Catalytic photooxidation of 4-methoxybenzyl alcohol with a flavin-zinc(n)-cyclen complex

Article abstract of DOI:10.1002/chem.200400232

Flavin-zinc(II)-cyclen 10 contains a covalently linked substrate binding site (zinc(II)-cyclen) and a chromophore unit (flavin). Upon irradiation, compound 10 effectively oxidizes 4-methoxybenzyl alcohol (11-OCH₃) to the corresponding benzaldeh

Full text of DOI:10.1002/chem.200400232

R. Cibulka, B. Kçnig, and R. Vasold
Chem. Eur. J. 2004, 10, 6223 – 6231
DOI: 10.1002/chem.200400232
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6223

FULL PAPER
Catalytic Photooxidation of 4-Methoxybenzyl Alcohol with a Flavin–Zinc(ii)-
Cyclen Complex
Radek Cibulka,*[a] Rudolf Vasold,[b] and Burkhard Kçnig*[b]
Abstract: Flavin–zinc(ii)-cyclen 10 con-
tains covalently linked substrate
binding site (zinc(ii)–cyclen) and
oxidation is based on photoinduced
electron transfer from the coordinated
benzyl alcohol to the flavin chromo-
phore. This intramolecular process pro-
vides a much higher photooxidation ef-
ficiency, with quantum yields 30 times
those of the comparable intermolecular
process with a flavin chromophore
a
without a binding site. For the reaction
in buffered aqueous solution a quan-
tum yield of F = 0.4 is observed. The
turnover number in acetonitrile is in-
creased (up to 20) by high benzyl alco-
hol concentrations. The results show
that the covalent combination of a
chromophore and a suitable binding
site may lead to photomediators more
efficient than classical sensitizer mole-
cules.
a
chromophore unit (flavin). Upon irra-
diation, compound 10 effectively oxi-
dizes 4-methoxybenzyl alcohol (11-
OCH3) to the corresponding benzalde-
hyde both in water and in acetonitrile.
In the presence of air, the reduced
flavin 10-H2 is reoxidized, and so cata-
lytic amounts of 10 are sufficient for al-
cohol conversion. The mechanism of
Keywords:
flavin
electron transfer
macrocyclic ligands
·
·
·
photooxidation · sensitizers
Introduction
act intramolecularly, thus increasing the efficacy of the elec-
tron-transfer process. The substrate–sensitizer interaction
through coordinative bond formation is reversible, allowing
a catalytic process.
Photoinduced electron transfer (PET) in reversible nonco-
valent assemblies of electron-donor and electron-acceptor
moieties is studied intensively[1,2] because of its importance
for related biological processes. However, utilization of PET
for catalysis or sensitization of chemical reactions has so far
mainly been restricted to diffusion-controlled collision pro-
cesses. Only a limited number of systems with a defined re-
versible interaction between the photosensitizer and the
substrate have been reported.[3,4] To improve the sensitizing
efficiency of a photoactive molecule for a chemical reaction
we have prepared a mediator containing a chromophore co-
valently tethered to a substrate binding site. In such a
system, the bound substrate and the photoactive unit inter-
As a model reaction for investigation of the sensitizing
synthetic receptor, the photooxidation of benzyl alcohols
has been used. The oxidation of alcohols to aldehydes is a
key transformation in the chemical industry and also in biol-
ogy, and numerous stoichiometric and catalytic reactions
and reagents to mediate the process have been developed.[5]
Investigations currently in progress are aimed at attempting
to increase the efficiency of the reaction and to make it
more environmentally benign.[6] New metal complexes[7] and
metal-free systems[8,9] that catalyze oxidations with oxygen
or hydrogen peroxide as the stoichiometric oxidant are
sought after. The flavin–zinc(ii)-cyclen complex reported
here fulfils these criteria.
[a] Dr. R. Cibulka
Institute of Organic Chemistry
Department of Chemical Technology, Prague
Technickµ 5, 16628 Prague 6 (Czech Republic)
Fax : (+420)224-354-288
Results and Discussion
Design and synthesis: Molecule 10 (Scheme 1) consists of
two covalently bound subunits: the flavin chromophore,
which upon irradiation provides the necessary redox energy
to transform the substrate into the product, and a coordina-
tive binding site for the substrate. Riboflavin was selected as
the chromophore because it becomes a strong oxidant upon
irradiation,[10] and riboflavin-2’,3’,4’,5’-tetraacetate has previ-
ously been used for intermolecular PET oxidation of benzyl
E-mail: radek.cibulka@vscht.cz
[b] Dr. R. Vasold, Prof. Dr. B. Kçnig
Institut für Organische Chemie
Universität Regensburg
Universitätsstrasse 31, 93040 Regensburg (Germany)
Fax : (+49)941-943-1717
E-mail: burkhard.koenig@chemie.uni-regensburg.de
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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DOI: 10.1002/chem.200400232
Chem. Eur. J. 2004, 10, 6223 – 6231

6223 – 6231
Scheme 1. Synthesis of flavin–zinc(ii)-cyclen complex 10. Reaction conditions: a) pyridine, reflux (82%); b) acetic acid, RT (52%); c) DCC, CH2Cl2, R T
(72%); d) K2CO3, DMF, RT (88%) e) CH2Cl2, RT (99%); f) 1. H2O, ion exchange (OHÀ); 2. CH3CN, 608C (39%).
alcohols.[9a,b,d] A diethylene glycol substituent was introduced
into the 10-position to increase the solubility in water and
thus to allow oxidation reactions in aqueous solutions.
Zinc(ii)–cyclen was chosen as the binding site for the sub-
strate, in view of the presence and functioning of Lewis acid
zinc(ii) ions in the active site of alcohol dehydrogenase.[11]
Cyclen has a high affinity (lg K = 16.2)[12] for zinc(ii) ions,
which precludes other binding equilibria. Moreover zinc(ii)–
cyclen is known to coordinate Lewis base substrates[13] such
as water, alcohols,[11b,14] imides, or phosphate anions weakly
and reversibly. Similar systems containing a crown ether
unit covalently linked to a flavin moiety have already been
reported.[15] In these cases, however, the crown ether is not a
binding site for a substrate but only coordinates alkali metal
or alkaline-earth metal ions, thus modulating the flavin
redox potential and resulting in changes in the oxidation
quantum yields of benzyl alcohol[15a] or alkali mandelates.[15b]
The flavin part of the molecule (compound 4) was pre-
pared by using the general method developed by Kuhn and
co-workers:[16] synthesis of 1,2-dimethyl-4,5-dinitrobenzene
(1), ipso-substitution of the nitro group with 2-(2-methoxy-
ethoxy)ethylamine (2) in pyridine, the reduction of the
second nitro group in 3 with dihydrogen and Pd/C, and sub-
sequent condensation of the obtained diamine with alloxane.
Triply tert-butyloxycarbonyl(Boc)-protected cyclen[17] was
treated with bromoacetic acid in the presence of dicyclohex-
ylcarbodiimide (DCC) to give 7. The alkylation of flavin 4
in its 3-position has to be performed quickly to avoid de-
composition of the molecule, so an excess of compound 7
Abstract in Czech: Flavin-Zn(II)-cyklen 10 obsahuje vazeb-
ˇ
nØ místo pro substrµt (Zn(ii)-cyklen), kterØ je kovalentne
ˇ
ˇ
vµzanØ k chromoforu (flavin). Po ozµrení oxiduje sloucenina
10 4-methoxybenzylalkohol 11-OCH3 na odpovídající alde-
ˇ
ˇ
hyd ve vode a v acetonitrilu. Provµdí-li se reakce v prítom-
´
nosti vzduchu, vznikající redukovany flavin 10-H2 se reoxi-
ˇ
duje a pro oxidaci alkoholu tak postacuje pouze katalytickØ
ˇ
ˇ
mnozství lµtky 10. Mechanismus oxidace je zalozen na fo-
ˇ
˚
toindukovanØm prenosu elektronu z benzylalkoholu koordi-
´
novanØho k cyklenovØ jednotce na flavinovy chromofor.
ˇ
ˇ
ˇ
Tento intramolekulµrní proces podstatne zvysuje fflcinnost
´
´
ˇ
ˇ
´
fotooxidace—její kvantovy vytezek je tricetinµsobny ve
´
´
ˇ
srovnµní s kvantovym vytezkem intermolekulµrní oxidace
flavinem bez vazebnØho místa. V pufrovanØm vodnØm pro-
ˇ
ˇ
´
ˇ
ˇ
stredí bylo dosazeno kvantovØho vytezku F = 0.4. Pocet
˚
ˇ
ˇ
´
´
dosazenych katalytickych cyklu v acetonitrilu se zvysuje s
ˇ
´
koncentrací benzylalkoholu az na hodnotu 20. Vysledky
ˇ
ukazují, ze kovalentní spojení chromoforu a vhodnØho va-
˚
˚
ˇ
zebnØho centra muze vØst k fotomediµtorum, kterØ jsou
ˇ ˇ
ˇ
ˇ
fflcinnejsí nez klasickØ fotosenzibilizµtory.
Chem. Eur. J. 2004, 10, 6223 – 6231
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6225

R. Cibulka, B. Kçnig, and R. Vasold
FULL PAPER
was used in DMF with K2CO3 as base. The reaction in DMF
was found to be ten times faster than that in acetonitrile.
Deprotection of 8 with trifluoroacetic acid in dichlorome-
thane gave the ammonium salt 9, and the free amine form
of 9 was obtained by use of a strong basic ion exchanger.
Because of its low stability[18] the free base was immediately
converted into the zinc(ii) perchlorate complex 10. All reac-
tions starting from compound 4 were performed with exclu-
sion of light to prevent photochemical degradation of the
flavin moiety. For comparison, the 3-methylflavin 5, which
lacks the cyclen binding site, was also prepared.
presence of benzyl alcohol (11-H), because the electron-
transfer process for this redox couple is endergonic. The
rate constants (kq) of flavin emission quenching by 4-me-
thoxybenzyl alcohol (11-OCH3) given in Table 1 were esti-
mated by use of the measured lifetimes and slopes (KS) of
Stern–Volmer plots (see Experimental Section for details).
The values of kq—of about 6”109 mÀ1 sÀ1—are virtually iden-
tical for all investigated flavins. However, the obtained
Stern–Volmer plots for dynamic emission quenching of 4
and 5 with 11-OCH3 are linear, while the nonlinear plot ob-
served for 10 indicates a static quenching mechanism
through a noncovalent interaction between 10 and 11-
OCH3, as shown in Scheme 3 (see Supporting Information
for data). An affinity constant of about 24 LmolÀ1 was esti-
mated from the emission quenching data (see Supporting In-
formation).
Cyclic voltammetry and fluorescence quenching: The abili-
ties of flavin derivatives 4, 5, and 10 to oxidize benzyl alco-
hols were estimated from the DG values for electron trans-
fer from the alcohol to the excited flavin. With the entropy
changes from ground to excited state neglected, DG was
calculated by the Rehm–Weller[19] equation [DG
=
96.4(Eo1 xÀE1red)Àe2/eaÀE0–0]. Redox potentials of flavin
=
2
=
2
reduction and alcohol oxidation were experimentally
determined by cyclic voltammetry (Table 1), and typical
Table 1. Estimated thermodynamic oxidation parameters (DG) and emis-
sion quenching rate constants (kq) for flavin derivatives 4, 5, and 10 in re-
action with benzyl alcohol (11-H) and para-methoxybenzyl alcohol (11-
OCH3).
Flavin
t
Er1ed
kq ”109 [mÀ1 sÀ1
]
DG [kJmolÀ1
]
=
2
[ns][a] of flavin[b] [V]
[d]
11-H 11-OCH3 11-H[c] 11-OCH3
10
4
5
5.4
7.1
6.4
À0.88
À1.08
À1.09
–
–
–
7.8
5.7
5.6
+11
+30
+31
À47
À28
À27
Scheme 3. Reversible complex of 10 and 11-OCH3.
[a] Fluorescence lifetimes (single-exponential decay; measured in aceto-
nitrile in the absence of benzyl alcohol). [b] Values obtained in acetoni-
trile versus ferrrocene/ferrocenium; cFlavin = 2”10À3 m. [c] E1ox (benzyl al-
Catalytic photooxidation of 4-methoxybenzyl alcohol in ace-
tonitrile: The photooxidation of 4-methoxybenzyl alcohol
(11-OCH3) to 4-methoxybenzaldehyde (12-OCH3) under
mediation by the flavin–zinc-cyclen 10 was investigated in
acetonitrile under atmospheric pressure of air. The presence
of oxygen is necessary to reoxidize the reduced flavin[9a,10a]
formed during photooxidation, so that catalytic amounts of
10 are sufficient. No aldehyde formation was observed after
60 min of irradiation of 11-OCH3 in acetonitrile in the ab-
sence of any flavin (Table 2, entry 1) even if the solution
contained an excess of hydrogen peroxide; hydrogen perox-
ide thus does not oxidize benzyl alcohol under these reac-
tion conditions. Flavin–zinc(ii)-cyclen 10 mediates the photo-
oxidation, providing 51% conversion after 1 h of irradiation
(Table 2, entry 2). The quantum yield of aldehyde formation
is F = 3.8”10À2.
=
2
cohol, 11-H) = 1.79 V versus ferrocene/ferrocenium. [d] Eo1 x (4-methoxy-
=
2
benzyl alcohol, 11-OCH3) = 1.19 V versus ferrocene/ferrocenium.
values[9a,b,10d] were used for the Coulombic term (e2/ea =
5.4 kJmolÀ1) and the flavin excitation energy (E0–0
=
241 kJmolÀ1). The reduction potential of flavin 10 with
zinc(ii)–cyclen substitution was found to be shifted in the
positive direction by 200 mV in relation to flavins 4 and 5,
which may be interpreted in terms of the electron-withdraw-
ing effect of the adjacent zinc(ii) ion.
The thermodynamic data reveal that 11-OCH3 (see
Scheme 2) is a suitable substrate for flavin-mediated photo-
oxidation. No flavin emission quenching is observed in the
In the presence of flavins 4 and 5, without the zinc(ii)–
cyclen unit, the oxidation proceeds very slowly, and after
60 min of irradiation only small amounts of products were
detected. The importance of covalently connected flavin and
zinc(ii)–cyclen for high photoconversion of benzyl alcohol is
shown by comparison of the data for 10 with those obtained
in the presence of equimolar amounts of 3-methylflavin 5[20]
and cyclen–zinc(ii) perchlorate (13). The more positive re-
duction potential makes 10 the better oxidant, but the esti-
mated change in DG is not sufficient to explain the observed
significant differences in reactivity.[21] The highly effective
Scheme 2. Benzyl alcohols 11 used for photooxidation, product of photo-
oxidation (12-OCH3), and zinc(ii) cyclen bisperchlorate (13).
6226
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Chem. Eur. J. 2004, 10, 6223 – 6231

Catalytic Photooxidation
6223 – 6231
Table 2. Quantum yields of the photooxidation of 4-methoxybenzyl alcohol (c = 2”10À3 molLÀ1) in acetoni-
trile[a] mediated by different flavins (c = 2”10À4 molLÀ1).
in the presence of 10 was moni-
tored analytically (Figure 2).
The data show a clean conver-
sion process that affords 90%
alcohol conversion after 2 h.
This corresponds to a theoreti-
cal turnover number of 9 for
the sensitizer 10, but in view of
its degradation during the
course of the reaction its actual
efficiency must be much higher.
To confirm the proposed
Entry
Flavin
Conversion
after 1 h of
irradiation [%]
Rel. abs. at 444 nm
after 1 h of
Quantum yield
of aldehyde
irradiation[b][%]
formationF[c]
1
2
3
4
5
6
none
10
2
8
0
51
1
1
2
–
–
45
54
54
53
52
3.8”10À2
1.3”10À3
9.1”10À4
1.8”10À3
1.1”10À1
5+13
10+Sc(TfO)3
[d]
70
[a] The data were obtained from irradiation (l>420 nm) of an oxygen-saturated solution of flavin and 11-
OCH3 in acetonitrile under atmospheric pressure. [b] Relative absorbance of the reaction mixture at 444 nm
after 60 min irradiation time relative to the absorbance at the beginning of the experiment. [c] Quantum yield
electron-transfer
mechanism,
calculated from the rate of aldehyde formation during irradiation for 5 min. [d] c(Sc(TfO)3) = 1”10À2 molLÀ1
.
the oxidation of 4-methoxyben-
zyl alcohol (11-OCH3) by flavin
intramolecular electron-transfer process from reversibly co-
ordinated 11-OCH3 to the excited flavin chromophore of 10
is clearly the origin of its photocatalytic function (Figure 1).
was performed under exclusion of oxygen. Irradiation of a
deaerated acetonitrile solution containing flavin 10 and 11-
OCH3 with visible light (l>420 nm) resulted in the forma-
*
Figure 2. Concentrations of 11-OCH3 (&), 12-OCH3
(
), and hydrogen
peroxide ( ) during photooxidation of 11-OCH3 (c = 2”10À3 molLÀ1) in
oxygen-saturated acetonitrile in the presence of a catalytic amount of 10
(c = 2”10À4 molLÀ1).
~
Figure 1. Proposed mechanism of catalytic photooxidation of benzyl alco-
hol 11-OCH3 mediated by flavin 10 in the presence of dioxygen.
Unfortunately, degradation of the flavin molecule in so-
lution was observed during irradiation by visible light, the
typical flavin absorbance maximum intensity decreasing
during the course of the reaction. Table 2 gives the relative
decrease after 1 h. A similar degradation of the flavin is ob-
served in the absence of benzyl alcohol (see Supporting In-
formation).
tion of 12-OCH3 and the reduced flavin 10-H2. The process
was monitored by H NMR, UV/Vis spectrophotometry, and
HPLC (see Supporting Information).
The efficiency of photooxidation by flavins 10 and 5 in-
creases with increasing alcohol concentration, reaching max-
imum turnover numbers of irradiation of about 20 and 1.6,
respectively, after 60 min. (Figure 3). The flavin–zinc(ii)-
cyclen conjugate 10 is thus also significantly more efficient
then flavin 5 at higher substrate concentrations.[22]
1
Fukuzumi et al.[9a] have reported the stabilization of ribo-
flavin-2,3,4,5-tetraacetate in the photocatalytic oxidation of
benzyl alcohols by rare-earth-metal ion coordination to the
carbonyl-oxygen donor atoms of the isoalloxazine ring. In
an attempt to increase the stability of 10 we performed the
oxidation of 11-OCH3 in the presence of an excess of scan-
dium(iii) triflate (Table 2, entry 6). The observed rate of al-
dehyde formation is slightly higher with scandium triflate,
but flavin decomposition is still severe, as indicated by a
drop in the flavin absorption intensity to 52% of its original
value (45% in the absence of scandium triflate) after 1 h of
reaction time.
Photooxidation of 4-methoxybenzyl alcohol in water: The
good solubility of 10 in water allows the mediated photooxi-
dation of 11-OCH3 to be studied in buffered aqueous solu-
tions (borate buffer, pH 7.2). Irradiation of a deaerated
aqueous solution of 10 and 11-OCH3 afforded the corre-
1
sponding benzaldehyde 12-OCH3, as detected by H NMR
and HPLC analysis. The photocatalytic efficiencies and
quantum yields of flavins 5 and 10 in dioxygen-saturated
aqueous solutions were compared by turnover number after
irradiation for 60 min (Table 3). Generally, the rates of pho-
tooxidation in water are higher than those in acetonitrile,
The formation of aldehyde 12-OCH3 and hydrogen perox-
ide and the conversion of 11-OCH3 during photooxidation
Chem. Eur. J. 2004, 10, 6223 – 6231
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R. Cibulka, B. Kçnig, and R. Vasold
FULL PAPER
oxidation of 4-methoxybenzyl alcohol to the corresponding
aldehyde. The oxidation mechanism is based on photoin-
duced electron transfer, both in acetonitrile and in water. In
the presence of dioxygen, the flavin is reoxidized and 10
acts efficiently in catalytic amounts. The role of the zinc(ii)-
cyclen unit is to coordinate the hydroxy group of the benzyl
alcohol in reversible fashion, thus facilitating intramolecular
electron transfer to the excited flavin. The efficiency of the
intramolecular process with 10 is significantly higher than
for flavins lacking the binding site. In summary, compound
10 operates as an efficient photosensitizer for the oxidation
of 4-methoxybenzyl alcohol by oxygen. The reaction pro-
ceeds in aqueous solutions at pH 7.2 and at ambient temper-
ature. The study demonstrates that sensitizers with increased
efficiency for photochemical processes can be obtained
through the combination of chromophores with suitable
binding sites. Such systems may find applications in photo-
oxidations or photoreductions of biomolecules, waste water
treatment, selective photochemical transformations, or the
chemical storage of light energy.
Figure 3. Dependence on the alcohol concentration of the flavin turnover
*
numbers for photooxidation of 11-OCH3 with 10 (&) and 5 ( ) in aceto-
nitrile (cflavin = 2”10À4 molLÀ1) after 60 minutes irradiation.
Table 3. Comparison of the efficiency of flavin mediators (c
10À4 molLÀ1
for the photooxidation of 4-methoxybenzyl alcohol in
borate buffer, pH 7.2.[a]
= 2”
)
Flavin
Concentration Turnover Rel. abs. at 444 nm Quantum
of 11-CH3
after 1 h
after 1 h
yield F[c]
[mmolLÀ1
]
irradiation
irradiation[b] [%]
Experimental Section
none
10
5
5+13
10
10, pH 9.0
2
2
2
2
20
20
0
7.6
8.3
8.3
10.9
12.7
–
–
General: 1,2-Dimethyl-4,5-dinitrobenzene (1),[16,24] 2-(2-methoxyethoxy)-
ethylamine (2),[25] and triply Boc-protected cyclen 6[17b] were synthe-
sized by known procedures. All other reagents used were commercially
available reagent grade. Solvents were distilled and dried by standard
procedures. Temperature data are uncorrected. 1H NMRand 13C NMR
spectra were recorded on a Bruker Avance instrument at 300 MHz and
75 MHz, respectively. Chemical shifts are reported in ppm, coupling con-
stants (J) in Hz. Elemental analyses were performed on a Vario EL III
analyzer. UV/Vis and fluorescence spectra were recorded on Varian
Cary 50 Bio and Varian Eclipse spectrometers. Mass spectra were
measured on Finnigan MAT 95, ThermoQuest Finnigan TSQ 7000, and
Finnigan MAT SSQ710A spectrometers. TLC analyses were carried out
on DC Alufolien Kieselgel 60H F254 (Merck). Gerundan Si 60 silica gel
(0.063–0.200 mm; Merck) was used for column chromatography.
57
79
73
76
84
0.23
0.11
0.14
0.40
0.40
[a] The data were obtained after irradiation of oxygen-saturated solutions
of the catalyst and substrate in water with light (l>420 nm) under at-
mospheric pressure. [b] Relative absorbance of the reaction mixture at
444 nm after irradiation for 60 min relative to the absorbance at the be-
ginning of experiment. [c] Quantum yield calculated from the rate of al-
dehyde formation during irradiation for 5 min.
which can be interpreted in terms of the higher polarity of
the medium.[23] At higher concentration of substrate (c =
20 mmolLÀ1), a photooxidation quantum yield of F = 0.40
was achieved with compound 10. The quantum yield was
not influenced by an increase in the pH from 7.2 to 9.0.
The turnover numbers for flavins 5 and 10 are fairly simi-
lar, in contrast with the results in acetonitrile. Flavin 10 suf-
fers from its lower photostability, as is evident from the rela-
tive absorbance of the reaction mixture after 60 min of irra-
diation. However, the quantum yield calculated from the in-
itial conversion in the first five minutes of the reaction indi-
cates a higher efficiency for 10 than for 5 in the catalytic
photooxidation. The observed difference in reactivity in
aqueous solutions is smaller than in acetonitrile. The compe-
tition of benzyl alcohol and water binding to the coordina-
tion site of 10 may hamper the formation of the alcohol/
flavin–zinc(ii)-cyclen complex necessary for efficient intra-
molecular oxidation.
N-[2-(2-Methoxyethoxy)ethyl]-4,5-dimethyl-2-nitroaniline (3): A solution
of 1,2-dimethyl-4,5-dinitrobenzene (1, 2.18 g, 11.11 mmol) and 2-(2-me-
thoxyethoxy)ethylamine (2, 2.10 g, 17.62 mmol) in pyridine (270 mL) was
heated to reflux for 15 h. A further portion of 2-(2-methoxyethoxy)ethyl-
amine (1.05 g, 8.81 mmol) was added, and the solution was heated to
reflux until all starting material had been converted into product (TLC,
chloroform/methanol 100:3). The overall reaction time was 65 h. The re-
action mixture was cooled, chloroform (400 mL) was added, and the so-
lution was extracted with aqueous citric acid (10%, 10”250 mL) and
with water (2”250 mL). The organic phase was dried over sodium sul-
fate, the solvents were evaporated in vacuum, and the crude product was
purified by column chromatography (chloroform/methanol 100:1) to give
compound 3 (2.45 g, 82%) as an orange oil. Rf = 0.23 (CHCl3/MeOH
100:1); 1H NMR(CDCl 3): d
= 2.17 (s, 3H; ArCH3), 2.26 (s, 3H;
3
ArCH3), 3.40 (s, 3H; CH3O), 3.51 (t, J(H,H) = 5.8 Hz, 2H; CH2N), 3.59
(m, 2H; CH3OCH2), 3.67 (m, 2H; CH3OCH2CH2), 3.77 (t, 3J(H,H) =
5.6 Hz, 2H; OCH2CH2N), 6.63 (s, 1H; ArH), 7.92 ppm (s, 1H; ArH); 13C
NMR(CDCl 3): d = 18.6 (+), 20.7 (+), 42.7 (À), 59.2 (+), 69.3 (À), 70.6
(À), 71.9 (À), 114.1 (+), 124.5 (Cquat), 126.5 (+), 130.0 (Cquat), 144.1
(Cquat), 147.2 ppm (Cquat); MS (70 eV, EI): m/z (%): 268 (24) [M]+, 179
(100) [MÀC4H9O2]+; elemental analysis calcd (%) for C13H20N2O4
(268.3): C 58.19, H 7.51, N 10.44; found: C 57.61, H 7.24, N 10.27.
10-[2-(2-Methoxyethoxy)ethyl]-7,8-dimethyl-10H-benzo[g]pteridine-2,4-
dione (4): Nitro compound 3 (1.30 g, 4.84 mmol) was dissolved in acetic
acid (20 mL) and after addition of Pd/C (10%) was stirred for 23 h in an
autoclave under dihydrogen atmosphere (600 kPa). The reaction mixture
was filtered through Celite, and boric acid (2.7 g, 43.7 mmol) and alloxan
monohydrate (2.48 g, 15.49 mmol) were added immediately. The mixture
Conclusion
The flavin–zinc(ii)-cyclen conjugate 10 has been prepared as
a new sensitizer with a substrate binding site for the photo-
6228
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 6223 – 6231

Catalytic Photooxidation
6223 – 6231
was stirred for 9 h under nitrogen at room temperature. The solution was
diluted with chloroform (250 mL) and water (50 mL), the separated
chloroform phase was washed with water (3”100 mL), and the organic
phase was dried over magnesium sulfate. After evaporation of solvent in
vacuo and recrystallization of the crude product from water, compound 4
(0.86 g, 52%) was obtained as orange crystals. M.p. 2298C; 1H NMR
(CDCl3): d = 2.45 (s, 3H; ArCH3), 2.54 (s, 3H; ArCH3), 3.28 (s, 3H;
CH3O), 3.42 (m, 2H; CH3OCH2), 3.60 (m, 2H; CH3OCH2CH2), 4.02 (t,
3J(H,H) = 5.6 Hz, 2H; OCH2CH2N), 4.93 (t, 3J(H,H) = 5.8 Hz, 2H;
CH2N), 7.77 (s, 1H; ArH), 8.03 (s, 1H; ArH), 8.46 ppm (s, 1H; NH); 13C
NMR(CDCl 3): d = 19.5 (+), 21.4 (+), 45.9 (À), 59.0 (+), 68.0 (À), 70.8
(À), 71.8 (À), 117.0 (+), 132.16 (+), 132.22 (Cquat), 135.0 (Cquat), 135.8
(Cquat), 137.1 (Cquat), 148.1 (Cquat), 150.3 (Cquat), 155.2 (Cquat), 159.6 ppm
(Cquat); UV/Vis (acetonitrile): lmax (e) = 225 (31000), 270 (31400), 345
(8400), 444 (11900); MS (70 eV, EI): m/z (%): 344 (2) [M]+, 242 (100)
[MÀC5H10O2]+, 171 (58) [MÀC7H11NO4]+, 156 (24) [MÀC7H12N2O4]+;
elemental analysis calcd (%) for C17H20N4O4 (344.4): C 59.29, H 5.85, N
16.27; found: C 58.98, H 5.78, N 16.20.
reduced pressure, and the remaining solid was dried in vacuo to yield 9
(0.26 g, 99%) as a yellow solid. M.p. 69–708C; 1H NMR(600 MHz,
CD3CN, TMS): d = 2.43 (s, 3H; ArCH3), 2.55 (s, 3H; ArCH3), 3.19 (s,
3
3H; CH3O), 3.22 (m, 8H; CH2N), 3.26 (m, 4H; CH2N), 3.37 (t, J(H,H)
=
4.7 Hz, 2H; CH3OCH2), 3.56 (t, 3J(H,H)
=
4.6 Hz, 2H;
CH3OCH2CH2), 3.69 (m, 2H; CH2NC=O), 3.88 (m, 2H; CH2NC=O),
3.91 (t, 3J(H,H) = 5.5 Hz, 2H; OCH2CH2N), 4.85 (t, 3J(H,H) = 5.5 Hz,
2H; CH2N), 4.89 (s, 2H; NCH2C=O), 7.84 (s, 1H; ArH6), 7.89 ppm (s,
1H; ArH9); 13C NMR(150 MHz, CD 3CN): d = 18.2 (+), 20.3 (+), 42.5
(À), 43.3 (À), 44.1 (À), 44.3 (À), 44.9 (À), 45.0 (À) 45.8 (À), 46.35 (À),
46.43 (À), 47.5 (À), 57.8 (+), 67.0 (À), 70.3 (À), 71.3 (À), 115.7 (q, CF3),
116.9 (+), 131.1 (+), 131.9 (Cquat), 135.07 (Cquat), 135.10 (Cquat), 137.5
(Cquat), 148.5 (Cquat), 149.1 (Cquat), 155.7 (Cquat), 160.1 (Cquat), 160.4 (q,
CF3COOÀ), 169.8 ppm (Cquat); MS (ESI, CH3CN): m/z (%): 557 (100)
2+
[M+HÀ3”CF3COOH]+, 279 (13) [M+H2 À3CF3COOH], 186 (24)
3+
[M+H3 À3CF3COOH]; MS (-ESI, CH3CN): m/z (%): 783 (59)
[MÀCF3COOHÀH]À, 669 (15) [MÀ2”CF3COOHÀH]À, 227 (100) [(2”
CF3COOHÀH)]À; elemental analysis calcd (%) for C33H43F9N8O11
(898.8): C 44.10, H 4.82, N 12.47; found: C 43.96 H 4.97, N 12.71.
Tri-tert-butyl 10-bromoacetyl-1,4,7,10-tetraazacyclododecane-1,4,7-tricar-
boxylate (7):
A
solution of triply Boc-protected cyclen
6
(2.5 g,
10-[2-(2-Methoxyethoxy)ethyl]-7,8-dimethyl-3-[2-oxo-2-(1,4,7,10-tetraaza-
cyclododec-1-yl)ethyl]-10H-benzo[g]pteridine-2,4-dione zinc(ii) bisper-
chlorate (10): Ammonium salt 9 (0.25 g, 0.28 mmol) was dissolved in
water and passed trough an ion exchanger (Merck Ion exchanger III,
OHÀ form) to obtain its free base form. The water was evaporated under
reduced pressure and the remaining solid was dried under vacuum to
yield 10-[2-(2-methoxyethoxy)ethyl]-7,8-dimethyl-3-[2-oxo-2-(1,4,7,10-tet-
5.29 mmol), bromoacetic acid (0.85 g, 6.11 mmol), and DCC (1.09 g,
5.28 mmol) in anhydrous dichloromethane (60 mL) was stirred for 20 h
under nitrogen at room temperature. White solid was filtered off, and the
filtrate was washed with a solution of sodium hydroxide (2m, 2”40 mL)
and water (2”40 mL) and dried over sodium sulfate. The crude product
obtained by evaporation of the solvents in vacuo was purified by column
chromatography (ethyl acetate/petroleum ether 3:1). Product 7 (2.26 g,
72%) was obtained as a white solid. RF = 0.70 (ethyl acetate/petroleum
ether 3:1); m.p. 558C; 1H NMR(CDCl 3): d = 1.46 (s, 18H; (CH3)3C),
1.49 (s, 9H; (CH3)3C), 3.37–3.57 (m, 16H; CH2N), 3.84 ppm (s, 2H;
CH2Br); 13C NMR(CDCl 3): d = 28.0 (+), 49.5–51.6 (À), 80.5 (Cquat),
80.7 (Cquat), 155.4 (Cquat), 157.3 (Cquat) ppm; MS (70 eV, EI): m/z (%): 592
and 594 (10) [M]+, 513 (63) [MÀBr]+, 492 and 494 (17)
raazacyclododec-1-yl)ethyl]-10H-benzo[g]pteridine-2,4-dione
(0.112 g,
72.3%), which was used without purification for the preparation of the
1
zinc complex. H NMR(CD CN): d = 2.44 (s, 3H; ArCH3), 2.53 (s, 3H;
3
ArCH3), 2.57 (m, 4H; CH2N), 2.64 (m, 4H; CH2N), 2.77 (m, 4H; CH2N),
3.19 (s, 3H; CH3O), 3.37 (m, 2H; CH3OCH2), 3.51 (m, 2H; CH2NC=O),
3.56 (m, 2H; CH3OCH2CH2), 3.59 (m, 2H; CH2NC=O), 3.91 (t, J =
5.8 Hz, 2H; OCH2CH2N), 4.83 (t, J = 5.8 Hz, 2H; CH2N), 4.96 (s, 2H;
NCH2C=O), 7.80 (s, 1H; ArH), 7.90 ppm (s, 1H; ArH); MS (ESI,
CH3CN): m/z (%): 557.3 (100) [M+H]+; HR-MS (C27H41N8O5): calcd.
557.3200; found 557.3206 Æ 0.0007.
[MÀC4H8ÀCO2]+,
413
(40)
[MÀC4H8ÀCO2ÀBr]+,
371
(58)
[MÀC7H10O3Br]+, 357 (37), 57 (100) [C4H9]+; elemental analysis calcd
(%) for C25H45N4O7 (593.6): C 50.59, H 7.64, N 9.44, Br 13.46; found: C
50.18, H 7.21, N 9.16, Br 13.20.
A solution of zinc(ii) bisperchlorate hexahydrate in acetonitrile (2 mL)
Tri-tert-butyl
10-(2-{10-[2-(2-Methoxyethoxy)ethyl]-7,8-dimethyl-2,4-
was added to the solution of the free amine base of 9 (0.119 g,
dioxo-4,10-dihydro-2H-benzo[g]pteridin-3-yl}acetyl)-1,4,7,10-tetraazacy-
clododecane-1,4,7-tricarboxylate (8): A mixture of 4 (0.34 g, 0.99 mmol),
7 (1.73 g, 2.91 mmol), and potassium carbonate (1 g) in dry dimethylfor-
mamide (50 mL) was stirred under nitrogen at room temperature until
all 4 had been converted into the product (ca. 4 h). The progress of the
reaction was monitored by TLC (ethyl acetate/methanol 5:2). A second
portion of 4 (0.16 g, 0.46 mmol) was added, and the mixture was stirred
at room temperature for 8 h, diluted with chloroform (200 mL), and
washed with water (4”150 mL), and the organic phase was dried over
magnesium sulfate. After evaporation of the solvents and purification of
the crude product by column chromatography (ethyl acetate/methanol
4:1), compound 8 (1.1 g, 88%) was obtained as a yellow solid. RF = 0.55
(ethyl acetate/methanol 4:1); m.p. 137–1398C; 1H NMR(CDCl 3): d =
1.48 (m, 27H; (CH3)3C), 2.43 (s, 3H; ArCH3), 2.53 (s, 3H; ArCH3), 3.29
(s, 3H; CH3O), 3.30–3.75 (m, 16H; CH2N), 3.42 (m, 2H; CH3OCH2),
0.21 mmol) in acetonitrile (3 mL) under nitrogen. The mixture was stir-
red for 30 min at 608C and after evaporation of solvents a red solid was
obtained. The crude product was dissolved in hot acetone (608C), and
after cooling impurities precipitated. The solids were filtered off and,
after evaporation of the solvent from the filtrate, compound 10 (0.09 g,
51%) was obtained as a red solid. M.p.>2608C, decomp. at 2608C; 1H
NMR(CD CN): d = 2.45 (s, 3H; ArCH3), 2.55 (s, 3H; ArCH3), 2.72 (m,
3
2H; CH2N), 2.85 (m, 2H; CH2N), 2.95 (m, 2H; CH2N), 3.00–3.25 (m,
6H; CH2N), 3.15 (s, 3H; CH3O), 3.34 (m, 2H; CH3OCH2), 3.53 (m, 2H;
CH3OCH2CH2), 3.71 (m, 2H; CH2NC=O), 3.89 (t, J
= 5.8 Hz, 2H;
OCH2CH2N), 3.94 (m, 2H; CH2NC=O), 4.88 (t, J = 5.6 Hz, 2H; CH2N),
4.90 (s, 2H; NCH2C=O), 7.88 (s, 1H; ArH6), 7.94 ppm (s, 1H; ArH9); 13C
NMR: d = 18.2 (+), 20.2 (+), 43.4 (À), 43.5 (À), 45.2 (À), 45.9 (À),
46.0(À), 57.7 (+), 66.9 (À), 70.2 (À), 71.2 (À), 117.7 (+), 131.0 (+), 132.3
(Cquat), 133.0 (Cquat), 136.1 (Cquat), 139.8 (Cquat), 150.2 (Cquat), 150.7 (Cquat),
157.8 (Cquat), 161.0 (Cquat), 173.8 ppm (Cquat); UV/Vis (acetonitrile): lmax
3.55 (m, 2H; CH3OCH2CH2), 4.00 (t, 3J(H,H)
=
5.8 Hz, 2H;
OCH2CH2N), 4.85–4.95 (m, 4H; CH2N, CH2Br), 7.75 (s, 1H; ArH),
(e) = 225 (27100), 270 (28600), 345 (7400), 444 (10100); MS (ESI,
8.04 ppm (s, 1H; ArH); 13C NMR(CDCl ): d = 19.5 (+), 21.3 (+), 28.42
CH3CN): m/z (%): 310.1 (100) [(MÀ2”ClO4À)]2+, 557.4 (50) [LH]+,
619.3 (16) [(MÀ2”ClO4ÀÀH+)]+, 679.4 (48) [(MÀ2”ClO4À+CH3-
COOÀ)]+, 719.2 (17) [(MÀClO4À)]+; HR-MS (C27H39N8O5Zn+): calcd.
619.2335; found 619.2375Æ0.0029.
3
(+), 28.46 (+), 28.58 (+), 45.4 (À), 50.03 (À), 50.06 (À), 50.29 (À), 51.50
(À), 59.0 (+), 68.1 (À), 70.7 (À), 71.7 (À), 80.2 (Cquat), 80.5 (Cquat), 116.9
(+), 132.1 (Cquat), 132.2 (+), 134.9 (Cquat), 135.5 (Cquat), 136.6 (Cquat), 146.7
(Cquat), 147.5 (Cquat), 149.0 (Cquat), 155.2 (Cquat), 155.6 (Cquat), 159.8 ppm
(Cquat); MS (ESI, CH2Cl2 +CH3OH+CH3COONH4): m/z (%): 858 (100)
[M+H]+, 758 (26) [MÀC4H7ÀCO2]+; elemental analysis calcd (%) for
C42H64N8O11 (857.0): C 58.86, H 7.53, N 13.07; found: C 58.65, H 7.28, N
12.68.
10-[2-(2-Methoxyethoxy)ethyl]-3,7,8-trimethyl-10H-benzo[g]pteridine-
2,4-dione (5): A mixture of flavin 4 (0.05 g, 0.15 mmol), sodium carbon-
ate (0.18 g, 1.7 mmol), and methyl iodide (0.78 g, 5.5 mmol) in dry DMF
(3 mL) was stirred at room temperature for 45 h. Water (1 mL) was then
added and the pH was adjusted to 7 with hydrochloric acid (1m). The so-
lution was extracted with chloroform (3”20 mL) and the organic phase
was dried with sodium sulfate. After evaporation of the solvents and re-
crystallization from water, the pure product was obtained as yellow nee-
dles (0.027 g, 52%). M.p. 162–1648C; 1H NMR(CDCl 3): d = 2.43 (s,
3H; ArCH3), 2.52 (s, 3H; ArCH3), 3.26 (s, 3H; CH3O), 3.39 (m, 2H;
CH3OCH2), 3.51 (s, 3H; CH3N), 3.56 (m, 2H; CH3OCH2CH2), 3.99 (t,
J = 5.2 Hz, 2H; OCH2CH2N), 4.93 (t, J = 5.2 Hz, 2H; CH2N), 7.74 (s,
10-[2-(2-Methoxyethoxy)ethyl]-7,8-dimethyl-3-[2-oxo-2-(1,4,7,10-tetraaza-
cyclododec-1-yl)ethyl]-10H-benzo[g]pteridine-2,4-dione·3CF3COOH (9):
A solution of 8 (0.25 g, 0.29 mmol) dissolved in dichloromethane (5 mL)
and trifluoroacetic acid (5 mL) was stirred for 3 h under nitrogen at
room temperature. The solvents were evaporated, and the remaining
solid was dissolved in water (10 mL). The aqueous solution of 9 was ex-
tracted with dichloromethane (10 mL), the water was evaporated under
Chem. Eur. J. 2004, 10, 6223 – 6231
www.chemeurj.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6229

R. Cibulka, B. Kçnig, and R. Vasold
FULL PAPER
1H; ArH), 8.03 ppm (s, 1H; ArH); 13C NMR: d = 19.5 (+), 21.4 (+),
28.7 (+), 45.3 (À), 59.0 (+), 68.1 (À), 70.8 (À), 71.7 (À), 116.8 (+), 132.1
(Cquat), 132.2 (+), 135.0 (Cquat), 135.4 (Cquat), 136.6 (Cquat), 147.4 (Cquat),
148.7 (Cquat), 156.0 (Cquat), 160.2 ppm (Cquat); MS (70 eV, EI): m/z (%):
[1] For reviews on photoinduced electron transfer in non-covalently
bonded assemblies see: a) M. D. Ward, Chem. Soc. Rev. 1997, 26,
365–375; b) D. G. Whitten, Acc. Chem. Res. 1980, 13, 83–90; c) I.
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Willner in Topics in Current Chemistry (Ed.: J. Mattay), Springer,
New York, 1991, Vol. 159, pp. 177–201.
[2] Recent examples of photoinduced electron transfer in non-covalent-
ly bonded assemblies: a) M. Braun, S. Atalick, D. M. Guldi, H.
Lanig, M. Brettreich, S. Burghardt, M. Hatzimarinaki, E. Ravanelli,
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J. Am. Chem. Soc. 2003, 125, 4068–4069; c) H. F. M. Nelissen, M.
Kercher, L. De Cola, M. C. Feiters, R. J. M. Nolte, Chem. Eur. J.
2002, 8, 5407–5414; d) R. Ballardini, V. Balzani, M. Clemente-Leꢁn,
A. Credi, M. T. Gandolfi, E. Ishow, J. Perkins, J. F. Stoddart, H.-R.
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124, 11541–11551; f) K. Lang, V. Krµl, P. Kapusta, P. Kubµt, P.
358 (4) [M]+, 256 (100) [MÀC5H10O2]+, 199.1 (38) [C11H9N3O]+, 171 (28)
+
[C10H9N3]+, 156 (12) [C10H8N]2
; elemental analysis calcd (%) for
C18H22N4O4 (358.4): C 60.32, H 6.19, N 15.63; found: C 59.98, H 5.76, N
15.45.
Photooxidations: Photooxidations were performed in quartz cuvettes (d
= 10 mm). Aliquot portions of stock solutions of reactants in acetonitrile
or in water were added direct into the cell to achieve concentrations of
flavin catalyst and benzyl alcohol of 2”10À4 molLÀ1 and 2”10À3 molLÀ1
,
respectively. The mixture was purged with oxygen for 5 min before the
reaction. The cell containing the reactants was irradiated with a lamp
(500 W) through a filter transmitting only light with l>420 nm. The
amount of hydrogen peroxide produced was determined by the sodium
iodide method:[26] The sample (50 mL) was diluted with excess sodium
iodide and the amount of I3À formed was determined spectrophotometri-
cally (lmax = 350 nm). The amount of the reactant, 4-methoxybenzyl al-
cohol and the product, 4-methoxybenzaldehyde, was determined by
HPLC: aliquot portions of the reaction mixture were diluted with the so-
lution of the internal standard (naphthalene, Fluka p. a. grade) and the
sample was analyzed on an Agilent 1100 HPLC System (column: Phe-
nomenex Luna C18 column, 150 mm, 5 ml) with use of spectrophotomet-
ric detection (Agilent 1100 Diode Array Detector). The amount of the
flavin catalyst was monitored by measurement of the absorbance of the
reaction mixture at the maximum of the flavin absorption (l = 444 nm).
Quantum yields of 4-methoxybenzyl alcohol photooxidation were deter-
mined from the kinetics of 4-methoxybenzaldehyde formation over irra-
diation for 5 min with the standard actinometer potassium ferrioxalate.[27]
The amount of 4-methoxybenzylaldehyde formed was monitored by
HPLC as described above.
ˇ
Vasek, Tetrahedron Lett. 2002, 43, 4919–4922; g) T. Kojima, T. Saka-
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3709–3711.
For reaction monitoring in deaerated water and acetonitrile by NMR
spectroscopy, solutions were prepared directly in the NMRtube. The so-
lution was then purged with argon for 15 min and the reaction mixture
was irradiated (l>420 nm) for 8 h. Signals of the resulting aldehyde
were correlated with signals of a standard.
[5] a) A. Sheldon, J. K. Kochi, Metal-Catalysed Oxidations of Organic
Compounds, Academic Press, New York, 1981, b) Organic Synthesis
by Oxidation with Metal Compounds (Eds.: W. J. Mijs, C. R. H. de
Fluorescence quenching: The relative fluorescence intensities were mea-
sured on a Varian Eclipse spectrometer in acetonitrile solutions contain-
ing 10 (c = 2”10À5 molLÀ1), 4 (c = 1”10À5 molLÀ1), or 5 (c = 1”
´
Jonge), Plenum, New York, 1986; c) M. Hudlicky, Oxidation in Or-
10À5 molLÀ1
)
and the corresponding benzyl alcohol (c
=
0–2”
ganic Chemistry, ACS, Washington, DC, 1990; d) J. Muzart, Tetrahe-
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10À2 molLÀ1) at 258C. Stern–Volmer plots (I0/I = 1+KS[Q]) were con-
structed, and constants KS were evaluated as the slope of the dependence
(in the case of 4 and 5) or as a slope of the linear part (in the case of 10).
[6] a) C. L. Hill, Nature, 1999, 401, 436–437; b) P. T. Anastas, J. C.
Warner, Green Chemistry: Theory and Practice, Oxford University
Press, Oxford, 1998.
Quenching rate constants (kq) were calculated from values of KS (KS
kq·t). Fluorescence lifetimes (t) of flavins 10, 4, and 5 in acetonitrile were
measured by use of a Hurricane (Spectra-Physics) laser source (lex
=
[7] Examples of alcohol oxidations catalyzed by metal ion complexes:
a) I. E. Markꢁ, P. R. Giles, M. Tsukazaki, S. M. Brown, C. J. Urch,
Science 1996, 274, 2044–2046; b) Y. Wang, J. L. DuBois, B. Hedman,
K. O. Hodgson, T. D. P. Stack, Science 1998, 279, 537–540; c) G-J.
ten Bring, I. W. C. E. Arends, R. A. Sheldon, Science 2000, 287,
1636–1639; d) I. E. Markꢁ, P. R. Giles, M. Tsukazaki, I. ChellØ-Reg-
naut, C. J. Urch, S. M. Brown, J. Am. Chem. Soc 1997, 119, 12661–
12662; e) P. Chaudhuri, M. Hess, T. Weyhermüller, K. Wieghardt,
Angew. Chem. 1999, 111, 1165–1168; Angew. Chem. Int. Ed. 1999,
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6826–6833; g) G. Csjernyik, A. H. ꢂll, L. Fadini, B. Pugin, J-E.
Bäckvall, J. Org. Chem. 2002, 67, 1657–1662; h) K. Yamaguchi, N.
Mizuno, Angew. Chem. 2002, 114, 4720–4724; Angew. Chem. Int.
Ed. 2002, 41, 4538–4542; i) Uozumi, R. Nakao, Angew. Chem.
2003, 115, 204–207; Angew. Chem. Int. Ed. 2003, 42, 194–197; j) P.
Chaudhuri, M. Hess, J Müller, K. Hildenbrand, E. Bill, T. Weyher-
muller, K. Wieghardt, J. Am. Chem. Soc. 1999, 121, 9599–9610;
k) N. Kakiuchi, Y. Maeda, T. Nishimura, S. Uemura, J. Org. Chem.
2001, 66, 6620–6625; l) G. Ragagnin, B. Betzemeier, S. Quici, P.
Knochel, Tetrahedron 2002, 58, 3985–3991; m) R. A. Sheldon,
I. W. C. E. Arends, A. Dijksman, Catalysis Today, 2000, 57, 157–
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Synth. Catal. 2004, 346, 109–119.
=
401.5 nm) and a high speed digital oscilloscope LeCroy 9360 analyzer
(200 ps).
Cyclic voltammetry: Cyclic voltammetry measurements were performed
with An Autolab PGSTAT 20 set-up at room temperature in acetonitrile
under argon with use of a conventional undivided electrochemical cell
and a platinum disc as the working electrode, platinum wire as the auxili-
ary electrode, and calomel as the reference electrode. Redox potentials
were referenced against ferrocenium/ferrocene. In all experiments, the
scan rate was 50 mVsÀ1 and Pr4NBF4 (tetrapropylammonium tetrafluoro-
borate) was used as supporting electrolyte (c = 0.1 molLÀ1).
Acknowledgement
We thank Prof. Penzkoffer for measurements of lifetimes and Miss
Brennan for photostability measurements. Financial support of this work
by the DAAD (International Quality Network—Medicinal Chemistry)
and the Volkswagen Stiftung (Schwerpunktprogramm Elektronentrans-
fer) is acknowledged.
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Received: March 11, 2004
Revised: July 28, 2004
Published online: October 14, 2004
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6231