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Addition alkene formation

The linear appearance of the plot shows that this reaction obeys a first-order rate law. Additional mechanistic studies suggest that alkene formation proceeds in a two-step sequence. In the first step, which is rate-determining, the C — Br bond breaks to generate a bromide anion and an unstable cationic intermediate, hi the second step, the intermediate transfers a proton to a water molecule, forming the alkene and H3 ... [Pg.1067]

The nickel addition in chromium oxide decreased the formation of alkenes which was smaller than the one observed in the presence of just chromium oxide. It is to be remarked that the decrease of alkene formation was independent of the quantity of nickel in the catalyst. However, the catalytic activity for the fluorination reaction decreased when the nickel content increased. Thus the addition of nickel in small quantities allowed to increase the selectivity for the fluorination reaction. We could suggest that nickel substitute... [Pg.384]

Our analysis of literature data will focus on two closely related questions about the influence of changes in the relative thermodynamic driving force and Marcus intrinsic barrier for the reaction of simple carbocations with Bronsted bases (alkene formation) and Lewis bases (nucleophile addition) on the values of ks/kp determined by experiment. [Pg.83]

Without additives, radical formation is the main reaction in the manganese-catalyzed oxidation of alkenes and epoxide yields are poor. The heterolytic peroxide-bond-cleavage and therefore epoxide formation can be favored by using nitrogen heterocycles as cocatalysts (imidazoles, pyridines , tertiary amine Af-oxides ) acting as bases or as axial ligands on the metal catalyst. With the Mn-salen complex Mn-[AI,AI -ethylenebis(5,5 -dinitrosalicylideneaminato)], and in the presence of imidazole as cocatalyst and TBHP as oxidant, various alkenes could be epoxidized with yields between 6% and 90% (in some cases ionol was employed as additive), whereby the yields based on the amount of TBHP consumed were low (10-15%). Sterically hindered additives like 2,6-di-f-butylpyridine did not promote the epoxidation. [Pg.443]

The major problem of the application of zeolites in alkane-alkene alkylation is their rapid deactivation by carbonaceous deposits. These either strongly adsorb on acidic sites or block the pores preventing the access of the reactants to the active sites. A further problem is that in addition to activity loss, the selectivity of the zeolite-catalyzed alkylation also decreases severely. Specifically, alkene formation through oligomerization becomes the dominant reaction. This is explained by decreasing ability of the aging catalyst to promote intermolecular hydride transfer. These are the main reasons why the developments of several commercial processes reached only the pilot plant stage.356 New observations with Y zeolites reconfirm the problems found in earlier studies.358,359... [Pg.261]

Palladium(0)-catalysed coupling reactions of haloarenes with alkenes, leading to carbon-carbon bond formation between unsaturated species containing sp2-hybridised carbon atoms, follow a similar mechanistic scheme as already stated, the general features of the catalytic cycle involve an oxidative addition-alkene insertion-reductive elimination sequence. The reaction is initiated by the oxidative addition of electrophile to the zero-valent metal [86], The most widely used are diverse Pd(0) complexes, usually with weak donor ligands such as tertiary phosphines. A coordinatively unsaturated Pd(0) complex with a formally d° 14-electron structure has meanwhile been proven to be a catalytically active species. This complex is most often generated in situ [87-91],... [Pg.409]

The reaction of alkenes with sulfur trioxide is an important synthesis of 1,2-oxathiolane 2,2-dioxides. The best conditions involve the use of a dioxane-sulfur trioxide complex at low temperature. The initial addition involves formation of the most stable cation followed by ring closure, as shown in Scheme 31 (74JOC2459). [Pg.775]

Chiral Auxiliary for Asymmetric Induction. Numerous derivatives of (—)-8-phenylmenthol have been utilized for asymmetric induction studies. These include inter- and intramolecular Diels-Alder reactions, dihydroxylations, and intramolecular ene reactions of a,p-unsaturated 8-phenylmenthol esters. These reactions usually proceed in moderate to good yield with high diastereofacial selectivity. a-Keto esters of 8-phenylmenthol (see 8-Phenylmenthyl Pyruvate) have been used for asymmetric addition to the keto group, as well as for asymmetric [2 -F 2] photoadditions and nucleophilic alkylation. Ene reactions of a-imino esters of 8-phenylmenthol with alkenes provide a direct route to a-amino acids of high optical purity. Vinyl and butadienyl ethers of 8-phenylmenthol have been prepared and the diastereofacial selectivity of nitrone and Diels-Alder cycloadditions, respectively, have been evaluated. a-Anions of 8-phenylmenthol esters also show significant diastereofacial selectivity in aldol condensations and enantiose-lective alkene formation by reaction of achiral ketones with 8-phenylmenthyl phosphonoacetate gives de up to 90%. ... [Pg.471]

The reaction of alkyllithium reagents with acyclic and cyclic tosylhydrazones can lead to mixtures of elimination (route A) and addition (route B) products (Scheme 22). The predominant formation of the less-substituted alkene product in the former reaction (Shapiro Reaction) is a result of the strong preference for deprotonation syn to the N-tosyl group. Nucleophilic addition to the carbon-nitrogen tosyl-hydrazone double bond competes effectively wiA a-deprotonation (and alkene formation) if abstraction of the a-hydrogens is slow and excess organolithium reagent is employed. Nucleophilic substitution is consistent with an Su2 addition of alkyllithium followed by electrophilic capture of the resultant carbanion. [Pg.377]

The methylenation of ketones and aldehydes by the Wittig reaction is a well-established and selective methodology. Unlike addition-elimination methods of alkene formation, the Wittig proceeds in a defined sense, producing an alkene at the original site of the carbonyl. The Wittig reaction is not considered here, but is used as the standard by which the methods discussed are measured. The topics covered in the methylenation sections include the Peterson alkenation, the Johnson sulfoximine approach, the Tebbe reaction and the Oshima-Takai titanium-dihalomethane method. [Pg.731]

Application of the Wittig reaction of a nonstabilized ylide to the synthesis of an ( )-alkene is practically and effectively carried out by the Schlosser modification. Alternatively, the use of a trialkylphos-phonium ylide can produce high ratios of ( )-alkene." Recently, Vedejs has developed a reagent using dibenzophosphole ylides (110) to synthesize ( )-disubstituted alkenes (111) fixnn rddehydes (equation 24). The initial addition of ylide occurs at -78 C, but the intermediate oxaphosphetane must be heated to induce alkene formation. The stereoselectivity in the process is excellent, particularly for aldehydes with branched substitution a to the reacting center. Both the ethyl and butyl yli s have b n utilized. [Pg.758]

In an application of (Z)-selective alkene formation to enolizable aldehydes, it was noted that the combination of LiCl and DBU was effective for deprotonation by lithium complexation of the Still phosphonate. In this example, the cyclopropyl aldehyde (176) reacted chemoselectively in the presence of the ketone (equation 43). In addition, the ( )-alkene could be synthesized by lithium coordination with a standard HWE methyl phosphonate. As this example illustrates, the trifluoroethyl phosphonate can fill an important void by providing trisubstituted alkenes with sensitive substrates in go< selectivity. From the examples of Marshall and Oppolzer it appears that the application of the reaction to higher order trisubstituted alkenes is selective for the (Z)-isomer. The magnitude of the selectivity is substrate specific and dependent on the rapid rate of eo firo-a-oxyphosphonate decomposition. [Pg.767]

The term Peterson alkenation has been used to describe the elimination of a functionalized organo-silicon compound with alkene formation for substrates synthesized by these methods. In accordance with the mechanistic definition outlined in the introduction to this cluq)ter, such topics are not considered in detail in this review. For the purpose of this discussion, the Peterson alkenation will be considered as the addition of an anion derivative to a carbonyl compound, followed by elimination to the alkene. [Pg.785]

The stereochemistry of trisubstituted alkene formation was demonstrated by Kingsbury and Cram who found that the ctyr/iro-sulfoxide (37) gave mainly the ( )-alkene (38 equation 19) with the (Z)-alkene (40 equation 20) being the major product from thermolysis of the r/treo-sulfoxide (39). The rate and stereochemistry of elimination was found to be independent of solvent, and so the concerted syn elimination mechanism outlined in equation (21) was proposed. The variation of the -deuterium isotope effect with temperature is consistent with a linear hydrogen transfer, and the elimination has been found to be reversible, with sulfenic acids being trapped both inter- and intra-molecularly by addition to alkenes to give sulfoxides (Schemes 2 and... [Pg.1017]


See other pages where Addition alkene formation is mentioned: [Pg.249]    [Pg.155]    [Pg.961]    [Pg.124]    [Pg.376]    [Pg.1086]    [Pg.658]    [Pg.352]    [Pg.179]    [Pg.155]    [Pg.443]    [Pg.155]    [Pg.128]    [Pg.454]    [Pg.256]    [Pg.74]    [Pg.722]    [Pg.564]    [Pg.68]    [Pg.120]    [Pg.916]    [Pg.938]    [Pg.1142]    [Pg.267]    [Pg.673]    [Pg.948]    [Pg.55]    [Pg.395]    [Pg.743]    [Pg.755]    [Pg.758]    [Pg.763]    [Pg.769]    [Pg.800]    [Pg.155]   
See also in sourсe #XX -- [ Pg.299 ]




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