Knoevenagel reaction competitive reactions

Various competitive reactions can reduce the yield of the desired Michael-addition product. An important side-reaction is the 1,2-addition of the enolate to the C=0 double bond (see aldol reaction, Knoevenagel reaction), especially with a ,/3-unsaturated aldehydes, the 1,2-addition product may be formed preferentially, rather than the 1,4-addition product. Generally the 1,2-addition is a kinetically favored and reversible process. At higher temperatures, the thermodynamically favored 1,4-addition products are obtained. 

With some methylene active compounds (NCCH2PO(OEt)2) competition between Knoevenagel and Wittig-Homer reactions has been observed the ratio of Knoevenagel and Wittig-Homer products is dependent upon the reaction conditions. 

The Knoevenagel condensation is the method of choice for the preparation of a,p-unsaturated dicarbonyl compounds and related compounds and only a few alternative methods have been developed. However, with the traditional Knoevenagel condensation there are problems with the reactivity of ketones, with the competitive Michael addition occuring in the reaction of some active methylene compounds. There is also a problem with steieocontrol in the synthesis of Knoevenagel products from unsymmetrical 1,3-dicarbonyl compounds. An alternative method is the addition of Grignard reagents to vinylogous carbamates (see Section 11.2.6). Another possibility is the reaction of a metal ketimate with malonodini-trile to yield ylidenemalonodinitriles (see Section 11.3.1.7). ... 

With unsymmetrical 1,3-diketones, the reactivity of the carbonyl groups determines the outcome of the reaction, which proceeds via an initial Knoevenagel condensation. The two cyanopyridines 24 and 26 are formed from l-ethoxypentane-2,4-dione 25 in a proportion of 15 75 because of the competition between the two different electrophilic carbonyl groups. In contrast, the diketo ester 27 yields exclusively the cyanopyridone 28, because it reacts via the more strongly activated carbonyl function adjacent to the ester group ... 

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