Leaving groups defined

Correlations with o in carboxylic acid derivative reactions have been most successful for variations in the acyl portion, R in RCOX. Variation in the alkyl portion of esters, R in RCOOR, has not led to many good correlations, although use of relative rates of alkaline and acidic reactions, as in the defining relation, can generate linear correlations. The failure to achieve satisfactory correlations with cr for such substrates may be a consequence of the different steric effects of substituents in the acyl and alkyl locations. It has been shown that solvolysis rates of some acetates are related to the pA", of the leaving group, that is, of the parent alcohol. The pK of alcohols has been correlated with but this relationship... 

Equations (7-66) and (7-67), or related versions, have been used by Hupes and Jencks and by Castro and co-workers to account for curvature. The quantity p/(S defines the center of eurvature of the plot and is expected to occur when the Y>Ka of the nucleophile is equal to the p/( of the leaving group." For weaker nucleophiles pKa < p/(S), breakdown of the tetrahedral intermediate will be rate determining, because the leaving group X is a stronger nucleophile than is N , so 2 < -li if, however, p/( > p/(S, the nucleophilic attack is rate determining. 

Kevin and co-workershave developed nucleophilicity scales based on the solvolysis of cationic substrates R-X, the leaving group being neutral rather than anionic. Their/V ,jO+ scale is defined in Eq. (8-72). 

This mechanism is the same as the simple electrophilic one shown on page 970 except that the charges are reversed (lUPAC An- -Ae or A +Ah). When the alkene contains a good leaving group (as defined for nucleophilic substitution), substitution is a side reaction (this is nucleophilic substitution at a vinylic substrate, see p. 428). 

Apparent dehydration reactions, thus, appears to require certain well-defined conditions, namely conjugation of the OH group to make it a good leaving group, and acidity of the vicinal proton. Such conditions are seldom met simultaneously, offering a possible explanation for why the reaction occurs so rarely. 

The central feature that defines all DUBs is that they recognize and act at the C-terminus of the ubiquitin or ubiquitin-like domain. All mature ubiquitin and ubiquitin-like proteins have a C-terminal gly-gly motif and DUB cleavage releases leaving groups attached to the carboxyl group of the C-terminal glycine. With the exception of the JAMM metalloproteases, DUB catalysis starts with the nucleophilic attack of the catalytic cysteine on the carbonyl carbon of the scissile bond to... 

The reliability and usefulness of a new empirical nucleofugality index, which is defined as the group electrophilicity of the leaving group embedded in the substrate that undergoes the nucleophilic attack, were tested against experimental kinetic data recorded for aminolysis reactions of variously substituted phenoxy- and thiophenoxy-carbonyl and the corresponding thiocarbonyl derivatives.50... 

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