Solvent effect alkene mixtures

Table 5-7. Observed solvent effects on the proportion of alkene formed in mono- and bimolecular / -eliminations in ethanol/water mixtures [16, 44],...

Small but significant effects of solvent polarity were found in the autoxidation of a variety of alkenes and aralkyl hydrocarbons [216-220] (styrene [216, 218, 219], ethyl methyl ketone [217], cyclohexene [218], cumene [218, 219], tetralin [219], etc.). An extensive study on solvent effects in the azobisisobutyronitrile (AIBN)-initiated oxidation of tetralin in a great variety of solvents and binary solvent mixtures was made by Kamiya et al. [220],... 

The palladium(II) catalyst, because of its Lewis acidity, may play a role in the addition of aUylic tin to the ketone however, acylation of crotyltin was not reported to form a tertiary alcohol using palla-dium(II). It appears that solvent effects dominate in these cases. As part of the same study, substitute vi-nylstannanes were shown to undergo acylation with retention of configuration however, the resulting a, -unsaturated ketones were not configurationally stable to the reaction conditions. Isomerically pure (Z)- 1-propenylstannane was acylated to afford a 50 50 mixture of alkenes (equation 87). The (Z)-a, un-saturated ketone was shown to isomerize to a mixture of (Z)- and ( )-isomers under the reaction conditions. Mixtures of (Z)- and ( )-2-substituted vinylstannanes were acylated to afford mainly the ( )-a, -unsaturated ketone (equation 88). ... 

Two mechanistic variations can be envisioned to be operative in some Wittig reactions. In some cases (Z-stereoselective reactions) a very asynchronous reaction with a 2-center "anti" pseudo betaine transition state is involved. The other mechanism involves a more synchronous concerted reaction with a 4-center "syn" transition state. These two mechanisms may compete, e.g., in the reactions of semistabilized ylides with aldehydes where mixtures of Z- and E-alkenes are obtained. Further details of these studies will be provided in future publications, as well as additional data about solvent effects. 

In some cases the addition of bromide to alkene radical cations is reversible. For example, the addition of bromide to the p-methyl-4-methoxystyrene radical cation occurs reversibly, as demonstrated by the formation of the radical cation when the P-bromo radical is generated independently by photolysis of l-(4-methoxyphenyl)-l,2-dibtomopropane (Eq. 18). An equilibrium constant of 2 x 10 M has been measured for the loss of bromide from this radical in acetonitrile. The apparent lack of reactivity of 1,3-dioxole radical cations with bromide ion in water has also been explained on the basis of reversible addition with rapid loss of bromide from the product radical. However, on the basis of the solvent effects noted above, it is also possible that the lack of reactivity in water is a solvent effect since decreases in reactivity of 4 to 5 orders of magnitude have been observed for reactions of bromide ion with styrene radical cations in largely aqueous solvent mixtures. - ... 

In absence of alkane solvent (Table 3), this highly non-ideal behavior leads even to a separation factor higher than 9 for a hexene/dodecene mixture allowing their very efficient separation. In practice, the above mentioned packing effects for alkane/alkene mixtures can be exploited in adsorptive separation or catalytic processes the relative selectivity for alkenes with different chain length can be adjusted by choosing different alkane solvents and different alkene concentrations. 

Structure (43) (QPCH2CH2PQ) (Q = mixture of 1,4- and 1,5-cyclooctanediyl) was reported to be a very effective ligand for the palladium catalyzed hydroformylation of internal and terminal alkenes using small amounts of NaCl or HC1 as an additive in PhOMe or diglyme as the solvent.138-141... 

By far the most commonly used - though not the most environmentally friendly -solvent is CCl (or more usually water-CCl ). In a classic paper Sharpless et al. showed that oxidation reactions of RuO (and other some Ru-based oxidants) were accelerated by addition of a little acetonitrile to the conventional water-CCl biphasic mixture. It was suggested that the CH3CN might function as a mild donor stabilising a lower oxidation state carboxylato Ru species which could be involved in the catalytic process [260]. A comparative study of CCl, acetone, ethyl acetate, cyclohexane and acetone for cleavage of alkenes and alkynes by RuClg/aq. IO(OH)3/solvent showed that cyclohexane was the most effective [216]. Other solvents sometimes... 

Alkylation with Carbonyl Compounds The Prins Reaction. Carbonyl compounds react with alkenes in the presence of Brpnsted acids to form a complex mixture of products known as the Prins reaction. The use of appropriate reaction conditions, solvents, and catalysts allows one to perform selective syntheses. Characteristically formaldehyde is the principal aldehyde used. Mineral acids (sulfuric acid, phosphoric acid), p-toluenesulfonic acid, and ion exchange resins are the most frequent catalysts. Certain Lewis acids (BF3, ZnCl2, SnCl4) are, however, also effective. 

Fluorous solvents proved to be highly effective in epoxidation of alkenes. H202 can be used in combination with trifluoroacetone,27 perfluoroacetone,28 or a mixture of perfluoroacetone and hexafluoro-2-propanol.29 In fluorinated alcohols as solvents uncatalyzed epoxidations with aqueous H202 are performed.30,31... 

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