Esters charge delocalized into

These conclusions are supported by data on the tertiary 2-norbornyl esters (723) 510 and (724)S11) in which the C(3)-D2 effect is nearly independent of configuration. The kH/kD of (724) is much smaller than that of (723), due to charge delocalization into the phenyl ring. 6,6-Dideuteration has a negligible effect on the solvolysis rate of 1,2-dimethyl-2-norbornyl p-nitrobenzoate (725)S12 Although the interpretation of secondary deuterium isotope effects is often difficult, comparison of the various norbornyl derivatives discussed here has uniformly been claimed to support alkyl participation. 

A number of factors combine to lower the susceptibility of phosphoric mono-, di-, and tri-esters to the type of nucleophilic attack required for intemucleotide-bond formation.8 Although a P=0 bond is present, the polarization of this bond is not so great as with an analogous C=0 bond, due to ptt - dn back-bonding into empty, phosphorus d-orbitals.150 Furthermore, the pKa values for the first dissociations of both phosphoric mono- and di-esters are approximately unity,8 which means that, in reactions in pyridine, the anions are involved. Partial delocalization of the formal, negative charge into phosphorus... 

Acylation has frequently been used to convert arsonium ylides into other more stabilized ylides, reagents used being acid chlorides ° acid anhyd-rides " or esters These examples all involve C-acylation but complications can arise, as shown in equation 17 It was suggested that reaction with benzoyl bromide gives a kinetically controlled product which, in the presence of acetate, is converted into the thermodynamically controlled product In this case delocalization of the negative charge leads to acylation at a site other than the ylidic carbon atom. 

Amides are already acylated amines, and have a low reactivity in nucleophilic reactions because the amide carbonyl group is more negatively charged than the carbonyl groups of normal esters (4) and of ketones, because of the delocalization of the lone pair of electrons on the nitrogen into the carbonyl group (equation (5)). 

---