Baldwin's rules
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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, 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 | ✓ | ✓ | ✗ |
| endo | ✗ | ✓ | ||
| 4 | exo | ✓ | ✓ | ✗ |
| endo | ✗ | ✓ | ||
| 5 | exo | ✓ | ✓ | ✓ |
| endo | ✗ | ✗ | ✓ | |
| 6 | exo | ✓ | ✓ | ✓ |
| endo | ✗ | ✓ | ✓ | |
| 7 | exo | ✓ | ✓ | ✓ |
| endo | ✓ | ✓ | ||
[edit] 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]:
[edit] 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
- ^ 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
- ^ 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
