Alkyl-oxygen fission

The designation Ac signifies acyl-oxygen fission, whereas A1 is alkyl-oxygen fission. When n-amyl alcohol was hydrolyzed in 0-enriched water, the 0 appeared in the product acid rather than the alcohol, showing that alkyl-oxygen fission could not have occurred. Carpenter gives many examples of isotopic studies. 

Al, as shown in structure 3), the molecularity (1 or 2), and the ionic form of the substrate [A for conjugate acid RC(OH)OR and B for conjugate base RCOOR]. Note that alkyl-oxygen fission constitutes nucleophilic substitution and is therefore equivalent to the classification ... 

In the hydration of compounds 2f and 2g, besides the expected ester, three other products (acetic acid, an alkene, and alcohol) were observed. These products were postulated to arise via a fragmentation of the intermediate vinyl cation, 6, as shown in Scheme II. The importance of the fragmentation path is presumably determined by the stability of the alkyl cation formed by the alkyl oxygen fission. 

Isotopes can also be used to solve mechanistic problems that are non-kinetic. Thus the aqueous hydrolysis of esters to yield an acid and an alcohol could, in theory, proceed by cleavage at (a) alkyl/ oxygen fission, or (b) acyl/oxygen fission ... 

The acid-catalysed hydrolysis of phosphinic esters (158) is relatively insensitive to the substituent (R = Me, Et, Pr1, or Bu1) attached to phosphorus this contrasts with the base-catalysed process.126 The acid hydrolysis of the phosphinic esters (159) (see Organophosphorus Chemistry , Vol. 7, p. 129 for the base hydrolysis) takes place with alkyl-oxygen fission in an SnI process.127... 

A c2 pathway, 241, 384 A,4 1 pathway, 241, 380 acid-catalysed, 240, 378 acyl-oxygen fission, 88, 240, 242 alkyl-oxygen fission, 240 isotope labels in, 88, 241 steric effects in, 242 Esters... 

Triphenylmethyl acetate67-84 and benzoate75 are solvolyzed with alkyl-oxygen fission even under neutral conditions, and the reaction is strongly acid-catalyzed in each case. The hydrolysis of triphenylmethyl acetate in 80% dioxan-water has been studied by Bunton and Konasiewicz 7. The reaction involves near-quantitative alkyl-oxygen fission in both acidic and initially neutral solution, as shown by the incorporation of l80 from enriched H20 into the triphenylmethyl alcohol produced, and by the lack of incorporation into the acetic acid. The rate of the acid-catalyzed reaction at 25°C is about 40% slower in 90% dioxan than in the more aqueous solvent. ... 

Since there are but two possibilities, acceptable evidence may be pos tive or negative. It is sufficient, for example, in order to establish acyl-oxygen fission, to show either that the acyl-oxygen bond is cleaved, or that the alkyl-oxygen bond is not, and both types of evidence have been used. Some of the evidence, for reactions involving alkyl-oxygen fission, has already been discussed in the sections dealing with the AA) 1 reaction (p. 87). 

In the case of alkyl-oxygen fission, on the other hand, the carboxylic acid has initially normal isotopic constitution, but the alochol is now labelled. 

Many of the esters which are hydrolyzed by the AalI mechanism in acid are also hydrolyzed with alkyl-oxygen fission under neutral condi-tions60,67 74 75 84 85 88 89. These reactions have the high enthalpies and entropies of activation characteristic of unimolecular reactions, and involve the ionization of (usually) tertiary alkyl esters, to the carbonium ion and a carboxylate anion in the rate-determining step, viz. 

A or B acid or base catalysis AC or AL acyl-oxygen or alkyl-oxygen fission 1 or 2 unimolecular or bimolecular. E10B designates unimolecular elimination through the conjugate base. 

The alcohol here undergoes alkyl-oxygen fission and the resulting car-bonium cation combines with anionic chlorine. This fission is facilitated... 

In 1897, it was observed889 that potassium iodide (in hot alcohol or acetone) converts ethyl p-bromobenzenesulfonate to ethyl iodide. Many years later, methyl and ethyl iodides 49 841 and ethyl a-iodopropionate808(b) were prepared in good yield by the action of the same iodide (in boiling water or alcohol) on the corresponding p-toluene-sulfonates. In these examples, the reaction obviously proceeded as an alkyl-oxygen fission, in accordance wi h the following equation ... 

The disconnection underlying this procedure is an alkyl-oxygen fission and not the acyl-oxygen fission as in the reactions discussed above. The non-nucleophilic base used in this reaction is l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) which converts the carboxylic acid into its carboxylate ion but does not interact in a competing substitution reaction with the alkyl halide. The ester-forming reaction may therefore be regarded as an SN2 reaction between the carboxylate ion and the alkyl halide.162... 

These sulphonate esters cannot be made by direct esterification of the sulphonic acids since further reaction of the ester with the alcohol gives rise to an ether by means of an alkyl-oxygen fission process. 

An ester can be split either at point A (acyl oxygen fission) or at point B (alkyl oxygen fission) ... 

H)-Oxazolones react readily with nucleophiles, C(5) and C(2) be ing possible sites for attack, and, in the case of unsaturated azlactones, C(a) as well (see 199-201). It has been proved by using water labelled with lsO that the acid-catalyzed hydrolysis of unsaturated azlactones proceeds by alkyl-oxygen fission (equation 42). The formation, hydrolysis and reduction of 4-methylene-5(4H)-oxazolones is a well-established method for the synthesis of a-amino acids, e.g. phenylalanine (equation 43). The addition of hydrazoic acid to 5(4H)-oxazolones without methylene groups at C(4) likewise occurs exclusively at C(2) to yield tetrazoles by ring-opening and recyclization (equation 44). 

Oxazolidinones are virtually neutral they do not give stable salts with acids but they can be alkylated at the nitrogen atom under basic conditions. They are rather resistant to hydrolysis vigorous treatment with alkali yields /3-amino alcohols whereas hydrochloric acid gives /3-chloroalkylamines (equation 82). The combined action of aromatic hydrocarbons and aluminum chloride on 2-oxazolidinones leads to /3-arylethylamines which are formed by alkyl-oxygen fission (equation 83) (80TL1719). 

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