Boric acid aldol condensations

Boric acid catalyzes the self-condensation of aldehydes and ketones to produce a,/l-unsaturated enones [6]. The yields are much higher than those reported with other acidic or basic catalysts. Under similar conditions, aldehydes which are not readily susceptible to aldol condensation, dismutate to form esters (Tischenko reaction) [7]. A catalytic amount of boric acid-sulfuric acid mixture has been used to synthesize aryl esters in good yields (Eq. 3) [8] this reaction was unsuccessful when mineral acids or boric acid alone were used. 

Condensation catalyst. Boric acid (also boron oxide and 10-hydroxy-10.9-boroxarophenanthrene)123 has been used as catalyst for aldol condensation and subsequent dehydration. The reaction is carried out in refluxing m-xylene under a Dean-Stark trap for removal of water. Examples126 ... 

Offenhauer and Nelsen discovered that boric acid is an excellent catalyst for aldol condensations treatment of heptanal with boric acid in refluxing toluene under a Dean-Stark trap gives the condensation product in quantitative yield (equation 10). The authors suggested an intermediate enol borate this was probably the flrst example of a boron enolate aldol reaction. 

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