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