Rearrangements of Electron-Deficient Intermediates

The reactions grouped in this sedion are all characterized by skeletal rearrangement. Some are generally considered to be concerted, and thus discrete electron-defident intermediates (carbonium ion, carbene, nitrene, etc.) are bypassed. Reactions in which migration is to carbon or nitrogen are the most important these are considered separately in the sections that follow. 

Diazoketones give rearranged products when decomposed thermally or photo-chemically. The reaction is known as the Wolff rearrangement and is of synthetic importance, since it constitutes a convenient method for one-carbon homologation 

In actual practice, the thermal reaction is often catalyzed by silver salts. Under these conditions, there is no detectable kinetic carbon-isotope effect when the 

CHAPTER 8 REACTIONS INVOLVING ELECTRON-DEFICIENT INTERMEDIATES 

The net structural change is the same for both mechanisms. The energy requirements of the cyclopropanone and semibenzilic mechanisms may be fairly closely balanced, as instances of the operation of the semibenzilic mechanism have been reported even 

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A re-evaluation of the Hofmann rearrangement in electron-deficient systems has been undertakenA detailed study of the discrete intermediates, and the sensitivity of the intermediates and products to reagents and to each other in the Hofmann rearrangement of A-a-tosylasparagine, has led to a process that produces 2-(S)-(tosylamino)-/ -alanine on a large scale." ... 

Steroidal 5a hydroxy-6-ketones are readily converted into 5a-halo-6-ketones by the action of hydrogen halide in chloroform [gi]. The mechanism is not clear, for it seems necessary to invoke a C(s) carbonium ion intermediate, yet the 6-ketone must exert a strong polar effect opposing development of electron deficiency at C(5). It is also curious, if a carbonium ion is indeed involved, that it does not suffer either skeletal rearrangement or loss of a proton from C(4> (see p. 258). These difficulties may be circumvented by a mechanism (Fig. 16) in which ionisation of a C(5)-hydroxyl/hydrogen halide complex... 

Irradiation of electron deficient arenes in the presence of cis-l,2-diphenylcyclopropane leads to formation of the trans isomer by an electron transfer mechanism. The reaction occurs by way of the radical cation of the cyclopropane which isomerises prior to back electron transfer. It has now been examined using menthyl and bornyl esters of benzene tetracarboxylic acid as chiral electron transfer sensitisers. °° Slight excesses of one of the enantiomers of the trans-1,2-diphenylcyclopropane were observed. The dicyanoanthracene sensitised reactions of 1,1,2,3-tetra-arylcyclopropanes have been studied.Depending on the substituents present on the arene rings these compounds rearrange to 1,1,3,3-tetra-arylpropenes. The rearrangement occurs in a ring opened radical cation intermediate. 

Undoubtedly, there are still innumerable rearrangements and synthetic applications of carbenes and nitrenes to be discovered. This, together with the flourishing chemistry of silylenes and the emerging chemistry of monovalent boron and phosphorus compounds gives the entire field of electron-deficient reactive intermediates enormous synthetic potential. 

Numerous examples of hypervalent iodine-promoted fragmentations or rearrangements at electron-deficient centers have been reported. Several examples of oxidative fragmentations are shown below in Schemes 3.139-3.142. A mild and efficient fragmentation reaction of p-amino alcohols 349 and a-amino acids 350 upon treatment with [bis(trifluoroacetoxy)iodo]pentafluorobenzene leading to N,0-acetals 351 has been developed (Scheme 3.139). This method has been utilized in an improved synthesis of the key intermediate of discorhabdins [455,456]. 

Gallagher et al. described the cycloaddition of electron-deficient azides with ketene-(S,S)-acetals 12 [ 13]. Depending on the azide compoimd, the im-stable intermediate 13 rearranged in order to afford 14 or 15 in moderate yields (Scheme 4). 

Structure and Reactivity of Carbenes Generation of Carbenes Addition Reactions Insertion Reactions Rearrangement Reactions Related Reactions Nitrenes and Related Intermediates Rearrangements to Electron-Deficient Nitrogen... 

Diazocycloheptatriene (1), generated from the sodium salt of tropone tosylhydrazone, reacts with electron-deficient acetylenes to give H- 1,2-benzodiazepines 4 in moderate yield. It is suggested that the primary adducts 2 rearrange via the intermediates 3 to the products 4.114... 

Rearrangements may also proceed via intermediates that are essentially cations, anions, or radicals, though those involving carbocations, or other electron-deficient species, are by far the most common. They may involve a major rearrangement of the carbon skeleton of a compound, as during the conversion of 2,3-dimethylbutan-2,3-diol (pinacol, 42) into 2,2-dimethylbutan-3-one (pinacolone, 43, cf. p. 113) ... 

The formation of an intermediate with electron-deficient oxygen is also one of the possible paths for the oxidation of alcohols. An intermediate such as LVI, or the chromate ester LVII which might behave in similar fashion, could rearrange to give "abnormal products or lose a proton to give the usual, expected product.889-882... 

A simplified mechanism for the Beckmann rearrangements and important related reactions is shown hi Scheme 9. Summarizing the mechanism section, the key step of the reaction is the migration of an a-carbon group to the electronically deficient nitrogen atom of the oxime. A nitrilium ion in some cases or an imidate in others are key intermediates in the reaction. Their destiny determines the course of the transformation. Basically, three different pathways may be possible and can be synthetically exploited ... 

In contrast to the somewhat limited synthetic utility of nitrenes, there is an important group of reactions in which migration occurs to electron-deficient nitrogen. One of the most useful of these reactions is the Curtius rearrangement 16 This reaction has the same relationship to acylnitrene intermediates that the Wolff rearrangement does to acylcar-benes. The initial product is an isocyanate, which can be isolated or trapped by a nucleophilic solvent. 

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