivity can be controlled almost absolutely without resorting to classical conditions used to favour either the kinetic or the thermodynamic products. In fact, the kinetic (alpha'-dianion) and the thermodynamic dianion (gamma-dianion) are preferentially formed in THF at 20-degrees-C with a slight excess of Li-TMP (alpha'-conditions) or of MeLi/TMEDA (gamma-conditions) respectively. While under gamma-conditions, alkylation in the gamma-position is almost exclusively observed, kinetic preference for the alpha'-dianion is dependent on the size of the alkyl group linked to the nitrogen atom. The preference for alkylation at the alpha'-position increases from methyl to the bulky tert-butyl group. Once formed, the alpha'-dianion very slowly interconverts into the more stable gamma-dianion, even in the presence of TMEDA, whereas, when the dianion is formed under gamma-conditions, the rate of equilibration is comparable with that of its formation, with the result that gamma-dianion is exclusively formed. Theoretical calculations (HF/6-31G//3-21G) on the more stable conformation of the monoanion, both 'naked' and as a lithium ion pair, provide convincing explanations of these findings.