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Phenyl acetate reaction mechanism

Only the hydrophobic and steric terms were involved in these equations. There are a few differences between these equations and the corresponding equations for cyclo-dextrin-substituted phenol systems. However, it is not necessarily required that the mechanism for complexation between cyclodextrin and phenyl acetates be the same as that for cyclodextrin-phenol systems. The kinetically determined Kj values are concerned only with productive forms of inclusion complexes. The productive forms may be similar in structure to the tetrahedral intermediates of the reactions. To attain such geometry, the penetration of substituents of phenyl acetates into the cyclodextrin cavity must be shallow, compared with the cases of the corresponding phenol systems, so that the hydrogen bonding between the substituents of phenyl acetates and the C-6 hydroxyl groups of cyclodextrin may be impossible. [Pg.79]

Quantitative structure-reactivity analysis is one of the most powerful tools for elucidating the mechanisms of organic reactions. In the earliest study, Van Etten et al. 71) analyzed the pseudo-first-order rate constants for the alkaline hydrolysis of a variety of substituted phenyl acetates in the absence and in the presence of cyclodextrin. The... [Pg.82]

The phenol ArOH is always a side product, resulting from some ArO that leaks from the solvent cage and abstracts a hydrogen atom from a neighboring molecule. When the reaction was performed on phenyl acetate in the gas phase, where there are no solvent molecules to form a cage (but in the presence of isobutane as a source of abstractable hydrogens), phenol was the chief product and virtually no o- or p-hydroxyacetophenone was found." Other evidence" for the mechanism is that... [Pg.726]

More recently, Kaiser and coworkers reported enantiomeric specificity in the reaction of cyclohexaamylose with 3-carboxy-2,2,5,5-tetramethyl-pyrrolidin-l-oxy m-nitrophenyl ester (1), a spin label useful for identifying enzyme-substrate interactions (Flohr et al., 1971). In this case, the catalytic mechanism is identical to the scheme derived for the reactions of the cycloamyloses with phenyl acetates. In fact, the covalent intermediate, an acyl-cyclohexaamylose, was isolated. Maximal rate constants for appearance of m-nitrophenol at pH 8.62 (fc2), rate constants for hydrolysis of the covalent intermediate (fc3), and substrate binding constants (Kd) for the two enantiomers are presented in Table VIII. Significantly, specificity appears in the rates of acylation (fc2) rather than in either the strength of binding or the rate of deacylation. [Pg.233]

Initial theoretical studies focused on steps (1) and (2). Several model systems were examined with ab initio calculations.1191 For the reaction of methyl amine with methyl acetate, it was shown that the addition/elimi-nation (through a neutral tetrahedral intermediate) and the direct displacement (through a transition state similar to that shown in Figure 5a) mechanisms for aminolysis had comparable activation barriers. However, in the case of methyl amine addition to phenyl acetate, it was shown that the direct displacement pathway is favored by approximately 5 kcal/mol.1201 Noncovalent stabilization of the direct displacement transition state was therefore the focus of the subsequent catalyst design process. [Pg.84]

At the start of this section the cleavage of meta- and para-substituted phenyl acetates by a- and )S-CD was discussed in detail and a variety of evidence was cited that is consistent with the mechanisms A and B, in Scheme 2. Further support for the view that para-substituents tend to force the phenyl group out of the cavity (Scheme 2B) comes from the different effects that neutral additives (potential inhibitors) have on the cleavage of m- and p-nitrophenyl acetate (mNPA and pNPA). In brief, species which bind to CDs, and inhibit the reaction of mNPA, do not necessarily inhibit that of pNPA (Tee and Hoeven, 1989 Tee et al., 1993b). [Pg.39]

Bruice and Sturtevant, 1959. The EM s are complicated by a mechanism change for the substituted compounds (see footnote 6) and are based on the reaction of the phenyl acetate with imidazole... [Pg.255]

Reactions of a wide range of substituted phenyl acetates with six a-effect nucleophiles have revealed little or no difference, compared with phenolate nucleophiles, in the values of the Leffler parameters. As a result, the case for a special electronic explanation of the a-effect is considered unproven. Studies of the kinetics and mechanism of the aminolysis and alkaline hydrolysis of a series of 4-substituted (21) and 6-substituted naphthyl acetates (22) have revealed that, for electron-withdrawing substituents, aminolysis for both series proceeds through an unassisted nucleophilic substitution pathway. [Pg.40]

If the photo-Fries reaction would occur via a concerted mechanism, the absence of solvent should be of minor importance for the formation of rearranged products. However, conclusive evidence supporting the radical pair mechanism arises from the experiments carried out with phenyl acetate (10) in the vapor phase. The major product in the irradiations of 10 is phenol (13), which accounts for 65% of the photoproducts. Under these conditions, less than 1% of ortho -hydroxyace-tophenone (11) appears to be formed [19,20]. Conversely, when a high cage effect is expected, as in rigid matrixes (i.e., polyethylene), the result is completely different, and phenol is practically absent from the reaction mixtures [29]. In the intermediate situation (liquid solution), both rearranged products and phenol are formed in variable amounts depending on solvent properties. These observations... [Pg.49]

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). [Pg.85]

Another convenient method for the preparation of functionalized cyclobutanol derivatives is by treatment of 1,2-diphenylethylene acetals containing a 1,3-dithiane moiety in the y-position, e.g. 14c. with butyllithium. The isolation of 2,2-(propane-l,3-diyldisulfanyl)cyclobutanol (15c) together with benzyl phenyl ketone in 90 and 92 % yield, respectively, indicates that the reaction mechanism should involve the intramolecular attack of the metalated dithiane on the acetal carbon atom with concomitant hydride shift at the acetal group.15... [Pg.68]

Even more surprising is the finding that the reactivity picture exhibited by the various ethoxide species in the ethanolysis of phenyl acetate is very similar to that found in the ethanolysis of the activated amide N-methyl-2,2,2-trifluoroacetanilide [Eq. (3)], despite the different mechanisms of the two reactions [13]. Schowen ef al. [14] showed that C-N bond breaking in the rate-limiting decomposition of the tetrahedral intermediate is assisted by proton transfer from a general acid to the leaving group. [Pg.115]

DFT was employed to study the mechanism of ammonolysis of phenyl formate in the gas phase, and the effect of various solvents on the title reaction was assessed by the polarizable continuum model (PCM). The calculated results show that the neutral concerted pathway is the most favourable one in the gas phase and in solution.24 The structure and stability of putative zwitterionic complexes in the ammonolysis of phenyl acetate were examined using DFT and ab initio methods by applying the explicit, up to 7H20, and implicit PCM solvation models. The stability of the zwitterionic tetrahedral intermediate required an explicit solvation by at least five water molecules with stabilization energy of approximately 35 kcalmol-1 25... [Pg.58]

Push-pull acid-base catalysis has been proposed to account for the proton switch mechanism which occurs in the methoxyaminolysis of phenyl acetate (Scheme 11.14) where a bifunctional catalyst traps the zwitterionic intermediate. A requirement of efficient bi-functional catalysis is that the reaction should proceed through an unstable intermediate which has p values permitting conversion to the stable intermediate or product by two proton transfers after encounter with the bifunctional catalyst the proton transfer with monofunctional catalysts should also be weak. [Pg.308]

The mechanism of the Montedison reaction has been studied in some detail, and tentative mechanisms have been offered. The proposed catalytic cycle is shown in Fig. 4.11. The biphasic reaction medium consists of a layer of diphenyl ether and that of aqueous alkali. In the presence of alkali, the precatalyst Co2(CO)8 is converted into 4.7. The sodium salt of 4.7 is soluble in water but can be transported to the organic phase, that is, a diphenyl ether layer by a phase-transfer catalyst. The phase-transfer catalyst is a quaternary ammonium salt (R4N+X ). The quaternary ammonium cation forms an ion pair with [Co(CO)4]. Because of the presence of the R groups, this ion pair, [R4N]+[Co(CO)4], is soluble in the organic medium. In the nonaqueous phase benzyl chloride undergoes nucleophilic attack by 4.7 to give 4.40, which on carbonylation produces 4.41. The latter in turn is attacked by hydroxide ion transported from the aqueous phase, to the organic phase again by the phase-transfer catalyst. The product phenyl acetate and 4.7 are released in the aqueous phase as the sodium or quaternary ammonium salts. [Pg.74]

An example of a few of these reactions that occur in our environment with several commonly used pesticides is illustrated in Figures 7-11. Fleck (15) has illustrated in Figure 7 that ultraviolet light catalyzes the decomposition of DDT. In the presence of air, one of the decomposition products is 4,4 -dichlorobenzophenone. However, when air is absent, 2,3-dichloro-l,l,4,4-tetrakis-(p-chlorophenyl)-2-butene is formed. This compound, through subsequent oxidation, may be converted into 4,4 -dichlorobenzophenone. In mammals 2,2-bis(p-chloro-phenyl) acetic acid (DDA) has been identified and shown to be excreted in the feces and urine. The mechanism of formation of DDA is believed to be an initial dehydrochlorination to DDE, which is then hydrolyzed to DDA as shown in Figure 8. Mattson et ah (29) found both DDT and DDE in most samples of human fat, and Walker et ah (44) noted low levels of these same compounds in restaurant meals. [Pg.241]

That a single solvent molecule clustered to a nucleophile can drastically change the reaction pathway has been demonstrated by studying the reaction of phenyl acetate with methoxide ion in the gas phase [671, 672]. Alkaline hydrolysis of esters in solution is known to proceed by attack of the nucleophile at the carbonyl carbon atom to form a tetrahedral intermediate, followed by cleavage of the acyl-oxygen bond (Bac2 mechanism) cf. Eq. (5-138a). [Pg.276]

The exact mechanism has still not been completely worked out. " Opinions have been expressed that it is completely intermolecular, ° completely intramolecular, and partially inter- and intramolecular. One way to decide between inter- and intramolecular processes is to run the reaction of the phenolic ester in the presence of another aromatic compound, say, toluene. If some of the toluene is acylated, the reaction must be, at least in part, intermolecular. If the toluene is not acylated, the presumption is that the reaction is intramolecular, though this is not certain, for it may be that the toluene is not attacked because it is less active than the other. A number of such experiments (called crossover experiments) have been carried out sometimes crossover products have been found and sometimes not. As in 11-17, an initial complex (68) is formed between the substrate and the catalyst, so that a catalyst/substrate molar ratio of at least 1 1 is required. In the presence of aluminum chloride, the Fries rearrangement can be induced with micro-wave irradiationSimply heating phenyl acetate with microwave irradiation gives the Fries rearrangement. " The Fries rearrangement has been carried out in ionic melts. [Pg.736]

The occurrence of general acid-catalyzed hydroxylaminolysis or methoxylaminolysis of thiol esters or amides has been described in Section IIB in terms of kinetically important tetrahedral intermediates. Two kinetically indistinguishable mechanisms for general acid-catalyzed aminolysis reactions are represented by transition states 42 and 43. Mechanism 42 involves a prior protonation of the ester followed by a general base-catalyzed aminolysis mechanism 43 is a general acid-assisted nucleophilic reaction of the amine. Mechanism 42 can be ruled out in the hydrazinolysis of phenyl acetates (Bruice and Benkovic, 1964) and in the hydrazinolysis of S-thiolvalerolactone (Bruice et al., 1963) on the basis of a calculated rate constant which is greater than the diffusion-controlled limit. Mechanism 43 is therefore correct. [Pg.320]

Analyzing the frequency factor v, Bursey43 was able to show that the details of the reaction mechanisms of the electron impact induced ketene elimination from ortho substituted phenyl acetates and from acetanilides, 36, are dramatically influenced by the nature of the substituent R = F, Cl, Br, J. The results demonstrate that the decrease of v going from the voluminous iodine to the small fluorine is not caused by steric effects (which should operate in an opposite direction) but is the result of an electronic interaction of both substituents, leading to a tighter transition state in the case of the more electronegative fluorine. Additional factors which are not yet completely understood play a decisive part in the fragmentation of the anilides. [Pg.240]

Neuvonen H. Kinetics and mechanisms of reactions of pyridines and imidazole with phenyl acetates and trifluoroacetates in aqueous acetonitrile with low content of water nucleophilic and general base catalysis in ester hydrolysis. J Chem Soc Perkin Trans II 1987 266 159-67. [Pg.244]

Especially, 2-phenyl-4,5,6,7-tetrahydro-l,2-benz- (121) and 5-methyl-2,4-diphenyl-isothiazolium salts 62 (R = H, 4-MeO, 2-C1, 2,6-Cl2) were studied with the HPLC-MS(MS) method to monitor the oxidation of isothiazolium salts with H202/acetic acid (96%) (03CG147). The strongly acidic reaction mixture was separated on a RP-18 column without any sample pretreatment and included intermediates, which were identified by API-MS(MS)-techniques. The aim of this work was to establish the reaction mechanism using several N-functionalized salts. [Pg.265]

The cleavage of esters in basic medium, mediated by CDs, is the system most studied. In some cases, such as that with meta-substituted phenyl acetates, cleavage is strongly accelerated on the contrary, the reaction of para-substituted isomers is accelerated only modestly. The observed effects depend on the chain length of the ester, the CD, and the position of the substituent on the phenyl group. The mechanism typically involves nucleophilic attack by the ionized secondary hydroxyl groups of CD, but in other cases, general base catalysis competes with nucleophile catalysis. [Pg.405]


See other pages where Phenyl acetate reaction mechanism is mentioned: [Pg.18]    [Pg.192]    [Pg.174]    [Pg.194]    [Pg.294]    [Pg.46]    [Pg.351]    [Pg.556]    [Pg.54]    [Pg.45]    [Pg.45]    [Pg.227]    [Pg.57]    [Pg.368]    [Pg.250]    [Pg.351]    [Pg.276]    [Pg.45]    [Pg.328]    [Pg.306]    [Pg.306]    [Pg.591]    [Pg.250]    [Pg.424]    [Pg.257]    [Pg.20]    [Pg.881]   
See also in sourсe #XX -- [ Pg.120 ]




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