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Macrocyclic Ligands: Functional Host Guest Systems.

Macrocyclic Ligands: Functional Host Guest Systems. Post Graduate Lecture Course February 2000 Lecture 3. Synthesis: Ring Closure. This is the key step Chemistry generally simple but product selectivity often poor Thermodynamically large rings are stable

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Macrocyclic Ligands: Functional Host Guest Systems.

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  1. Macrocyclic Ligands: Functional Host Guest Systems. Post Graduate Lecture Course February 2000 Lecture 3

  2. Synthesis: Ring Closure • This is the key step • Chemistry generally simple but product selectivity often poor • Thermodynamically large rings are stable • Kinetics favour 5-membered rings - macrocycles are not favourable

  3. Cyclisation vs Polymerisation • Competing Processes • Linear Polymerisation favoured by solvent free reactions • 1:1 and 2:2 cyclisation are slow; favoured by low temperatures and high dilution (<10-3 M). Ziegler et al., Liebigs Ann. Chem., 1933, 504, 95. • Polycyclisation (3:3, 4:4 etc) generally difficult to obtain high yields

  4. Synthetic Solutions • Two methodologies have been adopted to overcome ring closure difficulties • High Dilution Synthesis • Template-Mediated Synthesis

  5. Template-Mediated Cyclisation • Use of a metal (usually) ion • interacts with heteroatoms of reactants Two possible roles: • Pre-arrangement of functional groups so that orientation favours cyclisation (Kinetic Template Effect) • Extracting Cyclic product from reaction mixture, thus altering equilibrium position (Thermodynamic Template Effect)

  6. Examples • Curtis Synthesis: Curtis, J. Chem. Soc., 1960, 4409.

  7. Busch, J. Amer. Chem. Soc., 1964, 86, 3651. Inorg. Synth., 1975, 16, 220.

  8. Metal mediated cyclo-oligomerisation • Norman, J. Am. Chem. Soc., 1982, 104, 4700 • Edwards, J.Chem. Soc. Dalton Trans., 1999, 1024

  9. Metal-Mediated Schiff Base Condensation Example of thermodynamic template effect. Busch, 1966, J. Am. Chem. Soc.,1966, 88, 3170.

  10. Schiff Base Ligands: Other Substrates • Reaction usually involves an ,-diamine and a diketone or dialdehyde. • Some dialdehydes and diketones...

  11. Schiff Base Ligands: Other Substrates • Some typical diamines: • Some Schiff base macrocycles:

  12. High Dilution Synthesis • Relatively uncommon • Generally more difficult experimentally • Yields often lower • Less product selective • Hence separation of products necessary So why use it at all…...

  13. Some substrates not amenable to template methods • Sometimes metal template ion cannot be removed from macrocycle • Sometimes it just works well!

  14. High Dilution Apparatus

  15. Examples of high dilution syntheses • Lehn, Tetrahedron, 1973, 29, 1629 • At 0C - slower reaction but higher yield (Babb, J. Heterocyclic Chem., 1986,23,609) • Can speed up reaction by activating the acid chloride….

  16. Here the thiazolidine increases the activity of the carbonyl function, increasing reaction rate significantly.

  17. Note that many apparently high dilution syntheses are in fact template reactions. • For example… The alkali metal ion acts as template for the cyclisation

  18. Changing alkali metal ion alters the yield.. Solvent Template ion Li Na K Cs MeCN 6 44 27 15 dioxane - 16 24 - glyme - 8 24 15 MeOH - - <5 -

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