Preparation of Allenic Hydrocarbons

Very often the carbon framework of the future allene is already present in the substrate and often it is propargylic in nature. For example, base-catalyzed isomeriza-tions of acetylenic hydrocarbons - with the triple bond in a non-terminal (40) or terminal position - were often used to prepare allenic hydrocarbons in the early days of allene chemistry [8]. The disadvantage of this approach consists in the thermodynamic instability of the allenes produced if not prohibited for structural reasons, the isomerizations do not stop at the allene but proceed to the more stable conjugated diene stage. In practice, complex mixtures are often formed [9] (see also Chapter 1). 

Here an alkynyl sulfoxide 55 is first carbocuprated with an organocopper reagent 56 to provide a vinylcopper intermediate 57, which is then zinc homologated with the primary zinc sp3-carbenoid 58 to yield the allylzinc intermediate 59. This, in a spontaneous syw-/)-climination, gives the corresponding allene 60. This protocol could also be adopted to the preparation of chiral allenes. 

In another reduction, the propargylic phosphate 64 is reduced with samarium(II) iodide in the presence of tetrakis(triphenylphosphine)palladium and tert-butanol as a proton source the allene 65 is produced almost exclusively, 1% of the isomeric alkyne 66 being present in the product mixture [19]. 

In the second example, one carbon atom of the future allene group is contributed by the sulfoxide 72, which is first converted to the ketone 74 by LDA treatment followed by quenching of the resulting carbanion with methyl 3-phenylpropanoate (73). To convert the now complete carbon framework into a central allene unit, the enol triflate of 74, intermediate 75, is produced under the conditions given in the scheme. Ultimate butyllithium treatment of 75 then furnishes the alkylated allene 76 [23], 

The olefin metathesis reaction apparently has rarely been applied to allene synthesis so far, or at least the potential of this important process has not been exploited 

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As demonstrated in most of the other chapters of this book, there is a countless - and growing - number of applications of allenes in preparative organic chemistry. A need for an extra section on the use of allenic hydrocarbons in synthesis may therefore not be apparent. However, since this chapter concentrates on the construction and then preparation of novel jr-systems - neglecting all non-hydrocarbon substituents - some general remarks on the preparative usefulness of the parent systems seem to be in order. 

The number of fundamentally new preparative methods for synthesizing allenes has not increased significantly in the last two decades. Hence the by now classical routes summarized in Scheme 5.4 still form the main approaches to allenic hydrocarbons. 

In summary, a broad range of synthetic methods are available today to allow the synthesis of practically any required acyclic allenic hydrocarbon. We can therefore turn to the preparation of specific target molecules and we preferentially select these from the hydrocarbons collected in Schemes 5.1-5.3. Before doing that, however, some recent approaches to alkyl-substituted allenes are considered. 

Generally, cycloproylallenes are prepared by the same methods as employed for the synthesis of other allenic hydrocarbons. Thus, cyclopropylallene (17) itself has been obtained from vinylcyclopropane (139) via its dibromocarbene adduct 140 using the DMS method (Scheme 5.19) [54],... 

Why Allenic Hydrocarbons Are of Interest in Preparative Organic Chemistry... 

The amino acid 26, which has been isolated from various Amanita fungi [35], is one of the few examples of a natural product with an achiral allene moiety (Scheme 18.10) and was prepared inter alia by Strecker synthesis and also substitution reactions of allenic bromides and phosphates [36]. Recently, even unfunctionalized allenes have been found in nature seven allenic hydrocarbons 27 with chain lengths ranging from C23 to C31 were isolated from the skin of the Australian scarab beetle Anti-trogus consanguineus and related species (Scheme 18.10) [37]. Also these allenes do not occur in enantiomerically pure form, but with enantiomeric excesses of86-89% ec. 

The reaction sequence outlined in Scheme 20.30 for the preparation of the chlorinated enyne-allenes was successfully adopted for the synthesis of the C44H26 hydrocarbon 251 having a carbon framework represented on the surface of C60 (Scheme 20.50) [83]. Condensation of the monoketal of acenaphthenequinone (243) with the lithium acetylide 101 afforded the propargylic alcohol 244. On exposure to thionyl chloride, 244 underwent a cascade sequence of reactions as described in Scheme 20.30 to furnish the chloride 248. Reduction followed by deprotection produced 250 to allow a repeat of condensation followed by the cascade transformation and reduction leading to 251. 

Gem-Dihalocydopropanes belong to the most readily available cyclopropane derivatives known today. They have been shown to be extremely valuable starting materials for the preparation of cyclopropanes and cyclopropenes, they may be converted to bicyclobutane derivatives and spiropentanes, can lead to allenes and the higher cumulenes, cyclopentenes and cyclopentadienes, and many other classes of compounds, both hydrocarbon systems and derivatives with valuable functional groups. The article summarizes the preparative developments in the area of gem-dihalocyclopropane chemistry during the last decade. 

Petrow, H.G. and Allen, R.J., Control of the Interaction of Novel Platinum-on-Carbon Electrocatalysts with Fluorinated Hydrocarbon Resins in the Preparation of Fuel Cell Electrodes, U.S. Patent 4,166,143, August 28, 1979. 

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