Alkene isomerization reaction, solvent effects

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). ... 

In all the evaluations of Table 4, the solvent effects on the activation free enthalpies are positive, increase with increasing solvent dielectric constants and tend to be larger for the endo than for the exo adducts. This behaviour, in accord with the experimental trend, is due to the electrostatic contribution the CDS and CDR contributions, in fact, are rather independent of the isomeric reaction considered and, moreover, appear to obtain comparable values in every 1,3-dipolar cycloaddition. For the Tomasi parametrisation in water, for example, the CDR" contribution for the cycloadditions of diazomethane and nitrile oxides to substituted alkenes amounts to -1.85 0.14 kcal mok This finding can be traced back to the view that the CDR term is approximately proportional to the solvent accessible surface area (the cavity area) of solutes and to the feature of TSs of having very alike structures of the new forming pentatomic ring so that the changes of the cavity areas from reactants to TSs are similar. 

In low-polarity media, specific interaction with protic species (water) dramatically affects the reactivity (nucleophilicity or basicity) of anions with high charge density (OH. F. oxanions. carbanions, etc.). Basicity of OH in the Hofinann elimination reaction of (hexyl)4 N 0H /7H20 (Eq. 9), carried out in a chlorobenzene-water two-phase system, increases 50.000 times by reducing the hydration number n of the anion from 11 to 3. The enhancement is extrapolated to be more than nine powers of 10 for the hypothetical anhydrous hydroxide. This indicates that the largely dehydrated hydroxide, extracted in a low-polarity solvent (chlorobenzene) from concentrated alkaline soluiions. is an extremely powerful base. Results account for the dramatic effect produced by an increase of base on the rate of reactions promoted by alkali hydroxides under LL-PTC conditions, such as carbanion formation and alkylation, alkene isomerization, H/D exchanges in carbon acids, and acid-base equilibria ... 

Long-chain aliphatic olefins give only insufficient conversion to the acids due to low solubility and isomerization side reactions. In order to overcome these problems the effect of co-solvents and chemically modified /i-cyclodextrins as additives was investigated for the hydrocarboxylation of 1-decene [23], Without such a promoter, conversion and acid selectivity are low, 10% and 20% respectively. Addition of co-solvents significantly increases conversion, but does not reduce the isomerization. In contrast, the addition of dimethyl-/i-cyclodextrin increased conversion and induced 90% selectivity toward the acids. This effect is rationalized by a host/ guest complex of the cyclic carbohydrate and the olefin which prevents isomerization of the double bond. This pronounced chemoselectivity effect of cyclodextrins is also observed in the hydroformylation and the Wacker oxidation of water-insoluble olefins [24, 25]. More recent studies of the biphasic hydrocarboxylation include the reaction of vinyl aromatic compounds to the isomeric arylpropanoic acids [29, 30], and of small, sparingly water-soluble alkenes such as propene [31]. 

A Br0nsted acid HX may be used as a co-catalyst. Long-chain alkenes give only insufficient conversion due to low solubility and isomerization side reactions. In order to overcome these problems the addition of co-solvents (such as y0-cyclo-dextrins cf. Section 2.2.3.3) was recommended. Their advantageous effect was rationalized by a host-guest complex of the cyclic carbohydrate and the alkene feed which prevents isomerization of the double bond [5]. 

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