Preparation of Allenes

Aldehydes and ketones can be converted into terminal allenes by reaction with a-lithiovinylsilanes followed by elimination (9). 

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Chemistry of Ketenes, Allenes and Related Compounds", S. Patai (ed.), John Wiley and Sons, Chichester, New York, Brisbane, Toronto, (1980). lOe. H. Hopf, "The Preparation of Allenes and Cumulenes", Chapter 20 in "The Chemistry of Ketenes, Allenes and Related Compounds", S. Patai (ed.), John Wiley and Sons, Chichester, New York, Brisbane, Toronto, (1980). 

The acylation of Wittig reagents provides the most convenient means for the preparation of allenes substituted with various electron-withdrawing substituents. The preparation of o-allenic esters has been accomplished by the reaction of resonance-stabilized phosphoranes with isolable ketenes and ketene itself and with acid chlorides in the presence of a second equivalent of the phosphorane. The disadvantages of the first method are the necessity of preparing the ketene and the fact that the highly reactive mono-substituted ketenes evidently cannot be used. The second method fails when the a-carbon... 

This chapter has discussed the transition metal-catalyzed synthesis of allenes. Because allenes have attracted considerable attention as useful synthons for synthetic organic chemistry, effective synthetic methods for their preparation are desirable. Some recent reports have demonstrated the potential usefulness of optically active axially chiral allenes as chiral synthons however, methods for supplying the enantiomerically enriched allenes are still limited. Apparently, transition metal-catalyzed reactions can provide solutions to these problems. From the economics point of view, the enantioselective synthesis of axially chiral allenes from achiral precursors using catalytic amounts of chiral transition metal catalysts is especially attractive. Considering these facts, further novel metal-catalyzed reactions for the preparation of allenes will certainly be developed in the future. 

A richer structural variety is realized when the propargyl carbonates 90 are treated with cyclopropyl(cyano)thienyl cuprates [56] such as 141, allowing the preparation of allenes with up to four different substituents (Scheme 5.19) [57]. 

During the last decade, a substantial number of novel (sometimes even stereoselective) strategies for the preparation of allenic prostaglandins have been devised. The approach used by Patterson involves a three-component coupling via a 1,4-addi-tion of the organocopper compound 121 to the enone 120, followed by alkylation of the enolate formed with the bromide 122 (Scheme 18.40) [121]. However, due to the notoriously low reactivity in the alkylation of the mixed copper-lithium enolate formed during the Michael addition [122], the desired product 123 was obtained with only 28% chemical yield (the alkylation was not even stereoselective, giving 123 as a 1 1 mixture of diastereomers). 

Tius and co-workers elegantly applied a variant of the Nazarov reaction to the preparation of cyclopentenone prostaglandins (Scheme 19.39) [46]. Moreover, it was demonstrated that the chirality of non-racemic allenes is transferred to an sp3-hybridized carbon atom. Preparation of allenic morpholinoamide 214 and resolution of the enantiomers by chiral HPLC provided (-)- and (+)-214. Compound (-)-214 was exposed to the vinyllithium species 215 to afford a presumed intermediate which was not observed but spontaneously cyclized to give (+)- and (—)-216 as a 5 1 mixture. Compound (+)-216 was obtained with an 84% transfer of chiral information and (-)-216 was obtained in 64% ee. The lower enantiomeric excess of (—)-216 indicates that some Z to E isomerization took place. This was validated by the conversion of 216 to 217, where the absolute configuration was established. The stereochemical outcome of this reaction has been explained by conrotatory cyclization of 218 in which the distal group on the allene rotates away from the alkene to give 216. 

Cyclic allenes have previously been obtained only admixed with the isomeric acetylenes. The present two-step synthesis is a practical method for the preparation of cyclic allenes, and at the same time it describes a general method for the preparation of allenes. It is based on the original work of Doering and co-workers. Examples of the reaction sequence above are known in which allenes are not produced, or they represent only a part of the reaction products. A one-step synthesis of 1,2-cyclonona-diene has been reported. ... 

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. 

Preparation of Allenes by the Reaction of Acetylenic Alcohols with Hydrobromic Acid-Cuprous Bromide [73a]... 

Preparation of allenic acetals from unsaturated carbenes [137]. 

TABLE 3.8. Silver-Catalyzed Preparation of Allenic Sulfones... 

Finally, a key method for the preparation of allenes is the dehalogenation of cyclopropane geminal-dibromides using methyllithium. This procedure has been reviewed previously341 and a recent series of papers concerning the synthesis of branched triangu-lanes have used this methodology (equation 39)343,343. 

The [2,3]sigmatropic rearrangement of allylic selenides has proven to be a useful method for the preparation of allenic alcohols. Selenide 170 was obtained by a free-radical selenosulfonation of the corresponding enyne. Oxidation with mCPBA afforded the allenic alcohol 171 in 89% yield via an intermediate selenoxide (Scheme 49).295... 

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