Acetate, as leaving group

Ion pair collapse in the acyloxy migration is so rapid as to preclude nucleophilic trapping of the contact ion pair even by intramolecular nucleophiles, which essentially precludes the use of acetates as leaving groups in tandem rearrangement reactions of the types discussed below [111, 127]. 

St/2 nucleophilic attack with acetate as leaving group... 

This is followed by a 1,4-elimination, where pyridine acts as base and an acetate as leaving group. This elimination is particularly easy because it leads to an aromatic ring. 

Problem 7.10 Compare the effectiveness of acetate (CH,COO"), phenoxide (C H,0 ) and benzenesulfo-nate (C H SO,) anions as leaving groups if the acid strengths of their conjugate acids are given by the values 4.5, 10.0. and 2.6, respectively. M... 

The extent of this failure is evident from comparisons of experimental measurements of rate and equilibrium constants. One comparison in the literature is provided by Ritchie and coworkers study of the relatively stable cation, pyronin (the 3,6-bis(dimethylamino)xanthylium cation 71) with a series of nucleophiles.252 Another example is McClelland s measurements of rate and equilibrium constants for the reactions of halide and acetate ions with the trityl cation.19 As already mentioned fluoride and acetate are less reactive than bromide and chloride despite their equilibrium affinities being much greater. This is reflected indeed in the much lower rates of solvolysis of the fluoride and acetate than bromide or chloride as leaving groups... 

Half of an anhydride s acid units are lost as leaving groups. If the acid is expensive, we would not use the anhydride as an activated form to make a derivative. The acid chloride is a more efficient alternative, using chloride as the leaving group. Anhydrides are used primarily when the necessary anhydride is cheap and readily available. Acetic anhydride, phthalic anhydride, succinic anhydride, and maleic anhydride are the ones we use most often. Diacids commonly form cyclic anhydrides, especially if a five- or six-membered ring results. 

Putting on the phosphate is tricky too and more protection is necessary. This phosphorus compound with one chloride as leaving group and two benzyl esters as protecting groups proved ideal. The benzyl esters can be removed by hydrogenation (Chapter 24) and the acetal by treatment with dilute acid to give AMP. 

Many further examples of reactions in which phosphate anions act as leaving groups have been reported. Palladium-catalysed reactions of cyclic ketene acetal phosphates, e.g. 64, are reported to offer advantages over the corresponding... 

On p. 344 you saw a tosylate (we just called it a sulfonate ester then) being displaced by acetate in an Sn2 reaction. Acetate is not a very good nucleophile, and it is a testament to the power of the sulfonate esters that they are willing to act as leaving groups even with acetate, which is usually too weak to react by 5 2. 

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