Selectivity under microwave irradiation. Benzylation of 2-pyridone: an experimental and theoretical study

The reaction of 2-pyridone with benzyl bromide in the absence of base and under solvent-free conditions
has been studied experimentally and by computational methods. This reaction was one of the first reported
examples in which modification of selectivity under microwave irradiation was observed. C- and/
or N-alkylations were obtained depending on the benzyl halide and the heating system. N-Alkylation
through mechanism A (SN2 mechanism) is kinetically favoured while C-alkylation through an SN1-type
mechanism is thermodynamically favoured and is observed under microwave irradiation. Two SN1-type
mechanisms (mechanisms B and C) have been calculated, mechanism C being a kind of SNi. The influence
of the pyridone/benzyl bromide ratio was studied. A second molecule of pyridone stabilizes the transition
state and assists the leaving of the bromide ion. The occurrence of C-alkylation under microwave
irradiation is explained by the predominance of the thermodynamic control in these conditions. Under
microwave irradiation N-alkylation through an SN1-type mechanism (mechanism C) can also occur. The
dependence of the outcome of N-alkylation on the benzyl bromide ratio has been explained by a shift in
the mechanism from SN2 to SN1 under microwave irradiation. Computational calculations have shown to
be a useful tool for determination of the origin of the selectivity under microwave irradiation.

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