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.

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:


 * [[Image:Baldwinregel2.png|Reaction of methyl 4-hydroxy-2-methylenebutanoate according to Baldwin rules]]

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.

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


 * [[Image:5-exo-dig-reaction.png|500px|5-exo-dig reaction Li 2007]]

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


 * [[Image:6-endo-dig-reactionR.png|500px|6-endo-dig reaction Lin 2007]]