Acceleration of Base-Catalysed Reactions in Dipolar Aprotic Solvents

4 Acceleration of Base-Catalysed Reactions in Dipolar Aprotic Solvents 

Many reactions become possible only in such superbasic solutions, while others can be carried out under much milder conditions. Only some examples of preparative interest (which depend on the ionization of a C—H or N—H bond) will be mentioned here. The subsequent reaction of the resulting carbanion may involve electrophilic substitution, isomerization, elimination, or condensation [321, 322]. Systematic studies of solvent effects on intrinsic rate constants of proton-transfer reactions between carbon acids and carboxylate ions as well as amines as bases in various dimethyl sulfoxide/ water mixtures have been carried out by Bernasconi et al. [769]. 

Rates of isotopic H/D exchange and racemization of optically active 2-methyl-3-propionitrile in the presence of potassium methoxide can be increased by a factor of 5 10 on going from pure methanol (sr = 32.7) to dimethyl sulfoxide (sr = 46.5) [31, 231, 304]. 

The Wittig reaction proceeds more rapidly in dimethyl sulfoxide than in other customary solvents due to the enhanced deprotonation of the phosphonium salts to alkylidenephosphoranes [327]. Even camphor, a rather unreactive ketone, undergoes the Wittig reaction easily in base/CH3SOCH3 systems [327]. 

Similarly spectacular increases in rate were found for the transformation of ketone hydrazones into hydrocarbons using the Wolff-Kishner reduction. This is mainly due to the enhanced ionization of the N—H bond. This reaction occurs even at room temperature in tert-C4H90K/CH3S0CH3 and proceeds by solvent-mediated transfer of a proton from nitrogen to carbon [328, 329]. 

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