Photosensitized oxidation of 13C,15N-labeled imidazole derivatives

Article abstract of DOI:10.1021/ja012253d

An efficient synthesis of imidazoles with isotope labeling at different positions of the five-membered ring was developed. We carried out a detailed mechanistic study of the photosensitized oxidation of isotope-labeled imidazole derivatives. A new product, CO₂, was observed in the photooxidation of 2-H,N1-H imidazoles, but not in 2-substitituted imidazoles. The carbon of CO₂ derives from the 2C of imidazole. As shown by ¹⁸O experiments, both oxygen atoms of CO₂ originate mainly from one molecule of oxygen. Transient intermediates were detected by low-temperature NMR in the photosensitized oxidation of the isotope-labeled imidazoles. Quantitative analysis of the ¹³C NMR at different temperatures and times correlates the formation of one intermediate with the loss of another, thus allowing the complete decomposition pathway of the transient intermediates to be established. Singlet oxygen reacts with 4,5-diphenylimidazole via a [4 + 2] cycloaddition to form a 2,5-endoperoxide, which, upon warming, decomposes to a hydroperoxide. The hydroperoxide in one pathway loses water to form an imidazolone 7, which is hydrolyzed to a hydroxyimidazol-2-one 11. In another pathway, the hydroperoxide rearranges to diol 8. The diol rearranges to a carbamate 9 by opening and reclosing the five-membered ring. 9 decomposes to CO₂ and benzil diimine. A labile NH in the imidazole is crucial for the decomposition of the initially formed endoperoxide, otherwise the endoperoxide decomposes to regenerate starting material. Many similarities exist between the photooxidations of imidazole and guanosine in organic solvent, suggesting that the two reactions share a similar reaction mechanism with singlet oxygen.

Full text of DOI:10.1021/ja012253d

Published on Web 07/19/2002
Photosensitized Oxidation of 13C,15N-Labeled Imidazole
Derivatives
Ping Kang and Christopher S. Foote*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of California,
Los Angeles, Los Angeles, California 90095-1569
Received September 27, 2001. Revised Manuscript Received February 26, 2002
Abstract: An efficient synthesis of imidazoles with isotope labeling at different positions of the five-membered
ring was developed. We carried out a detailed mechanistic study of the photosensitized oxidation of isotope-
labeled imidazole derivatives. A new product, CO2, was observed in the photooxidation of 2-H,N1-H
imidazoles, but not in 2-substitituted imidazoles. The carbon of CO2 derives from the 2C of imidazole. As
shown by 18O experiments, both oxygen atoms of CO2 originate mainly from one molecule of oxygen.
Transient intermediates were detected by low-temperature NMR in the photosensitized oxidation of the
isotope-labeled imidazoles. Quantitative analysis of the 13C NMR at different temperatures and times
correlates the formation of one intermediate with the loss of another, thus allowing the complete
decomposition pathway of the transient intermediates to be established. Singlet oxygen reacts with 4,5-
diphenylimidazole via a [4 + 2] cycloaddition to form a 2,5-endoperoxide, which, upon warming, decomposes
to a hydroperoxide. The hydroperoxide in one pathway loses water to form an imidazolone 7, which is
hydrolyzed to a hydroxyimidazol-2-one 11. In another pathway, the hydroperoxide rearranges to diol 8.
The diol rearranges to a carbamate 9 by opening and reclosing the five-membered ring. 9 decomposes to
CO2 and benzil diimine. A labile NH in the imidazole is crucial for the decomposition of the initially formed
endoperoxide, otherwise the endoperoxide decomposes to regenerate starting material. Many similarities
exist between the photooxidations of imidazole and guanosine in organic solvent, suggesting that the two
reactions share a similar reaction mechanism with singlet oxygen.
Scheme 1
Introduction
Imidazole structures exist in many purines such as guanosine,
xanthine, theophylline, and 8-methylcaffeine, and the reactions
with singlet oxygen take place by attack on the imidazole
portion.1 Studies of imidazole model systems should also help
to clarify the mechanism of imidazole breakdown in enzyme
deactivation by photosensitized oxidation. It has been shown
that histidine (His) destruction by singlet oxygen can be
correlated with loss of biological activity of His containing
enzymes.2 Mechanistic studies strongly indicate that the photo-
oxidation of His involves singlet oxygen and that the target of
the oxidation is the imidazole ring of the His residue.
proposed major reaction pathway with singlet oxygen is shown
in Scheme 1. In these studies, CH3OH was the reaction medium
as well as the trapping agent for intermediates. The endoperoxide
was proposed to be the initially formed product. The final
product was the methanol adduct.
Products from the photosensitized oxidation of various
imidazole derivatives and their yields were reported previously3,5-7
and have been compiled by Wasserman et al.4 The total yields
of the isolated products from photooxidation of 2-H-imidazole
derivatives are quite low (9-35%), which implies that many
products are not identified in such reactions. Irie and co-workers
reported 17 compounds from the photosensitized oxidation of
Wasserman and co-workers carried out extensive studies on
a variety of alkyl and aryl-substituted imidazoles.3,4 The
* To whom correspondence should be addressed. E-mail: foote@
chem.ucla.edu.
(1) Matsuura, T.; Saito, I. J. Chem. Soc. Chem. Commun. 1967, 693; Matsuura,
T.; Saito, I. Tetrahedron Lett. 1968, 29, 3273; Matsuura, T.; Saito, I.
Tetrahedron 1968, 24, 6609; Matsuura, T.; Saito, I. Tetrahedron 1969, 25,
541-547; Matsuura, T.; Saito, I. Tetrahedron 1969, 25, 549-556;
Matsuura, T.; Saito, I. Tetrahedron 1969, 25, 557-564.
(2) Chang, S. H.; Teshima, G. M.; Milby, T.; Gillece-Castro, B.; Canova-
Davis, E. Anal. Biochem. 1997, 244, 221-227; Morikawa, S. Y.; Kanatani,
A.; Yoshimoto, T.; Tsuru, D. Agric. Biol. Ch. 1991, 55, 2099-2103; Kuno,
S.; Fukui, S.; Toraya, T. Arch. Biochem. Biophys. 1990, 277, 211-217;
Henschen-Edman, A. H. Cell. Mol. Life Sci. 1997, 53, 29-33; Micanovic,
R.; Procyk, R.; Lin, W. H.; Matsueda, G. R. J. Biol. Chem. 1994, 269,
9190-9194.
(3) Wasserman, H. H.; Druckrey, E. J. Am. Chem. Soc. 1968, 90, 2440.
(4) Wasserman, H. H.; Lipschutz, B. H. In Singlet Oxygen; Wasserman, H.
H., Murray, R. W., Eds.; Academic Press: New York, 1979; pp 430-510.
(5) Wasserman, H. H.; Stiller, K.; Floyd, M. B. Tetrahedron Lett. 1968, 29,
3277-3280.
(6) White, E. H.; Harding, M. J. C. J. Am. Chem. Soc. 1964, 86, 5686.
(7) Sonnenberg, J.; White, D. M. J. Am. Chem. Soc. 1964, 86, 5685.
9
10.1021/ja012253d CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 9629-9638
9629

A R T I C L E S
Kang and Foote
Scheme 2
leading to CO2 is a novel reaction pathway and that more
detailed studies of the photooxidation of imidazoles are needed.
In this contribution, we report the synthesis of 13C,15N-labeled
4,5-diphenylimidazole derivatives and their low-temperature
photosensitized oxidation. A series of transient intermediates
was detected using low-temperature NMR. A detailed reaction
mechanism was deduced from this study. A preliminary account
of the synthesis of a few of these compounds and a partial study
of their reactions has appeared.15
Scheme 3
Results
CO2 Formation. Since no CO2 formation was reported in
previous studies of photosensitized oxidation of imidazole
derivatives but we found CO2 to be a major product of the
oxidation of guanosine, our first attempt was to look for CO2
formation. The photosensitized oxidation of imidazole deriva-
tives was carried out in a 5-mm NMR tube at -80 °C in
acetone-d6 with ca. 5 × 10-5 M of 2,9,16,23-tetra-tert-butyl-
19H,31H-phthalocyanine as sensitizer and a Cermax 300-W
xenon lamp as the light source. A chromium glass filter was
used to cut off wavelengths below 547 nm. After 1 h of
photolysis, the reaction mixture was allowed to warm to room
temperature. CO2 formation, was shown by GC/MS.
Several imidazole derivatives were tested for CO2 formation
and the results are shown in Table 1. These compounds can be
categorized into two groups. The first group consists of 4,5-
diphenylimidazole (entry 1), 4-methylimidazole (entry 2), and
imidazole (entry 3), which are 2-H,N1-H-derivatives. Compris-
ing the second group are 2-substituted (entry 6) or N1-substituted
(entries 4, 5) or 2,N1-disubstituted (entry 7). CO2 was detected
from all the imidazoles in the first group, while very little or
none was detected from the second group. These results strongly
suggest that the rearrangement of N1-H and 2-H of the imidazole
ring are necessary for the formation of CO2 in the photosensi-
tized oxidation of imidazole derivatives.
N-benzoyl histidine at pH 11.8 In contrast, the total yields of
the products from 2-H-substituted imidazole derivatives are
much higher (42%-97%).
Foote and co-workers reported low-temperature NMR studies
of photosensitized oxidation of imidazoles and observation of
2,5-endoperoxides as initial oxidation products.9 The intermedi-
ates have been shown to undergo several subsequent reactions,
depending on the substrates. In some cases, the initial adduct
lost oxygen to regenerate starting material. (Scheme 2)
The photosensitized crosslinking of proteins is involved in
the photodynamic therapy (PDT) of tumors and other diseases.10
His-His crosslinking plays a role in the photosensitized cross-
linking of proteins mediated by singlet oxygen.11 His can also
crosslink with Lys, Cys, Trp, Tyr, and even Arg on photosen-
sitized treatment. Kopecek et al. reported the formation of His-
His crosslinks in the photosensitized oxidation of N-benzoyl-
l-histidine (Bz-His).12 A mechanism for the formation of the
crosslink was proposed in which the first step was the 1,4-
cycloaddition of singlet oxygen to the Bz-His imidazole ring
to give an unstable endoperoxide. The endoperoxide rearranged
by nucleophilic addition and the elimination of one water
molecule to give the crosslinked product. (Scheme 3)
In a previous mechanistic study of the photosensitized
oxidation of guanosine,13 we detected two transient intermediates
from an 8-13C-labeled guanosine derivative by low-temperature
NMR. However, no precursors to the two observed transient
intermediates could be detected directly even down to -100
°C, which suggests that any such precursors are very unstable.
We also identified CO2 as a major product that comes from the
8-position (C2 of the imidazole ring; CO2 was also formed from
other carbons, probably C6) of guanosine. Because of the
structural similarity of imidazole and guanosine, we looked for
reports of CO2 formation in the photosensitized oxidation of
imidazole but found none,5,9,14 which suggests the reaction
The carbon atom in CO2 originates from the C2 of the
imidazole ring. 13CO2 (m/z 45) was shown by GC/MS from the
photosensitized oxidation of 2-13C-4,5-diphenylimidazole. For-
mation of 13CO2 was also confirmed by observation of a 13C
NMR peak at 124.0 ppm.
The origin of the two oxygen atoms of CO2 was studied in
an 18O experiment. The three substrates used in this experiment
were 4,5-diphenylimidazole, 4-methylimidazole, and imidazole.
The resulting CO2 was analyzed by MS, and the distributions
of CO2 isotopes are shown in Table 2. A substantial amount of
48[CO2] was observed in the photosensitized oxidation of all
three substrates. The experimental distributions do not exactly
match the calculated values for either the case where both O
atoms of CO2 derive from the same O2 molecule (2-O in Table
2) or the case where they come from two different O2 molecules
(1-O in Table 2). However, they are closer to the values for
2-O than 1-O. The calculated 48[CO2] values are 1.9-2.6 times
larger than experimental 48[CO2] for 2-O for all three substrates,
but 7-13 times smaller for 1-O. If the two O atoms of CO2
(8) Tomita, M.; Irie, M.; Ukita, T. Tetrahedron Lett. 1968, 47, 4933-4936.
(9) Ryang, H.-S.; Foote, C. S. J. Am. Chem. Soc. 1979, 101, 6683-6687.
(10) Henderson, B. W.; Dougherty, T. J., Eds. Photodynamic Therapy; Marcel
Dekker: New York, 1992; Ochsner, M. J. Photochem. Photobiol., B 1997,
39, 1-18.
(11) Shen, H. R.; Spikes, J. D.; Kopeckova, P.; Kopecek, J. J. Photochem.
Photobiol., B 1996, 35, 213-219; Shen, H. R.; Spikes, J. D.; Kopecekova,
P.; Kopecek, J. J. Photochem. Photobiol., B 1996, 34, 203-210; Balasubra-
manian, D.; Du, X.; Zigler, J. S. Photochem. Photobiol. 1990, 52, 761-
768.
(12) Shen, H. R.; Spikes, J. D.; Smith, C. J.; Kopecek, J. J. Photochem.
Photobiol., A 2000, 130, 1-6.
(13) Kang, P.; Foote, C. S. J. Am. Chem. Soc. 2002, 124, 4865-4873.
(14) Wasserman, H. H.; Wolff, M. S.; Stiller, K.; Saito, I.; Pickett, J. E.
Tetrahedron 1981, 37S1, 191-200; Wasserman, H. H.; Vinick, F. J.; Chang,
Y. C. J. Am. Chem. Soc. 1972, 94, 7180-7182; Wasserman, H. H.; Scheffer,
J. R.; Cooper, J. C. J. Am. Chem. Soc. 1972, 94, 4991-4996; Wasserman,
H. H. Heterocycles 1979, 12, 133-134; Lipshutz, B. H.; Morey, M. C. J.
Org. Chem. 1983, 48, 3745-3750; Graziano, M. L.; Iesce, M. R.; Scarpati,
R. J. Chem. Soc., Chem. Commun. 1979, 7-8.
(15) Kang, P.; Foote, C. S. Tetrahedron Lett. 2000, 41, 9623-9626.
9
9630 J. AM. CHEM. SOC. VOL. 124, NO. 32, 2002

Photosensitized Oxidation of Imidazole Derivatives
A R T I C L E S
Table 1. CO2 Formation in Photosensitized Oxidation of Imidazole Derivatives
0 °C, where 46[CO2] increases much faster than 48[CO2], as a
result of the exchange of 48[CO2] with water. These observations
suggest that intermediates in the reaction decompose to form
mainly 48[CO2] but not 46[CO2] as the temperature rises. After
2 days at room temperature, almost all 48[CO2] exchanges with
water to form 46[CO2]. The 46[CO2] also exchanges with water
at room temperature as the total isotope abundance decreases.
The 18O experiments suggest that both O atoms of CO2 in the
photosensitized oxidation of imidazole derivatives originate
mainly from one O2 molecule but that both mechanisms
contribute to the CO2 formation. The increase in 48[CO2] with
temperature implies that a low-temperature direct CO2 formation
pathway by a different mechanism exists.
Synthesis of 13C,15N-Labeled 4,5-Diphenylimidazole De-
rivatives. For unlabeled 4,5-diphenylimidazole, no transient
intermediates could be detected at low temperature due to low
solubility and low NMR sensitivity of the compound. To
increase the NMR sensitivity we synthesized 13C,15N-labeled
imidazoles. Various synthetic routes were evaluated.16-18 Be-
cause of the limitations on the availability of labeled starting
material, most of the methods are not suitable. We chose a
synthesis based on a procedure reported by Bakibaev et al.16
and Novelli.18 The synthesis involves first heating a mixture of
15N-urea and 13C-formic acid at 150 °C for 4 h. Benzoin was
then introduced, and the reaction mixture was heated at 180 °C
for 3 h. The product was suspended in water. After filtration,
the powder was resuspended in 5% HCl and heated to 80-90
°C. The filtrate was treated with an excess of NH3 to give pure
4,5-diphenylimidazole in 80% yield (Scheme 4). 13C-labeled
benzoin was synthesized by coupling 2 mol of 13C benzaldehyde
in the presence of NaCN (Scheme 4).
Table 2. CO2 Isotope Distribution in Photosensitized Oxidation of
Imidazole Derivativesa.
substrate
44[CO ]:45[CO ]:46[CO ]:48[CO ]
2
2
2
2
4,5-diphenylimidazolec
expb
100:1.19:1.87:2.08
100:1.10:0.44:3.97
100:1.98:7.94:0.16
100:1.21:3.04:2.06
100:1.10:0.50:5.44
100:2.10:10.88:0.29
100:1.16:2.66:2.66
100:1.10:0.46:5.99
100:2.02:11.98:0.36
2-Of,h
1-Og
expb
4-methylimidazoled
imidazolee
2-Of,h
1-Og
expb
2-Of,h
1-Og
a Reaction mixture at rt after photolysis at -80 °C, after correction for
CO2 in air. b exp: experimental results. c Using 18O-enriched oxygen gas
32[O2]:34[O2]:36[O2] ) 100:0.44:3.97. d Using 18O-enriched oxygen gas
32[O2]:34[O2]:36[O2] ) 100:0.50:5.44. e Using 18O-enriched oxygen gas
32[O2]:34[O2]:36[O2] ) 100:0.46:5.99. f 2-O: theoretical CO2 isotope dis-
tribution using experimental O2 isotope distribution based on the assumption
that both O atoms of CO2 are derived from one O2 molecule. g 1-O:
theoretical CO2 isotope distribution using experimental O2 isotope distribu-
tion based on the assumption that the O atoms of CO2 are derived from
two O2 molecules. h According to the supplier, 34[O2] consists of two 17O.
If 34[O2] consists of one 16O and one 18O 2-O would be the same as above
and 1-O are not much different from the above. See Experimental Section
for the details.
This synthetic method is versatile and selective with different,
sometimes labeled, starting materials. We synthesized 2-13C-
4,5-diphenylimidazole (1), 1,3-15N-4,5-diphenylimidazole (2),
2-13C-1,3-15N-4,5-diphenylimidazole (3), and 2,4,5-13C-1,3-15N-
4,5-diphenylimidazole (4). The structures of labeled imidazoles
Figure 1. Change of 46[CO2] and 48[CO2] isotope abundance with
temperature and time. Photooxidation of 4-methylimidazole in acetone. 18
O
1
were confirmed by H NMR, 13C NMR, and high-resolution
enriched oxygen (32[O2]:36[O2] ) 100:33.7) was used for the photooxidation.
MS. In all four cases, the incorporation of isotope into imidazole
is almost 100%. NMR couplings between these isotopes in the
(*) Two days later.
came only from two different O2 molecules, we would not
expect such a significant amount of 48[CO2] (the highest
calculated value for 48[CO2] is 0.36% for this scenario).
One probable reason that the 18O incorporation is lower than
theoretical is that the initially formed 48[CO2] reacts with water,
exchanging one 18O for 16O. The exchange rate should depend
on temperature and should increase at higher temperatures.
Figure 1 shows that the 48[CO2] isotope abundance continues
to increase as the temperature of the reaction increases. The
46[CO2] isotope abundance does not change much until above
(16) Bakibaev, A. A.; Yagovkin, A. Y.; Filimonov, V. D. Zh. Org. Khim. 1991,
27, 1512-1519.
(17) Bredereck, H.; Theilig, G. Chem. Ber. 1953, 86, 88-96; Bredereck, H.;
Gompper, R.; Hayer, D. Chem. Ber. 1959, 92, 338-343; Chimishkyan, A.
L.; Svetlova, L. P.; Leonova, T. V.; Gulyaev, N. D. Zh. Obshch. Khim.
1984, 54, 1477-1481; Cohen, L. A.; D’sa, A. J. Heterocycl. Chem. 1991,
28, 1819-1820; Grimmett, M. R. AdV. Heterocycl. Chem. 1970, 12, 103-
183; Grimmett, M. R. In Imidazoles and Their Benzo DeriVatiVes (iii)
Synthesis and Applications; Katritzky, A. R., Rees, C. W., Potts, K. T.,
Eds.; Pergamon Press: New York, 1984; pp 457-497; Grimmett, M. R.
In Imidazoles; Katritzky, A. R., Rees, C. W., Scriven, E. F. V., Eds.;
Pergamon Press: New York, 1996; pp 79-220.
(18) Novelli, A. Anales Asoc. Quim. Argentina 1952, 40, 112-114; Chem. Abstr.
1953, 147, 9321c.
9
J. AM. CHEM. SOC. VOL. 124, NO. 32, 2002 9631

A R T I C L E S
Kang and Foote
Table 3. Chemical Shifts of the Transient Intermediates.
Scheme 4
imidazole were observed. For example, we observed the splitting
1
of the H peak by 13C (J2H-2C ) 207.0 Hz) and 15N (J2H-N
)
)
10.3 Hz, J1H-1N ) 96.4 Hz), of the 13C peak by H (J2C-2H
1
207.1 Hz) and 15N (J2C-N ) 5.0 Hz) in 2-13C-1,3-15N-
diphenylimidazole, of the 15N peak by H (JN1-H ) 97.2 Hz)
1
in 1,3-15N-diphenylimidazole, of the 13C peak by an adjacent
13C (JC4-C5 ) 71.9 Hz) in 2,4,5-13C-1,3-15N-diphenylimidazole.
The splitting of the 15N peak by 13C was not observed in the
starting material but was detected in the products. These
magnetic interactions are very useful in the identification of
the transient intermediates in the photosensitized oxidation of
imidazole (see next section).
solvents such as acetone-d6 and a mixture of dimethyl sulfoxide
(DMSO-d6) and CD2Cl2 were also used. Some intermediates
are more stable in one solvent system than the other, which
allowed us to make solutions enriched in one or another
intermediate.
1. Characterization of Transient Intermediates. Photosen-
sitized oxidation of 2-13C-4,5-diphenylimidazole at -100 °C
gave one intermediate at -100 °C. There was only one 13C
(C2) peak in the reaction mixture with a chemical shift of 102.2
ppm (Tables 3 and 4). It is a tertiary carbon split into a doublet
by a directly bonded hydrogen (JC-H ) 194.1 Hz). 1H NMR of
the first intermediate shows that the 2-H peak at 6.94 ppm was
also split into a doublet (JH-C ) 195.0 Hz). HMQC also shows
a cross-peak between the C2 peak and the 2-H peak (see
Supporting Information). Introduction of 1,3-15N splits C2 into
a doublet of doublets (JC-N1 ) 5.1 Hz, JC-N3 ) 4.9 Hz), which
suggests C2 is bonded to two nonequivalent 15N. The two 15N
peaks were also split into two doublets (JN1-C ) 5.6 Hz,
JN3-C ) 4.3 Hz). The chemical shifts for the two 15N are 111.1
ppm for N1 and 307.0 ppm for N3, which indicates that the N3
15N is an sp2 nitrogen and the N1 15N is an sp3 nitrogen. The
N1 peak was split into a doublet (JN1-H ) 70.0 Hz) by one
directly bonded hydrogen (1H chemical shift 5.11 ppm). The
Photosensitized Oxidation of Isotope-Labeled Imidazole
Derivatives. To take advantage of the synthetic imidazoles with
isotope labeling at various positions of the five-membered ring,
we designed our experiments according to the following
approach. First, the singly labeled 2-13C-diphenylimidazole was
used in the photosensitized oxidation. The formation of the initial
intermediate and its subsequent decomposition to other inter-
mediates were followed by 13C NMR. Since under our NMR
acquisition conditions only the peaks of 13C-labeled compounds
show up in the carbon spectrum of the reaction mixture, each
individual peak corresponds to one compound, which makes it
easier to identify each transient intermediate and follow its
changes. Using singly labeled 2-13C-diphenylimidazole also
allows us to determine the conditions (solvent, temperature)
under which each individual intermediate is stable. We could
then choose the ideal conditions to study that intermediate in
more detail. Next, we oxidized 2-13C-1,3-15N-4,5-diphenylim-
idazole, which provides additional spectroscopic information
such as 15N chemical shifts and 13C-15N and 15N-1H coupling
constants. Finally, we used 2,4,5-13C-1,3-15N-4,5-diphenylim-
idazole, which yielded more spectroscopic data and allowed us
to fully characterize these transient intermediates. This approach
of using imidazoles with successive isotope labeling allowed
us to obtain increasing structural information on the transient
intermediates without increasing the complexity of interpreting
the spectroscopic data.
1
corresponding splitting of H by N1 (JH-N1 ) 69.7 Hz) was
1
1
also seen in the H NMR of the intermediate. The H-15N
HMQC also confirmed the N1-H structure. The chemical shifts
of C4 and C5 were obtained from the photosensitized oxidation
of 2,4,5-13C-1,3-15N-4,5-diphenylimidazole. Both carbons are
quaternary carbons. The chemical shift of C4 at 172.6 ppm
suggests a CdN structure in the intermediate. The chemical
shift of C5 is 100.3 ppm. C4 and C5 were split by each other
into two doublets (JC4-C5 ) 46.2 Hz). The chemical shifts of
102.2 ppm for C2 and 100.3 ppm for C5 are characteristic of
sp3 carbons directly bonded to two or more heteroatoms. On
the basis of the above spectroscopic information, the initially
formed intermediate is assigned to be a 2,5-endoperoxide 5.
When the reaction mixture was warmed to -88 °C, the 2,5-
endoperoxide began to decompose, and several other intermedi-
Photosensitized oxidation of 13C- or 15N-labeled or 13C,5N-
labeled 4,5-diphenylimidazole was carried out in a 5-mm NMR
tube at -100 °C with ca. 5 × 10-5 M of 2,9,16,23-tetra-tert-
butyl-19H,31H-phthalocyanine as sensitizer and a Cermax
300-W xenon lamp as the light source. A chromium glass filter
was used to cut off wavelengths below 547 nm. The starting
material was dissolved in a solvent of acetone-d6 and trichlo-
rofluoromethane (1:5, ca. 0.04 M). After 30 min of photolysis,
the NMR tube was transferred to a precooled NMR probe. Other
1
ates were formed. (Tables 3 and 4) H NMR of the second
intermediate showed a hydrogen at 11.85 ppm, consistent with
an OOH proton. The chemical shift of C2 is 117.5 ppm, which
9
9632 J. AM. CHEM. SOC. VOL. 124, NO. 32, 2002

Photosensitized Oxidation of Imidazole Derivatives
A R T I C L E S
Table 4. Coupling Constants of the Transient Intermediates
is split into a doublet by a directly bonded hydrogen at 6.82
for the intermediate. However, the dioxirane would have to have
sp3 N-H nitrogens to be at the correct oxidation state, and the
chemical shifts are in the range for sp2 nitrogen and are unsplit
ppm (JC2-H ) 161.2 Hz). The hydrogen at 6.82 ppm is a doublet
1
(JH-C2 ) 160.6 Hz). H-13C HMQC also showed the con-
1
nectivity of this hydrogen to C2 (see Supporting Information).
The C2 peak becomes a triplet in the presence of 15N1 and 15N3
(JC2-N ) 4.7 Hz), which shows that C2 is connected to two
equivalent N. The equivalence of N1 and N3 is confirmed by
the identical chemical shift of 15N1 and 15N3 (both 356.6 ppm).
The 15N peak was also split into a doublet by C2 (JN-C2 ) 4.7
Hz). The chemical shifts of C4 and C5 are also identical (167.0
ppm), showing two CdN structures in the second intermediate.
The 13C intensity ratio of 167.0 ppm (C4 and C5) to 117.5 ppm
(C2) is 2:1. No N1-H signal was visible in the 1H NMR of this
intermediate. The chemical shift of C2 (117.5 ppm) suggests
an sp3 carbon connected to three heteroatoms. We assigned the
second intermediate as hydroperoxide 6 with C2V symmetry.
The third intermediate was formed as the temperature was
raised to -80 °C. It has a symmetry plane, as evidenced by the
identical chemical shifts of C4 and C5 (181.4 ppm) and of N1
and N3 (308.5 ppm). (Tables 3 and 4) These chemical shifts
show there are two CdN structures in the third intermediate.
The chemical shift of C2 is 177.8 ppm, a quaternary carbon,
split into a triplet by 15N1 and 15N3 (JC2-N ) 6.5 Hz). The
corresponding splittings were also seen in 15N1 and 15N3
(JN-C2 ) 6.5 Hz). Both 2-H and N1-H signals were absent in
the third intermediate. The chemical shift of 177.8 ppm suggests
a carbonyl carbon for C2. On the basis of the above information,
the structure of the third intermediate is assigned to be 7.
The fourth intermediate appeared as the reaction was warmed
to -80 °C. It is also symmetrical, since the chemical shifts for
C4 and C5 are at 162.9 ppm and those for N1 and N3 are at
364.2 ppm (Table 3, Table 4). The chemical shifts also suggest
two CdN structures in this intermediate. C2, a quaternary car-
bon at 127.3 ppm, was split into a triplet by 15N1 and 15N3
(JC2-N ) 1.5 Hz), showing the connectivity of C2 to both 15N1
and 15N3. The C2 chemical shift is characteristic of an sp3
carbon directly bonded to four heteroatoms. On the basis of
the above spectroscopic information either a dioxirane or a diol
from the ring opening of dioxirane would be possible structures
by hydrogen. A H-13C HMBC experiment showed a cross-
peak between C2 and two hydrogens at 8.25 ppm (see
Supporting Information). This hydrogen was not directly bonded
to either a carbon or a nitrogen, as no cross-peaks were found
1
1
in either H-13C HMQC or H-15N HMQC. This hydrogen
was thus assigned to be OH. From the spectroscopic evidence,
the fourth intermediate is diol 8.
The fifth intermediate appeared at around -60 °C. It is not
symmetrical. The chemical shift of C2 is 163.7 ppm (Tables 3
and 4). It is a quaternary carbon, split into a doublet by either
15N1 or 15N3 (JC2-N ) 2.3 Hz), indicating that one C2-N bond
is broken in the fifth intermediate. The corresponding splitting
of 15N by C2 is found only in N3 (chemical shift at 288.2 ppm,
JN3-C2 ) 2.3 Hz) but not in N1 (chemical shift at 44.9 ppm).
The chemical shifts of N3 and C5 (189.1 ppm) show a CdN
double bond in the intermediate. N1 is an amino group, since
it is a triplet split by two bonded hydrogens (JN1-H ) 74.0 Hz).
N1-H has a chemical shift of 4.59 ppm and is a doublet
(JH-N1 ) 74.2 Hz). The chemical shift of C4 (100.6 ppm) is
characteristic of an sp3 carbon connected to two heteroatoms,
which indicates C4 is bonded to an oxygen and the N1 nitrogen.
C4 and C5 are split by each other into two doublets (JC4-C5
)
39.0 Hz). The above spectroscopic information suggests that
the five-membered imidazole ring was opened and reclosed to
form the fifth intermediate, assigned to be 9.
2. Transformation of One Intermediate to Another. The
transformation of one intermediate to another was followed by
quantitative analysis of C2 13C NMR peak changes with
temperature and time. When the reaction mixture was warmed
to 253 K (-20 °C), only two intermediates were left, 7 and 9.
Further warming from 253 to 273 K (0 °C) caused the 9 (163.7
ppm) peak to decrease and that of CO2 (124.0 ppm) to increase.
Intermediate 9 decomposed completely to CO2 at 273 K (Figure
2, panel A). As a check, the total intensity of 9 and CO2
remained unchanged. The absorption of 7 (177.8 ppm) did not
change during this process. This analysis clearly shows that most
9
J. AM. CHEM. SOC. VOL. 124, NO. 32, 2002 9633

A R T I C L E S
Kang and Foote
Figure 2. Correlation of decomposition of one intermediate to the formation of another intermediate. (Panel A) Decomposition of intermediate 9 to CO2
and 1,2-diphenylethanediimine. (Panel B) Decomposition of intermediate 8 to form 9. (Panel C) Decomposition of endoperoxide 5 to hydroperoxide 6. (*)
The reference of the relative intensity is acetone-d6 at 206 ppm.
Scheme 5
of the CO2 originates from 9. Intermediate 7 is not involved in
CO2 formation. 9 decomposes to CO2 and another product, 1,2-
diphenylethanediimine (benzil diimine), 10 (Scheme 8). A low-
temperature FAB/MS spectrum shows the molecular ion for 9
(m/z 253.1 [M + H]+) and for benzil diimine (m/z 209.1 [M +
H]+). Benzil diimine was fully characterized by 13C,1H,15N
NMR and 1H-15N HMQC (see Experimental Section for
details). Benzil diimine is not stable at room temperature and
decomposes to a complex mixture, probably via hydrolysis of
compounds did not change. Since we have shown that CO2 is
derived from 9, it is clear that the decomposition of 8 leads to
9, which further decomposes to form CO2.
the diimine and subsequent reactions between these hydrolyzed
products. However, we were able to isolate benzil from the
reaction mixture at room temperature by column chromatog-
raphy. The complexity of the decomposition of benzil diimine
from the photosensitized oxidation of 4,5-diphenylimidazole
may explain why the isolated yields of product from photosen-
sitized oxidation of 2-H imidazoles are usually low.
Intermediate 7 decomposed as we warmed the reaction
mixture to room temperature. 11 was formed from attack of
adventitious water on intermediate 7. 11 is not stable at room
temperature and decomposes to benzil and urea. When we added
methanol at low temperature after the photosensitized oxidation
and warmed it to room temperature, we detected methanol
adduct 12 (Scheme 5) and the water adduct (11) was suppressed.
By following the 13C peak changes of the reaction mixture
from 200 K (-73 °C) to 243 K (-30 °C), we found that
decomposition of intermediate 8 (127.3 ppm) leads to the
formation of intermediate 9 (163.7 ppm). Figure 2 panel B
shows that the intensity decrease of 8 was accompanied by an
increase of those of 9 and CO2. The total intensities of the three
Quantitative analysis of the NMR of the reaction mixture from
180 K (-93 °C) to 195 K (-78 °C) revealed that the
endoperoxide decomposed to form the intermediate with C-2
at 117.5 ppm. Over this temperature range, only these two
intermediates exist in the reaction mixture. Figure 2 panel C
shows that as the 102.2 ppm peak decreases, the 117.5 ppm
peak increases, and the total intensity remains constant.
The lifetimes of the intermediates depend on the solvent
system used. When the photosensitized oxidation was done in
a mixture of CD2Cl2 and DMSO-d6, a predominant amount of
intermediate 8 and only a very small amount of intermediate 6
were formed. 8 was more stable in a mixture of CD2Cl2 and
DMSO-d6 than in either acetone-d6 or CD2Cl2. When the
reaction was done in acetone-d6, we saw a large amount of 6
and a very small amount of 8. The polarity of the solvent or
the water impurity in DMSO-d6 may be responsible for these
differences.
9
9634 J. AM. CHEM. SOC. VOL. 124, NO. 32, 2002

Photosensitized Oxidation of Imidazole Derivatives
A R T I C L E S
Scheme 6
Scheme 8
Scheme 7
Photosensitized Oxidation of N-Methyl-4,5-diphenylim-
idazole. The decomposition of the endoperoxide is triggered
by the rearrangement of the N1-H, since this peak disappeared
as the endoperoxide decomposed. This observation suggested
that substitution of the N-H by N-methyl would stabilize the
endoperoxide. The photosensitized oxidations of N-methyl-4,5-
diphenylimidazole, N-methyl-1,3-15N-4,5-diphenylimidazole, and
N-methyl-2-13C-1,3-15N-4,5-diphenylimidazole were carried out
under similar conditions. The endoperoxide 17 (see Experi-
mental Section for characterization) is much more stable than
the unmethylated endoperoxide and does not decompose until
-28 °C. The decomposition of the endoperoxide regenerated
the starting material (Scheme 6). Adding a methyl to the 1N
position of the imidazole totally changes the reaction pathway
of the endoperoxide rearrangement, which suggests that proton
transfer from the N-H is much more facile than the extrusion
of oxygen.9
Scheme 9
since 7 is not on that pathway, and this reaction would not
explain the oxygen isotope composition of the CO2. A dioxirane
intermediate was suggested in a previous study of photooxida-
tion of an 8-13C guanosine derivative,13 however there is no
evidence for its involvement in this reaction. We cannot provide
a completely satisfactory mechanism at present. One possibility
is shown in Scheme 9. Cleavage of the C2-H bond in the
presence of a trace amount of base gives carbanion 18, which
can be stabilized by several resonance structures and the 6-π-
electron aromatic ring system. 1,2-Shift of the carbanion results
in 19. Protonation of 19 gives diol 8. This mechanism is
consistent with the fact that the two oxygen atoms of CO2 come
from one molecular oxygen. Dehydration of 8 to 7 would be
inhibited by the fact that 7 is a cyclopentadienone.
Discussion
On the basis of the structures of the various intermediates
and the information on transformation of one intermediate to
another, we propose a mechanism for the photosensitized
oxidation of 4,5-diphenylimidazole (Scheme 8). Singlet oxygen
reacts with 4,5-diphenylimidazole via a [4 + 2] cycloaddition
to form the initial product 2,5-endoperoxide 5. Upon warming
to 185 K (-88 °C), the 2,5-endoperoxide decomposes to form
hydroperoxide 6.
A hydroperoxide of 2,4,5-triphenylimidazole (lophine) was
shown to be the precursor to the light emitter for the chemilu-
minescence in the reaction of lophine with air in the presence
of base.7 The hydroperoxide was also prepared from the
photosensitized oxidation of lophine in the presence of meth-
ylene blue and oxygen and underwent decomposition with
chemiluminescence through the dioxetane6 (Scheme 7). We did
not observe dioxetane formation in the decomposition of
hydroperoxide 6, which indicates that 6 decomposes by a
different reaction pathway. Another difference is the position
of the peroxidic group. In lophine hydroperoxide the peroxidic
group is on the C4 position of the imidazole ring, whereas in 6
the peroxidic group is on the C2 position.
The rearrangement of the diol to 9 probably proceeds via
the formation of a ring-opened carbamate intermediate that
recloses the ring to form 9, which finally decomposes to CO2
and benzil diimine.
The mechanistic studies of photosensitized oxidation of
guanosine and 4,5-diphenylimidazole show many similarities.
(1) In both reactions, CO2 is generated, and the carbon of CO2
originates from the C2 of the five-membered imidazole ring.
The oxygen atoms of CO2 come from one molecule of oxygen.
(2) CO2 was only observed in the photosensitized oxidation of
the 2-H imidazoles. There was no CO2 formation in the
photosensitized oxidation of 2-substituted imidazoles such as
2-methyimidazole. Substitution of the 8-H of guanosine by a
methyl group did not yield any CO2 in the photosensitized
oxidation.19 (3) The final products from reactions in organic
solvent both have an imidazole ring-opened diimine structure
or its tautomer: benzil diimine 10 in the reaction of imidazole
The hydroperoxide in one pathway loses water to form 7,
which is hydrolyzed to 11. In another pathway, the hydroper-
oxide 6 decomposes to diol 8. As evidence for this suggestion,
the 2,5-endoperoxide decomposes quantitatively to form the
hydroperoxide before the appearance of other intermediates, and
the hydroperoxide always decomposes before the diol.
The decomposition of the hydroperoxide to diol is unprec-
edented. The diol does not come from addition of water to 7
(19) Sheu, C.; Foote, C. S. J. Am. Chem. Soc. 1993, 115, 10446-10447.
9
J. AM. CHEM. SOC. VOL. 124, NO. 32, 2002 9635

A R T I C L E S
Kang and Foote
Scheme 10
Scheme 11
and an imidazole ring-opened product 13 in the reaction of
guanosine (Scheme 10). (4) The intermediates that decompose
to CO2 in the photosensitized oxidation of guanosine and in
that of imidazole both have carbamate structures (15 and 16 in
guanosine and 9 in imidazole, Scheme 10) Whether the carbamic
acid intermediates in the reaction of guanosine is the ring-opened
or -closed form is not clear at present. (5) An endoperoxide
was proposed to be the initially formed product in the photo-
sensitized oxidation of guanosine, and the endoperoxide was
directly detected in the reaction of imidazole. These similarities
suggest that the mechanism of photosensitized oxidation of
guanosine in nonpolar organic solvent probably proceeds
similarly to that of 4,5-diphenylimidazole in part.
A major product from photosensitized oxidation of guanosine
in nonpolar organic solvent is 13, whereas the main products
from photosensitized oxidation of 2′-deoxyguanosine in aqueous
Conclusions
A detailed mechanistic study of the photosensitized oxidation
of isotope-labeled imidazole derivatives was carried out, and a
mechanism was proposed on the basis of this study. Singlet
oxygen initially reacts with 4,5-diphenylimidazole via a [4 +
2] cycloaddition to form a 2,5-endoperoxide. Upon warming,
the 2,5-endoperoxide decomposes to form the hydroperoxide
which decomposes in two pathways. In one pathway, the
hydroperoxide loses water to form 7, which is hydrolyzed to
11. In another pathway, the hydroperoxide decomposes to diol
8. The diol rearranges to 9 by opening and reclosing the five-
membered ring. 9 decomposes to CO2 and benzil diimine 10.
A labile NH in the imidazole is required for the decomposition
of the initially formed endoperoxide. Otherwise the endoper-
oxide decomposes back to starting material. A comparison with
the reaction of guanosine derivatives in organic solvent shows
many similarities between the two reactions such as the
structures of the final relatively stable products, structures of
carbamate intermediates that decompose to CO2 and so forth,
suggesting that the two reactions share a similar reaction
mechanism with singlet oxygen.
20-22
medium are the diastereomers of spiroiminodihydantoin.
In both cases, the yields are less than 50%. The overall reaction
is analogous to the photosensitized oxidation of 4,5-diphen-
ylimidazole, where the common intermediate is the 2,5-
endoperoxide and two reaction pathways lead to two different
products: benzil diimine and 4,5-diphenyl-4-hydroxyimidazol-
2-one. The reaction pathway to benzil diimine is probably
similar to the pathway to 13; there are many similarities between
the two pathways. If the reaction pathway to 4,5-diphenyl-4-
hydroxyimidazol-2-one is the same in the guanosine reaction,
4,8-dihydro-4-hydroxy-8-oxo-guanosine (D, Scheme 11) or 5,8-
dihydro-5-hydroxy-8-oxo-guanosine (E, Scheme 11) or both
would be formed. E has been proposed to be the intermediate
in both the peroxynitrite oxidation22 and in the one-electron
oxidation21 of 7,8-dihydro-8-oxo-guanosine, which contracts the
six-membered ring to the spiroiminodihydantoin. D and E can
be formed by water attacking the CdN at 4 (pathway d) or 5
(pathway c) position of an intermediate C which has a structure
similar to that of 7. D can also be formed directly from the ring
opening of the endoperoxide A (pathway a) as previously
proposed by Cadet.
Experimental Section
1H NMR and 13C NMR spectra were recorded on Bruker ARX-400,
ARX-500, and Avance 500 spectrometers. Low-temperature NMR
spectra were taken in a pre-cooled probe maintained at the desired
temperature. The solvents for low-temperature NMR were acetone-d6,
a mixture of DMSO-d6 and CD2Cl2, and CCl3F with several drops of
acetone-d6 added as locking solvent. GC/MS spectra were recorded on
a HP G1800 A GCD with HP-5 GC column (crosslinked 5% PH ME
siloxane, 30 m × 0.25 mm × 0.25 µm). Positive ion FAB mass spectra
were collected on a VG ZAB-SE reverse geometry, extended mass
range, magnetic sector mass spectrometer. Thin-layer chromatography
(TLC) was done using either DC-Fertigplatten Kieselgel 60F254 or
DC-Plastikfolien Kieselgel 60 F254 from E. Merck. Column chroma-
tography was performed on silica gel 60, 70-230 mesh or 230-400
mesh from E. Merck.
An important difference between the imidazole and guanosine
oxidations is that in imidazole, the labile proton is the N1-H,
whereas in the guanosine, it is the C8-H; this leads to the major
difference in the first ring-opened intermediate (B in guanosine),
since, like the N-methyl derivatives of histidine, the endoper-
oxide from the 8-methyl derivative of guanosine is relatively
stable and only loses oxygen.19
(20) Ravanat, J. L.; Cadet, J. Chem. Res. Toxicol. 1995, 8, 379-388; Luo, W.
C.; Muller, J. G.; Rachlin, E. M.; Burrows, C. J. Org. Lett. 2000, 2, 613-
616; Adam, W. Personal communication, 2001.
(21) Luo, W. C.; Muller, J. G.; Rachlin, E. M.; Burrows, C. J. Chem. Res.
Toxicol. 2001, 927-938.
(22) Niles, J. C.; Wishnok, J. S.; Tannenbaum, S. R. Org. Lett. 2001, 3, 963-
966.
9
9636 J. AM. CHEM. SOC. VOL. 124, NO. 32, 2002

Photosensitized Oxidation of Imidazole Derivatives
A R T I C L E S
Materials. 13C-Carbonyl benzaldehyde, urea-15N2, and formic acid-
13C were from Isotec Inc. Deuterated solvents were from Cambridge
Isotope Laboratory. NaCN, HCl, NH4OH, K2CO3 were from Fisher
Scientific. Dimethyl carbonate, 18-crown-6-imidazole, 1-methylimid-
azole, 1,2-methylimidazole, 4-methylimidazole, and 2,4,5-triphenylim-
idazole were from Aldrich. 4,5-diphenylimidazole was from Lancaster.
Preparation of 1,2-13C-Benzoin. To 0.65 mL of ethanol (95%), 0.5
mL of water, and 0.5 g of NaCN in a 10-mL round-bottomed flask
was added 0.5 g of 13C-Carbonyl benzaldehyde (0.5 g).. The reaction
mixture was heated to boiling (90-95 °C). After 0.5 h reaction, the
solution was cooled with an ice bath and filtered with suction. The
resulting solid product was washed with water and dried to give 1,2-
13C-benzoin 0.33 g (66% yield).
1H NMR (CDCl3 δ 7.32 ppm) 7.89 (m, 4H, phenyl ortho-H), 7.49
(m, 2H, phenyl para-H), 7.39 (m, 4H, phenyl meta-H), 5.93 (dd,
1JC-H ) 146.7 Hz, 3JC-H ) 2.91 Hz, 1H, HO-C-H). 13C NMR (CDCl3,
δ 77.0 ppm) 198.9 (d, 1C, CdO, JC-C ) 39.1 Hz), 76.2 (dd, 1C, C-OH,
JC-H ) 147.3 Hz, JC-C ) 39.1 Hz). HRMS-EI (m/z): 214.0899
(Calculated 214.0904).
General Procedure for Preparation of Isotope-Labeled 4,5-
Diphenylimidazole. A mixture of urea (1.2 g) and formic acid (1.2 g
95%) in a 10-mL round-bottomed flask was heated in an oil bath at
150 °C for 4 h. After cooling it down to 70 °C, 1.05 g of benzoin was
added, and the reaction mixture was heated at 180 °C for 3 h. The
reaction mixture was poured into 70 mL of water and stirred vigorously
to dissolve the gummy product. The resulting powder was filtered,
washed with water, and suspended in 5% HCl (50 mL). The solution
was heated to 80-90 °C and filtered hot, and the acidic filtrate was
treated with an excess of NH4OH to form a white precipitate, which
was filtered and dried to give 0.86 g of 4,5-diphenylimidazole (80%
yield based on starting benzoin).
2-13C-4,5-Diphenylimidazole, 1. The starting materials were urea,
formic acid-13C, and benzoin. 1H NMR (CDCl3, δ 7.32 ppm) 7.98, 7.46
(d, 1H, C2-H, JC2-H ) 206.9 Hz), 7.51 (m, 4H, phenyl), 7.32 (m, 6H,
phenyl), 3.36 (s, broad, 1H, N1-H). 13C NMR (acetone-d6, δ 206.0 ppm)
134.6 (d, 1C, C2, JC2-H ) 207.0 Hz), 127.8, 127.2, 125.5 (phenyl).
HRMS-EI (m/z): 221.1032 (Calculated 221.1034).
1H NMR (CDCl3, δ 7.31 ppm) 7.62 (s, 1 H, C2-H), 7.58-7.17 (m,
10 H, phenyl), 3.54 (s, 3H, NCH3). 13C NMR (CDCl3, δ 77.0 ppm)
138.28 (C5), 137.39 (C2), 134.64 (C4), 130.64, 130.57, 128.96, 128.88,
128.56, 128.08, 126.58, 126.27, (phenyl), 31.11 (NCH3).
N-Methyl-2-13C-1,3-15N-4,5-diphenylimidazole. The starting mate-
rial was 1,3-15N-4,5-diphenylimidazole. 1H NMR (CDCl3, δ 7.33 ppm)
7.58 (dd, 1 H, C2-H, JC2-H ) 206.0 Hz, JN1-H ) 11.2 Hz, JN3-H ) 8.1
Hz), 7.54-7.17 (m, 10 H, phenyl), 3.54 (s, 3H, NCH3). 13C NMR
(CDCl3, δ 77.0 ppm) 138.28 (C5), 137.39 (ddd, C2, JC-H ) 206.0 Hz,
JC-N1 ) 11.4 Hz, JC-N3 ) 0.5 Hz), 134.64 (C4), 130.69, 129.01, 128.62,
128.14, 126.64, 126.62, 126.34, 32.22 (dd, JC-N1 ) 11.1, JC-N3 ) 1.9
Hz). 15N NMR (acetone-d6, urea-15N2 δ 73.4 ppm) 252.7 (N3), 162.8
(d, N1, JN-C ) 11.7 Hz). HRMS-EI (m/z): 237.1126 (Calculated
237.1132).
Photosensitized Oxidation of Imidazole Derivatives. Photosensi-
tized oxidations of ca. 4 mg of isotope-labeled 4,5-diphenyimidazole
derivative were carried out in a 5-mm NMR tube at -100 °C with ca.
5 × 10-5 M of 2,9,16,23-tetra-tert-butyl-19H,31H-phthalocyanine as
sensitizer and a Cermax 300-W xenon lamp as the light source. A
chromium glass filter was used to cut off wavelengths below 547 nm.
The starting material was dissolved in a solvent of acetone-d6 and
trichlorofluoromethane (1:5). Dry O2 was bubbled into the solution
continuously. After 30 min of photolysis, the NMR tube was transferred
to a precooled NMR probe. Other solvents such as acetone-d6 and a
mixture of dimethyl sulfoxide (DMSO-d6) and CD2Cl2 were also used
in the reaction. Low-temperature NMR spectra were taken on the Bruker
ARX 500 or Avance 500. The temperature range was -100 °C to room
temperature. Photooxidations were also carried out in acetone-d6,and
a mixture of CD2Cl2 and DMSO-d6 at -80 °C.
Quantitative analysis of the 13C NMR spectrum was carried out by
integrating 1H-coupled 13C NMR peaks. A control experiment showed
that intensity of CO2 did not change much from -90 to -10 °C, which
suggests that CO2 was still dissolved in the solvent. Since we were
1
following the same carbons in the reaction mixture, integrating H-
coupled 13C NMR peaks can provide reliable information about
concentration changes in the reaction mixture.
1
1,3-15N-4,5-Diphenylimidazole, 2. The starting materials were urea-
15N2, formic acid, and benzoin. 1H NMR (acetone-d6, δ 2.07 ppm) 7.76
(t, 1H, C2-H, JN-H ) 10.3 Hz), 7.55 (m, 4H, phenyl), 7.27-7.36 (m,
6H, phenyl), 5.13 (d, 1H, N1-H, JH-N1 ) 88.4 Hz). 15N NMR (DMSO-
Intermediate 5: H NMR (acetone-d3, δ 2.06 ppm) 6.94 (d, 1 H,
C2-H, JC2-H ) 195.0 Hz), 7.77-7.34 (m, 10 H, phenyl), 5.11 (d, 1
H, N1-H, JN1-H ) 69.7 Hz). 13C NMR (acetone-d3, δ 206.0 ppm) 102.2
(ddd, C2, JC-H ) 194.1 Hz, JC-N1 ) 5.1 Hz, JC-N3 ) 4.9 Hz), 100.3
(d, C5, JC4-C5 ) 46.2 Hz), 172.6 (d, C4, JC5-C4 ) 46.2 Hz). 15N NMR
(urea-15N2 δ 73.4 ppm) 307.0 (d, N3, JN-C ) 5.6 Hz), 111.1 (dd, N1,
JN1-H ) 70.0 Hz, JN3-C ) 4.3 Hz).
Intermediate 6: 1H NMR (acetone-d3, δ 2.06 ppm) 11.85 (s, OOH),
6.82 (d, 1 H, C2-H, JC2-H ) 160.6 Hz), 7.77-7.34 (m, 10 H, phenyl).
13C NMR (acetone-d3, δ 206.0 ppm) 117.5 (dt, C2, JC-H ) 161.2 Hz,
JC-N ) 4.7 Hz), 167.0 (s, C4, C5). 15N NMR (urea-15N2 δ 73.4 ppm)
356.6 (d, N1, N3, JN-C ) 4.7 Hz).
d6, urea-15N2 δ 73.4 ppm) 261.5 (1N, N3), 170.1 (1N, N1, JN1-1H
97.2 Hz). HRMS-EI (m/z): 222.0939 (Calculated 222.0941).
)
2-13C-1,3-15N-4,5-Diphenylimidazole, 3. The starting materials were
1
urea-15N2, formic acid-13C, and benzoin. H NMR (DMSO-d6, δ 2.49
ppm) 12.46 (d, 1H, N1-H, JN1-H ) 78.9 Hz), 7.96, 7.55 (dt, 1H, C2-H,
JC2-H ) 206.7 Hz, JN-H ) 10.2 Hz), 7.43 (m, 4H, phenyl), 7.32, 7.25
(m, 6H, phenyl). 13C NMR (DMSO-d6, δ 39.5 ppm, H-decoupled) 135.6
(t, 1C, C2, JC2-N ) 5.0 Hz), 128.5, 127.5, 126.4 (phenyl). 15N NMR
(DMSO-d6, urea-15N2 δ 73.4 ppm) 263.6 (1N, N3), 170.9 (1N, N1).
HRMS-EI (m/z): 223.0968 (Calculated 223.0976).
Intermediate 7: 1H NMR (acetone-d3, δ 2.06 ppm) 7.77-7.34 (m,
10 H, phenyl). 13C NMR (acetone-d3, δ 206.0 ppm) 177.8 (t, C2,
JC-N ) 6.5 Hz), 181.4(s, C4, C5. 15N NMR (urea-15N2 δ 73.4 ppm)
308.5 (d, N1, N3, JN-C ) 6.5 Hz).
2,4,5-13C-1,3-15N-4,5-Diphenylimidazole, 4. The starting materials
were urea-15N2, formic acid-13C, and 1,2-13C-benzoin. 1H NMR (DMSO-
d6, δ 2.50 ppm) 12.28, 12.09 (d, 1H, N1-H, JN1-H ) 93.0 Hz), 7.68 (d,
1H, C2-H, JC2-H ) 206.9 Hz), 7.53-7.25 (m, 10H, phenyl H). 13C
NMR (DMSO-d6, δ 39.5 ppm) 134.6 (d, 1C, C2, JC2-H ) 207.0 Hz),
135.9 (d, 1C, C5, JC4-C5 ) 91.4 Hz), 126.1 (d, 1C, C4, JC5-C4 ) 91.0
Hz). 15N NMR (DMSO-d6, urea-15N2 δ 73.4 ppm) 261.7 (1N, N3), 170.1
(1N, N1). HRMS-EI (m/z): 225.1042 (Calculated 225.1042).
Preparation of N-Methyl-4,5-diphenylimidazole. A mixture of 1.0
g of K2CO3, 0.2 g of 4,5-diphenylimidazole, 2.5 mL of dimethyl
carbonate, and 12 mg of 18-crown-6 in a 10-mL round-bottomed flask
was heated at 100 °C for 10 h. Dimethyl carbonate was removed by
evaporation under vacuum. The residue was extracted with ether, and
rotary evaporation gave 0.18 g of N-methyl-4,5-diphenylimidazole (85%
yield).
1
Intermediate 8: H NMR (acetone-d3, δ 2.06 ppm) 8.25 (s, 2 H,
OH), 7.77-7.34 (m, 10 H, phenyl). 13C NMR (acetone-d3, δ 206.0
ppm) 127.3 (t, C2, JC-N ) 1.5 Hz), 162.9 (s, C4, C5). 15N NMR (urea-
15N2 δ 73.4 ppm) 364.2 (N1, N3).
1
Intermediate 9: H NMR (acetone-d3, δ 2.06 ppm) 4.59 (d, 1 H,
N1-H, JH-N1 ) 74.2 Hz), 7.77-7.34 (m, 10 H, phenyl). 13C NMR
(acetone-d3, δ 206.0 ppm) 163.7 (d, C2, JC-H ) 2.3 Hz), 100.6 (d, C5,
JC5-C4 ) 39.0 Hz), 189.1 (d, C4, JC4-C5 ) 39.0 Hz). 15N NMR (urea-
15N2 δ 73.4 ppm) 288.3 (d, N3, JN-C ) 2.3 Hz), 44.9 (t, N1, JN1-H
)
74.0 Hz).
1,2-Diphenylethanediimine 10: 1H NMR (acetone-d3, δ 2.06 ppm)
10.31 (d, 1 H, N-H, JH-N ) 54.3 Hz), 7.77-7.34 (m, phenyl H). 13C
9
J. AM. CHEM. SOC. VOL. 124, NO. 32, 2002 9637

A R T I C L E S
Kang and Foote
NMR (acetone-d3, δ 206.0 ppm) 174.5 (C4, C5). 15N NMR (urea-15N2
δ 73.4 ppm) 302.1 (d, N1, N3, JN-H ) 54.0 Hz). FAB/MS (m/z [M +
H]+): 209.1.
4,5-Diphenyl-4-hydroxyimidazol-2-one: 13C NMR (acetone-d3, δ
206.0 ppm) 11a 166.6 (dd, C2, JC2-N1 ) 15.2 Hz, JC2-N3 ) 8.9 Hz),
182.9 (d, C4, JC4-C5 ) 38.1 Hz), 98.7 (dt, C5, JC5-C4 ) 39.2 Hz,
2-O: 4,5-diphenylimidazole 44[CO2]:45[CO2]:46[CO2]:48[CO2] )
100:1.19:1.87:2.08
4-Methylimidazole 44[CO2]:45[CO2]:46[CO2]:48[CO2] )
100:1.10:0.50:5.44
Imidazole 44[CO2]:45[CO2]:46[CO2]:48[CO2] )
100:1.10:0.46:5.99
1-O is the same as above except for the 46[CO2] abundance.
JC4-N ) 10.3 Hz); 11b 165.8 (dd, C2, JC2-N1 ) 16.7 Hz, JC2-N3
)
10.7 Hz), 181.0 (d, C4, JC4-C5 ) 31.6 Hz), 90.8 (dt, C5, JC5-C4 ) 31.6
Hz, JC4-N ) 11.1 Hz).
4,5-Diphenyl-4-methoxyimidazol-2-one 12: 13C NMR (acetone-d3,
δ 206.0 ppm) 165.8 (dd, C2, JC2-N1 ) 17.4 Hz, JC2-N3 ) 9.8 Hz),
186.8 (d, C4, JC4-C5 ) 37.7 Hz), 95.7 (dt, C5, JC5-C4 ) 37.8 Hz,
JC4-N ) 11.8 Hz). 15N NMR (urea-15N2 δ 73.4 ppm): 303.2 (N3), 120.3
(N1, JN1-C2 ) 16.6 Hz).
1-O: 4,5-diphenylimidazole 44[CO2]:45[CO2]:46[CO2]:48[CO2] )
100:1.19:8.38:0.17
4-Methylimidazole 44[CO2]:45[CO2]:46[CO2]:48[CO2] )
100:1.10:11.38:0.32
2,5-Endoperoxide of N-Methyl-2-13C-1,3-15N-4,5-diphenylimida-
zole 17: (acetone-d3, δ 2.07 ppm) 7.61-7.22 (m, 10H, phenyl-H), 6.97,
6.58 (dd, 1H, C2-H JC2-H ) 194.6 Hz, JN-H ) 3.9 Hz), 3.84 (d, 3H,
N-CH3, 3JN-H ) 11.7 Hz). 13C NMR (acetone-d3, δ 206.0 ppm): 174.6
Imidazole 44[CO2]:45[CO2]:46[CO2]:48[CO2] )
100:1.10:12.44:0.39
1
Acknowledgment. We thank the NSF for support from Grant
No. CHE-9730386. The Avance 500 NMR was supported by
NSF CHE-9974928.
(d, 1C, C4, JC-N ) 6.5 Hz), 105.2 (dd, 1C, C2, JC2-H ) 194.2 Hz,
JC2-N ) 4.4 Hz), 101.3 (s, 1C, C5), 130.8, 129.2,128.7,128.6,128.3,
128.2, 127.6, 127.4, 126.8, 126.7, 125.5 (12C, phenyl C), 67.7 (s, 1C,
N-CH3). 15N NMR (urea-15N2 δ 73.4 ppm): 310.6 (N1), 104.6 (N3).
Supporting Information Available: Detailed CO2 distribution
CO2 Distribution and 18O2 Tracer Study. Photosensitized oxidation
was carried out in the same manner as above. After the solution in a
5-mm NMR tube was purged with argon, O2 gas was introduced into
the solution by a gastight syringe every 15 min. The NMR tube was
capped with a septum rubber cap. After completion of photolysis, the
NMR tube was slowly warmed to the desired temperature. A gas sample
above the solution was taken with a 25-µL gastight syringe and analyzed
by GC/MS. The data shown in Table 2 are the mean values of 4-5
determinations that are within (5% from the mean value. See
Supporting Information for the detailed calculation of the theoretical
CO2 distribution.
1
1
calculations, H-15C HMQC of 5 and 6, H,13C NMR of 1,2-
13C-benzoin, 1H,13C,15N NMR of 2,4,5-13C-1,3-15N-4,5-diphen-
1
ylimidazole, H,13C,15N NMR of N-methyl-2-13C-1,3-15N-4,5-
diphenylimidazole, 13C,15N NMR of reaction mixture of 2,4,5-
13C-1,3-15N-4,5-diphenylimidazole at 193 K, 15N NMR of 2,5-
endoperoxide 5, H-15N HMQC of 5 and 9, H-13C HMBC
of 2,5-endoperoxide 5 and diol intermediate 8, 13C NMR and
1H-15N HMQC of 1,2-diphenylethanediimine 10, 13C,15N NMR
of 4,5-diphenyl-4-methoxyimidazol-2-one 12, 13C NMR of 4,5-
diphenyl-4-hydroxyimidazol-2-one 11 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
1
1
In the above calculations we assume that 34[O2] consists of two 17O.
34[O2] could also consist of one 16O and one 18O. In that case the 2-O
result is the same as above.
JA012253D
9
9638 J. AM. CHEM. SOC. VOL. 124, NO. 32, 2002