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Cycloalkenes dimerization

The structure of the dimer can be derived simply by evaluation of the cross signals in the HH COSY plot. The cycloalkene protons form two AB systems with such small shift differences that the cross signals lie within the contours of the diagonal signals. [Pg.208]

Metathesis of a cycloalkene initially yields a cyclic dimer, i.e. the size of the ring is doubled ... [Pg.134]

Transfer of carbene 3a from 19 to a variety of alkenes and cycloalkenes has been achieved under catalysis by copper(I) chloride74,79 - 82. However, with the exception of cyclohexene69,70 (72% yield), only moderate yields could be obtained. In all cases, the cyclopropanation was stereospecific with respect to the double bond configuration of the alkene and gave the sterically less crowded cyclopropane diastereomer in excess. As in the photochemical cyclopropanation, the formal carbene dimer trans-1,2-bis(trimethylsilyl)ethene is often formed as the major by-product. Cyclopropanation of fraws-but-2-ene with 19 with copper(II) chloride as catalyst was found to be even less... [Pg.741]

Pseudonitrosites and a-nitro ketone oximes were identified as intermediates in this process. This route has since been developed to provide access to mono- and poly-cyclic furoxans from readily-available alkenes and cycloalkenes. Treatment of the alkene with dinitrogen trioxide affords the nitro-nitroso adduct, which is readily isolated as its nitroso dimer (pseudonitrosite) thermal isomerization to the a-nitro ketone oxime, followed by dehydration with cyclization leads to the furoxan (Scheme 19). [Pg.421]

Under an atmosphere of ethylene, the ROM-RCM of cycloalken-ynes, when the cycloalkene bears the alkyne moiety at the olefinic Cl position, produce dimeric and/or bicyclic compounds [22] (Scheme 9). The expected bicyclic product with a cyclooctadiene ring is obtained with a low yield, whereas the dimeric macrocycle is the major compound. [Pg.300]

In 1977, McMurry and Kees [152] developed a titanium-induced intramolecular coupling procedure to form cycloalkenes from dicarbonyl compounds. Mechanistically, as shown in Scheme 85, the coupling reaction proceeds by an initial pinacol dimerization of the dicarbonyl 253 to 254, followed by titanium-induced deoxygenation to afford alkene 255. [Pg.162]

It should be noted that under some conditions reaction of internal perfluoroalkenes and -cycloalkenes with tributylphosphane and triphenylphosphane results in dimerization, preceded by defluorination and formation of a 1,3-diene and an appropriate difluoro-A -phosphane. For example, reaction of perfluoroalkenes containing three perfluoroalkyl groups at the double bond w ith a phosphane[tributylphosphane, triphenylphosphane, tris(diethylamino)phosphane] results in reductive dimerization of these alkenes, to give 1,3-dienes and difluoro-A -phosphanes. [Pg.430]

Formation of dimer products seems to be due to increased electrophilicity of the multiple bond of the fluoroalkeiie. Thus, the initial stage of the reaction of triphenylphosphane with perfluoro(l-methylcyclopentenc) is addition to the multiple bond. The intermediate (7/8) attacks a second molecule of cycloalkene, and difluorotriphenyl-/ -phosphanc is eliminated producing perfluoro[l-methyl-2-(2-melhylcyclopent-l-enyl)cyclopentenc] (9) in 75% yield. [Pg.432]

Tetrahexylammonium phthalimide, generated in situ from potassium phthalimide and tetra-hcxylammonium bromide, aminates the racemic 3-(acyloxy)cycloalkenes 9 to imides (S)-10 with extremely high enantioselectivity in the presence of a palladium(0) complex prepared from dimeric n-allylpalladium chloride and the chiral ligand (/ ,/ )-G78. (5)-10c has been converted to a protected derivative of (S)-2-aminopimelic acid. [Pg.1168]

The extent and types of interfering reactions during hydroalumination of alkynes were examined in Section 3.11.2.2 (equations 18-24). To a lesser degree two of these reactions are also encountered in A1—H additions to alkenes and cycloalkenes. Analogous to the reductive dimerization of alkynes (equation 23) is the dimerization of a-alkenes by Bu 2AlH, which can be conducted catalytically in hydride to give high yields of dimer (equation 30). ... [Pg.744]

In examining the cycloalkenes, one must first recognize that a double bond has considerable inherent strain. For example, the dimerization of ethylene to give cyclobutane is fairly exothermic (—18 kcal mol" ) and if there were a way to readily overcome orbital symmetry restrictions, cyclobutane would be a very common reagent. However, in the following, we will take the conventional view that ethylene is unstrained. Then, in comparing cycloalkanes and cycloalkenes it is helpful to define olefinic strain (OS) as the difference in strain between the alkene and the corresponding alkane ... [Pg.15]

Sensitized irradiation of cyclohexenes and cycloheptenes in protic media results in protonation. This phenomenon, which is not shared by other acyclic or cyclic olefins, has been attributed to ground-state protonation of a highly strained tran -cycloalkene intermediate. In aprotic media, either direct or triplet-sensitized irradiation of cyclohexene produces a stereoisomeric mixture of (2 -I- 2] dimers 49-51 as the primary products, with 50 predominating. The reaction apparently involves an initial cis-trans photoisomerization of cyclohexene followed by a nonstereospecific nonconcerted ground-state cycloaddition, promoted by the high degree of strain involved. In contrast, cycloheptene undergoes only a slow addition to the p-xylene used as sensitizer. [Pg.407]

In a laser flash-photolysis study, 2-phenyladamantene was generated in benzene at room temperature from 3-noradamantyl(phenyl)diazomethane. This strained cycloalkene decays with second-order kinetics to give a dimer, and reacts much faster with O2 and Bu3SnH than with methanol, thus revealing a substantial radical character. Diphenyldiazomethanes possessing stable /er -butylaminoxyl and Ullman s nitronyl nitroxide radicals, e.g. (25), have been prepared by photolysis of the parent diazomethanes. Analysis of ESR fine structures showed that the carbene and radical centres couple ferromagnetically in these molecules, as expected. [Pg.301]

Photochemical decomposition of diazo(trimethylsilyl)methane (1) in the presence of alkenes has not been thoroughly investigated (see Houben-Weyl Vol. E19b, p 1415). The available experimental data [trimethylsilylcyclopropane (17% yield) and la,2a,3j8-2,3-dimethyl-l-trimethylsilylcyclopropane (23% yield)] indicate that cyclopropanation occurs only in low yield with ethene and ( )-but-2-ene. In both cases the formal carbene dimer is the main product. In reactions with other alkenes, such as 2,3-dimethylbut-2-ene, tetrafluoroethene or hexafluoro-propene, no cyclopropanes could be detected.The transition-metal-catalyzed decomposition of diazo(trimethylsilyl)methane (1) has been applied to the synthesis of many different silicon-substituted cyclopropanes (see Table 3 and Houben-Weyl Vol.E19b, p 1415) 3.20a,b,2i.25 ( iQp. per(I) chloride has been most commonly used for carbene transfer to ethyl-substituted alkenes, cycloalkenes, styrene, and related arylalkenes. For the cyclopropanation of acyl-substituted alkenes, palladium(II) chloride is the catalyst of choice, while palladium(II) acetate was less efficient, and copper(I) chloride, copper(II) sulfate and rhodium(II) acetate dimer were totally unproductive. The cyclopropanation of ( )-but-2-ene represents a unique... [Pg.821]

As mentioned above, some arylalkenes, such as stilbene, form complexes with Ag+. Such complexes are also formed between Ag+, as added silver perchlorate, and simpler alkenes. Typical of this is the interaction with 1-methylenecyclohexane when a crystalline complex is formed. Irradiation of this complex in the polycrystalline state or in solution in methanol affords isomerization to 1-methylcyclohexene by a 1,3-hydrogen migration path. Further irradiation brings about the stereospecific formation of the exo,trans,exo-dimer (24) of 1-methylcyclohexene. Less specific photodimerization is also reported for the irradiation of the /S-pinene (25)/silver perchlorate complex69 The mechanism was thought to involve a silver/cyclohexenyl radical similar in type to that observed in the y-radiolysis of silver/cycloalkene complexes70,71. [Pg.362]

Anionic dimerization initiated by addition of a cyclopropenyl anion to the double bond of the free cycloalkene [16]. [Pg.56]

Similar to the benzynezirconocene, cyclohexyne, cyclopen-tyne, alkyne, alkene, cycloalkene zirconocenes, and related species insert various substrates such as alkynes, alkenes, aldehydes, ketones, nitriles or phosphaalkynes. They lead in general five-membered zirconacycles, which can be converted by transmetalation or exchange reactions into fused-ring aromatic or heterocyclic compounds. The extension of this chemistry to heterobenzyne complexes can be realized, for instance, in phosphinine compounds.Consequently, under mild conditions, -phosphabenzyne-zirconocene complexes are formed and can be isolated either as PMeg adducts or as dimers when the elimination reaction is carried out without added phosphane (Scheme 28). [Pg.5310]

The focus of the present review has been on oligomerization of alkenes using solid catalysts. Cyclic compounds such as cycloalkanes and cycloalkenes have not been considered. For a variety of olefin molecules and a variety of solid catalytic systems, the reaction selectivity to dimers, trimers, etc. was found to be determined by at least three factors, namely ... [Pg.279]


See other pages where Cycloalkenes dimerization is mentioned: [Pg.259]    [Pg.85]    [Pg.845]    [Pg.650]    [Pg.845]    [Pg.214]    [Pg.103]    [Pg.451]    [Pg.137]    [Pg.2318]    [Pg.464]   
See also in sourсe #XX -- [ Pg.181 , Pg.182 , Pg.183 , Pg.184 ]




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