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Baldwin’s Rules

Baldwin’s Rules. Nadia Fleary-Roberts 02/05/12. Baldwin’s rules for ring closure. Proposed in 1976 by Sir Jack Baldwin Guidelines for ring closing reactions of 3- to 7 -membered rings- empirical rules derived from experimental observations. J. Chem. Soc. Chem. Commum., 1976 , 734-736

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Baldwin’s Rules

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  1. Baldwin’s Rules Nadia Fleary-Roberts 02/05/12

  2. Baldwin’s rules for ring closure • Proposed in 1976 by Sir Jack Baldwin • Guidelines for ring closing reactions of 3- to 7-membered rings- • empirical rules derived from experimental observations J. Chem. Soc. Chem. Commum., 1976, 734-736 Advanced organic chemistry, March p 212 Clayden 1st Edn p 1140 2nd Edn p 810 Dr. F. Pradaux, Sheffield University

  3. Nomenclature 1. Ring size 3, 4,......7 Number Indicates the size of the ring being formed

  4. Nomenclature 2. Bond • The bond broken as the ring is formed is outsidethe new ring The bond broken as the ring is formed is inside the ring

  5. Nomenclature 3. Geometry geometry of the carbon undergoing the ring closing can be describes as: sp3 tetrahedral carbon tet sp2 trigonal carbon trig sp digonal carbon dig

  6. For example.....

  7. Reactions can be favoured or disfavoured • kinetic favourability of a reaction • A disfavoured reaction does not mean that the reaction will not occur -just that it is more difficult • Disfavoured cases require severe distortion of bond angles and distances

  8. Ideal bond angles • Favoured cyclisations enable terminal atoms to achieve proper geometries Trigonal carbons (Bürgi-Dunitz) Tetrahedral carbons (Walden inversion) Digonal carbons

  9. Tetrahedralsystems all exo-tet cyclisations are favoured

  10. Trigonal systems All exo-trig cyclisatons are favourable 5-endo trig are disfavoured!

  11. Favoured exo-tet and exo-trig Exo-tet Exo-trig

  12. Disfavoured 5-Endo-trig

  13. Disfavoured 5-Endo-trig...iodocyclisation • Iodocyclisation forms the disfavoured “anti-Baldwin” endo-trig product • WHY? • Overwhelming attraction between nucleophile and electrophile • No kinetically favoured alternative pathways- 4-exo-tet would be too strained Knight et.al. Tetrahedron Lett. 1998, 39, 8909

  14. however • 5-exo-trig strongly favoured over the 5-endo-trig

  15. Digonalsystems All endo-dig are favoured

  16. Endo-dig rationale • Alkyne has two π* orbitals-one in the plane of the ring

  17. Disfavoured 3- and 4-exo-dig • Nucleophile cannot attack at the required 120° angle

  18. Exceptions to the Baldwin’s rules • second row elements Why? • Long C-S bond • Sulfur has empty 3d orbitals • cations

  19. Summary

  20. Intramolecular endocyclic alkylation of ketone enolates Nomenclature enolexo or enolendo orientation of the enolate C-C bond exo-tet or exo-trig with respect to the C-Y terminus 6to 7 Favoured 3 to 5 Disfavoured 3 to 7 Favoured Baldwin, Kruse, JACS Chem Commun. 1977, 233

  21. Oxygen alkylation vs. Carbon alkylation (Compare with disfavoured 5-endo trig) (Compare with favoured 6-endo-trig)

  22. Oxygen alkylation vs. Carbon alkylation • Oxygen alkylation -electrophile approaches in plane of the enolate • Carbon alkylation in 6-enolendo possible • Perpendicular approach to the enolate

  23. Intramolecular aldol condensations Favoured 3 to 7 disfavoured 3 to 5 favoured 6 to 7 Baldwin, Tetrahedron, 1982, 38, 2939

  24. 5-enolendo-exo-trig vs. 6-enolendo exo-trig • 6-enolendo-exo-trig exclusively formed.

  25. Summary

  26. Just remember..... 1. Exo-tet and endo-trig favoured 2. 5-endo trig disfavoured* 3. Endo-dig favoured 1. Enolexo endo-tet and trig are favoured Endocyclic reactions

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

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