Lone pairs 1,2-anionic rearrangements

While the lone pair on the hydroxyl oxygen of hydroxamic acid 77 would be tightly bound resulting in a weak noH-cr N Qr interaction, the anion 78 would possess a high energy pair of electrons that would enhance the anomeric effect and drive the rearrangement. 

A plausible mechanism for the formation of 4 is rationalized on the basis that photolysis of 3 results in [2-1-2] cyclization to thietane 4 and is subsequently followed by rearrangement to thiolactone 5 (Scheme 6). Ring opening of the initially formed thietane 4 leads to a zwitterion, which is facilitated by lone pair electrons of nitrogen and oxygen atoms, and nucleophilic reaction of the thiolate anion to carbonyl carbon gives 5. For the tricyclic thietane 4a, nucleophilic addition of the thiolate anion is difficult, and results in the formation of stable thietane 4a. 

The fact that the w orbital drops drastically on bending is, of course, related to the fact that the vinyl anion (sp2 lone pair) is more stable than the ethyl anion (higher energy sp3 lone pair). That is, upon bending, more s character is mixed into the LUMO of the alkyne, which ultimately becomes the sp2 anion lone pair upon addition of a nucleophile. However, our treatment also explains why nucleophilic attack on alkynes is preferred even for early transition states or for those reactions, such as cycloadditions and thermal rearrangements, in which anions are not formed. 

Nucleophilic Addition to a Polarized Multiple Bond Beta Elimination from an Anion or Lone Pair Rearrangement of a Carbocation Rearrangement with Loss of Leaving Group... 

Claisen rearrangements introduce the complication of oxygen lone pairs within the rearranging system, rather than as substituents on the perimeter. They may be ignored and the transition structure treated as benzene-like, or they may interrupt the conjugation, and the transition structure is then like a heptatrienyl anion.915 Predictions based on the simple theory above, whichever of these models is taken, match most of the substituent effects, and more elaborate treatments with calculations account for the anomalous accelerating effect of a donor substituent at C-6.916... 

Figure 9.19 Lone pair/radical stabilization dramatically accelerates the anionic oxy-Cope rearrangement.

Appropriately positioned anions, atoms with lone pairs of electrons to share, and jt-bonds can act to assist the departure of a leaving group, forming a temporary ring structure, which is then reopened by an external nucleophile. Such processes can result in retention of stereochemistry via two inversions or structural rearrangements. 

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