Some Illustrative Examples of Effects on Selectivity

Very few results are available on selectivity effects, and in which kinetic details of reactions have been described, because of a lack of strict comparisons between MW and A activation. Reports of the effect of MW on selectivity have been reviewed by De la Hoz et al. [89, 91] and a discussed in Chapter 5 of this book. 

Regiospecific N- or C-benzylations of 2-pyridone have been conducted under solvent-free conditions in the absence of base. The regioselectivity was controlled by the activation method (MW or A) or, vhen using microwaves, by the emitted power level or the leaving group of the benzyl halides [90] (Eq. 93)  

To explain these results it can be assumed that the TS leading to C-alkylation is more polar than that responsible for N-alkylation. This assumption, however, assumes the occurrence of kinetic control (which is not known for certain in this example). 

On microwave irradiation, vinylpyrazoles react with N-trichloroethyhdene carbamate, undergoing addition to the imine system through the conjugated vinyl group [173] (Eq. 94)  

Use of microwave irradiation as an energy source is crucial to performing the reaction and avoiding decomposition or dimerization of the starting pyrazole, which are observed in the absence of MW irradiation. 

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