Cyclopentadienes cyclization

Hydrosilylation of I-vinyl-1-cyclohexene (77) proceeds stereoselectively to give the (Z)-l-ethylidene-2-silylcyclohexane 78, which is converted into (Z)-2-ethylidenecyclohe.xanol (79)[74]. Hydrosilylation of cyclopentadiene affords the 3-silylated 1-cyclopentene 80. which is an allylic silane and used for further transformations[75.75a]. Cyclization of the 1,3,8, lO-undecatetraene system in the di(2.4-pentadienyl)malonate 69 via hydrosilylation gives the cyclopentane derivative 81. which corresponds to 2.6-octadienylsilanc[l8,76]. 

Reactions of acetylene and iron carbonyls can yield benzene derivatives, quinones, cyclopentadienes, and a variety of heterocycHc compounds. The cyclization reaction is useful for preparing substituted benzenes. The reaction of / fZ-butylacetylene in the presence of Co2(CO)g as the catalyst yields l,2,4-tri-/ f2 butylbenzene (142). The reaction of Fe(CO) and diphenylacetylene yields no less than seven different species. A cyclobutadiene derivative [31811 -56-0] is the most important (143—145). 

Methane, chlorine, and recycled chloromethanes are fed to a tubular reactor at a reactor temperature of 490—530°C to yield all four chlorinated methane derivatives (14). Similarly, chlorination of ethane produces ethyl chloride and higher chlorinated ethanes. The process is employed commercially to produce l,l,l-trichloroethane. l,l,l-Trichloroethane is also produced via chlorination of 1,1-dichloroethane with l,l,2-trichloroethane as a coproduct (15). Hexachlorocyclopentadiene is formed by a complex series of chlorination, cyclization, and dechlorination reactions. First, substitutive chlorination of pentanes is carried out by either photochemical or thermal methods to give a product with 6—7 atoms of chlorine per mole of pentane. The polychloropentane product mixed with excess chlorine is then passed through a porous bed of Fuller s earth or silica at 350—500°C to give hexachlorocyclopentadiene. Cyclopentadiene is another possible feedstock for the production of hexachlorocyclopentadiene. 

On transforming the cyclopentenone 1431 into the trimethylsilyloxy diene 1432 the ensuing Diels-Alder cyclization gives rise to 69% tricychc compound 1433 [10] (Scheme 9.8). For Diels-Alder-reactions of thioaldehydes, selenoaldehydes, or unsaturated nitroso compounds with cyclopentadiene, see the reactions of 602 to 603 and of 605 to 606 in Scheme 5.48 and of 1092 to 1093 in Scheme 7.43. For Diels-Alder-reactions of silyloxyazadienes such as 510 with maleic anhydride to give 511, see Scheme 5.29. 

In addition to the reaction shown in Scheme 53, some other related reactions that are thought to proceed via cyclic carbometallation have also been reported (Scheme 54). In the cyclization reaction of 2-ethenyl-2 -ethynylbiphenyl, both Cr and W carbyne complexes must undergo the same cyclic carbometallation as that shown in Scheme 53 to give the corresponding metallacyclohexadiene intermediates, but the final products obtained were different.256 Some tungsten-carbyne complexes have been shown to undergo a stepwise [2 + 2 + 2]-cyclization via formal cyclic carbometallation that can be followed by reductive elimination to produce cyclopentadiene-tungsten complexes.2... 

Depending on the types of substituents and the precise reaction conditions (l,3-butadien-l-yl)carbene complexes can undergo direct cyclization to yield cyclo-pentadienes [337,350]. As mentioned in Section 2.2.5.1, cyclopentadiene formation occurs particularly easily with aminocarbene complexes [351]. Alternatively, in particular at higher reaction temperatures, CO-insertion can lead to the formation of a vinylketene complex, which, again depending on the electronic properties of the substituents and the reaction conditions, can cyclize to yield cyclobutenones, furans [91,352], cyclopentenones, furanones [91], or phenols (Dotz benzannulation) [207,251,353]. 

Pyridine can also be prepared from cyclopentadiene by ammoxidation, or from 2-pentenenitrile by cyclization and dehydrogenation. Furfuryl alcohol or furfural reacts with ammonia in the gas phase to give pyridine (Scriven et al., 1996). 

The dihalide cyclization method has been used to make the two isomeric tetrahydrodithiocins (394) and (395), as well as the benzo derivative (396). Oxidation of (394) and (395) gives rise to crystalline disulfones (m.p. 227 and 130 °C respectively), while (395) behaves as a dienophile towards cyclopentadiene (64ZC302) (cf. 255, Section 5.19.4.4). 

Although the majority of studies in this area involve five-membered ring cyclic anhydrides, a few pyrolytic reactions involving acyclic anhydrides have been reported. Thus, for example, FVP of 278 gives the alkylideneketene 279 with loss of trifluoroacetic acid142, while at 650 °C 280 loses both trifluoroacetic acid and cyclopentadiene to afford the indenylideneketene 281, which cyclizes by way of 282 to give 283143. FVP of... 

A new synthesis of isoxazoles is by successive treatment of a ketoxime with butyllithi-um, the ester of a carboxylic acid and sulfuric acid, e.g. 1 -> 2 (94S989). Hitrovinyl oximes 3 (R1, R3 = alkyl or aryl) undergo oxidative cyclization to isoxazoles 4 by the action of DDQ or iodine/potassium iodide (94JHC861). Flash-vacuum pyrolysis of the 1,3-dipolar cycloadduct 5 of acrylonitrile oxide to norbornadiene results in a retro-Diels-Alder reaction to give cyclopentadiene and 3-vinylisoxazole 6 (94CC2661). 

When the reactant molecules have built-in restrictions, certain alternatives are precluded. A conrotatory cyclization is thermally allowed for butadiene but is not possible in cyclopentadiene, because it would lead to a trans ring junction photochemical excitation must... 

The electrophilic cyclization of vinylallenes delivers cyclopentadienes with interesting substitution patterns.35 The anellated cyclopentadienes shown in Scheme 12.18 can be prepared in this way. Silver and gold catalysts have been compared using this substrate. 

Although some nucleophilic alkenes and cyclopentadiene [1] upon treatment with DIB readily added two acetoxy groups, generally isolated double bonds are inert under mild conditions. An interesting reaction of preparative importance was the intramolecular cyclization of 1,5-cyclooctadiene. 

With sodium methoxide as base, cyclopentadiene reacts with benzalacetophenone to give a mixture of the diphenyl dihydropentalenes 146 and 147 (Scheme 25).227 Once Michael addition occurs, a second deprotonation generates a new cyclopenta-dienide anion which is ideally constructed for intramolecular cyclization and dehydration. 

Scheme 3, Cyclopentadienes via cyclization of 2-amino-l-metalla-l,3,5-trienes.

In contrast to 2-ammo-l-metalIa-l, 3,5-trienes 7 and 11, which are cyclized thermally to give cyclopentadienes in smooth reaction (Schemes 3 and 4), 2-ethoxy-l-metalla-l,3,5-hexatrienes exhibit a high tendency to undergo cyclization reactions involving nucleophilic attack at the carbene carbon atom. 

Cyclopentenones. l-Ethynyl-2-propenyl acetates (ly in the presence of this Pd(ll) salt and acetic acid (1 equiv.) cyclize to cyclopentenones (2) in 50-90% yield. The cyclopentadiene a has been identified as an intermediate by trapping with a dienophile. 

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