The Baldwin rules for ring closure follow the earlier ones by Bürgi and Dunitz in stating the preferred angles of nucleophilic (and electrophilic) attack in bond forming reactions, and are as famous for the interest in their exceptions as for their adherence. Both sets of rules fundamentally explore the geometry of the transition states involved in the reaction, as reflected in the activation free energies. Previous posts exploring the transition states for well-known reactions have revealed that the 4th dimension (the timing of the bond formations/breakings) can often spring surprises. So this post will explore a typical Baldwin ring formation in the same way.
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