Time-resolved electronic absorption spectroscopy has been
successfully applied to clarify the
mechanism of the “abnormal” photochemical cleavage of 4-nitrophenyl
piperidinoalkyl ethers induced
by controlled Coulombic disturbance of the “normal” electronic
distribution of the radical anion
intermediate. Thus, photolysis of
1-piperidino-2-(2-methoxy-4-nitrophenoxy)ethane (a system
with
an amine with limited freedom of motion) in acetonitrile leads to C−O
bond photocleavage in a
relatively slow process (k ≈ 4 × 105
s-1) from intermediate species that show radical-ion
pair
behavior. Systems with higher freedom of motion of the amine
moiety, such as 1-piperidino-5-(2-methoxy-4-nitrophenoxy)pentane or 4-nitroveratrole + triethylamine,
show the intermediate radical-ion pairs mainly evolving to reduction products, probably a result of
intermediates with geometries
not allowed for the system with limited freedom of motion of the
amine.
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