Doering-Moore-Skattebol

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). 

At temperatures above —100 °C the lithiohalocyclopropanes are converted to cyclopropylidenes by formal loss of one molecule of lithium halide. The main route of stabilization of these carbenacyclopropanes consists in the formation of allenes. The process which has been termed the Doering-Moore-Skattebol allene synthesis ( DMS-synthesis ) [66-68], has been developed into the most general method for the preparation of these reactive compounds which especially during the last decade have been used in organic synthesis with growing success [69, 70],... 

The successful application of the Doering-Moore-Skattebol reaction to the preparation of relatively small-ring heteroorganic cycloallenes is of recent origin. A specific example is provided by 6,6-dichloro- and 6,6-dibromo-3-oxa-bicyclo-[3.1.0]hexane, respectively, which yield l-oxa-3,4-cyclohexadiene when treated with n-butyllithium at low temperatures. The formation of the allene was proven by various trapping experiments [89],... 

Cyclohexa-1,2-diene (76) generated in this way can be trapped with styrene, - hexa-2,4-diene, 2,3-dimethylbuta-1,3-diene, cyclopenta-1,3-diene, furan and 2-methylfuran to give [2 - - 2] cyclo-adducts and the product distribution was independent of the mode of preparation. 1-Methylcyclohexa-1,2-diene (79), also prepared by the Doering-Moore-Skattebol method, readily dimerized to give 80 and formed [2 - - 2] cycloadducts with styrene (81). ... 

Stable nine-membered rings or higher homologs of endocyclic allenes are routinely prepared by the Doering-Moore-Skattebol method (Table 6). 

Table 7. Vinylallene vs. Cyclopentadiene Formation in the Doering-Moore-Skattebol Reaction Br Br...

Bis-allenes can be prepared by the Doering-Moore-Skattebol method from bisfdibromocyclo-propyl) derivatives, e.g. 136 ° and 141. ° These reactions appear to proceed by a stepwise rather than a one-step mechanism as evident from byproducts which are derived from intermediate cyclopropylideneallenes via intramolecular insertion processes (C-H or double bond insertions). These reactions often predominate to the exclusion of bis-allene formation, e. g. in the case of 139. °°... 

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. 

Cumulenes can be prepared by the Doering-Moore-Skattebol method using methylene-substituted dihalocyclopropanes. In principle, such reactions can be used in succession to produce extended cumulenes however, limitations include the stability of the cumulene and the non-regioselectivity of the carbene insertion reactions. 

For reviews on the transformation of dibromocyclopropanes to allenes (see so-called Doering-Skattebol-Moore reaction) see (a) Hopf H (1980) In Patai S (ed) The Chemistry of Ketenes, Allenes and Related Compoimds Wiley, New York, Part 2, Chapter 2, p 779 ... 

Doering and LaFlamme [10b] were the first to report that sodium and magnesium metal are capable of converting substituted gem-dibromocyclo-propanes to allenes in varying yield. However, it was found that sodium reacts best in the form of a high surface dispersion on alumina. At a later date, Moore and Ward [11a] and then Skattebol [12] reported that methyllithium or n-butyl-lithium reacts with gem-dibromocyclopropanes to give allenes in high yield. The related dichloro compounds were found to be inert to methyllithium but reacted slowly with -butyllithium. Several examples of the preparation of allenes from gem-dibromocyclopropanes are shown in Table I. 

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