Allenes, cyclic

Reduction of allenes. Cyclic and acyclic allenes are reduced stereospecifically by diimide (generated from hydrazine, H2Oa, CuS04) to cis-alkenes.6... 

Since an allenic system is built of only three collinear carbon atoms, whereas an acetylenic unit requires two bond angles of 180°, it is expected that the allene isomers of cyclic alkynes will become increasingly more stable than their acetylenic analogs with decreasing ring size. This has been demonstrated for the ring sizes 9-11 by Moore and Ward [61] (Table 8-5). In their experiments, the equilibrium compositions of cyclic allene/cyclic acetylene mixtures were... 

Because of the linear geometry required of cumulated dienes cyclic allenes like cycloalkynes are strained unless the rings are fairly large 1 2 Cyclononadiene is the smallest cyclic allene that is sufficiently stable to be isolated and stored conveniently... 

Because the fluorine substituents both inductively and hyperconjugatively withdraw electron density from the C(2)-C(3) tt bond, the LUMO is located there, and Diels-Alder reactions take place exclusively with this bond [25] 1,1 -Difluoro allene and fluoroallene reaet readily with a large selection of cyclic and acyclic dienes, and acyclic dienes, [2+2] cycloadditions compete with the Diels-Alder processes As shown in the example in equation 79, a significantly different regiochemistry is observed for the [2+4] cycloaddition compared with the [2+2]... 

The thermal [2 + 2] cycloaddition is limited to certain activated alkenes. For instance tetrafluoroethylene, tetrachloroethylene, allenes e.g. 17, ketenes and ena-mines can form cyclic dimers or react with other alkenes ... 

To explain this extremely high anti selectivity, a cyclic transition state has been proposed in which the a-substituent of the inline and the methyl group of the allenic reagent are aligned in a less sterically congested anti relationship15. 

The reaction of propargylic chiral acetals with a catalytic copper reagent (RMgX/5% CuX) provides the expected alkoxy allenes in quantitative yield (Table 3)61. The diastereomeric excess is highly dependent on the size of the ring of the acetal and on the type of substituents it contains. The best diastereomeric excess is 85% with the acetal derived from cyclooctanediol. The use of lithium dimethylcuprate results in 1,2-addition lo the triple bond and the resulting lithium alkenyl cuprate bearing a cyclic acetal does not eliminate even at reflux temperature ( + 35°C). 

Reaction of alkynols 156 with benzenesulphenyl chloride afforded either the vinylacety-lene sulphoxides 157 or the allene sulphoxides 158 depending upon the substitution pattern of alkynols 156. Vinylacetylene sulphoxides 157 result from a [2,3]-allylic rearrangement of the sulphenate ester 159 (equation 88). In the case of the cyclic... 

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." ... 

The allenic stannanes can be transmetallated by treatment with SnCl4, a reaction that results in the formation of the a propargyl stannane. If the transmetallation reaction is allowed to equilibrate at 0°C, an allenic structure is formed. These reagents add stereospecifically to the aldehyde through cyclic TSs.194... 

Further exploitation of the molecular shape feature has led more recently to the design of another series of clathrate hosts by substituting on the allene rigid backbone bulky groups 22>, Representative compounds of this new host family are 20 and 21. The allene 20 (R = t-butyl) shows an interesting clathration behaviour upon crystallization from various environments, including alicyclic and aromatic compounds, heterocycles, cyclic ketones and eyclohexaneamine 26>. 

The unique combination of double bonds in the molecules of those compounds, each with different reactivity along with the easy preparation, makes phosphorylated allenes useful substrates for the synthesis of different cyclic and noncyclic organophosphorus compounds. Recent investigations increase the scope of application of phosphorylated allenes as precursors in organic syntheses. Most of them are accompanied by the formation of five- or six-membered phosphorus heterocycles, which in many cases demonstrate certain biological activity. 

All the synthetic protocols described above have limitations to some extent and the yields of the products were modest. In some cases the formation of 1,2-alkadi-enephosphonate derivatives is essential for obtaining the final cyclic products. This is the reason why many authors have used the higher reactivity of 1,2-alkadi-enephosphonates, discovered by Mark [42] in 1962 for the preparation of 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives. Since then, the oxaphospholic cyclization of 1,2-alkadienephosphonate system of double bonds has become the easiest method for the synthesis of these compounds. The special structure of phosphorylated allenes is responsible for their special properties, which has attracted the attention of chemists for a long time [43 16],... 

Macomber has described the oxidation of 3-methyl-l,2-butadienephosphonic acid with 3-chloroperbenzoic acid to the corresponding cyclic allenic oxide, which undergoes rearrangement to produce the corresponding unstable 2,5-dihydro-1,2-oxaphosphole-2-oxide derivative (Scheme 25) [66],... 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

In a later report, Schmidt and Allen (1970) extended their measurement to 38 pure liquids and mixtures at room temperature and to 5 liquids as a function of temperature. The free-ion yields are arranged by the alkanes and their isomeric and cyclic counterparts, which show considerable differences in the results. Thus, the free-ion yield in neopentane (NP) is about seven times that in n-pentane. Some of the results are shown in Table 9.1. In mixtures of NP with CC1, or CS, the observed decrease of Gf with the additive concentration has been interpreted by Mozumder and Tachiya (1975) as due to epithermal electron scavenging (vide infra). 

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