Alkyl substituted allenes

Nonfluonnated allenes also readily react with fluoroalkenes to give diverse fluonnated alkylidenecyclobutanes [727, 12S, 129, 130] (equations 55 and 56), except for tetramelhylallene, which rearranges to 2,4-dimethyl 1,3-pentadiene under the reaction conditions prior to cycloaddition (equation 57) Systematic studies of l,l-dichloro-2,2-difluoroethylene additions to alkyl-substituted allenes establish a two-step, diradical process for alkylidenecyclobutane formation [131, 132, 133]... 

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. 

The electrophilic addition of alkyl-substituted allenes may afford terminal attack and center attack products, depending on the structures of allenes and electrophiles (Scheme 10.1). 

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 ... 

Propargyl bromides can react with alkyl Grignard reagents [64a-e] to give good yields of alkyl-substituted allenes only if the final hydrolysis is effected with the minimum amount of pure water rather than with acid [64a-e]. 

The different reaction course followed by the silyl- and alkyl-substituted allenes is fully consistent with the hyperconjugation model, which predicts that a /i-silyl group stabilizes carbenium ions better than a /3-alkyl group, and with previous conclusions that a-silyl substitution in carbocations is destabilizing relative to a-methyl substitution. The larger space requirements of the trimethylsilyl groups compared with methyl groups may also contribute to the kinetic stability of 413. 

The central carbon atom (=C=) of alkyl-substituted allenes absorbs in the range of about 200-215 ppm, whereas the terminal atoms (C=C=C) absorb in the range of about 75-97 ppm. 

Besides simple enones and enals, less reactive Michael acceptors like /3,/3-disubstituted enones, as well as a,/3-unsaturated esters, thioesters, and nitriles, can also be transformed into the 1,4-addition products by this procedure.44,44a,46,46a The conjugate addition of a-aminoalkylcuprates to allenic or acetylenic Michael acceptors has been utilized extensively in the synthesis of heterocyclic products.46-49 For instance, addition of the cuprate, formed from cyclic carbamate 53 by deprotonation and transmetallation, to alkyl-substituted allenic esters proceeded with high stereoselectivity to afford the adducts 54 with good yield (Scheme 12).46,46a 47 Treatment with phenol and chlorotrimethylsilane effected a smooth Boc deprotection and lactam formation. In contrast, the corresponding reaction with acetylenic esters46,46a or ketones48 invariably produced an E Z-mixture of addition products 56. This poor stereoselectivity could be circumvented by the use of (E)- or (Z)-3-iodo-2-enoates instead of acetylenic esters,49 but turned out to be irrelevant for the subsequent deprotection/cyclization to the pyrroles 57 since this step took place with concomitant E/Z-isomerization. 

Dilithiation of allene leads to the acetylene LiCH2C=CLi. Alkyl-substituted allenes also react, e.g. ... 

The silyl-cupration of allenes has been shown to be a reliable source of both allyl and vinyl silanes (Scheme 49). Simple alkyl-substituted allenes lead predominantly to allyl silanes, while phenyl substitution on the allene, and allene itself, react to give vinyl silanes. The intermediate organocopper derivatives (175) and (176)... 

A series of arylynamides with alkyloxy groups at the ortho position of the aryl group has been prepared by an alkylation/cross-coupling/amidation sequence and a subsequent gold-catalysed reaction provides benzofurans with amine functionalities at the 2-position and alkyl groups at the 3-position. The mechanism of gold(III)-catalysed isomerization of alkyl-substituted allenes to conjugated dienes in the presence of a... 

Allenes (C=C= CH2) absorb strongly at 2000-1900 cm" as a result of the out-of-phase CCC stretch. The band is sometimes double. The terminal =CH2 wagging vibration gives rise to a strong band near 850 cm" with its overtone near 1700 cm". In the Raman spectra of mono -alkyl-substituted allenes a strong doublet appears near 1130 and 1100 cm" presumably involving the in-phase stretch of the allene double bands. ... 

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