Olefines as intermediates

Condensation of thiophenoxide anions with various fluonnated polychloro- or polybromoethanes gives fluoroalkyl phenyl sulfides [57, 52, 55], These formal substitutions involve fluonnated olefins as intermediates [52, 55], In the case of perhalogenated ethanes, the mechanism shows a similarity with that of dihalo-genodifluoroinethane [52] (equations 46 and 47)... 

Methanol is first dehydrated to dimethylether (DME). The equilibrium mixture thereof is then converted to light olefins. In the final steps of the reaction path, the Cj-Cg olefins are converted to paraffins, aromatics, naphthenes and higher olefins by polycondensation and alkylation reactions. The importance of light olefins as intermediates in the conversion of methanol to gasoline was soon recognized. As a result, several attempts were made to selectively produce light olefins from methanol on zeolite catalysts, not only on medium-pore zeolites but also on small-pore... 

The other type of reaction involves a concerted elimination process [reactions (2) through (7) in Table I]. It should be noted that there are other lyases which catalyze elimination reactions with the formation of olefins as intermediates, but these intermediates are further degraded into other nonolefin unsaturated end products. An example of this kind of reaction is the deamin-... 

Olefins and cycloolefins give unsaturated ketones with acyl hahdes (the Nenitzescu reaction) (170). Saturated chloroketones are formed as intermediates followed by elimination of HCl. Similar products may be obtained by using acid... 

Sulfitation and Bisulfitation of Unsaturated Hydrocarbons. Sulfites and bisulfites react with compounds such as olefins, epoxides, aldehydes, ketones, alkynes, a2iridines, and episulftdes to give aHphatic sulfonates or hydroxysulfonates. These compounds can be used as intermediates in the synthesis of a variety of organic compounds. 

The synthetic procedure described is based on that reported earlier for the synthesis on a smaller scale of anthracene, benz[a]anthracene, chrysene, dibenz[a,c]anthracene, and phenanthrene in excellent yields from the corresponding quinones. Although reduction of quinones with HI and phosphorus was described in the older literature, relatively drastic conditions were employed and mixtures of polyhydrogenated derivatives were the principal products. The relatively milder experimental procedure employed herein appears generally applicable to the reduction of both ortho- and para-quinones directly to the fully aromatic polycyclic arenes. The method is apparently inapplicable to quinones having an olefinic bond, such as o-naphthoquinone, since an analogous reaction of the latter provides a product of undetermined structure (unpublished result). As shown previously, phenols and hydro-quinones, implicated as intermediates in the reduction of quinones by HI, can also be smoothly deoxygenated to fully aromatic polycyclic arenes under conditions similar to those described herein. 

Perfluorodecalin is transformed to octakis(phenylthio)naphthalene by phenyl thiolate in dipol ar aprotic solvents The crucial first step of this reaction may involve an elimination of the tertiary fluorine atoms, which would lead to a symmetric perfluorobicyclic olefin as an intermediate [65] (equation 58). 

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

Dehydration of prednisolone acetate (175b) yields the corresponding 9,11 olefin. As a variation on the chemistry we have seen previously, this olefin is allowed to react with chlorine in the presence of lithium chloride. If this addition is assumed to proceed by the customary mechanism, the first intermediate should be the 9a,11a-chioronium ion. Axial attack by chloride anion from the 110 position will lead to the observed stereochemistry of the product dichlorisone (240). ... 

Hydrogenation of aromatics under mild conditions gives mainly the all-cis isomer as if hydrogen addition takes place from only one side of the molecule (23,24). Reductions under more vigorous conditions may give other isomers by isomerization of the initially formed all-cis product. Under mild conditions, other isomers are accounted for by desorption and readsorption in a new orientation of intermediate olefins, as well as by double-bond migration in the... 

Natural gas and crude oils are the main sources for hydrocarbon intermediates or secondary raw materials for the production of petrochemicals. From natural gas, ethane and LPG are recovered for use as intermediates in the production of olefins and diolefms. Important chemicals such as methanol and ammonia are also based on methane via synthesis gas. On the other hand, refinery gases from different crude oil processing schemes are important sources for olefins and LPG. Crude oil distillates and residues are precursors for olefins and aromatics via cracking and reforming processes. This chapter reviews the properties of the different hydrocarbon intermediates—paraffins, olefins, diolefms, and aromatics. Petroleum fractions and residues as mixtures of different hydrocarbon classes and hydrocarbon derivatives are discussed separately at the end of the chapter. 

In Europe naphtha is the preferred feedstock for the production of synthesis gas, which is used to synthesize methanol and ammonia (Chapter 4). Another important role for naphtha is its use as a feedstock for steam cracking units for light olefins production (Chapter 3). Heavy naphtha, on the other hand, is a major feedstock for catalytic reforming. The product reformate containing a high percentage of Ce-Cg aromatic hydrocarbons is used to make gasoline. Reformates are also extracted to separate the aromatics as intermediates for petrochemicals. 

Diels-Alder cycloadditions of enaminothiones with electrophilic die-nophiles (83AHC145, section III,G) have been widely used for the preparation of various thiopyran derivatives. In addition to expected 2H-thiopyrans, 4//-isomers were trapped as intermediates or even as final products. Nitroolefines (90T1951), olefinic carbonyl derivatives [80JH405 85JOC1545 92JCS(P1)2603] allenedicarboxylic esters... 

Like the examples above, dihydroxyacetanilide epoxidase (DHAE) uses an olefin as the substrate for epoxidation. Its mechanism, however, is fundamentally different from those of cytochrome P450 or flavin-dependent enzymes. Dihydroxyacetanilide is an intermediate in the biosynthesis of the epoxyquinones LL-C10037a, an antitumor agent produced by the actinomycete Streptomyces LL-C10037 [75, 76], and MM14201, an antibiotic produced by Streptomyces MPP 3051 (Scheme 10.20) [77]. The main structural difference between the two antibiotics lies in the opposite stereochemistry of the oxirane ring. 

Epoxides are often encountered in nature, both as intermediates in key biosynthetic pathways and as secondary metabolites. The selective epoxidation of squa-lene, resulting in 2,3-squalene oxide, for example, is the prelude to the remarkable olefin oligomerization cascade that creates the steroid nucleus [7]. Tetrahydrodiols, the ultimate products of metabolism of polycyclic aromatic hydrocarbons, bind to the nucleic acids of mammalian cells and are implicated in carcinogenesis [8], In organic synthesis, epoxides are invaluable building blocks for introduction of diverse functionality into the hydrocarbon backbone in a 1,2-fashion. It is therefore not surprising that chemistry of epoxides has received much attention [9]. 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

Pd/P(t-Bu)., in the presence of Cy2NMe, is an unusually mild and versatile catalyst for Heck reactions of aryl chlorides (Tables 1 and 2) (as well as for room-temperature reactions of aryl bromides).21 22 23 Example A, the coupling of chlorobenzene with butyl methacrylate, illustrates the application of this method to the stereoselective synthesis of a trisubstituted olefin a-methylcinnamic acid derivatives are an important family of compounds that possess biological activity (e.g., hypolipidemic24 and antibiotic25) and serve as intermediates in the synthesis of pharmaceuticals (e.g., Sulindac, a non-steroidal anti-inflammatory drug26). Example B, the coupling of 4-chlorobenzonitrile with styrene, demonstrates that Pd/P(t-Bu). can catalyze the Heck reaction of activated aryl chlorides at room temperature. 

Reaction of Unsaturated Compounds with Dialkyl Phosphites The diesters of phosphorous acid are somewhat resistant to oxidation and used as intermediates for the reaction with olefins forming phosphonic acids. The reaction takes place in the presence of free radical initiators, such as di-/-bu-tyl peroxide [104,105]. 

The current-potential relationship indicates that the rate determining step for the Kolbe reaction in aqueous solution is most probably an irreversible 1 e-transfer to the carboxylate with simultaneous bond breaking leading to the alkyl radical and carbon dioxide [8]. However, also other rate determining steps have been proposed [10]. When the acyloxy radical is assumed as intermediate it would be very shortlived and decompose with a half life of t 10" to carbon dioxide and an alkyl radical [89]. From the thermochemical data it has been concluded that the rate of carbon dioxide elimination effects the product distribution. Olefin formation is assumed to be due to reaction of the carboxylate radical with the alkyl radical and the higher olefin ratio for propionate and butyrate is argued to be the result of the slower decarboxylation of these carboxylates [90]. 

The Markovnikov regioselectivity of the gem-alkenes is associated with a chemoselectivity. in favour of methanol attack, significantly greater than that observed for the other alkenes. If no sodium bromide is added to the reaction medium, no dibromide is observed for this series. Therefore, these alkenes behave as highly conjugated olefins, as regards their regio- and chemo-selectivity. In other words, the bromination intermediates of gem-alkenes resemble P-bromocarbocations, rather than bromonium ions. Theoretical calculations (ref. 8) but not kinetic data (ref. 14) support this conclusion. 

It has been shown (ref. 21) that a solvent which is both protic and nucleophilic, assists the formation of the bromination intermediates of moderately reactive olefins as styrenes in two ways, (Scheme 7). Firstly, the solvent initiates bromide ion formation electrophilically and, secondly, favours... 

Phosphinyl radicals, obtained by hydrogen abstraction from dialkyl phosphites, have been trapped with t-nitrosobutane and the resultant nitroxyl radicals examined by e.s.r, The reaction of phosphinyl radicals, e.g. (5) and (6), with olefins has been shown to occur with retention of configuration at phosphorus. " These radicals have also been postulated as intermediates in the reactions of dialkyl disulphides and diaryl disulphides with phosphinates. " From the reaction of diphenyl disulphide... 

To test this theory, a mixture of n-hexane and Relabeled 1-hexene was reacted in hydrogen over the catalyst at various space velocities. The specific activity of each of the products (the n-hexenes were lumped together) are shown in Figure 2. The important observation is made at zero conversion. When extrapolated to Infinite space velocity, the benzene has approximately the same specific activity as the hexene, thus clearly indicating that essentially all the benzene is formed in a reaction sequence that involves equilibrium with gaseous n-hexenes. It may then be concluded that olefins are intermediates in the aromatiza-tion process. 

We have recently reported that the addition of Ni results in a promotion of the isomerization activity of sulfated zirconia [10] comparable to that obtained by the addition of Fe and Mn. It has been previously observed that the presence of H2 causes a decrease in isomerization activity, a result consistent with the mechanism that involves olefins as reaction intermediates Here, we... 

It also explains the /Z selectivity of products at low conversions (kinetic ratio. Scheme 19). In the case of propene, a terminal olefin, E 2-butene is usually favoured (E/Z - 2.5 Scheme 19), while Z 3-heptene is transformed into 3-hexene and 4-octene with EjZ ratios of 0.75 and 0.6, respectively, which shows that in this case Z-olefins are favoured (Scheme 20). At full conversion, the thermodynamic equilibriums are reached to give the -olefins as the major isomers in both cases. For terminal olefins, the E olefin is the kinetic product because the favoured pathway involved intermediates in which the [ 1,2]-interactions are minimized, that is when both substituents (methyls) are least interacting. In the metathesis of Z-olefins, the metallacyclobutanes are trisubstituted, and Z-olefins are the kinetic products because they invoke reaction intermediates in which [1,2] and especially [1,3] interactions are minimized. 

Kharasch and coworkers were the first to show that thiols and olefins cooxidize in an atmosphere of oxygen at room temperature to yield substituted 2-sulphinylethanols 81 (equation 45). Later on, it was demonstrated that a-mercapto-substituted hydroperoxides are formed as intermediates. Thus, Oswald found that cooxidation of thiophenol with styrene gave the corresponding 8-mercaptohydroperoxide 82 which subsequently underwent rearrangement to 2-phenylsulphinyl-a-phenylethanol 83 (equation 46). 

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