Isomerization solvent effects

Isomerization.—Solvent effects have been used to distinguish between intramolecular twist and one-end-off mechanisms for isomerization of -diketone complexes of... 

The evidence presented so far excludes the formation of dissociated ions as the principal precursor to sulfone, since such a mechanism would yield a mixture of two isomeric sulfones. Similarly, in the case of optically active ester a racemic product should be formed. The observed data are consistent with either an ion-pair mechanism or a more concerted cyclic intramolecular mechanism involving little change between the polarity of the ground state and transition state. Support for the second alternative was found from measurements of the substituent and solvent effects on the rate of reaction. 

Taskinen and Nummelin (op. cit.) reported many other isomer equilibria in their paper. Most of these used cyclohexane as the solvent and I2 as the catalyst and so are not confounded by solvent effects. However, these authors noted that hydrogen atom transfer induced disproportionation (to form the aromatic benzene) dominates this reaction for the case of 49/50 isomerization and so they needed alternative reaction conditions. 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Pappalardo, R. R., Sanchez Marcos, E., Ruiz-Lopez, M. F., Rinaldi, D. and Rivail, J. L. Solvent effects on molecular geometries and isomerization processes A study of push-pull ethylenes in solution, JAm.Chem.Soc., 115 (1993), 3722-3730... 

The thermal isomerization of tert. -butyl-diisobutylborane to triisobutylborane and the stepwise isomerization of triisopropyl and tri-sec.-butyl boranes to the corresponding straight-chain isomers have been studied over a range of temperature, both neat and using either the end product trialkylborane or diglyme as solvent117 . In all cases the reactions were first order and showed no solvent effect. 

Meso-dl isomerization of [47] was described by Koch (Koch et ah, 1975 Koch, 1986 Olson and Koch, 1986. The intermediate radical [48] is in equilibrium with the dimer and can be easily recognized by esr spectroscopy. The thermodynamic parameters for bond homolysis, as a function of medium, are reported in Table 18. A strong solvent effect is observed, in contrast to Riichardt s example ([24], [25]) reported above. This is interpreted as a manifestation of the polar character of the intermediate radical. The easy detection of [48] by esr spectroscopy is traced back, at least in part, to its captodative character. However, the strong solvent effect on homolysis of [47] need not necessarily be related to the captodative character of radicals [48]. 

The photochemical isomerization 41B — 41A was observed in nonpolar solvents (83AJC1603). Equilibrium 1) was attained in CD3OD solution after several days, but following concentration and cooling of the solution, only the cyclic isomer 41B, solvated by methanol, was isolated in the solid state. Alcoholic solvents effectively quench the photochemical isomerization. 

However, this process requires a reaction time of 2 days and is inapplicable to unsymmetrical couplings (two different epoxides). As the study described in equation 24 revealed a dramatic solvent effect on similar Brook isomerizations in the adduct of lithio dihalo(triaIkylsilyl)methanes with epoxides (isomerization did not occur following metalation and initial alkylation in THF but proceeded readily upon addition of HMPA), the effect of HMPA for promoting the Brook isomerization was studied once the first alkylation was complete (equation 28) . ... 

To verify this mechanistic possibility, the solvent dependence of the ene products derived from the photooxygenation of the isomeric a,-unsaturated esters 93-E and 93-Z was examined (Scheme 23). For 93-E, the two ene products are formed from two different perepoxides. When the oxygen atom of the perepoxide intermediate is placed syn to the ester group, 93b is produced, whereas 93a is formed from the opposite case. For isomer 93-E, the expected solvent effect was found (93a/93b = 85/15 in CCI4 or benzene, and 70/30 in DMSO). On the other hand, for the isomer 93-Z, both products are formed from the same intermediate (the perepoxide oxygen is placed anti to the ester functionality), and no solvent dependence on the ene products was found (93a/93b = 95/5 in CCI4 and 93/7 in DMSO). 

Even for a simple reaction, involving just one reactant species and one product species, such as a keto-enol tautomerism or a cis-trans isomerization, the above equation for a given solvent is complicated. StUl, in specific cases it is possible to unravel the solvent effects of cavity formation, for the solute species have different volumes, polarity/polarizability if the solute species differ in their dipole moments or polarizabilities, and solvent Lewis acidity and basicity if the solutes differ in their electron-pair and hydrogen-bond acceptance abilities. 

Thermodynamic Equilibrium, Kinetics, Activation Barriers, and Reaction Mechanisms for Chemical Reactions in Karst Terrains (White, 1997) Solvent Effects On Isomerization Equilibria—an Energetic Analysis in the Framework of Density Functional Theory (Lelj and Adamo, 1995)... 

No evidence was found of isomerization of the CH to the COMe adduct on addition of methanol to the DMSO solution,41 although a solvent effect of this kind was observed in carbocyclic series.48 However, methanol seems to have a depressing effect on the stability of the CH adduct and to promote its decomposition. Slow ring opening has been suggested to occur in MeOH-DMSO (1 9) starting from the 4-methoxy isomer.41... 

More important, all of the 4 -substituted trans-9-styrylanthracenes 69b-k undergo photochemical trans- cis isomerization. The effect of solvent polarity is obvious from the greatly enhanced quantum yields of trans-+cis isomerization observed for most substituted 9-styrylanthracenes in acetonitrile solution. In some cases, the cis-isomers are actually favored at the photostationary state (see Table 10). 

Isomer stabilities and activation energies have been calculated for keto-enol tautomerization of simple carbonyl compounds, MeC(R)=X (X = O R = H, Me) 129 both specific and bulk solvent effects have been analysed. Related isomerizations of acid derivatives (R = F, CN) and other related structures (R = H X = CF12, NH, S) are compared. 

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