Cumulenes, preparation

After the air in the flask had been replaced completely with nitrogen, 100 ml of dry diethyl ether, 0.20 mol of the cumulenic ether (see Chapter V, Exps. 7, 8 and 11) and 1 g (note 1) of copper(l) bromide were placed in it. A solution of the Grignard-reagent, prepared from 0.50 mol of the chloride (see Chapter II,... 

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). 

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

Cumulenes have also been prepared by treating alkynyl epoxides with boron trifluoride. 1,4 Elimination of BrC C=C—CBr has been used to prepare conjugated dienes (C=C—C=C). ... 

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

Radialenes represent the biggest and best known subset of the radialene family this is not surprising in view of the fact that more methods to prepare them exist than for any other class of radialenes. The major strategies are the transformation of appropriate cyclobutane derivatives, the thermal or Ni(0)-catalyzed cyclodimerization of butatrienes or higher cumulenes and the cyclotetramerization of (l-bromo-l-alkenyl)cuprates. 

Another [5]radialene ketone, the tetraallene 102a, has been prepared in 74% yield by treating the [5]cumulene 98a with stoichiometric amounts of Ni(CO)2(PPh3)267 the sterically very shielded ketone 102b is produced analogously in 32% yield from the corresponding terminally bridged [5]cumulene 98b (equation 11). In both cases the dimers of the [5]cumulenes, the peralkylated [4]radialenes are formed as well (see Section II.B, Scheme 11). 

In a search of jr-donor systems for the preparation of compounds having a metallic conductivity, the bis-thioxanthene cumulene 56 was obtained. It was oxidized by cone. H2SO4 to the acetylenic dication 57 rather than undergoing the expected protonation of the multiple bonds (equation 22)36. 

Cp=Cy bond of indenyl-allenylidene complexes 30 which leads to the stereoselective formation of cationic amino-allenylidenes 31. When R = Ph, complexes 31 can be transformed into the secondary derivatives 32 via treatment with LiBHEts and subsequent purification on sUica-gel column. Further insertions of MeC=CNEt2 into 32 allow the preparation of polyunsaturated cumulene chains (related insertion reactions will be discussed in the reactivity section). 

Preparation and Reactivity of Higher Metal Cumulenes Longer than Allenylidenes... 

Preparation and Reactivity of Higher Metai Cumulenes Longer than Aiienyiidenes... 

Many of the compounds of this type have been prepared by interaction of cumulene ylides witli heterocumulenes and these are covered in a review by Bestmann <77AG(E)349>. The oxetane system 5 results from reaction of Ph3P=C=C=S with aryl isocyanates, ArNCO <77AG(E)349>. An unusual approach to an oxetane ylide is the reaction of DMAD with CO2 and two equivalents of methyl phosphite which gives 6 <81CC607>. 

Enyne ethers HC=CCH=CHOR are useful synthetic intermediates. They can be prepared by base-catalysed addition of alcohols to diacetylene. The required conditions are rather forcing and not very attractive for laboratory scale preparations. A much more convenient way to prepare the enyne ethers (in these cases more than 80 rel.% of the -isomer is obtained) consists in treatment of the easily accessible 1,4-dialkoxy-2-alkynes with two equivalents of alkali amide in liquid ammonia. The first step in this elimination is the (transient) formation of an "anion RO-fiH-C CCH OR, which eliminates ROH (143). The resulting cumulenic ether ROCH=C=C=CH2 is immediately converted into the metallaied enyne ether. 

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. 

The homologous cumulene 44 prepared by an analogous vinylidene insertion reaction is spontaneously transformed to the dimer 45.46 By contrast the stable tert-butyl derivative 46 undergoes thermal cycloaddition at one of the inner double bonds giving 47. The structure of this dimer was confirmed by X-ray diffraction.47... 

Cyclodimerization of unsymmetrically substituted butatricncs such as 12 give both head-to-head and hcad-to-tail cycloaddition products. The structure of the head-to-head dimer was confirmed by its independent synthesis from the mixed cycloaddition of cumulenes 8 and 10,21 22 These dimerizations proceed by discrete nickel cyclopentanes which was established by the isolation of the 2-bispyridinenickel complex of the l.l,4,4-tetramelhylbuta-l,2,3-triene dimer.23 4-Radialenes with extended conjugation, potential organic conductors and semiconductors, have been prepared by similar methods as illustrated by the examples below.24,25... 

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