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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)

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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
3	endo		X	√

4	exo	√	√	X
4	endo		X	√

5	exo	√	√	√
5	endo	X	X	√

6	exo	√	√	√
6	endo	X	√	√

7	exo	√	√	√
7	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

^ 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