Baldwin's rules

As Baldwin's rules is defined as the set of rules that allow predictions about the kinetic preference of chemical reactions with cyclic transition states, mostly ring-closure reactions. They were named after their discoverer, Jack Baldwin.

Nomenclature

The Baldwin's rules make use of specific terminology in which a ring closure is characterized by three parameters:

Above are for the nomenclature of the ring closure mechanism called features - each separated by hyphens - called in the order listed.

Preferred and disadvantaged mechanisms

Using Baldwin's rules can be judged whether it is likely that a certain mechanism leads to a ring closure. It is notably not absolute statements, so you do not divided into "allowed " and " forbidden", but in " preferred" and " disadvantaged " reactions. However, the rules are applicable only if the nucleophilic reaction center may also engage in their preferred angle of attack: For tet centers of the optimal angle is 180 ° (similar to the Walden inversion ) for trig - centers employ about 107 ° (see also Bürgi- Dunitz trajectory) and for dig - centers 120 °. Is not accessible due to the present molecular geometry this angle of attack, no ring closure is carried out, even if the reaction after Baldwin was supposed to take place preferentially.

The Baldwin's rules are based on the principle that all those ring-closing mechanisms take place at a disadvantage, a strong distortion of the bond angles and lengths in the molecule would be necessary for its expiration. In particular, these are:

• 3 -exo-dig and 4 -exo-dig
• 3 -endo -trig, 4 -endo-trig and 5-endo -trig

Caution! 5-endo -trig reactions like drawn appear conclusive, but are not plausible and therefore do not take place.

Endo- tet reactions do not lead to ring circuits. Nevertheless, they can be predicted by the Baldwin rules. Disadvantaged are:

• 5-endo - tet and 6 -endo- tet

In the table to the right of the Baldwin rules for three-to seven-membered rings are shown; this green marked reaction mechanisms expire preferred, while disadvantaged in red. With their help it is possible to predict the favored product when multiple pathways are conceivable as in the example.

The following points should also be followed:

• The Baldwin's rules make a prediction about kinetic probabilities, not on thermodynamics (eg equilibria ). In preferred reactions so the reverse reaction is also kinetically preferred. Just can be made thermodynamically favored reactions in port or instead.
• Cations and molecules with elements of the 3rd period often do not obey the Baldwin rules.
• Reactions that seem to contradict the Baldwin rules may have come into existence by intermolecular mechanisms.
• Nucleophiles sp2 centers, such as Enolates must be considered separately, since the orbital symmetry of π orbitals is different.