Skattebol rearrangment

If the double bond is connected directly to the three-membered ring, i.e. if the substrate is a dihalo-vinylcyclopropane derivative, a vinylcyclopropylidene to cyclopentadiene isomerization, the so-called Skattebol rearrangement, takes place upon treatment with alkyllithium. Again, this ring-forming step competes with a ring-destruction process leading to allenic hydrocarbons (vinylallenes,... 

Principally, also the classification of eliminations with multiple migrations is possible according to the developed criteria. Thus, in the so called Skattebol rearrangement , which, if there is suitable substitution, results in the migration of... 

The generation and chemistry of cyclopropylidene has been extensively reviewed Of the unimolecular processes, the conversion of cyclopropylidene to allene represents a synthetically important type of reaction. However, this transformation is accompanied by competitive intra-and intermolecular processes such as C-H insertions and spiro[2.2]pentane formation from insertion to alkenes. Furthermore, vinylcyclopropylidenes undergo a concerted ring expansion to cyclopentenylidene (Skattebol rearrangement). Although cyclopropylidene to allene rearrangements can occur at temperatures below — 50 C, activation enthalpies of competing processes may be lower and therefore temperature effects can play an important role in the maximization of yields. 

The diazotization route is frequently accompanied by products derived from solvolysis of the initially formed cyclopropylidene or the rearranged cyclopentenylidene (Skattebol rearrangement) in the case of vinylcyclopropylidene with alcohol solvent, although allenes still account for the major products in the case of vrc-disubstituted cyclopropylidenes. It is noteworthy that the stereochemistry of the ring substituents (Table 2, entries 2 and 3) is an important factor in affecting yields of allenes. gcw-Disubstitution of vinylcyclopropanes diminishes formation of allenes in favor of products from the Skattebol rearrangement (entry 6). 

Conjugated bis-allenes separated by vinyl, aryl or heteroatomic groups can be prepared from their corresponding bisdihalocyclopropanes by the Doering-Moore-Skattebol method. However, competing processes such as the Skattebol rearrangement and other intramolecular processes often represent predominant pathways, for example, reactions of 157 and 159 further examples can be found in refs 116-120. 

Skattebol rearrangements in more complex dibromo-vinylcyclopropanes have also been reported. Thus, either a system consisting formally of two double bonds and one dibromocyclopropane unit or an educt containing two dibromo-cyclopropane moieties and one double bond undergo several carbene-carbene isomerizations upon treatment with methyllithium. In both cases, complex product mixtures arise. 

The Doering-Moore-Skattebol method including a cyclopropylidene-allene rearrangement is often used for the synthesis of allenes. However, the reaction conditions applied are often not compatible with acceptor substituents. One of the rare exceptions is the transformation 76 —> 77 (Scheme 7.11) [122]. The oximes 77 are not accessible by the classical route starting from allenyl ketone and hydroxylamine (see Section 7.3.2). 

This chapter will cover only reactions in which the isomerization to the allene starts from a stable molecule and not from a reactive intermediate generated in situ by reactions which are not isomerizations, such as the Doering-Moore-Skattebol reaction or free carbenes. Metallotropic rearrangements also will not be covered many of these reactions can be found in Chapter 9. Furthermore, the allene should be the final product of the reaction and not only a transient species leading to other products (see, for example, Chapters 6 and 20). 

Mechanistically this process is interesting since it involves a carbene-carbene rearrangement as was shown by Skattebol [110], Baird [111] and Brinker [112] using isotopically labeled substrates. 

Skattebol L (1967) Tetrahedron 23 1107. The rearrangement is profoundly effected by substituents Holm KH, Skattebol L (1984) Acta Chem. Scand. B38 783... 

A Skattebol-type rearrangement of a l-bromo-l-(trimethylstannyl)cyclopropane derivative has been reported in which a dialkenylcyclopropane was formed. Thus, the endo-isomer of 7-bromo-7-(trimethylstannyl)bicyclo[4.1.0]hept-2-ene (1), when heated in benzene, gave bromotrimethyl-... 

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