Related Rearrangements

The subjects of this section are two reactions that do not actually involve carbo-cation intermediates. They do, however, result in carbon to carbon rearrangements that are structurally similar to the pinacol rearrangement. In both reactions cyclic intermediates are formed, at least under some circumstances. In the Favorskii rearrangement, an a-halo ketone rearranges to a carboxylic acid or ester. In the Ramberg-Backlund reaction, an a-halo sulfone gives an alkene. 

The Favorskii Rearrangement. When treated with base, a-halo ketones undergo a skeletal change that is similar to the pinacol rearrangement. The most commonly used bases are alkoxide ions, which lead to esters as the reaction products. This reaction is known as the Favorskii rearrangement.84 

Ring expansion of cyclic ketones using diazo compounds 

There is evidence that the rearrangement involves cyclopropanones or their open 1,3-dipolar equivalents as reaction intermediates.86 

Reactions Involving Carbocations, Carbenes, and Radicals as Reactive Intermediates 

CHAPTER 10 REACTIONS INVOLVING CARBOCATIONS, CARBENES, AND RADICALS AS REACTIVE INTERMEDIATES 

This reaction is known as the Favorskii rearrangement,37 If the ketone is cyclic, a ring contraction occurs. 

There is considerable evidence that the rearrangement involves cyclopropanones and/or the 1,3-dipolar isomers of cyclopropanone as reaction intermediates.39 

There is also a related mechanism that can operate in the absence of an acidic a. hydrogen, which is called the semibenzilic rearrangement. 

The net structural change is the same for both mechanisms. The energy requirements of the cyclopropanone and semibenzilic mechanisms may be fairly closely balanced. Cases of operation of the semibenzilic mechanism have been reported even for compounds with a hydrogen available for enolization. Among the evidence that the cyclopropanone mechanism usually operates is the demonstration that a symmetrical intermediate is involved. The isomeric chloroketones 7 and 8, for example, lead to the same ester. 

It has been shown that the two ketones are not interconverted under the conditions of the rearrangement. A common intermediate in the rearrangements of the two ketones can explain these observations, and the cyclopropanone intermediate would meet this requirement. The involvement of a symmetrical intermediate has also been demonstrated by labeling in he case of a-chlorocyclohexanone  

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Arylanthranils (388) on thermolysis give acridones (389) (81AHC(28)23l). 3-Phenylan-thranils (390) also form acridones (391) on treatment with nitrous acid (67AHC(8)277). Related rearrangements are found with 3-heteroarylanthranils (e.g. 392 393) (81AHC(28)23l). 

Iodide and thiocyanate ion are effective catalysts for inducing a related rearrangement (62AG(E)S28). This reaction can be envisioned as proceeding by nucleophilic attack on the lesser substituted aziridinyl carbon atom by iodide ion to give an iodoethyl intermediate such as (132) which is subsequently converted to the final product. 

A related rearrangement developed by Kukolja and coworkers (b-77MI51001) gives 3-methylenecepham (43) which is a key intermediate for the production of cefaclor (see... 

Scheme 10.3 gives some examples of pinacol and related rearrangements. Entry 1 is a rearrangement done under strongly acidic conditions. The selectivity leading to ring expansion results from the preferential ionization of the diphenylcarbinol group. Entry 2, a preparation of 2-indanone, involves selective ionization at the benzylic alcohol, followed by a hydride shift. 

The Ramberg-Backlund Reaction. a-Halosulfones undergo a related rearrangement known as the Ramberg-Backlund reaction.91 The carbanion formed by deprotonation gives an unstable thiirane dioxide that decomposes with elimination of sulfur dioxide. This elimination step is considered to be a concerted cycloelimination. 

A related scrambling of groups in a silene has also been reported by Eaborn (143) to explain the structure of compounds isolated from the thermolysis of tris(trimethylsilyl)fluorodiphenylsilylmethane at 450°C, where Me and Ph groups freely interchange between silicon atoms [Eq. (19)]. A related rearrangement is probably also involved in the photochemical silene-to-silene isomerizations derived from acylpolysilanes described earlier. 

Armesto, Horspool and coworkers have extensively studied the related rearrangement of /hy-LinsaUiraled imine derivatives, the so-called aza-di-7r-methane rearrangement37. A particularly interesting example is seen by the rearrangement of 75 to 76 (Scheme 19). Adduct 76 furnished chrysanthemic acid following a simple series of steps. It has been... 

A ring enlargement process was used effectively to access the enantiopure pyrrolo [ 1,4]oxazepine-9a(7//)-carboxylate derivatives 142 and 143. The sequence involved copper (Il)-catalysed decomposition of an a-diazocarbonyl derivative attached to a chiral morpholinone, and a carbenoid, spiro-[5,6]-ammonium ylide, Stevens [1,2] rearrangement sequence. The Stevens and related rearrangements have considerable further potential for novel heterocyclic syntheses <00TA3449>. 

Yates, B.F. Radom, L. Intramolecular Hydrogen Migration in Ionized Amines a Theoretical Study of the Gas-Phase Analogues of the Hofmann-Loffler and Related Rearrangements. J. Am. Chem. Soc. 1987,109,2910-2915. 

Chapter 1, by Armesto et al., summarizes the recent results on di-ir-methane and related rearrangements. Many of the reactions reported in this chapter were discovered by Armesto s group and expand the potential of this versatile reaction often known as the Zimmerman reaction. This chapter complements many of the reactions discussed previously in this series. 

Whereas related rearrangements involving hydrogen migration have been observed in the transformation of cyclo-X -phosphazadienes 179 to cyclo-X -phosphaza-trienes 180 (Schmidpeter and Ebeling ), no such processes have been observed for 1,2-dihydro-X -phosphorins, i. e. 181 fails to rearrange io 182. 

Other related rearrangements have been reported to occur by heating di-arylaminoacetylenic tertiary alcohols with potassium hydroxide [12]. (See Table VI.)... 

More interesting from the preparative point of view are related rearrangements of substituted car-2-enes. On benzene sensitized photolysis (450-W mercury lamp, 6 h) of 4a-hydroxymethyl-car-2-ene in acetic acid, a mixture of 15% unchanged starting material, ( + )-endo-( 16a, 13%) and ( — )-e.vo-7-hydroxymethyl-l,4,4-trimethylbicyclo[3.2.0]hept-2-ene (17a, 72%) was obtained.75 Several more examples are listed. Several of these compounds have been used in natural product syntheses.74,76... 

In a related rearrangement with simultaneous oxidation by palladium on carbon in toluene at 1103C, dihydroazepines 13 were formed in 80-100% yield.82... 

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