Ambident reactivity, HSAB

Aminopyridine, and ambident reactivity, 63 bromination of. 79 by Chichibabin reaction, 12 diazotization of. 67 and HSAB, 63 nJtration of. 72... 

Horticultural uses, 134 HSAB theory, and ambident reactivity. 5, 61,62... 

Michael addition, on A-2-thiazoline-5-one. with diethylazodicarboxylate, 433 with maleic anhydride, 433 4-phenyl-1,2,4-triazolin-3,5-dione, 433 Microreversibility principle, 85 Miticide, 440 Mitodepressive, 149, ISO Mn complexes, 104 Molecular orbital calculations. 17 and ambident reactivity, 61, 62 and amino-imino equilibrium, 18 and electrophilic substitution, 18 and HSAB, 1 8... 

We cannot, then, expect this approach to understanding chemical reactivity to explain everything. We should bear in mind its limitations, particularly when dealing with reactions in which steric effects are likely to be important, and in which solvent effects are involved. Solvent effects are well known, for example, to be a large part of the explanation of ambident reactivity and other manifestations of the principal of HSAB.246 Some mention of all these factors will be made again in the course of this book. Arguments based on the interaction of frontier orbitals are powerful, as we shall see, but they must not be taken so far that we forget these important limitations. 

HSAB is particularly useful for assessing the reactivity of ambident nucleophiles or electrophiles, and numerous examples of chemoselective reactions given throughout this book can be explained with the HSAB principle. Hard electrophiles, for example alkyl triflates, alkyl sulfates, trialkyloxonium salts, electron-poor car-benes, or the intermediate alkoxyphosphonium salts formed from alcohols during the Mitsunobu reaction, tend to alkylate ambident nucleophiles at the hardest atom. Amides, enolates, or phenolates, for example, will often be alkylated at oxygen by hard electrophiles whereas softer electrophiles, such as alkyl iodides or electron-poor alkenes, will preferentially attack amides at nitrogen and enolates at carbon. 

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