Olefin formation

If desired, the alcohol may be identified as the 3 5-dinitrobenzoate (Section 111,27) it is then best to repeat the experiment on a larger scale and to replace the dilute hydrochloric acid by dilute sulphuric acid. It must, however, be pointed out that the reaction is not always so simple as indicated in the above equation. Olefine formation and rearrangement of the alcohol sometimes occur thus n-prop3 lamine yields n-propyl alcohol, isopropyl alcohol and propylene. 

Dehydration. Dehydration of amyl alcohols is important for the preparation of specialty olefins and where it may produce unwanted by-products under acidic reaction conditions. Olefin formation is especially facile with secondary or tertiary amyl alcohols under acidic conditions. The reverse reaction, hydration of olefins, is commonly used for the preparation of alcohols. 

Olefin formation (preferentially less substituted) from alcohols via xanthate pyrolysis. 

The product composition from these reactions is influenced by the location of the functional group in the substrate. Olefin formation is the most common side reaction and in certain cases, especially with reductions of tosyl-hydrazones (section IV-B), it may become dominant so that the reaction can be used for the preparation of mono-labeled olefins. 

Two serious drawbacks of this method are the extensive deuterium scrambling around the reaction site and the occasional formation of olefinic side products, which are hard to separate by conventional means. The extent of olefin formation may depend on the nature of the Raney nickel since it is known that desulfurization with deactivated Raney nickel can yield olefins. Best results are obtained when the deuterated Raney nickel is prepared very rapidly and used immediately after preparation. 

The extent of olefin formation depends on the position of the functional group, " on the degree of a-substitution and on the concentration of the hydride (or deuteride). Usually olefin formation can be largely suppressed by increasing the concentration of lithium aluminum deuteride. With certain tosylhydrazones, however, such as the C-17 derivative (103), olefin (104) is a major product irrespective of the quantity of the reagent used. ... 

A mechanism which is consistent with the various experimental results for olefin formation involves the initial abstraction of the hydrazone proton (103->106) In this case, however, expulsion of the tosylate anion is associated with the abstraction of a second hydrogen from C-16 instead of hydride attack on the C=N bond (compare 97 98 and 106 107). Ex-... 

The elimination from sulfonates of secondary alcohols is frequently easier than more direct methods applied to the free alcohols. As with the latter, there are the possibilities of isomeric olefin formation and rearrangement reactions. In addition, displacement and hydrolysis may occur, but these side reactions can usually be suppressed. 

In certain cases this reduction (with lithium aluminum hydride) takes a different course, and olefins are formed. The effect is dependent on both the reagent concentration and the steric environment of the hydrazone. Dilute reagent and hindered hydrazone favor olefins borohydride gives the saturated hydrocarbon. The hydrogen picked up in olefin formation comes from solvent, and in full reduction one comes from hydride and the other from solvent. This was shown by deuteriation experiments with the hydrazone (150) ... 

Olefin formation by elimination from sulfonate esters, 331... 

Olefin formation by reduction of keto derivatives via tosylhydrazones, 354 Olefin formation by reduction of thioketals, 356... 

Olefin formation by reduction of a,/3-un-saturated ketones with diborane, 347 Olefin formation by reductive eliminations, 343... 

The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

A common consequence of migration in complex molecules is that tetrasubstituted olefins result, which can be hydrogenated only with difficulty, if at all. It is easier to try to prevent hindered olefin formation than it is to correct it. Attempted hydrogenation of the exocyclic methylene group in 15 proved difficult when using an aged 10% Pd-on-C catalyst there was a... 

A higher steam/hydrocarhon ratio favors olefin formation. Steam reduces the partial pressure of the hydrocarbon mixture and increases the yield of olefins. Heavier hydrocarbon feeds require more steam than gaseous feeds to additionally reduce coke deposition in the furnace tubes. Liquid feeds such as gas oils and petroleum residues have complex polynuclear aromatic compounds, which are coke precursors. Steam to hydrocarbon weight ratios range between 0.2-1 for ethane and approximately 1-1.2 for liquid feeds. 

O Brien et al. provided the first examples of olefin formation by reductive alkylation of aziridines [97]. Treatment of aziridine 267 with s-BuLi gave olefin 270 in 76% yield (Scheme 5.68). For the formation of olefin 270 they suggest a reaction pathway that proceeds in a manner analogous to that proposed for epoxides [36] namely, nucleophilic attack of s-BuLi on lithiated aziridine 268 to form dilithiated species 269, which eliminates Li2NTs (TsNH2 was observed as a product of this reaction) to yield olefin 270. 

If the energetic criterion is applicable, we must conclude from the large endothermicity that Reaction 19 does not occur, but then we are at a loss to explain the formation of the C4H8 + ion. We can write reasonable mechanistic schemes for the olefin formation which rationalize the data and fail only with respect to the energetic criterion. Thus, we can write for the C4H8 + ion formed from a paraffin with a tert-butyl end structure... 

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]. 

This route was limited to R = Me or Et, since the use of higher alkyl groups resulted in HX elimination and olefin formation ... 

In addition to the above olefin formation, another although generally slow El reaction can take place, yielding a tertiary amine ... 

Silvestri, Naro, and Smith 142) have shown that cyclization reactions are strongly poisoned by adsorbed sulfur, although the activity for olefin formation was not much affected. This agrees with the conclusions of Shephard and Rooney 136). 

Reactions over chromium oxide catalysts are often carried out without the addition of hydrogen to the reaction mixture, since this addition tends to reduce the catalytic activity. Thus, since chromium oxide is highly active for dehydrogenation, under the usual reaction conditions (temperature >500°C) extensive olefin formation occurs. In the following discussion we shall, in the main, be concerned only with skeletally distinguished products. Information about reaction pathways has been obtained by a study of the reaction product distribution from unlabeled (e.g. 89, 3, 118, 184-186, 38, 187) as well as from 14C-labeled reactants (89, 87, 88, 91-95, 98, 188, 189). The main mechanistic conclusions may be summarized. Although some skeletal isomerization occurs, chromium oxide catalysts are, on the whole, less efficient for skeletal isomerization than are platinum catalysts. Cyclic C5 products are of never more than very minor impor-... 

Gray, M. R., and McCaffrey, W. C., Role of Chain Reactions and Olefin Formation in Cracking, Hydroconversion, and Coking of Petroleum and Bitumen Fractions. Energy Fuels, 2002. 16(3) pp. 756-66. 

In an intramolecular version of ketocarbenoid a-C/H insertion, copper-promoted decomposition of l-diazo-3-(pyrrol-l-yl)-2-propanone (258a) or l-diazo-4-(pyrrol-l-yl)-2-butanone (258b) resulted in quantitative formation of the respective cycli-zation product 259 242 >. The cyclization 260 -> 261, on the other hand, is a low-yield reaction which is accompanied by olefin formation. The product ratio was found to vary with the copper catalyst used, but the total yield never exceeded 35 % 243>. 

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