Rearrangement reactions 2,3 -sigmatropic rearrangements

Aromatic Substitution (Carey Sundberg, Chapter 11) Intramolecular Wittig Reaction Sigmatropic Rearrangements... 

Keywords Diels-Alder reactions, dipolar cycloadditions, electrocyclic reactions, ene reactions, pericyclic reactions, sigmatropic rearrangements... 

The three principal types of pericyclic reactions are cycloaddition, electro-cyclic rearrangement, and sigmatropic rearrangement ... 

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. 

Pericyclic processes comprise a broad and important class of concerted reactions of both theoretical and practical interest. These transformations, which are especially useful in the construction of carbon-carbon bonds,93 include electrocyclic reactions, sigmatropic rearrangements, and cycloadditions. Because they are not typically subject to general acid-general base chemistry but can be highly sensitive to strain and proximity effects, they are attractive targets for antibody catalysis. 

Keywords Thiocarbonyl compounds, Thiophilic addition, Cycloaddition reactions, Enethioli-sation, Eschenmoser reaction, Sigmatropic rearrangement. 

So far, cycloadditions have been our only examples of pericyclic reactions. There are several other classes of pericyclic reactions, of which the most notable are cheletropic reactions, sigmatropic rearrangements and electrocyclic reactions. In essence, frontier orbital theory treats each of them as a cycloaddition reaction. 

Pericyclic reactions are concerted processes that occur by way of a cyclic transition state in which more than one bond is formed or broken within the cycle. The classic example of such a process is the Diels-Alder cycloaddition reaction, one of the most common and useful synthetic reactions in organic chemistry. Cycloaddition reactions, sigmatropic rearrangements and electrocyclic reactions all fall into the category of pericyclic processes, representative examples of which are given in Schemes 3.1-3.3. This chapter will discuss these reactions and their use in synthesis. 

Electrocyclic reactions. Sigmatropic rearrangements. Reactions of carbocations. 

To a mixture of 65 ml of dry benzene and 0.10 mol of freshly distilled NN-di-ethylamino-l-propyne were added 3 drops of BFa.ether and 0.12 mol of dry propargyl alcohol was added to the reddish solution in 5 min. The temperature rose in 5-10 min to about 45°C, remained at this level for about 10 min and then began to drop. The mixture was warmed to 60°C, whereupon the exothermic reaction made the temperature rise in a few minutes to B5 c. This level was maintained by occasional cooling. After the exothermic reaction (3,3-sigmatropic rearrangement) had subsided, the mixture was heated for an additional 10 min at 80°C and the benzene was then removed in a water-pump vacuum. The red residue was practically pure acid amide... 

Anomalous Fischer cyclizations are observed with certain c-substituted aryl-hydrazones, especially 2-alkoxy derivatives[l]. The products which are formed can generally be accounted for by an intermediate which w ould be formed by (ip50-substitution during the sigmatropic rearrangement step. Nucleophiles from the reaction medium, e.g. Cl or the solvent, are introduced at the 5-and/or 6-position of the indole ring. Even carbon nucleophiles, e.g. ethyl acetoacelate, can be incorporated if added to the reaction solution[2]. The use of 2-tosyloxy or 2-trifluoromethanesulfonyloxy derivatives has been found to avoid this complication and has proved useful in the preparation of 7-oxygen-ated indoles[3]. 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

These reactions will be discussed in more detail under the topic of 3,3-sigmatropic rearrangements in Chapter 11. For the present we simply want to focus on the fact that the reaction is stereospecific-, the -isomer gives one diastereomeric product whereas the related Z-isomer gives a different one. The stereochemical relationship between reactants and products can be explained if the reaction occurs through a chairlike transition state in... 

The most important sigmatropic rearrangements from the synthetic point of view are the [3,3] processes involving carbon-carbon bonds. The thermal rearrangement of 1,5-dienes by [3,3] sigmatropy is called the Cope rearrangement. The reaction establishes equilibrium between the two 1,5-dienes and proceeds in the thermodynamically favored direction. The conversion of 24 to 25 provides an example ... 

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