Aldol reaction, self solvent effects

Aldol reactions. Self-condensation of aldehydes as well as Robinson annulation are effected. The latter process is conducted in the presence of molecular sieves 4A in aprotic solvents such as toluene. 

Thus, various kmds of bases are effective in inducing the Henry reaction The choice of base and solvent is not crucial to carry out the Henry reaction of simple nitroalkanes v/ith aldehydes, as summarized in Table 3 1 In general, sterically hindered carbonyl or nitro compounds are less reactive not to give the desired ni tro-aldol products in good yield In such cases, self-condensation of the carbonyl compound is a serious side-reaction Several mochfied procedures for the Henry reaction have been developed... 

The crystalline aluminosilicate-catalyzed aldol condensation of acetophenone to form dypnone has been reported (27). As shown in Table XXIII, hydrogen zeolites were the most effective catalysts for this conversion. Operation at low temperatures in the liquid phase is critical for this reaction, to avoid both coke formation and condensation with aromatic solvents. Catalyst aging was rapid, however. Only transient conversions of acetone to mesitylene were obtained over REX or H-mordenite at 315° owing to rapid intracrystalline self-condensation and coke formation. 

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