Simplest Self-Catalyzed Reaction

A reaction, speeded up by one of its products, is called self-catalyzed. Classic example of self-catalysis is a reaction of acetone iodination in acid medium, described by total scheme 

Reaction consists of two stages. On its first stage ketone transforms into enol (slowed stage)  

Further enol reacts with iodide to form final products. As reaction passes, hydronium content increases, which speeds up the reaction. Thus, rate of the reaction is defined by rate of acetone enolization  

This is one of the forms of so-called equation of logistic growth, and its graph has a very characteristic form with plateau at the beginning and a part with quick 

Graph of time-dependence of self-catalyzed reaction rate, given in Fig. 1.18, shows the presence of maximum. Location of maximum point could be found from the condition equality of second derivative (fCB(f)/dt to zero  

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Acetaldehyde 34, which is the simplest of all enolizable carbonyl compounds but highly reactive as an electrophile, is an inexpensive and versatile two-carbon nucleophile in enamine-based Mannich reactions. Mannich reactions of acetaldehyde as a donor with aryl or alkyl substituted N-Boc-imines 90 are effectively catalyzed by (S) -proline (13) in moderate yield but excellent enantioselectivity (Table 28.6, entries 1 and 2) [47]. Chemical yields are improved up to 87% when N-benzoyl (Bz)-imine is employed in the presence of diaryl prolinol silyl ether 85 with p-nitrobenzoic acid (entry 3) [48]. To suppress side reactions, such as self-aldol reactions, the moderate nucleophilicity of the axially chiral amino sulfonamide 23 is particularly useful for this type of Mannich reaction these conditions give the corresponding adducts 91 in good yield and excellent stereoselectivity (entries 4 and 5) [49]. 

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