Observation of imidazolidine intermediates in the deprotonation route to 2-azaallyl anions

When the reaction in Scheme 1 was followed by GC, the expected first-order kinetics were not observed; in fact, evidence for an intermediate was obtained. Performing the deprotonation in [2H8]THF allowed observation of the rapid buildup of two new compounds by 1H NMR, which were assigned as the lithioimidazolidines 5 and 6 (Scheme 2). These were slowly converted to the bicyclic pyrrolidine 4 after several hours at room temp. (after quenching with water). Apparently, the initial deprotonation is slow enough to allow intermolecular cycloaddition with the imine portion of a molecule of the starting material. Cycloadditions of 2-azaallyl anions with imines to give imidazolidines has been observed by others [9-11]. The reverse process, namely the anionic cycloreversion of the lithioimidazolidines to the 2-azaallyl anion, must be occurring. This process has not been previously observed, although similar anionic cycloreversions are known [9, 12-14].

In a separate experiment, workup at partial conversion allowed the isolation of the protio derivatives 7 and 8 of undefined stereochemistry.

Resubjection of these imidazolidines to the reaction conditions (LDA, THF, room temp.) also gave 4. Hence, for the first time, it has been demonstrated that lithioimidazolidines are subject to anionic cycloreversion to 2-azaallyl anions. This may allow a new route to 2-azaallyl anions which does not rely on imine deprotonation, thereby perhaps obviating the need for a non-enolizable imine. For example, imidazolidines may be synthesized from carbonyl compounds and vicinal diamines, then subjected to base to produce N-lithioimidazolidines and thus 2-azaallyl anions (see later).

1. Introduction
2. Observations of Imidazolidine Intermediates in the Deprotonation Route to 2-Azaallyl Anions (This page)
3. Deliberate Generation of Imidazolidines from 2-Azaallyl Anions, and Their Use as 2-Azaallyl Anion Precursors
4. Synthesis of Imidazolidines from Diamines, and Their Use in 2-Azaallyl Anion Generation
5. An Unusual Stereochemical Complementarity
6. How General is the Imidazolidine Fragmentation Route to 2-Azaallyl Anions? and Conclusion
7. Experimental Section
8. References