Alkyl oxygen cleavage

Oxiranones (a-lactones) (Section 5.05.3.2.1), e.g. (6), in accord with their putative facile opening to zwitterions (Scheme 18), usually react readily with nucleophiles by alkyl-oxygen cleavage (Scheme 41) (71JA557), rather than the acyl-oxygen cleavage common for esters. 

Therefore, transesterification reactions frequently fail when R is tertiary, since this type of substrate most often reacts by alkyl-oxygen cleavage. In such cases, the reaction is of the Williamson type with OCOR as the leaving group (see 10-14). With enol esters, the free alcohol is the enol of a ketone, so such esters easily... 

In the special case of 3-lactones, where small-angle strain is an important factor, alkyl-oxygen cleavage is observed (Bal2 mechanism, as in the similar case of hydrolysis of P-lactones, 10-10), and the product is not an amide but a P-amino acid ... 

Esters, RC02R, where the alkyl group R can form a relatively stable carbocation, e.g. (177) from (178), have been shown—by lsO labelling experiments—to undergo alkyl-oxygen cleavage ... 

Propagation follows in a similar manner with alkyl-oxygen cleavage... 

An outstanding example of the effect of substitution is to be found in the reactions of diketene, where alkyl-oxygen cleavage is repressed by the 4-methylene group, but a number of new reactions become possible because of carbanionoid character of this methylene group, as well as other changes. The reader is referred to an excellent recent review of the subject (74ACR265). 

There is some doubt as to whether the formic acid-2-propanol system reacts according to reaction (72). Tiedemann and Riveros (1974) observed reaction (75) in their icr study claiming that this process occurs by alkyl-oxygen cleavage in the alcohol. This is a slow reaction, and due to the low abundance of the product ion, the lsO experiment (Pau et al., 1978) was inconclusive. 

The results of analysis of the samples prepared via reactive extrusion of PMMA with dimethylamine are shown in Tables 1 and 2, and Figure 1. It is apparent from these results that the main product of the reaction was methacrylic anhydride, confirming the hypothesis that alkyl-oxygen cleavage was involved in the reaction of PMMA with amines (at least with dimethylamine). However, additional subtleties of the reaction are apparent from a more detailed analysis of the reaction products vs. degree of conversion and reaction conditions. 

A second type of anomalous cryoscopic behaviour is exhibited by f-butyl esters ". The benzoate and acetate both give varying / -values between 3 and 4, and by extrapolating the value for f-butyl benzoate to zero time Kuhn1" obtained a value close to 3.0. This he interpreted as evidence for alkyl-oxygen cleavage, and the formation of the f-butyl cation, viz-... 

The reactions of protonated carboxylic esters in the absence of significant amounts of water have been discussed above. Two types of cleavage are possible. Alkyl-oxygen cleavage is a very fast reaction for the conjugate acids of tertiary alcohols... 

The final increase in the rate of hydrolysis of the esters of primary aliphatic alcohols, in 85-100% H2SO, is presumably due to acyl-oxygen cleavage, since methyl acetate reacts faster than the ethyl ester, which would not be the case for alkyl-oxygen cleavage. So also may be the rapid hydrolysis of aryl acetates at moderate acidity, since aryl-oxygen cleavage is not expected to be so rapid a reaction. 

This suggests that the hydrolysis of the ethyl ester involves the Aac1 mechanism, but that protonated isopropyl benzoate cleaves by alkyl-oxygen cleavage. A change in mechanism at this point would explain the order of reactivity towards hydrolysis of the various alkyl benzoates, observed by Kuhn18. 

Since the great majority of studies of ester hydrolysis involving the AAi 1 mechanism have been with tertiary alkyl esters, strongly acidic media have not been necessary indeed some esters undergo alkyl-oxygen cleavage even in the absence of strong acid (see p. 146). 

PERCENTAGE OF ALKYL-OXYGEN CLEAVAGE OF /-BUTYL ACETATE IN AQUF.OUS HCI, AS MEASURED BY ikO-IN CORPORATION"... 

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