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Baldwin's rules



Baldwin's Rules in organic chemistry are a series of guidelines outlining the relative favourabilities of ring closure reactions in alicyclic compounds. They were first proposed by Jack Baldwin in 1976 [1] [2].

The rules classify ring closures in three ways:

  • the number of atoms in new ring formed
  • into exo and endo ring closures, depending whether the bond broken during the ring closure is inside (endo) or outside (exo) the ring which is formed
  • into tet, trig and dig, depending whether the electrophilic carbon is tetrahedral/sp3 (tet), trigonal/sp2, (trig) or digonal/sp, (dig)

Additional recommended knowledge

Thus, a ring closure reaction could be classified as, eg. 5-exo-trig. It should be noted that 5-endo-trig is disfavoured, but look "nice" on the paper:


Baldwin discovered that orbital overlap requirements for the formation of bonds favour only certain combinations of ring size and the exo/endo/dig/trig/tet parameters.

It should be noted that Baldwin's rules are general guidelines, and that exceptions do occur. Eg. cations often disobey Baldwin's rules, and reactions where a second-row atom are included in the ring.

The Baldwin disfavoured ring closures and Baldwin favoured ring closures: are listed below.


tet trig dig
3 exo X
endo X
4 exo X
endo X
5 exo
endo X X
6 exo
endo X
7 exo
endo

Examples

In one study seven-membered rings are constructed in a tandem 5-exo-dig addition reaction / Claisen rearrangement [3]:


A 6-endo-dig pattern is observed in a allene - alkyne 1,2-addition / Nazarov cyclization tandem catalysed by a gold compound [4]:

References

  1. ^ Baldwin, J. E., Rules for Ring Closure, J. Chem. Soc., Chem. Commun. 1976, 734. doi:10.1039/C39760000734
  2. ^ Baldwin, J. E., et al, Rules for Ring Closure: Ring Formation by Conjugate Addition of Oxygen Nucleophiles, J. Org. Chem., 42(24), 3846 (1977). doi:10.1021/jo00444a011
  3. ^ Synthesis of Seven-Membered Carbocyclic Rings via a Microwave-Assisted Tandem Oxyanionic 5-exo dig Cyclization-Claisen Rearrangement Process Xin Li, Robert E. Kyne, and Timo V. Ovaska J. Org. Chem., 72 (17), 6624 -6627, 2007. doi:10.1021/jo0710432
  4. ^ Gold-Catalyzed Synthesis of Bicyclo[4.3.0]nonadiene Derivatives via Tandem 6-endo-dig/Nazarov Cyclization of 1,6-Allenynes Guan-You Lin, Chun-Yao Yang, and Rai-Shung Liu J. Org. Chem. 2007, 72, 6753-6757 doi:10.1021/jo0707939
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Baldwin's_rules". A list of authors is available in Wikipedia.
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