Synthesis of imidazolidines from diamines, and their use in 2-azaallyl anion generation

Since imidazolidines may be prepared from diamines and aldehydes, we sought to explore this method as a route to 2-azaallyl anions. Initially, we prepared the diamines 14 and 15 from the imidazolidines 10 and 11, since we had them in hand (Scheme 6). The imidazolidines used in this case were derived from the benzene experiment in Scheme 4. Thus the diamine 15 was formed as the major product upon acidic hydrolysis. Condensation of 15 with hex-5-enal in a Dean-Stark apparatus afforded the imidazolidine 16, which was treated with LDA. Cycloreversion to the 2-azaallyl anion and N-benzylidinehexylamine occurred, leading to the bicyclic pyrrolidine 4. Unfortunately, the N-benzylidinehexylamine by-product dimerized to the imidazolidines 10 and 11, making the workup and isolation of 4 more difficult. Nonetheless, this experiment showed that diamines may be used as starting materials for 2-azaallyl anion chemistry.

To avoid the dimerization of the imine by-product of the imidazolidine cycloreversions, we synthesized the N-isopropyl diamine 19 from (d,l)-stilbenediamine 17 [15] by a reductive amination approach (Scheme 7). We hoped that imidazolidines derived from 19 would form N-benzylidineisopropylamine as a by-product, which might not dimerize due to steric considerations. Scheme 7 also shows the successful conversion of 19 to the imidazolidines 20 and 21. The stereochemistry at C(2) of these compounds is assumed based on molecular modelling (MM2), assuming that the most stable diastereomer would be formed under the condensation conditions.

With the imidazolidines 20 and 21 in hand, we subjected these compounds to deprotonation with butyllithium (Scheme 8). Smooth cycloreversion of the N-lithioimidazolidines occurred, leading to the cycloadducts 4 and 22. No messy by-products were observed from the dimerization of N-benzylidineisopropylamine, which was observed in nearly quantitative amounts by GC.

  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 (This page)
  5. An unusual stereochemical complementarity
  6. How general is the imidazolidine fragmentation route to 2-azaallyl anions? and Conclusion
  7. Experimental section
  8. References