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Baldwin s Rules for Ring Closure

Baldwin's Rules. Qualitative set of generalization on the probability of a ring closure (RC)Empirical

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Baldwin s Rules for Ring Closure

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    1. Baldwins Rules for Ring Closure Dr Fabienne Pradaux Wednesday 2nd February 2005

    3. Terminology/Classification (1) Prefix exo when the breaking bond is exocyclic to the smallest ring formed. Prefix endo when the breaking bond is endocyclic to the smallest ring formed.

    5. Rules for Ring Closure (1) Tetrahedral Systems 3 to 7-exo-Tet are all favoured processes 5 to 6-endo-Tet are disfavoured

    6. Rules for Ring Closure (2) Trigonal systems 3 to 7-exo-Trig are all favoured processes 3 to 5-endo-Trig are disfavoured; 6 to 7-endo-Trig are favoured

    7. Rules for Ring Closure (3) Digonal Systems 3 to 4-exo-Dig are disfavoured processes; 5 to 7-exo-Dig are favoured 3 to 7-endo-Dig are favoured

    8. Other Than Nucleophilic Cases Radical Processes (homolitic): Cationic Processes:

    9. Attack Trajectories Favoured paths to transition states are:

    10. Favoured and Disfavoured Favoured RC will be those in which length and nature of linking chain enables terminal atoms to achieve required trajectory to form final ring bond Disfavoured RC would require severe bond angle and distances distortion, so the desired RC will be difficult (if available, alternative pathways will dominate)

    11. Tetrahedral carbon (1) All exo-Tet cyclisations are favoured

    12. Tetrahedral carbon (2) Why all endo-Tet cyclisations are disfavoured?

    13. Trigonal Carbon (1) Why all exo-Trig cyclisations are favoured?

    14. Trigonal Carbon (2) Example of disfavoured 5-endo-Trig

    15. Trigonal Carbon (3)

    16. Trigonal Carbon (4) 5-endo-Trig exceptions :

    17. Trigonal Carbon (5) 5-endo-Trig versus 5-exo-Trig; Nitrogen analogue

    18. Trigonal Carbon (6) 5-endo-Trig versus 5-exo-Trig; Nitrogen analogue (MO explanation)

    19. Digonal carbon All endo-Dig cyclizations are favoured

    20. Intramolecular Alkylations of Ketone Enolates (1)

    21. Intramolecular Aldol Condensations (1)

    22. Stereoelectronic Constraints

    23. Intramolecular Alkylations of Ketone Enolates (1) Why 5-(Enolendo)-exo-tet disfavoured?

    24. Intramolecular Alkylations of Ketone Enolates (2) Why 6-(enolendo)-exo-tet favoured?

    25. Intramolecular Aldol Condensations (2)

    26. Intramolecular Aldol Condensations (3)

    27. Baldwin Rules in Harrity Group (1)

    28. Baldwin Rules in Harrity Group(2)

    29. Conclusion Only give information about whether processes are favoured or disfavoured and not allowed and forbidden. Nucleophilic RC feasibility strongly depends on ring size, geometry of reacting atom and exo or endo nature of reaction. Structural modification can dramatically affect the cyclization mode. If favoured trajectory of attack valid, then reaction will follow the Baldwins rules.

    30. Summary (1)

    31. Summary (2)

    32. References J. E. Baldwin, J. Chem. Soc., Chem. Commun. 1976, 734. J. E. Baldwin, J. Cutting, W. Dupont, L. Kruse, L. Silberman, R. C. Thomas, J. Chem. Soc., Chem. Commun. 1976, 736. J. E. Baldwin, R. C. Thomas, L. Kruse, L. Silberman, J. Org. Chem, 1977, 42, 3846. J. E. Baldwin, L. Kruse, J. Chem. Soc., Chem. Commun. 1977, 233. J. E. Baldwin, M. J. Lusch, Tetrahedron, 1982, 38, 2939. C. D. Johnson, Acc. Chem. Res. 1993, 26, 476. J. Clayden, N. Greeves, S. Warren, P. Wothers, Oxford, chapter 42, p 1140. D. A. Evans, internet course.

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