Polarity of the solvent

Theoretical work by the groups directed by Sustmann and, very recently, Mattay attributes the preference for the formation of endo cycloadduct in solution to the polarity of the solvent Their calculations indicate that in the gas phase the exo transition state has a lower energy than the endo counterpart and it is only upon introduction of the solvent that this situation reverses, due to the difference in polarity of both transition states (Figure 1.2). Mattay" stresses the importance of the dienophile transoid-dsoid conformational equilibrium in determining the endo-exo selectivity. The transoid conformation is favoured in solution and is shown to lead to endo product, whereas the cisoid conformation, that is favoured in the gas phase, produces the exo adduct This view is in conflict with ab initio calculations by Houk, indicating an enhanced secondary orbital interaction in the cisoid endo transition state . 

Assuming London-dispersion and electrostatic interactions to be dominant in arene - arene interactions (see Section 3.2.7), the solvent effect on the enantioselectivify is anticipated to be influenced by the polarisability and polarity of the solvent. The arene -arene interaction is inferred to... 

As regards sulphuric acid, there is here again an increase in polarity and an increase in rates of nitration when comparison is made with other solvents in the series. This gross fact would be difficult to reconcile with any mechanism, such as the one, which contains an essential forward step which would be retarded by increased polarity of the solvent. 

It seems now established by NMR spectroscopic investigations that a change can take place in electronic structures and atomic configuration of the dyes depending on the polarity of the solvent. Parameters describing the transition from one single bond to more double bond character vary according to the nature of the solvent (107). 

The reaction medium plays a very important role in all ionic polymerizations. Likewise, the nature of the ionic partner to the active center-called the counterion or gegenion-has a large effect also. This is true because the nature of the counterion, the polarity of the solvent, and the possibility of specific solvent-ion interactions determines the average distance of separation between the ions in solution. It is not difficult to visualize a whole spectrum of possibilities, from completely separated ions to an ion pair of partially solvated ions to an ion pair of unsolvated ions. The distance between the centers of the ions is different in... 

Solubility. At long oil lengths, the aUphatic hydrocarbon chains of the fatty acids constitute the major portion of the mass of the reski molecules therefore, the reski is soluble ki nonpolar aUphatic solvents. Conversely, as the oil length decreases and the phthaUc content kicreases, the aromaticity of the reski molecules kicreases, and the aromaticity and/or the polarity of the solvent must be kicreased ki order to dissolve the reski effectively. 

Acylation. Aliphatic amine oxides react with acylating agents such as acetic anhydride and acetyl chloride to form either A[,A/-diaLkylamides and aldehyde (34), the Polonovski reaction, or an ester, depending upon the polarity of the solvent used (35,36). Along with a polar mechanism (37), a metal-complex-induced mechanism involving a free-radical intermediate has been proposed. 

This section contains a brief review of the molecular version of Marcus theory, as developed by Warshel [81]. The free energy surface for an electron transfer reaction is shown schematically in Eigure 1, where R represents the reactants and A, P represents the products D and A , and the reaction coordinate X is the degree of polarization of the solvent. The subscript o for R and P denotes the equilibrium values of R and P, while P is the Eranck-Condon state on the P-surface. The activation free energy, AG, can be calculated from Marcus theory by Eq. (4). This relation is based on the assumption that the free energy is a parabolic function of the polarization coordinate. Eor self-exchange transfer reactions, we need only X to calculate AG, because AG° = 0. Moreover, we can write... 

With enamines of cyclic ketones direct C alkylation occurs with allyl and propargyl as well as alkyl halides. The reaction is again sensitive to the polarity of the solvent (29). The pyrrolidine enamine of cyclohexanone on reaction with ethyl iodide in dioxane gave 25% of 2-ethylcyclohexanone on hydrolysis, while in chloroform the yield was increased to 32%. 

The enamines derived from cyclic ketones give the normal alkylated products, although there is some evidence that unstable cycloadducts are initially formed (55b). Thus the enamine (28) derived from cyclohexanone and pyrrolidine on reaction with acrylonitrile, acrylate esters, or phenyl vinyl sulfone gave the 2-alkylated cyclohexanones (63) on hydrolysis of the intermediates (31,32,55,56). These additions are sensitive to the polarity of the solvent. Thus (28) in benzene or dioxane gave an 80% yield of the... 

Tomasi s Polarized Continuum Model (PCM) defines the cavity as the union of a series of interlocking atomic spheres. The effect of polarization of the solvent continuum is represented numerically it is computed by numerical integration rather... 

Alkali metal alkoxides, r-butyl acetate neat, 45°, 30 min, 98% yield of r-butyl ester from methyl benzoate. The rate constant for the reaction increases with increasing ionic radius of the metal and with decreasing polarity of the solvent. Equilibrium for the reaction is achieved in <10 sec. Other examples eire presented. " ... 

This method is not effective for tertiary alcohols. It has a strong rate dependence on the polarity of the solvent, with less polar solvents giving faster rates. 

The polarity of the C—-OH bond, i.e., the basicity of the carbinol-amine, depends on its structure, particularly on the stability of the ring system (degree of aromatic character), and the electron affinity of the substituents on nitrogen and carbon. Of course, external factors also play an important role in the equilibrium temperature, polarity of the solvent, and presence or absence of catalysts (the solvent can also act as a catalyst). 

The effect of the polarity of the solvent is obvious. Any particular carbinolamine is more dissociated in a more polar solvent than in a less polar one. 

The 0,N-dideuterated enol was formed by hydrolysis of the O-trimethylsilyl ether 123 (R = TMS) (in 80% [D6]DMSO/20% D2O with 5. lO " M DCl). N-Methylindoxyl (formed by hydrolysis of its acetate) exists in the solid state as a mixture of the enol and the keto tautomers (34% enol/66% keto). The NMR spectrum of freshly prepared solution in DMSO demonstrated signals of both enol and keto forms. However, at equilibrium (reached in 18 h at RT) the ratio of enol to ketone depends strongly on the polarity of the solvent used thus, in [Dg]DMSO the tautomeric mixture contains 92% enol, while in CDCI3 the keto form predominates (97%). A solution with 100% enol could be generated by hydrolysis of its O-trimethylsilyl ether [conditions 80% [Dfi]DMSO/20% D2O with 5 10" M DCl at 32°C (86TL3275 87PAC1577 88TL250)]. 

The NMR spectra of the corresponding A-aryl derivatives (168d, 169d) are fairly complex (several E/Z isomers), but it was concluded that tautomers 168 and 169 are both present. The equilibrium strongly depends on the polarity of the solvent, the concentration, the temperature, and the nature of the substituent R. 

Usually, the cychc isomers 272 are preferred in the sohd state. X-Ray structural evidence was obtained for compound 272 [R = COPh, R = Me, R = R" = H, R = CF3 X = O (92KK1184)]. In solution, the cyclic and ring-opened isomers are found in equihbria in ratios depending on polarity of the solvent and structural factors (see 88KGS3 95ZOB705 96AHC(66)1 for reviews). 

The ring-chain tautomerism of the imidazolidines 279 (80H1313) is of interest (Scheme 99). The isomer ratio is determined by the nature of the substituents and is hardly affected by the polarity of the solvent (CCI4, DMSO). 

Recently a definitive study of several isoxazol-5-ones using infrared and ultraviolet spectroscopy (Table I) has shown that the balance between the various tautomers is a delicate one and that all three of the structural types can predominate depending upon the nature of the substituents and the conditions of the experiment. However, the hydroxy form is only found when it is stabilized by chelation (i.e., a carbonyl substituent in the 4-position). The other compounds exist in the CH form in nonpolar media increasing polarity of the solvent stabilizes increasing amounts of the more polar NH forms. 

A study of the relative stability of tra 5-2-azidothiazole 32b with respect to the corresponding cis conformer 32a showed that 32b is destabilized in the gas phase but appears to be the more stable conformer in solution (Scheme 23). The stability increases with increasing polarity of the solvent [98JA4723]. 

In solution, the position of the tautomerie equilibrium of 1,2-dihydropyrimidines is highly infiueneed by the polarity of the solvent The eontent of the more polar 1,2-dihydro tautomer is greatly inereased in DMSO-iie due to strong intermoleeular bonding with the solvent eompared to that in CDCI3, as shown in Table I. 

Tautomerism of simple monosubstituted 1,4-dihydropyrimidines in solution has been studied on an example of 2-phenyldihydropyrimidine 48, prepared by condensation of benzamidine with acrolein [84H(22)657]. IR and H and NMR spectra at -60°C in specially purified solvents showed that this compound exists as a tautomeric mixture of 1,4- and 1,6-dihydro tautomers (Scheme 17), with the relative amount of 1,4-dihydro isomer 48a increasing with the polarity of the solvent. 

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