J . Org. Chem. 1997, 62, 6903-6910
6903
Effect of Top ologica lly Con tr olled Cou lom bic In ter a ction s on th e
Dyn a m ic Beh a vior of P h otoexcited Nitr op h en yl Alk yl Eth er s in
th e P r esen ce of Ter tia r y Am in es w ith Lim ited Motion F r eed om
Roberto Gonzalez-Blanco, J ose´ L. Bourdelande,* and J ordi Marquet*
Department of Chemistry, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Barcelona, Spain
Received September 24, 1996X
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.
The electron distribution in reactive intermediates
largely determines the outcome of a chemical or photo-
chemical reaction. However, the effect of the counterion
on the outcome of a chemical process (“metal ion cataly-
sis”, “electrophilic catalysis”, etc.) has been traditionally
attributed1 to Lewis acid complexation, ignoring the
important associated electrostatic effect. Only recently
has this electrostatic effect been recognized as responsible
for the lowest energy conformation of the radical anion/
cation pair in alkyl aryl ethers2 and the acceleration of
the electrocyclic reactions by metal cation complexation.3
ates9 although more recent literature5,7 showed that, in
most cases, this is an unnecessary hypothesis.
Intermediates of the ROAr•- type share many common
features with the aryl and benzyl halide radical anions
important in SRN1 reactions.10 Symons,11 Bunnett,12
Rossi,13 Save´ant,14 and others15 have proposed that
cleavage of C-X bonds in halogeno-aromatic radical
anions may be seen as the result of electron transfer from
the π* radical anion to the σ* arylnucleofugal bond by
an orbital crossing. Efficient fragmentation in aryl and
benzyl halides depends on a delicate balance. Thus,
electron-attracting groups that stabilize the π* orbitals
making easier the initial electron transfer can also
prevent the π*-σ* crossing by increasing the energy gap.
When a cyano group replaces the nitro group either in
p-nitroaryl or p-nitrobenzyl halides, the rate of dissocia-
tion of the radical-anion, as measured by pulse radioly-
sis,16 increases by at least five orders of magnitude.
For alkyl aryl ether radical anions, the efficiency and
selectivity of the fragmentation depends on the prob-
ability of transition from the π* state to the σ* state. In
addition a new feature, called the “spin regioconservation
principle”, must be taken into account.17 Guthrie and
Maslak proposed such a concept based on fragmentation
studies of aryl nitrobenzyl and benzyl nitroaryl ethers.
These authors state that the fission of alkyl aryl ether
In 1991, we predicted that the Coulombic alteration
of the “normal” electron distribution of a charged inter-
mediate could lead to previously unknown processes.4a
The idea was applied to achieve the previously unknown
reductive cleavage of certain ethers.4
The reductive cleavage of alkyl aryl ethers5 is an
important process in chemistry that has lately received
attention from both the synthetic6 and mechanistic7
points of view. The first reaction step leads to radical
anions, ROAr•-, known since 1968 from ESR studies.8
Dianions were also discussed in the past as intermedi-
X Abstract published in Advance ACS Abstracts, August 15, 1997.
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Dunn, E. J .; Moir, R. Y.; Buncel, E.; Purdon, J . G.; Bannard, R. A. B.
Can. J . Chem. 1990, 68, 1837. (c) Breslow, R.; Guo, T. J . Am. Chem.
Soc. 1988, 110, 5613. (d) Casaschi, A.; Desimoni, G.; Faita, G.;
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Am. Chem. Soc. 1989, 111, 8640.
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