Subject allene intermediates

This system was described in one report and has been synthesized by a copper-assisted cycloisomerization of alkynyl imines. The authors proposed the following mechanism at first, 372 could undergo a base-induced propargyl-allenyl isomerization to form 373 next, coordination of copper to the terminal double bond of the allene (intermediate 374) would make it subjected to intramolecular nucleophilic attack to produce a zwitterion 375. The latter would isomerize into the more stable zwitterionic intermediate 376, which would be transformed to the thiazole 377 (Scheme 55) <2001JA2074>. 

Photo-addition of allene to the enone (90) yield adduct (91) in 75 % yield, which was subjected to ketalization in 77% yield. Epoxidation of (92) with perbenzoic acid followed by chromatography on alumina afforded two expoxides (93) and (94). Both (93) and (94) could be converted separately through (95) and (96) respectively which was the common intermediate leading to isoishwarane (98) and ishwarane following a deketalization-retroaldol-aldol process to furnish the keto-alcohol (97) (99) 30>. 

Reaction with a first aldehyde transforms 176 into the vinylphosphonium chloride 177, which for practical reasons is subjected to an anion-exchange process, leading to the phosphonium salt 178. From this, phenyllithium treatment liberates the allenic phosphorane 179, an intermediate that has previously been used to prepare allenes from aldehydes [69], in the present case providing the products 180. The same protocol has also been applied to o-alkynylbenzaldehydes to yield allenes of interest as model compounds for the study of Schmittel and Myers-type cyclization reactions [70]. 

An early synthesis of allenylzinc reagents employed a two-step procedure in which monosubstituted allenes were subjected to lithiation in THF with tBuLi at -90 °C and the resulting allenyllithium intermediates were treated with ZnCl2. The allenylzinc reagents thus generated react in situ with aldehydes to afford mainly anti homopropargyl alcohols (Table 9.46) [98],... 

Cyclic allenes are the subject of considerable interest as strained unsaturated cyclic compounds168-170. lerl-Butyl-substituted cycloocta-1,2-diene is known as the smallest isolable cyclic allene170,171. Seven- and six-membered cyclic allenes are also well known as important intermediates in organic synthesis, and many synthetic methods have been... 

A detailed, extensive review of cyclopropene has appeared. Cyclopropene is the last of the small strained ring hydrocarbons to have its thermal decomposition subjected to intensive investigation.158 Cyclopropenes can decompose by a variety of mechanisms involving diradicals, vinylcarbenes, and vinylidenes. Cyclopropene itself has been shown to be an intermediate in the allene-propyne rearrangement. 

Reductive deoxygenation-rearrangement of 2-yne-l,4-diols to 1,3-dienes is a useful synthetic procedure since a large variety of ynediols are available in a few steps by sequential reaction at both ends of acetylene with aldehydes. Acetylenic 1,4-diols can be deoxygenated reductively by lithium aluminium hydride to form conjugated dienes of high stereoisomeric purity (equation 9). A modification to this procedure is the use of acetylenic 1,4-diol mono-THP derivative. Allenic tertiary alcohols which are intermediates in the reaction can be separated and subjected to reductive elimination rearrangement... 

Monomers Not Polymerizable by Plasma Initiation. When styrene and a-methy1styrene were subjected to plasma treatment, the monomers became yellowish and only trace amounts of insoluble films were formed. The discoloration was intensified and extensive formation of dark films were observed if carbon tetrachloride was added as the solvent. No post-polymerization was detectable for these monomers. Generally styrene and a-methylstyrene readily undergo thermal polymerization. However, no thermal polymerization was possible for these monomers after having been subjected to plasma treatment for one minute or less. It has been demonstrated from the emission spectra of glow discharge plasma of benzene (6) and its derivatives (7 ) that most of the reaction intermediates are phenyl or benzyl radicals which subsequently form a variety of compounds such as acetylene, methylacetylene, allene, fulvene, biphenyl, poly(p-phenylenes) and so forth. It is possible that styrene and a-methylstyrene also behave similarly, so that species from the monomer plasma are poor initiators for polymerization. 

The proposed mechanism based on product observation is depicted in (Scheme 16). Regioselective insertion of the proximal double bond of allene 31 into the Si-[Rh] bond of the hydrosilane-[Rh] oxidative adduct, HlRhJSiRs, affords intermediate D-L Subsequent allqme insertion forming the cyclic vinyl-Rh complex D-II followed by silane promoted reductive elimination provides the product 32 and the active Rh-Si complex. When allenynes that did not have substitutions on the alkyne terminus or on the distal allene double bonds were subjected to the optimized conditions, complex product mixtures were obtained. These... 

In this latter reaction mode, which is observed much more rarely than /3-dehydropal-ladation, a wide variety of ligands can be coupled to each other with the formation of new C—C, C—H, C—N, C—O, and C—Hal bonds. This section does not cover the numerous cascade couplings in which a number of successive intramolecular additions of 2 onto double bonds is eventually completed by /3-dehydropalladationt as well as the numerous [2 + 2 -l- 2] and [4 -I- 2] cyclotri- and cyclodimerizations of alkynes, enynes, and related compounds.The Pd(0)-catalyzed Cope rearrangement also will not be considered here, as it proceeds via bis(i7 -allyl)palladium(II) intermediates. The carbopalladation reactions of allenes, which have been reviewed recently, are covered in Sect. IV.7. (For new examples see also refs. [10]-[12]). On the other hand, the numerous Pd-catalyzed formal [3 + 2] cycloadditions of trimethylenemethane (TMM) complexes may be classified as carbopalladations of alkenes without subsequent dehydropalladation. As the subject of this section has partially been covered in several newly published reviews, " the attention here will be on the most recent and interesting communications. 

Liu and coworkers have recently published the synthesis of (3) and (4), as well as several other fluorinated pyrrole analogues (not shown), by aminofluorination of allenes the synthetic approach is shown in Scheme 3 [15], This strategy takes advantage of a selective, silver-catalyzed intramolecular fluorination reaction, but the approach works best with substrates that possess electron-withdrawing R -groups on the 3-position. Because of this limitation, the yield of (3) is significantly higher than (4), 80 % versus 28 %, respectively. 4-Fluoro-pyrrole-2-carboxylic acid (5a, Fig. 2), synthesized from a fluorinated proline, was explored as a potential intermediate on the route to (3), but only extensive decomposition products were observed when (5a) was subject to flash pyrolysis [16]. 

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