C–OMe Bond Formation and N–OMe Bond Fragmentation
minimum amount of chloroform and subjected to column
chromatography with hexane/ethyl acetate (from 4:1 to 1:1) as the
mobile phase. The photodeoxygenated products (6, 9, 10, 11, 13,
Conclusions
The structure of molecules that take part in ET processes
is often altered to the extent of causing bond dissoci- 15, 16) were identified by comparison with pure samples synthe-
sized in accordance with a previously reported procedure.[4a]
ation.[21] Competitive coupling of ET and bond cleavage
can take place by concerted or stepwise pathways.[22] The
concerted pathway is the only feasible choice in the absence
of an intermediate cation radical, but the stepwise pathway
may not occur even in its presence. In any case, the reaction
proceeds by the energetically most favourable pathway. The
bichromophoric AsD systems studied here take part in two
photochemical processes: intercomponent and intracom-
ponent methoxylation. Homolytic N–OMe bond fragmen-
tation upon irradiation is a characteristic process of this
group. However, the overall photoprocess in the AsD sys-
tems studied is controlled by the donating ability (redox
potential) of the donor subunit. N–OMe bond fragmenta-
tion in the radicals formed by the one-electron reduction of
N-methoxyisoquinolinium compounds results in the aro-
matic photomethoxylation of the electron-donor moiety.
This radical process is regioselectively guided by the resid-
ual spin density of the resulting cation radical on the elec-
tron-donor component. On the other hand, intracompon-
ent methoxylation is the intrinsic photochemical behaviour
of the N-methoxyisoquinolinium chromophore. The inser-
tion pathway for the reacting radicals can be adjusted by
the donating ability (i.e. the redox potential). This is consis-
1-Benzyl-4-methoxyisoquinoline (7): Colourless oil. Rf = 0.7 (Ac-
1
OEt/hexane, 1:1). H NMR (CDCl3): δ = 4.05 (s, 3 H), 4.57 (s, 2
H), 7.13–7.24 (m, 5 H), 7.51 (t, J = 8.5, 1.9 Hz, 1 H), 7.62 (t, J =
8.2, 1.3 Hz, 1 H), 8.03–8.06 (m, 2 H), 8.20 (d, J = 8.7 Hz, 1 H)
ppm. 13C NMR (CDCl3): δ = 41.5, 55.9, 103.8, 119.6, 121.56,
121.68, 122.7, 125.5, 126.1, 127.5, 128.4, 128.5, 129.1, 139.8, 140.0,
149.7, 152.4 ppm. MS: m/z (%) = 249 (35) [M]+, 248 (100). HRMS
(FAB): calcd. for C17H15NO3 [M]+ 249.1154; found 249.1156.
1-Benzyl-3-methoxyisoquinoline (8): Colourless oil. Rf
= 0.7
1
(AcOEt/hexane, 1:1). H NMR (CDCl3): δ = 4.00 (s, 3 H), 4.58 (s,
2 H), 6.86 (s, 1 H), 7.11–7.32 (m, 6 H), 7.49 (t, J = 6.8, 1.2 Hz, 1
H), 7.66 (d, J = 8.2 Hz, 1 H), 8.05 (d, J = 8.1, 1.0 Hz, 1 H) ppm.
13C NMR (CDCl3): δ = 41.5, 54.3, 99.0, 121.0, 123.8, 124.3, 125.83,
126.2, 126.4, 127.6, 127.9, 128.4, 128.7, 129.9, 131.0, 141.3,
152.6 ppm. MS: m/z (%) = 249 (61) [M]+, 248 (100), 233 (23).
HRMS (FAB): calcd. for C17H15NO3 [M]+ 249.1154; found
249.1159.
1-(2,4,5-Trimethoxybenzyl)isoquinoline (12): Solid. M.p. 110–
111 °C. Rf = 0.13 (AcOEt/hexane, 3:7); 1H NMR (CDCl3): δ = 3.61
(s, 3 H), 3.83 (s, 3 H), 3.88 (s, 3 H), 4.58 (s, 2 H), 6.54 (s, 1 H),
6.64 (s, 1 H), 7.46–7.54 (m, 2 H), 7.53 (t, J = 6.7 Hz, 1 H), 8.24 (d,
J = 7.9 Hz, 1 H), 8.44–8.49 (m, 2 H) ppm. 13C NMR (CDCl3): δ
= 40.7, 54.6, 56.1, 56.3, 112.9, 116.2, 120.2, 125.2, 126.0, 127.9,
tent with the emission observed upon excitation of the CT 130.7, 136.1, 141.6, 157.7, 158.1 ppm. MS: m/z (%) = 309 (4)
[M]+, 279 (21), 278 (100). HRMS (FAB): calcd. for C19H19NO3
state in systems that exhibit intermethoxylation, where
methoxy group transfer from the acceptor to the donor is
[M]+ 309.1365; found 309.1369.
the energetically most favourable pathway.
1-(2,4,5-Trimethoxybenzyl)-6,7-dimethoxyisoquinoline (14): Solid.
M.p. 128–129 °C. Rf = 0.35 (CH2Cl2/MeOH, 9.6:0.4). 1H NMR
(CDCl3): δ = 3.63 (s, 3 H), 3.83 (s, 3 H), 3.89 (s, 3 H), 3.92 (s, 3
H), 3.97 (s, 3 H), 4.50 (s, 2 H), 6.53 (s, 1 H), 6.73 (s, 1 H), 6.99 (s,
1 H), 7.38 (d, J = 5.5 Hz, 1 H), 7.55 (s, 1 H), 8.32 (d, J = 6.1 Hz,
1 H) ppm. 13C NMR (CDCl3): δ = 34.1, 55.8, 56.0, 56.1, 56.3, 56.9,
97.6, 104.6, 105.0, 113.7, 118.7, 118.9, 122.8, 133.5, 139.8, 143.3,
148.1, 149.9, 152.8, 158.4 ppm. MS: m/z (%) = 369 (23) [M]+, 338
(100), 280 (39). HRMS (FAB): calcd. for C21H23NO5 [M]+
369.1576; found 369.1581.
Experimental Section
General Methods: All starting material and reagents were used as
received. Solvents were spectrophotometric or HPLC grade and
1
used without further purification. H and 13C NMR spectra (200
and 50 MHz, respectively) were recorded from CDCl3 solutions by
using the solvent residual proton signal as standard. TLC analyses
were performed with silica gel 60 F256 plates, and column
chromatography was carried out with silica gel 60 (70–230 mesh).
Melting points were obtained in open capillaries. Samples for UV/
Vis and emission spectra were prepared in spectroscopic grade sol-
vents and adjusted to a linear range response. Molar absorption
coefficients were determined by using compound concentrations of
10–4 or 10–5 m. No fluorescent contaminants were detected upon
excitation in the wavelength region of experimental interest.
Fluorescence quantum yields were determined by comparison with
0.1 m quinine sulfate in 0.5 m sulfuric acid as a reference (Φ =
0.546)[23] and corrected for the refractive index of the solvent. The
samples were prepared with an absorbance between 0.1 and 0.2
a.u. at the excitation wavelength.
Supporting Information (see footnote on the first page of this arti-
cle): Details of the synthesis and photolysis studies of 1–5; UV
spectra and fluorescence emission/excitation spectra of compounds
1–5.
Acknowledgments
The authors wish to acknowledge funding from Spain’s Ministerio
de Ciencia
e Innovación and FEDER contribution (Project
CTQ2010-20303).
[1] a) A. Sengupta, I. Chakraborty, A. Ghosh, S. Lahiri, Tetrahe-
dron 2011, 67, 1689–1695; b) M. Oelgemöller, A. G. Griesbeck,
in Handbook of Organic Photochemistry and Photobiology
(Eds.: W. M. Horsepool, F. Lenci), CRC, Boca Raton, 2004; c)
V. Balzani, Electron Transfer in Chemistry, Wiley-VCH,
Weinheim, 2001; d) S. S. Isied Electron Transfer Reactions, ACS
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2008, 108, 2180–2237; f) J. Lappe, R. J. Cave, M. D. Newton,
I. V. Rostov, J. Phys. Chem. B 2005, 109, 6610–6619.
General Procedure for Irradiation: Photolyses were performed un-
der argon. A Pyrex immersion-well photoreactor equipped with a
medium-pressure mercury lamp (150 W) was used. Magnetically
stirred solutions of the N-methoxyisoquinolinium salts at a 10–3
m
concentration in the appropriate solvent were irradiated for 10 min.
The photolysates were washed with aqueous NaHCO3 and H2O
and dried with anhydrous MgSO4. The solvent was evaporated un-
der reduced pressure and the resulting product dissolved in the
Eur. J. Org. Chem. 2012, 1800–1808
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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