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Boric acid aldol reactions

When aldehydes, with or without a hydrogen, are treated with aluminum ethoxide, one molecule is oxidized and another reduced, as in 9-69, but here they are found as the ester. The process is called the Tishchenko reaction. Crossed Tishchenko reactions are also possible. With more strongly basic alkoxides, such as magnesium or sodium alkoxides, aldehydes with an a hydrogen give the aldol reaction. Like 9-69, this reaction has a mechanism that involves hydride transfer.751 The Tishchenko reaction can also be catalyzed752 by ruthenium complexes.753 by boric acid,754 and, for aromatic aldehydes, by disodium tetracarbonylferrate Na2Fe(CO)4,755 OS I, 104. [Pg.1235]

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. [Pg.90]

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 ... [Pg.20]

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. [Pg.138]

Another route involving 1,3-dipoIar addition was next attempted (54). The key feature of this route is a cycloaddition of nitrile oxide (55) 88 with citraconate 87 (Scheme 16). Treatment of 89 prepared from 3-bromopropanol with phenyl isocyanate produced the nitrile oxide 88 which was then subjected to cycloaddition with dimethyl citraconate 87 to afford the adducts 86a and 86b as a 1 1 mixture. In this reaction, as expected, 86a, in which the less hindered oxygen is substituted at the quaternary carbon, was the major product (56). Conveniently, however, the undesired adduct 86b could be separated in the following dehydration step. These adducts were next subjected to various reduction conditions (57) in order to obtain intermediate 85 (eq.9). Unfortunately, we could not obtain any of the desired compound even under Curran s conditions (Raney-Ni (W-2), boric acid) which are mild enough to suppress undesired side reactions. All compounds produced in this reduction were retro-aldol products 90-93 probably derived from 85 or its imine form. This... [Pg.287]


See other pages where Boric acid aldol reactions is mentioned: [Pg.389]    [Pg.240]    [Pg.240]    [Pg.8]    [Pg.82]    [Pg.154]    [Pg.240]   


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