A kinetic study of
the hydrogen atom transfer (HAT) reactions from
nitrogen-containing heterocycles (secondary and tertiary lactams,
2-imidazolidinones, 2-oxazolidinones, and succinimides) to the cumyloxyl
radical has been carried out employing laser flash photolysis with
ns time resolution. HAT occurs from the C–H bonds that are
α to nitrogen, activated by hyperconjugative overlap with the
N–C=O π system. In the lactam series, the second-order
HAT rate constant (
k
H
) was observed to
decrease by a factor of ∼4 going from the five- and six-membered
ring derivatives to the eight-membered ones, a behavior that was rationalized
on the basis of a reduced extent of hyperconjugative activation associated
to the greater flexibility of the larger rings compared to the smaller
ones. In the five-membered-ring substrate series, the
k
H
values were observed to increase by >3 orders of
magnitude
on going from succinimide to 2-imidazolidinones, a behavior that was
explained in terms of the divergent contribution of hyperconjugative
activation and deactivating electronic effects determined by ring
functionalities. The results are discussed in the framework of the
development of HAT-based C–H bond functionalization procedures.
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