Esters solvent effects

Esters can also be synthesized by an acid-catalyzed nucleophilic acyl substitution reaction of a carboxylic acid with an alcohol, a process called the Fischer esterification reaction. Unfortunately, the need to use an excess of a liquid alcohol as solvent effectively limits the method to the synthesis of methyl, ethyl, propyl, and butyl esters. 

There have been numerous studies on the kinetics of decomposition of A IRK. AIBMe and other dialkyldiazenes.46 Solvent effects on are small by conventional standards but, nonetheless, significant. Data for AIBMe is presented in Table 3.3. The data come from a variety of sources and can be seen to increase in the series where the solvent is aliphatic < ester (including MMA) < aromatic (including styrene) < alcohol. There is a factor of two difference between kA in methanol and k< in ethyl acetate. The value of kA for AIBN is also reported to be higher in aromatic than in hydrocarbon solvents and to increase with the dielectric constant of the medium.31 79 80 Tlic kA of AIBMe and AIBN show no direct correlation with solvent viscosity (see also 3.3.1.1.3), which is consistent with the reaction being irreversible (Le. no cage return). 

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. 

Lipase-catalyzed polymerizahon of oxyacid esters was reported. PPL catalyzed the polymerizahon of methyl 6-hydroxyhexanoate. ° The polymer with DP up to 100 was synthesized by polymerization in hexane at 69°C for more than 50 days. The PPL-catalyzed polymerization of methyl 5-hydroxypentanoate for 60 days produced the polymer with DP of 29. Solvent effects were... 

Aldol Condensation of Zinc Ester Enolates with Benzaldehyde Anil Solvent Effects (Scheme 16) (76)... 

We reported in the previous papers [8, 9] that the effect of the operational factors such as temperature and solvents on the polymorphic crystallization of a thiazole derivative - 2-(3 -Cyano-4-(2-methylpropoxy)-phenyl)-4-methyl-thiazole-5-car-boxylic acid (BPT) - which is an enzyme inhibitor. In this paper, we synthesized the esters of BPT and studied the effect of the molecular structure on polymorphic nucleation systemically, and at the same time we also examined the solvent effect on the polymorphic nucleation of the ester. 

The pyridinium chlorochromate (PCC) oxidations of pentaamine cobalt(III)-bound and unbound mandelic and lactic acids have been studied and found to proceed at similar rates.Free-energy relationships in the oxidation of aromatic anils by PCC have been studied. Solvent effects in the oxidation of methionine by PCC and pyridinium bromochromate (PBC) have been investigated the reaction leads to the formation of the corresponding sulfoxide and mechanisms have been proposed. The major product of the acid-catalysed oxidation of a range of diols by PBC is the hydroxyaldehyde. The reaction is first order with respect to the diol and exhibits a substantial primary kinetic isotope effect. Proposed acid-dependent and acid-independent mechanisms involve the rapid formation of a chromate ester in a pre-equilibrium step, followed by rate-determining hydride ion transfer via a cyclic intermediate. PBC oxidation of thio acids has been studied. ... 

Many mechanisms had been proposed in the past to rationalize this selectivity (tri-oxanes, perepoxide, exciplex, dipolar or biradical intermediates) however, it is now generally accepted that the reaction proceeds through an intermediate exciplex which has the structural requirements of a perepoxide. This assumption is supported by (a) the lack of stereoselectivity in the reactions with chiral oxazolines and tiglic acid esters (b) the comparison of the diastereoselectivity of dialkyl substituted acrylic esters with structurally similar non-functionalized aUtenes (c) the intermolecular isotope effects in the photooxygenation of methyl tiglate and (d) the solvent effects on regioselectivity. ... 

In the case of O2 oxidation of a,-unsaturated esters, there is a small but significant solvent effect on the variation of the ene products . Hydrogen abstraction from the methyl group which is geminal to the ester functionality in compound 79 (equation 2), producing 79a, decreases substantially as the solvent polarity increases. For example, the ratio of ene products 79a/79b decreases by a factor of 5 in going from carbon tetrachloride to the more polar solvent DMSO. 

The observed solvent effect on the stereoselectivity of singlet oxygen with ester 79 was rationalized by examination of the possible transition states of this reaction (Scheme 22). 

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). 

Solvation effects on the conformation of esters of three /i-snbstituted 1-phenyletha-nols with 2-flnoro-2-phenyl acetic acid (FCDA) were studied both experimentally (in five solvents ranging from CDCb to DMSO) and quantum mechanically. Semi-empiri-cal (AMI of MJS Dewar and PM3 of JJP Stewart) and ab initio (RHF/3-21 G) calculations were undertaken. Energy maps for the conformers of the esters as a function of the dihedral angles alpha (F-C-alpha acid-C=0) and beta (CO-O-C-alcohol-H) were obtained. Solvent effect calculations, through the self-consistent reaction field on the most stable conformers, were also carried out (Hamman et al., 1996). 

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