Reaction types rearrangement

Several processes are unique to ions. A common reaction type in which no chemical rearrangement occurs but rather an electron is transferred to a positive ion or from a negative ion is tenued charge transfer or electron transfer. Proton transfer is also conunon in both positive and negative ion reactions. Many proton- and electron-transfer reactions occur at or near the collision rate [72]. A reaction pertaining only to negative ions is associative detaclunent [73, 74],... 

The breaking of a strategic bond and the generation of synthesis precursors defines a synthesis reaction. In the simplest case, the reaction is already known from literature. In most cases, however, the rcaaion step obtained has to be generalised in order to find any similar and successfully performed reactions with a similar substituent pattern or with a similar rearrangement of bonds. One way of generalizing a reaction is to identify the reaction center and the reaction substructure of the reaction. This defines a reaction type. 

An unusual reaction involving s-triazine (247) and ethyl acetoacetate with sodium ethoxide leads eventually to the pyrido[4,3-ring opening and Dimroth-type rearrangement of the intermediate (248) (80JHC389>. 

Joly s method (or modifications) is the best procedure for preparing A " -3-ketones and can be extended to the elimination of hydrogen bromide from a-bromo ketones of all types. Rearrangement is sometimes observed but is not often serious. Selectivity can be improved in some instances by lowering the reaction temperature. The method has been found useful for the preparation of A" -3-ketones from 6-halo-A" -3-ketones ... 

The Bamford-Stevens decomposition of tosylhydrazones by base has been applied to steroids, although not extensively. It has been demonstrated that the reaction proceeds via a diazo compound which undergoes rapid decomposition. The course of this decomposition depends upon the conditions in proton-donating solvents the reaction has the characteristics of a process involving carbonium ions, and olefins are formed, often accompanied by Wagner-Meerwein-type rearrangement. In aprotic solvents the diazo compound appears to give carbene intermediates which form olefins and insertion products ... 

The reaction of tnflic acid with 2-hydroxy-2-adamantanecarboxylic acid results in decarbonylation to adamantanone [84] However, in the presence of carbon monoxide, the same reactants give 4,3 homoadamantanedione as the product of a pinacolone-type rearrangement (equation 35)... 

Reaction of nitromalon-bis-A -methylanilide (105) with sulfuric acid gives A -methylisatin- -oxime (107) and not 4-methylquinoxalin-3-one 1-oxide (108) as originally suggested. This transformation may involve a Beckmann-type rearrangement of the protonated aci-nitro compound (106) prior to dehydrative ring closure. ... 

This methodology has been used for the synthesis of the C3-C14 segment 24 of the antitumor agent laulimalide 23 (Scheme 4.22) [35]. The constrained chiral BOX ligand 21c in combination with Cu(OTf)2 afforded dihydropyrane 6f by a cycloaddition reaction in good yield and ee this was converted to the C3-C14 segment 24 via a Ferrier-type rearrangement in several steps. 

The first step in the nonreversible degradation reactions is the formation of a reactive a-dicarbonyl species through the p-elimination of a hydroxide ion. The subsequent reaction pathways to all degradation products can be described by just five reaction types, namely, p-elimination, benzilic acid rearrangement, a-dicarbonyl cleavage, aldol condensation, and retro-aldol condensation (see Fig. 7).31 Retro-aldol condensation and a-dicarbonyl cleavage involve C-C bond... 

In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

The reaction of crotonaldehyde and methyl vinyl ketone with thiophenol in the presence of anhydrous hydrogen chloride effects conjugate addition of thiophenol as well as acetal formation. The resulting j3-phenylthio thioacetals are converted to 1-phenylthio-and 2-phenylthio-1,3-butadiene, respectively, upon reaction with 2 equivalents of copper(I) trifluoromethanesulfonate (Table I). The copper(I)-induced heterolysis of carbon-sulfur bonds has also been used to effect pinacol-type rearrangements of bis(phenyl-thio)methyl carbinols. Thus the addition of bis(phenyl-thio)methyllithium to ketones and aldehydes followed by copper(I)-induced rearrangement results in a one-carbon ring expansion or chain-insertion transformation which gives a-phenylthio ketones. Monothioketals of 1,4-diketones are cyclized to 2,5-disubstituted furans by the action of copper(I) trifluoromethanesulfonate. ... 

En gros, the N insertion reactions can be subdivided into a Beckmann type and a Schmidt-type rearrangement part. Furthermore, some photochemical rearrangements of chiral oxaziridines are known to generate a range of optically active lactams. 

En grosy the C insertion reactions can be classified as C3 type and as C4 type rearrangements. Furthermore, a two step C4-type process involving chromium carbene addition and a CO insertion has been reported. 

The potential for solvent-free synthesis is relatively large, with examples of many well-known reaction types proceeding quite well under this type of regime these include transesterification, condensation and rearrangement reactions. Many workers have moved away from conventional thermal sources for providing the energy needed for these reactions -... 

Under the reaction conditions the initially formed thiosulfinate (143) is quantitatively transformed into the disulfide 144 by a Pummerer-type rearrangement. ... 

In a related example, reaction of N-hydroxy-N-methylthiophene-2-carboximidamide 56 with DMAD gave a double Michael addition product 57, which when heated at reflux in xylenes, afforded hydroxypyrimidinone 60 in 57% overall yield (Scheme 6.21) [9f]. The mechanism invoked was opening of the oxa-diazole 57 to 58, followed by a [3,3]-Claisen-type rearrangement to 59, which, after tautomerization and cyclization, afforded 60. 

N-benzylaniline with phosgene, and then with sodium azide to produce carbonyl azide 52. On heating, nitrogen is evolved and a separable mixture of nitrene insertion product 53 and the desired ketoindazole 54 results. The latter reaction appears to be a Curtius-type rearrangement to produce an N-isocyanate (54a), which then cyclizes. Alkylation of the enol of 54... 

Reactions involving alkoxy- or aryloxyphosphines with amines might also be expected to yield phosphazane oligomers/polymers. However, PhP(0Ph)2 with 1,2-(NH2)2c6Hit produced only the Arbuzov type rearrangement product CgHi, (NH) 2P(0)Ph. 

The chemical reactions of sulphonyl nitrenes include hydrogen abstraction, insertion into aliphatic C—H bonds, aromatic substitution , addition to olefinic double bonds, trapping reactions with suitable nucleophiles, and Wolff-type rearrangement. Hydrogen-abstraction from saturated carbon atoms is usually considered to be a reaction typical of triplet... 

Thus reaction of the 1-propyl cation (13) with water (reaction type a) will yield propan-l-ol (14), elimination of a proton from (13) will yield propene (15, reaction type b), while rearrangement of (13, reaction type d)—in this case migration of He—will yield the 2-propyl cation... 

The elimination reactions of carbocations (type b) will be discussed in more detail subsequently (p. 248), but the rearrangement reactions (type d) are of sufficient interest and importance to merit further study now. 

Carbanions can take part in most of the main reaction types, e.g. addition, elimination, displacement, rearrangement, etc. They are also involved in reactions, such as oxidation, that do not fit entirely satisfactorily into this classification, and as specific—ad hoc—intermediates in a number of other processes as well. A selection of the reactions in which they participate will now be considered many are of particular synthetic utility, because they result in the formation of carbon-carbon bonds. 

Scheme 2.146. Synthesis of several natural products via Pummerer-type rearrangement/ 1,3-dipolar cycloaddition/ring-opening reactions.

We have intentionally selected example reactions (Figs. 29-33) that would not usually be immediately obvious to a chemist. The examples chosen have all been concerned with rearrangements of various types, since their courses are frequently difficult to predict. It remains to emphasize that the reactivity functions contained in EROS perform perfectly well with other types of reaction. This is true, for example, with reactions that a chemist could derive directly from an analysis of the functional groups in a molecule. Thus, EROS predicts addition reactions to carbonyl compounds, nucleophilic substitutions, and condensation reactions, to name just a few examples. In all these reaction types, the possibility of assigning a quantitative estimate to the reactivity at the various sites via the reactivity functions is of particular merit. It... 

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