Allenes terminal

Like alkenes, methoxyallene undergo [5 + 2]-cycloaddition with the oxidopyrylium ion formed from the precursor 254 and triethylamine. The allenic terminal C=C bond adds from its sterically less encumbered face to afford the [5 + 2]-cycloadduct [186],... 

The possibility of Jt-allylpalladium complex formation through carbopalladation is excluded from the observation that no four- and/or six-membered rings are produced. The reaction apparently proceeds via an alternative pathway which involves a sequence of Jt-coordination of PhPdl to an allenic terminal double bond, oxypallada-tion and ensuing reductive elimination (Scheme 16.9). 

Type I (fast homodimerization) Terminal olefins, allylsilanes" Terminal olefins, allylsilanes," 1° allylic alcohols, ethers, and esters, " allyl boronate esters, allyl halides, alkyl-substituted allenes Terminal olefms, allyl boronate esters, 1° allylic alcohols, ethers, and esters,styrenes (no large ortho substit.), " " allyl allylsilanes, allyl sulfides, allyl phosphonates, " allyl phosphine oxides, protected allylamines ... 

Competition between the Myers-Saito and Schmittel cyclization is also observed for phosphorus-substituted allenes. Terminal alkynes 18 give naphthalene derivatives 19. 

Note 1. The lithiation of monoalky1al 1 enes is not completely regiospecific. The ratio of a- to ylithiated allene varies from about 80 20 for methyl-allene to 93 7 for hexylallene. tert.-Butylallene, however, is metallated exclusively on the terminal carbon atom. 

Note 2. The allenic ether is extremely sensitive towards oxygen. All operations during the work-up must be carried out under nitrogen. After termination of the distillation nitrogen should be admitted to the distillation apparatus. Too strong heating during the distillation involves the risk of an explosion. 

The three carbons of allene he in a straight line with relatively short carbon-carbon bond distances of 131 pm The central carbon because it bears only two substituents is sp hybridized The terminal carbons of allene are sp hybridized... 

Structural studies show allene to be nonplanar As Figure 10 7 illustrates the plane of one HCH unit is perpendicular to the plane of the other Figure 10 7 also portrays the reason for the molecular geometry of allene The 2p orbital of each of the terminal car bons overlaps with a different 2p orbital of the central carbon Because the 2p orbitals of the central carbon are perpendicular to each other the perpendicular nature of the two HCH units follows naturally... 

Allenes 2000-1915 (m-s) Two bands when terminal allene or when bonded to electron-attracting groups... 

Electrophilic additions to allenes represent an interesting reaction type which is related to additions to both alkenes and alkynes. An allene could, for example, conceivably be protonated at either a terminal s[p- carbon or the central sp carbon. 

Strong acids in aqueous solution convert allenes to ketones via an enol intermediate. This process also involves protonation at a terminal carbon. 

Conjugated terminal fluoroenynes add water to the double bond at -5 C to form a-fluoro allemc acid fluondes. At 20 C, the allenic acid fluorides and concentrated sulfunc acid give y- and 8-laaones (equation 1) In the presence of concentrated sulfuric acid, conjugated nontermmal fluoroenynes add water to the triple bond and then to the double bond to form a-fl uoro-P-diketones and exocyclic a-fluoroenones [/] (equation 2). 

Unsaturated substituents of dioxolanes 36-38 and dioxanes 39-41 are prone to prototropic isomerization under the reaction conditions. According to IR spectroscopy, the isomer ratio in the reaction mixture depends on the temperature and duration of the experiment. However, in all cases, isomers with terminal acetylenic (36, 39) or allenic (37, 40) groups prevail. An attempt to displace the equilibrium toward the formation of disubstituted acetylene 41 by carrying out the reaction at a higher temperature (140°C) was unsuccessful From the reaction mixture, the diacetal of acetoacetaldehyde 42, formed via addition of propane-1,3-diol to unsaturated substituents of 1,3-dioxanes 39-41, was isolated (74ZOR953). 

Allerie, P12C — C = CH2> is somewhat unusual in that it has two adjacent double bonds. Draw a picture showing the orbitals involved in the u and -n- bonds of allene. Is the central carbon atom sp2- or sp-hybridized What about the hybridization of the terminal carbons What shape do you predict for allene ... 

Analogous to the allylsilane cyclizations, alkynylsilanes can also be used to synthesize 1,2-fused bicyclic compounds. The intramolecular addition of 4-[5-(trimethylsilyl)-3-pentynyl]-2-cyclo-hexenone proceeded smoothly in the presence of various Lewis acids, yielding functionalized cw-fused octahydro-5/f-inden-5-ones containing the synthetically useful terminal allene unit45. 

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

After extraction of the neutral oil from the AOS sample, the neutral oil is made up volumetrically to at least a 10% solution in hexane. Of this solution 4 pi is spotted onto a silica gel TLC plate, together with terminal 5-sultone standard in the range 0.4-4 pg (equivalent to 0.1-1% sultone in the neutral oil). It is twice developed in a chamber saturated with 2-propyl ether. The solvent is completely evaporated and the spots visualized by vapor phase sulfuric acid charring using the technique described by Martin and Allen [139]. Humidity is not critical (10-30% is optimum) and activation of the plates has not been found necessary, but it might be required under conditions of high humidity. The level of sultone can be estimated by visual comparison with the standards or by the use of a densitomer. 

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