An unusual stereochemical complementarity

Scheme 9 summarizes the stereochemical outcome of the imidazolidine method and the deprotonation method for 2-azaallyl anion synthesis. Note that in each case, the two routes give the opposite stereochemical outcomes. Pyrrolidines 4b and 22a are the likely result of cycloaddition through the 'W'-geometry of the 2-azaallyl anion, while pyrrolidines 4a and 22b must proceed through a 'sickle'-geometry. It is certainly believable that the imidazolidine method and the deprotonation method may give different anion geometries. However, there are several curious aspects to this hypothesis. First, as determined above, the deprotonation of 1 goes through imidazolidines! Why, then, would these imidazolidines and 20 give different results? Possible explanations include
  1. Imidazolidines 7 and 8 (Scheme 3) each produce different 2-azaallyl anion geometries
  2. The stereochemistry of 20 [particularly at C(2)] is different from the stereochemistry of imidazolidine 7 (Scheme 3), perhaps leading to a different 2-azaallyl anion geometry.
Where the explanation becomes difficult is with the results shown at the bottom of Scheme 9. Note that 21, an imidazolidine analogous to 20, gives products resulting from the sickle- rather than W-anion geometry. How can the simple change of placing a phenyl group on the anionophile cause such a reversal? Also surprising is the fact that the deprotonation of 23 gives opposite results to the deprotonation of 1. However, no evidence for imidazolidine intermediates was obtained in the deprotonation of 23, so these two experiments are not entirely comparable.
The stereochemistry of the bicyclic pyrrolidines was vouchsafed by difference NOE NMR experiments.

1. Introduction
2. Observations of Imidazolidine Intermediates in the Deprotonation Route to 2-Azaallyl Anions
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 (This page)
6. How General is the Imidazolidine Fragmentation Route to 2-Azaallyl Anions? and Conclusion
7. Experimental Section
8. References