Lone pairs rearrangements

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... 

These examples serve to illustrate the fact that, in reactions in which carbenium ions are formed in proximity to the acetal lone pairs, unexpected rearrangements may occur. 

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. 

The dicyclopenta[a,characteristic element of some terpenoids was obtained in 100% yield by a very facile Cope rearrangement of the highly functionalized divinylcyclobutane derivative 533 on heating in benzene at 55 °C for 4 h. The mild conditions can be due to participation of the lone pair of the sulfur atom or to the strain energy of the divinylcyclobutane fragment (equation 206)259. 

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. 

Many different types of 1,3-dipoles have been described [Ij however, those most commonly formed using transition metal catalysis are the carbonyl ylides and associated mesoionic species such as isomiinchnones. Additional examples include the thiocar-bonyl, azomethine, oxonium, ammonium, and nitrile ylides, which have also been generated using rhodium(II) catalysis [8]. The mechanism of dipole formation most often involves the interaction of an electrophilic metal carbenoid with a heteroatom lone pair. In some cases, however, dipoles can be generated via the rearrangement of a reactive species, such as another dipole [40], or the thermolysis of a three-membered het-erocycHc ring [41]. 

In this configuration, the alkoxyl oxygen lone-pair involvement promotes not only heterolytic Sjvl and 8 2 reactions at nitrogen and homolysis of the A—OAcyl bond, but molecular rearrangements. 

The imido complex [Mo2(cp)2(/r-SMe)3 (/u.-NFl)]" " 25+ undergoes an irreversible one-electron (EC) reduction [70]. Controlled potential electrolysis afforded the amido analog [Mo2(cp)2(/x-SMe)3(/x-NH2)] 26 almost quantitatively after the transfer of IF mol 25+. The amido complex was not the primary reduction product the latter was assigned as a rearranged imide radical (Sch. 18), which is able to abstract a FI-atom from the environment (supporting electrolyte, solvent, or adventitious water) on the electrolysis timescale. In the presence of protons, the reduction of 25+ became a two-electron (ECE) process. This is consistent with the protonation at the nitrogen lone pair of the primary reduction product, followed by reduction of the resulting amido cation... 

Partially unsaturated azepines, like the fully conjugated systems, tend to be unstable in acid solution and undergo either rearrangement or ring contraction, generally to an aromatic system. Protonation takes place at nitrogen or, as in those hydroazepines in which delocalization of the nitrogen lone pair is possible, at the /3 -carbon of the enamine system. 

The electrons which need to be rearranged in order to generate the set of VB configurations include the lone pair on the base, B, the C—H bonding pair, and the N— -bond. The reactant configuration is illustrated in [44]. 

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