Model anisole

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

It has been shown that it is possible to compel regiospecific para substitution by enclosing the substrate molecules in a cavity from which only the para position projects. Anisole was chlorinated in solutions containing a cyclodextrin, a molecule in which the anisole is almost entirely enclosed (see Fig. 3.4). With a high enough concentration of cyclodextrin, it was possible to achieve a para/ortho ratio of 21.6 (in the absence of the cyclodextrin the ratio was only 1.48). This behavior is a model for the regioselectivity found in the action of enzymes. 

Shell Chemical Company), exhibits a maximum at 300 nm, corresponding to that of the model chromophore anisole. The fluorescence intensity decreases monotonically with increasing concentration of 2,4-dihydroxybenzophenone (DHB) and, furthermore, decreases with time on continued excitation (274 nm) in the spectrophotometer. The fluorescence loss with time may be resolved into two exponential decays. Initially, a relatively rapid fluorescence loss is observed within 20 sec, followed by a slower loss. Loss constants for the initial (k ) and secondary (kj) exponential decays for 1.5 ym films (on glass slides) containing varying concentrations of DHB are provided in Table I (entries 1-3). The initial loss constants are seen to decrease more markedly with increasing DHB concentration than the secondary constants. 

Feldman and Eastman have suggested that the kinamycins may by reductively activated to form reactive vinyl radical (25) and orf/to-quinone methide (26) intermediates (Scheme 3.2c) [16]. The authors provided convincing evidence that the alkenyl radical 25 is generated when the model substrate dimethyl prekinamycin (24) is exposed to reducing conditions (tri-n-butyltin hydride, AIBN). Products that may arise from addition of this radical (25) to aromatic solvents (benzene, anisole, and benzonitrile) were isolated. The ort/io-quinone methide 26 was also formed,... 

A series of [V3 (/x -0)(/c -02CR)6L3]Cl04 complexes (R = Me, Et, 4-anisole L = pyridine, 4-picoline, lutidine) has been prepared as models for V-containing impurities in crude oils [49]. These complexes feature octahedral geometry around each... 

Hirose, M., Tanaka, H., Takahashi, S., Futakuchi, M., Fukushima, S. Ito, N. (1993b) Effects of sodium nitrite and catechol, 3-methoxy catechol, or butylated hydroxy anisole in combination in a rat mulhorgan carcinogenesis model. Cancer Res.. 53, 32-37... 

Deuterated Model Compounds and Protodedeuteration Rate Measurements. Protodedeuteration rate studies were outlined for 2,4,6-trideuterophenol and 2,4,6-anisole, 3,5-dideutero- and 4,6-dideutero-guaiacols, 3,5-dideuteroveratrole, 5-deutero-4-hydroxy-3-methoxytoluene, and 2,6-dideutero-4-hydroxy-3-methoxytoluene (6). Using appropriate combinations of acid- and base-catalyzed deuterium exchange reactions, the following deuteroderivatives were prepared in this study 3,5-dideutero-and 4-deutero-2,6-dimethoxyphenols, 4,6-dideutero- and 5-deutero-1,2,3-trimethoxybenzenes, and 2,6-dideutero-4-hydroxy-3,5-dimethoxytoluene. 

Several model reactions were studied by Sordo et al. at the MP2/6-31G //RHF/ 6-31G, B3LYP/6-31G, and RHF/6-31G //RHF/3-21G levels of theory, both in the gas phase and in anisole (e = 4.33) and (V,(V-dimethylformamide (e = 37.0) solution, using SCRF procedures. Under these conditions, both concerted [n2s + n2a] and stepwise mechanisms were found, although only the latter were obtained in solution (Scheme 20). 

This correlation of anisoles using the above descriptors demonstrates that broader ranges of compounds can be compared with QSAR models. Selecting descriptors that allow for the comparison of these compounds across wider classes is the key to developing useful QSAR models. By experimenting, it was also found that a better correlation could be achieved by replacing... 

Readion of anisole (1) with acetic anhydride was chosen as a model, and ytterbium trifluoromethanesulfonate (ytterbium triflate, Yb(OTf)3) was the first RE(OTf)3 representative used. Several reaction conditions were examined the results are summarized in Table 1. When acetic anhydride, acetonitrile, or nitromethane was used as a solvent (entries 4—10), the reaction mixture became homogeneous and the acylation reaction proceeded smoothly. Nitromethane gave the highest yield of4-methoxyaceto-phenone (2) (entries 7-10). On the other hand, in carbon disulfide, dichloroethane, or nitrobenzene (entries 1-3), the reaction mixture was heterogeneous and the yield of 2 was low. It was noted that the acylation proceeded quantitatively when a catalytic amount of Yb(OTf)3 was used (0.2equiv., entry 9). Even when 0.05 equiv. of the catalyst was employed, 2 was obtained in 79 % yield (entry 10). 

In a pioneering work, Breslow and his colleagues have used a-cyclodextrins as enzyme models in the chlorination of anisole and obtained almost exclusive para chlorination1004-1006. Another approach is the application of micellar solutions1007. 

The transfer constants in styrene polymerizations are higher than predicted from model studies using diarylmethylium cations at -70° C (cf., Chapter 2) [289], In the model studies, (p-MeOPh)PhCH+ adds to styrene approximately 300 times faster than it reacts with anisole, 107 times faster than with toluene, and 10 ° times faster than with benzene. However, if the slightly higher activation enthalpy of reaction with anisole compared to styrene (AAHt = 12 kJ/mol) [289,322] is taken into account, extrapolation of their relative rate constants from - 70 to 0° C still indicates that anisole is 30 times more reactive than styrene. This is in contrast to the similar reactivities (Cx 1 at 0° C) calculated from polymerization studies [317]. That is, the p-methoxydiphenylcarbenium ion is apparently... 

Much less information is available on transfer constants in polymerizations of other alkenes. It appears that the transfer constant to anisole in isobutene polymerizations is smaller than in styrene polymerizations, and much closer to values predicted by model studies Cx 5 x 10 3 [323], Calculated Cx values are relatively independent of temperature. 

X-Substituted Benzenes. As before (p. 49), we use the benzyl anion as a model for X-substituted benzenes like anisole. The three lowest-energy orbitals (Fig. 3-10) are, like those of styrene, very similar to those of benzene. The HOMO is ij/A, and this, like the corresponding orbital in the allyl system, has nodes on the alternate atoms. For this reason, it is a non-bonding orbital, and its 7r-energy is zero. 

To start our investigations, we examined the conversion of 2,3-dimethyl-2-butene (1) into 3,3,4-trimethyl-4-penten-2-one (2) as a model reaction (eq. 1). The choice of acetic anhydride as the acetylating agent was made in the light of related studies on the acylation of aryl ethers. Our work in this field had shown that acetic anhydride was the most effective reagent for the Friedel-Crafts acylation of anisole in the presence of Hp zeolite. A lower degree of conversion was achieved with acetyl chloride, while hardly any reaction occurred with ethyl acetate or acetic acid [6]. 

In a closely related paper [121] the calculated oxidation potentials for eight phenolic compounds for which experimental results are known were correlated to develop a calibration curve. From these data the oxidation potentials of coniferyl alcohol, sinapyl alcohol, anisole, guaiacol, and a pinoresinol dimer were predicted. This paper applied B3LYP/6-31G(d) optimizations to both gas phase and solvated models, and compared the results to experimental data at pH = 0. Based on a correlation coefficient of 0.93 for the calibration curve, the oxidation potentials of the nnknowns were determined. The relative results from both of these papers are similar, with dimethoxy compounds having lower oxidation potentials than the mono-methoxy compounds. 

---