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)
Thus, a ring closure reaction could be classified as, for example, a 5-exo-trig.
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.
There are sometimes exceptions to Baldwin's rules. For example, cations often disobey Baldwin's rules, as do reactions in which a second-row atom is included in the ring.
| Baldwin dis/favoured ring closures | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 3 | 4 | 5 | 6 | 7 | ||||||
| type | exo | end | exo | end | exo | end | exo | end | exo | end |
| tet | ✓ | ✓ | ✓ | ✗ | ✓ | ✗ | ✓ | |||
| trig | ✓ | ✗ | ✓ | ✗ | ✓ | ✗ | ✓ | ✓ | ✓ | ✓ |
| dig | ✗ | ✓ | ✗ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
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
- ^ Baldwin, J. E., Rules for Ring Closure, J. Chem. Soc., Chem. Commun. 1976, 734. doi:10.1039/C39760000734
- ^ 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
- ^ Li, X.; Kyne, R. E.; Ovaska, T. V. (2007). "Synthesis of Seven-Membered Carbocyclic Rings via a Microwave-Assisted Tandem Oxyanionic 5-exodigCyclization−Claisen Rearrangement Process". The Journal of Organic Chemistry 72: 6624. doi:.
- ^ 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
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