Mechanistic investigations into the photochemistry of 4-allyl-tetrazolones in solution: A new approach to the synthesis of 3,4-dihydro-pyrimidinones

Article abstract of DOI:10.1021/jo060164j

Photolysis (λ = 254 nm) of 4-allyl-tetrazolones 2a-c was carried out in methanol, 1 -propanol, 1-hexanol, acetonitrile, and cyclohexane. The sole primary photochemical process identified was molecular nitrogen elimination, with formation of pyrimidinones 6a-c. Following the primary photocleavage, secondary reactions were observed in acetonitrile and cyclohexane, leading to phenyl-isocyanate (7), aniline (9), and 1-phenylprop-1-enyl-isocyanate (10a). In alcoholic solutions, the primary products, 6a-c, remained photostable even under extended irradiation, making possible the isolation of 3,4-dihydro-pyrimidinones as stable compounds in very high yields. The observed photostability of pyrimidinones 6a-c in alcohols is ascribed to the excited state quenching via reversible proton transfer, facilitated by the solvent cage stabilization due to formation of hydrogen bonds. The viscosity of alcohols is directly related to the cage effects observed. The photocleavage of 4-allyl-tetrazolones leads probably to a caged triplet radical pair. This hypothesis is confirmed by the solvent viscosity effect on the photolysis quantum yields. Additionally, dissolved molecular oxygen sensitizes the formation of pyrimidinones, as should be expected for a triplet intermediate that can only form the product molecule after T-S conversion, which is accelerated by oxygen.

Full text of DOI:10.1021/jo060164j

Mechanistic Investigations into the Photochemistry of
4-Allyl-tetrazolones in Solution: A New Approach to the Synthesis
of 3,4-Dihydro-pyrimidinones
Lu´ıs M. T. Frija,† Igor V. Khmelinskii,‡ and M. Lurdes S. Cristiano*,†
DQB, F.C.T., and CCMAR, UniVersidade do AlgarVe, Campus de Gambelas, 8005-039 Faro, Portugal,
and DQB, F.C.T., and C.M.Q.A., UniVersidade do AlgarVe, Campus de Gambelas,
8005-039 Faro, Portugal
mcristi@ualg.pt
ReceiVed January 24, 2006
Photolysis (λ ) 254 nm) of 4-allyl-tetrazolones 2a-c was carried out in methanol, 1-propanol, 1-hexanol,
acetonitrile, and cyclohexane. The sole primary photochemical process identified was molecular nitrogen
elimination, with formation of pyrimidinones 6a-c. Following the primary photocleavage, secondary
reactions were observed in acetonitrile and cyclohexane, leading to phenyl-isocyanate (7), aniline (9),
and 1-phenylprop-1-enyl-isocyanate (10a). In alcoholic solutions, the primary products, 6a-c, remained
photostable even under extended irradiation, making possible the isolation of 3,4-dihydro-pyrimidinones
as stable compounds in very high yields. The observed photostability of pyrimidinones 6a-c in alcohols
is ascribed to the excited state quenching via reversible proton transfer, facilitated by the solvent cage
stabilization due to formation of hydrogen bonds. The viscosity of alcohols is directly related to the cage
effects observed. The photocleavage of 4-allyl-tetrazolones leads probably to a caged triplet radical pair.
This hypothesis is confirmed by the solvent viscosity effect on the photolysis quantum yields. Additionally,
dissolved molecular oxygen sensitizes the formation of pyrimidinones, as should be expected for a triplet
intermediate that can only form the product molecule after T-S conversion, which is accelerated by
oxygen.
Introduction
in research areas of major interest.3 The tetrazolic ring is also
featured in the structure of many highly efficient drugs.4-6
wide range of tetrazole derivatives has been patented for
A
Tetrazole (CN4H2)1 and its derivatives have drawn widespread
attention as a result of their practical applications. The tetrazolic
acid fragment, -CN4H, has similar acidity to the carboxylic
acid group, -CO2H, and is almost isosteric with it but is more
stable metabolically.2 Hence, replacement of -CO2H by -CN4H
groups in biologically active molecules has been widely applied
(2) (a) Singh, H.; Chawla, A. S.; Kapoor, V. K.; Paul, D.; Malhotra, R.
K. Prog. Med. Chem. 1980, 17, 151. (b) Kimura, T.; Shuto, D.; Hamada,
Y.; Igawa, N.; Kasai, S.; Liu, P.; Hidaka, K.; Hamada, T.; Hayasshi, Y.;
Kiso, Y. Bioorg. Med. Chem. Lett. 2005, 15, 211.
(3) Noda, K.; Saad, Y.; Kinoshita, A.; Boyle, T. P.; Graham, R. M.;
Husain, A.; Karnik, S. S. J. Biol. Chem. 1995, 270, 2284.
(4) Mavromoustakos, T.; Kolocouris, A.; Zervou, M.; Roumelioti, P.;
Matsoukas, J.; Weisemann, R. J. Med. Chem. 1999, 42, 1714.
(5) Toney, J. H.; Fitzgerald, P. M. D.; Groversharma, N.; Olson, S. H.;
May, W. J.; Sundelof, J. G.; Vanderwall, D. E.; Cleary, K. A.; Grant, S.
K.; Wu, J. K.; Kozarich, J. W.; Pompliano, D. L.; Hammond, G. G. Chem.
Biol. 1998, 5, 185.
* Corresponding author. Phone: (+351) 289800100, ext. 7642. Fax: (+351)
289819403.
† CCMAR, Universidade do Algarve.
‡ C.M.Q.A., Universidade do Algarve.
(1) Bugalho, S. C. S.; Mac¸oˆas, E. M. S.; Cristiano, M. L. S.; Fausto, R.
Phys. Chem. Chem. Phys. 2001, 3, 3541.
10.1021/jo060164j CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/31/2006
J. Org. Chem. 2006, 71, 3583-3591
3583

Frija et al.
antihypertension activity and as angiotensin II receptor antago-
nists, useful for treating congestive heart failure and in prevent-
ing cardiac hypertrophy. The tetrazolic ring has also been used
to modify the structure of the heme environment in myoglobin
through incorporation of the N-tetrazol-5-yl histidine unit.7 The
3′-tetrazolo-3′-deoxythymidines were the first approved drugs
for AIDS treatment, and several tetrazole derivatives have been
explored for antituberculotic activity.8,9 Furthermore, a wide
range of compounds with the tetrazol-1-yl acetic acid structure
have been claimed as aldose reductase inhibitors for the
treatment and prevention of diabetes complications. Tetrazoles
are also used as artificial sweeteners, plant growth regulators,
herbicides and fungicides,10 in photography,11 and as gas-
generating agents in airbags.12
FIGURE 1. Hydrogenolysis and thermal isomerization of 5-allyloxy-
The relevance of tetrazolyl compounds stimulated research
in their structure and reactivity. Special attention has been
devoted to heteroaromatic ethers derived from tetrazole, with
important practical uses as intermediate compounds in the
transformation of alcohols.13-15 Compounds bearing an allylic
alcohol function are often vital structural units of biologically
active systems and have also attracted widespread attention as
key intermediates in synthesis.16 Previously, we have reported
that selective hydrogenolysis of the C-OH bond of allyl
alcohols can be achieved conveniently by first reacting the
alcohol with a 5-chloro-1-aryltetrazole17 so as to form the
heteroaromatic allyl ether 1, Figure 1, which will then undergo
smooth heterogeneously catalyzed transfer hydrogenolysis to
form the alkene 3, corresponding to the allyl group, and the
aryltetrazolone 4.18 In ethers 1, which can be regarded as
imidates, the heteroaromatic group together with an oxygen atom
from the original allyl alcohol acts as an excellent leaving group
in catalyzed ipso substitutions.
1-aryl-tetrazoles.
N-allyl isomers 2.19 Thermal O to N migration of the allyl group
in a series of 5-allyltetrazolyl compounds 1, Figure 1, proceeds
exclusively in a [3,3] sense through a polar chairlike transition
state, to give N-allyl-tetrazolones 2 as sole products.19 This
behavior is similar to the notionally comparable general Cope
rearrangement, which proceeds through an allowed [3,3] mech-
anism.20
Tetrazoles are known to exhibit a very interesting and rich
photochemistry. A literature survey revealed that photolysis of
1,4-dihydro-5H-tetrazole derivatives, both in solution and
isolated in low-temperature matrixes, generally results in the
elimination of N2. However, in several cases, it is unknown
whether the loss of N2 and formation of the final products occurs
in a concerted process or whether biradicals or zwitterions are
involved as intermediates.21 In other substituted tetrazoles, the
nature of the substituents present in the tetrazole ring was found
to strongly affect the final products. However, as for the
unsubstituted tetrazole, the main photoreaction path is either
the molecular nitrogen elimination or the ring-cleavage [3+2]
cycloelimination, leading to the production of azides.21-27
A selective heterogeneous catalytic transfer reduction of such
allyl ethers is quite remarkable, because it successfully competes
with hydrogenation of the double bond and also with the
relatively easy [3,3]-sigmatropic rearrangement to give the
In view of the widespread interest in tetrazoles and allylic
compounds, the photochemistry of the allylic derivatives of
tetrazole is now under investigation in our laboratories. In the
present work, we describe the photochemistry (λ ) 254 nm) of
4-allyl-tetrazolones, 2a-c, Figure 2, in solution. A mechanism
of photocleavage for these tetrazolyl derivatives is proposed.
(6) Hashimoto, Y.; Ohashi, R.; Kurosawa, Y.; Minami, K.; Kaji, H.;
Hayashida, K.; Narita, H.; Murata, S. J. CardioVasc. Pharmacol. 1998, 31,
568.
(7) Desarro, A.; Ammendola, D.; Zappala, M.; Grasso, S.; Desarro, G.
B. Antimicrob. Agents Chemother. 1995, 39, 232.
(8) Tamura, Y.; Watanabe, F.; Nakatani, T.; Yasui, K.; Fuji, M.;
Komurasaki, T.; Tsuzuki, H.; Maekawa, R.; Yoshioka, T.; Kawada, K.;
Sugita, K.; Ohtani, M. J. Med. Chem. 1998, 41, 640.
(9) Abell, A. D.; Foulds, G. J. J. Chem. Soc., Perkin Trans. 1 1997, 17,
2475.
(19) (a) Cristiano, M. L. S.; Johnstone, R. A. W. J. Chem. Soc., Perkin
Trans. 2 1997, 489. (b) Cristiano, M. L. S.; Johnstone, R. A. W. J. Chem.
Res. 1997, 164.
(10) Sandmann, G.; Schneider, C.; Boger, P. Z. Naturforsch., C: Biosci.
1996, 51, 534.
(11) Koldobskii, G. I.; Ostrovskii, V. A.; Poplavskii, V. S. Khim.
Geterotsikl. Soedin. 1981, 10, 1299.
(12) Zhao-Xu, C.; Heming, X. Int. J. Quantum Chem. 2000, 79, 350.
(13) Arau´jo, N. C. P.; Brigas, A. F.; Cristiano, M. L. S.; Frija, L. M. T.;
Guimara˜es, E. M. O.; Loureiro, R. M. S. J. Mol. Catal. A: Chem. 2004,
215, 113.
(14) Frija, L. M. T.; Cristiano, M. L. S.; Guimara˜es, E. M. O.; Martins,
N. C.; Loureiro, R. M. S.; Bikley, J. J. Mol. Catal. A: Chem. 2005, 242,
241.
(15) Arau´jo, N. C. P.; Barroca, P. M. M.; Brigas, A. F.; Cristiano, M. L.
S.; Johnstone, R. A. W.; Loureiro, R. M. S.; Pena, P. C. A. J. Chem. Soc.,
Perkin Trans. 1 2002, 9, 1213.
(20) (a) Woodward, R. B.; Hoffmann, R. The ConserVation of Orbital
Symmetry; Verlag Chemie, GmbH: Weinheim, Germany, 1970. (b) Ito,
H.; Taguchi, T. Chem. Soc. ReV. 1999, 28, 43. (c) Ziegler, F. E. Chem.
ReV. 1988, 88, 1423. (d) Dessolin, M.; Eisenstein, O. K.; Golfier, M.; Prange,
T.; Sautet, P. G. N. J. Chem. Soc., Chem. Commun. 1992, 132. (e) Boudjabi,
S.; Dewynter, G.; Montero, J. L. Synlett 2000, 5, 716. (f) Metz, P.; Mues,
C.; Schoop, A. Tetrahedron 1992, 48, 1071.
(21) Dunkin, I. R.; Shields, C. J.; Quast, H. Tetrahedron 1989, 45, 259.
(22) Chae, Y. B.; Chang, K. S.; Kim, S. S. Taehan Hwahakhoe Chi 1967,
11, 85.
(23) Awadallah, A.; Kowski, K.; Rademacher, P. J. Heterocycl. Chem.
1997, 34, 113.
(16) (a) Kumar A.; Dittmer, D. C. J. Org. Chem. 1994, 59 (9), 4760. (b)
Adam, W.; Peters, K.; Reuz, M. Angew. Chem., Int. Ed. Engl. 1994, 33,
1107. (c) Bergmeier, S. C.; Stanchina, D. M. Tetrahedron Lett. 1995, 36,
4533.
(17) Bugalho, S. C. S.; Lapinskii, L.; Cristiano, M. L. S.; Frija, L. M.
T.; Fausto, R. Vib. Spectrosc. 2002, 30, 213.
(24) Go´mez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, M. L.; Fausto,
R. J. Phys. Chem. A 2005, 109, 7967.
(25) Go´mez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, M. L.; Fausto,
R. J. Mol. Struct. 2005, doi: 10.1016/j.molstruc.2005.08.019.
(26) Go´mez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, M. L.; Fausto,
R. J. Photochem. Photobiol., A 2005, doi: 10.1016/j.jphotochem.2005.08.021.
(27) Go´mez-Zavaglia, A.; Reva, I. D.; Frija, L.; Cristiano, M. L. S.;
Fausto, R. J. Photochem. Photobiol., A 2005, 179, 243.
(18) Cristiano, M. L. S.; Johnstone, R. A. W.; Price, P. J. J. Chem. Soc.,
Perkin Trans. 1 1996, 1453.
3584 J. Org. Chem., Vol. 71, No. 9, 2006

Photochemistry of 4-Allyl-tetrazolones in Solution
FIGURE 2. B3LYP/6-311G(d) optimized structures of 1-phenyl-4-(prop-2-enyl)-tetrazole-5-one (2a), 4-(1-methylprop-2-enyl)-tetrazole-5-one (2b),
and 1-phenyl-4-(1-phenylprop-2-enyl)-tetrazole-5-one (2c) with the atom numbering. Tetrazolyl and phenyl rings are coplanar.
TABLE 1. Evolution of Substrate Concentrationa in the
Photolysisb of 4-Allyl-tetrazolones 2a-c, Using Different Alcohols as
Solvents
Results and Discussion
Compounds 2 are obtained from the corresponding tetrazolyl
ethers 1 (Figure 1), with quantitative conversion, by heating a
neat sample.19a Allyloxytetrazoles 1 were synthesized by the
initial reaction of allylic alcohol with sodium hydride followed
by the reaction with 5-chloro-1-aryltetrazole.
substrate concentration (%)
methanol
1-propanol
1-hexanol
irradiation time
(min)
2a
2b
2c
2a
2b
2c
2a
2b
2c
0.25
0.50
1.0
2.0
4.0
8.0
12
20
40
60
90
93
90
87
83
74
56
43
32
14
3
92
89
87
81
75
61
49
38
19
6
94
91
89
84
78
62
52
41
23
8
95
92
89
85
78
64
53
40
21
10
3
96
94
92
89
84
75
65
51
36
17
5
97
95
92
90
85
78
67
55
40
26
14
5
98
95
92
88
82
75
68
56
41
29
23
17
10
3
98
95
91
87
84
77
67
57
44
32
26
20
13
7
99
97
95
92
89
85
80
71
61
53
45
37
31
23
15
To assess the reaction mechanisms involved, two major
aspects were addressed: (i) Structural elucidation of the
photoproducts obtained after UV exposure (λ ) 254 nm) of
the tetrazolyl derivatives 2a-c in methanol, 1-propanol, 1-hex-
anol, acetonitrile, and cyclohexane. (ii) Evaluation of structural
effects, the nature of the solvent, and the effect of oxygen on
the photoreactivity of compounds 2a-c.
UV spectra of 4-allyl-tetrazolones 2a-c show absorptions
around 250 nm. These compounds were irradiated, and the
progress of the photoreactions was monitored by HPLC.
Quantum yields for photocleavage were determined separately.
The results obtained are presented and discussed below.
Identification of the Photoproducts. Photolysis of 1-Phen-
yl-4-(prop-2-enyl)-tetrazole-5-one (2a). Photolysis of 4-allyl-
tetrazolone 2a was initially conducted in methanol. In all
experimental conditions tested, namely, in air-equilibrated
solutions, in solutions saturated with oxygen, or after purging
with argon, a single photoproduct, identified as 3,4-dihydro-3-
phenylpyrimidin-2(1H)-one 6a (m/z ) 174), was formed.
Complete conversion of 2a was observed after 65 min (Table
1). Extension of the irradiation time to 3 h did not modify the
concentration of the primary photoproduct, and no signs of
secondary photoproducts were registered.
120
150
180
210
a Relative yields determined by HPLC. b λ ) 254 nm. Initial concentra-
tion of the substrates: 1.0 × 10-4 M.
(92%, isolated yield). The isolated dihydropyrimidinone 6a is
a stable compound, as no degradation was registered during
storage. Interestingly, this molecule belongs to a novel structural
class of pyrimidinones.28
The photoreactivity of 4-allyl-tetrazolone 2a was also assessed
in other alcohols. Irradiation of 4-allyl-tetrazolone 2a in
Isolation of the photoproduct 6a was carried out simply by
solvent evaporation under reduced pressure in mild conditions
(28) Frija, L. M. T.; Khmelinskii, I. V.; Cristiano, M. L. S. Tetrahedron
Lett. 2005, 46, 6757.
J. Org. Chem, Vol. 71, No. 9, 2006 3585

Frija et al.
FIGURE 3. Extinction coefficients (ꢀ) as a function of wavelength
for tetrazolone 2a (black line) and pyrimidinone 6a (blue line) in
methanolic solutions.
TABLE 2. Extinction Coefficient Values for Tetrazolones 2a-c
a
and Pyrimidinones 6a-c Calculated at λmax
ꢀ/105
compound
λ
max (nm)
(L‚mol-1‚cm-1
)
2a
2b
2c
6a
6b
6c
249
249
250
235
233
231
0.63
0.61
0.89
0.79
0.69
0.82
a Results obtained in methanol solution.
FIGURE 4. Proposed photodegradation pathways of 4-allyl-tetrazo-
lones 2a-c in solution.
1-propanol and 1-hexanol also led to formation of 3,4-dihydro-
3-phenylpyrimidin-2(1H)-one 6a as the sole photoproduct. These
results point to a single photochemical channel for the photo-
degradation of 2a involving molecular nitrogen extrusion. The
time for complete conversion of the substrate increased with
the carbon chain length of the alcohol (65 min in methanol, 90
min in 1-propanol, and 190 min in 1-hexanol; Table 1).
A single mechanistic route is proposed for the formation of
phenyl-isocyanate, involving the exclusive ring photocleavage
of pyrimidinone 6a, Figure 4, route a. This mechanistic approach
was confirmed when irradiation experiments using the pure
pyrimidinone 6a (isolated previously from the alcoholic solu-
tions) were done in acetonitrile and cyclohexane. In the course
of these experiments, formation of phenyl-isocyanate and aniline
was detected, confirming that these two secondary photoproducts
are a result of pyrimidinone photodegradation.
In contrast, two reactive channels could be possible for aniline
formation: (i) photocleavage of the pyrimidinone ring to give
directly the phenyl-nitrene (Ph-N:) intermediate, which will
then abstract hydrogen from the solvent to form aniline and
(ii) initial formation of phenyl-isocyanate that could undergo
subsequent photocleavage to phenyl-nitrene, with elimination
of CO, which would then abstract hydrogen to form aniline.
However, in view of the experimental observations, we consider
that aniline and phenyl-isocyanate are produced via two
independent photochemical channels of degradation of pyrimi-
dinone 6a, Figure 4, routes a and b.
Throughout the irradiation experiments, the appearance of
these two compounds was detected at the same time, indicating
that aniline is formed exclusively by photocleavage of the
pyrimidinone ring and does not derive from phenyl-isocyanate.
This hypothesis was confirmed by HPLC analysis of irradiated
solutions of phenyl-isocyanate in acetonitrile and cyclohexane,
in conditions similar to those used for allyltetrazolone 2a.
Phenyl-isocyanate remained photostable upon prolonged ir-
radiation at 254 nm, and no aniline formation was ever detected.
Figure 3 shows the spectra of compounds 2a and 6a. Note
that the primary product 6a has some absorbance at the
excitation wavelength and, thus, would reduce the apparent
photolysis quantum yields at higher conversions, even in
alcoholic solvents. The product interference becomes a more
serious issue in nonalcoholic solvents, as the respective second-
ary photoproducts have even higher extinction coefficients. The
extinction values for compounds 2a-c and 6a-c are given in
Table 2. Note that the three starting compounds have similar
extinction coefficients and positions of the peak maxima. The
values of the extinction coefficients are typical for states of ππ*
nature.
The observed photoreactivity for tetrazolone 2a in acetonitrile
and cyclohexane was similar to that in alcohols, but the primary
photoproduct 6a was in turn photolyzed. In both solvents, three
photoproducts were identified after irradiation: a primary
photoproduct (m/z ) 174) that coincides with the sole photo-
product obtained in alcoholic solutions, and two secondary
photoproducts (m/z ) 93 and 119). Analysis of the photolyzed
samples taken every 2 s allowed for the identification of
pyrimidinone 6a as the primary photoproduct. The other
(secondary) photoproducts started to be detectable at irradiation
times exceeding 20 s and were identified as phenyl-isocyanate
(7, m/z ) 119) and aniline (9, m/z ) 93) by comparing the
HPLC and GC-MS traces with those of standard samples.
The formation of phenyl-isocyanate and aniline by photo-
cleavage of 6a will be concomitant with the formation of two
3586 J. Org. Chem., Vol. 71, No. 9, 2006

Photochemistry of 4-Allyl-tetrazolones in Solution
the tetrazolyl ring system through photoinduced cleavage of the
weakest of the N-N formally single bonds (N(7)-N(8) and N(9)-
N(10) for 2a,b; N(1)-N(2) and N(3)-N(4) for 2c), Figure 2, leading
to the corresponding 3,4-dihydro-6-substituted 3-phenylpyri-
midin-6-ones as sole products. Thus, this methodology repre-
sents a new synthetic strategy to these compounds, from
5-allylloxytetrazoles 1, in excellent yields. Chemical diversity
may be introduced easily by changing substituents in the allylic
and the tetrazolic systems.19,28
FIGURE 5. Solvation of pyrimidinone 6a in alcoholic solutions.
other possible secondary products, 1-(amino) propene (8a) and
1-propenyl-isocyanate (10a), respectively. However, these pho-
toproducts were never detected chromatographically throughout
the irradiation experiments in any of the solvents. This could
be explained by their high volatility, their fast photodecompo-
sition into low molecular weight products, or their very rapid
elution in the chromatographic conditions used.
The higher photostability of the primary photoproduct 6a in
alcohols against cyclohexane and acetonitrile can be explained
based on solvent stabilization. In alcohols, there might be a
strong association between the solvent and the photoproduct
involving formation of relatively strong hydrogen bonds.
Pyrimidinone 6a bears several putative atoms capable of forming
hydrogen bonds with solvent molecules, Figure 5. In such
conditions, reversible proton transfer could be a fast and efficient
mechanism of the excited-state quenching,29 facilitated by steric
limitations resulting from the stable cage enclosing the pyri-
midinone molecule and preventing its photodecomposition. Also,
the absorbed energy is more efficiently dissipated through the
solvated complex, preventing relaxation through pathways
leading to photocleavage.
In acetonitrile and cyclohexane, the photoreactivity of tetra-
zolones 2b,c is also similar to that observed for 2a. In these
solvents, three photoproducts were identified for compound
2b: a primary photoproduct (m/z ) 188) coinciding with the
sole photoproduct in methanol and two secondary photoproducts
identified as phenyl-isocyanate (7, m/z ) 119) and aniline (9,
m/z ) 93). The other possible secondary species formed via
photodecomposition of 6b are 2-amine-2-butene (8b) and
1-methylprop-1-enyl-isocyanate (10b), Figure 4. However, as
for compounds 8a and 10a, these predicted photoproducts were
never detected in any of the conditions tested during photolysis.
Upon photolysis of tetrazolone 2c, using the same solvents, four
photoproducts were identified: the primary photoproduct (m/z
) 250) that coincides with the sole photoproduct in methanol
and three secondary photoproducts identified as phenyl-isocy-
anate (7, m/z ) 119), aniline (9, m/z ) 93), and 1-phenylprop-
1-enyl-isocyanate (10c, m/z ) 159), Figure 4.
Hence, the results obtained for the photolysis of compounds
2b,c are analogous to those obtained for the photolysis of 2a,
unequivocally demonstrating that their photoreactivity is com-
parable and that substituent effects on the photoproduct distribu-
tion during the photolysis of the 4-allyl-tetrazolones in solution
are negligible. However, as a result of an increase in the steric
effects (it becomes more difficult for the fragments to separate
with growing molecular size) from 2a to 2c, the time required
for complete photodegradation of the three tetrazolones increases
in the same order. As will be shown, similar conclusions follow
from analysis of the quantum yield determinations.
1H NMR spectra for compound 6a were obtained in deuter-
ated chloroform, acetonitrile, and methanol. In CDCl3 and CD3-
OD, all protons connected to carbon atoms show a similar
chemical shift. However, in CD3CN, the resonances for the same
protons are shifted downfield by 0.25 to 0.35 ppm. The signal
due to the resonance of the proton connected to the nitrogen
(N-H) appears at 10.2 and 9.1 ppm in CDCl3 and CD3CN,
respectively, and does not appear in CD3OD.
Photolysis of 4-(1-Methylprop-2-enyl)-tetrazole-5-one (2b)
and 1-Phenyl-4-(1-phenylprop-2-enyl)-tetrazole-5-one (2c).
The photochemistry of 4-(1-methylprop-2-enyl)-tetrazole-5-one
(2b) and 1-phenyl-4-(1-phenylprop-2-enyl)-tetrazole-5-one (2c)
in solution was investigated using the methodology described
above for tetrazolone 2a. In methanol (air-equilibrated, oxygen-
saturated, and argon-purged solutions), photolysis of these two
tetrazolones resulted in the exclusive formation of the corre-
sponding pyrimidinones 6b and 6c (Figure 4), identified as 3,4-
dihydro-6-methyl-3-phenylpyrimidin-2(1H)-one (m/z ) 188) and
3,4-dihydro-3,6-diphenylpyrimidin-2(1H)-one (m/z ) 250),
respectively. The same behavior was observed when using other
alcohols. These pyrimidinones were isolated from the alcoholic
solution in excellent yields (97% for 2b and 90% for 2c), and
no evidence of secondary photoproducts was ever observed
when extending the irradiation time. The time required for total
photodegradation of 4-allyl-tetrazolones 2a-c in the same
alcohol varied for the three compounds (Table 1), increasing
from RdH to RdCH3 and then to RdPh. As was noted for
compound 2a, the time required for complete conversion also
increased with the viscosity of the alcohol.
Photoinduced loss of nitrogen from compounds 2a-c may
involve a cycloelimination leading to a diaziridinone that reacts
further to give the observed 3,4-dihydro-6-substituted 3-phe-
nylpyrimidin-2(1H)-ones 6a-c or may occur through a biradical
intermediate that subsequently cyclizes to give the same
compounds 6a-c. According to the literature, detection of
biradical intermediates is not trivial and could not be achieved
in this work. In several related investigations, such species were
detectable only in a limited number of cases.21 However, based
on the experimental results obtained, it is our conviction that a
radicalar mechanism operates in this case (Figure 4). Generally,
the transition state in the concerted mechanism (cleavage of
two N-N formally single bonds and formation of one new N-N
formally single bond and the transformation of NdN double
bond into NtN triple bond) has a higher polarity as compared
to that of the initial substrate. Considering the variation in
polarity associated with the five solvents used in this work, it
appears that the solvent polarity effects are weak or absent
during tetrazolone photolysis, contrasting with what could be
expected for a concerted process occurring via a polar transition
state. No production of urea, via N2 elimination followed by
hydrogen abstraction, has ever been detected. However, this fact
is not sufficient to discard the formation of a biradical
intermediate in our experiments, because the reactive channel
The present investigation shows that the only photoproduct
of 2a-c formed in alcohols results from N2 elimination from
(29) Solntsev, K. M.; Huppert, D.; Agmon, N.; Tolbert, L. M. J. Phys.
Chem. A 2000, 104, 4658.
J. Org. Chem, Vol. 71, No. 9, 2006 3587

Frija et al.
TABLE 3. Quantum Yields (λ ) 254 nm, 25 °C) for the
Photodegradation of 4-Allyl-tetrazolones 2a-c, in the Range of
Solvents Used
SCHEME 1
solvent
(air-eq.)
viscosity, η
(mPa‚s)
2a, Φobs
2b, Φobs
2c, Φobs
acetonitrile
methanol
cyclohexane
1-propanol
1-hexanol
0.37
0.54
0.89
1.95
4.58
0.38
0.36
0.34
0.29
0.23
0.35
0.34
0.31
0.26
0.20
0.31
0.30
0.27
0.23
0.17
tion of these results with the tetrazolone photoreactivity
exhibited in the range of solvents used provides valuable
information toward an interpretation of the mechanism that
operates through the photolysis.
The observed viscosity dependence clearly points to the
importance of the cage effect in the primary reaction mecha-
nism.31 We believe that the caged radical pair involving species
2-2 and N2 (see Figure 4) is formed as the primary photo-
product, which may either recombine, reconstituting the parent
molecule 2 (kr) or escape from the cage (kd), eventually yielding
the photolysis product 6. The importance of cage effects depends
on the relative velocity (and, thus, the kinetic energy and the
mass)31 of the two fragments, as well as on the solvent viscosity.
Radical cage effects have an enormous impact on chemical
reactivity in solution, and in fact, they are necessary to explain
many fundamental reaction phenomena. These phenomena
include rate-viscosity correlations, variations in products and
product yields, and variations in quantum yields, as a function
of medium. Expression (3) for the product quantum yield results
from Scheme 1, in the absence of dissolved oxygen:
FIGURE 6. Plot of 1/φ vs solvent viscosity (η) for the photolysis of
4-allyl-tetrazolones 2a ([), 2b (9), and 2c (2) in argon-purged
solutions. Values of viscosity at 25 °C: 0.369 (acetonitrile), 0.544
(methanol), 0.894 (cyclohexane), 1.945 (1-propanol), and 4.580 (1-
hexanol) mPa‚s.30
for radical recombination could be much more favorable than
the hydrogen abstraction route.
kd + kST
pairkd + kST + kr
φ ) φ
(3)
Photolysis Quantum Yields and the Mechanism of Pri-
mary Reactions. The quantum yields obtained are rationalized
on the basis of the following mechanism of primary reactions
occurring upon photoexcitation of tetrazolones 2a-c:
In Scheme 1, Substrate denotes the starting compound 2a-
c, Product denotes the reaction product 6a-c, and [RT‚‚‚N2*]
denotes the caged radical pair, including the triplet biradical
2-2. This reaction scheme is not quite complete, as it should
additionally include re-encounters between RT and N2* that
escaped the solvent cage and the reaction between RT and O2
in the bulk. However, the complete scheme would be too
cumbersome for the analysis and unnecessary for the present
purpose, given the lack of time-resolved kinetic data.
Because no fluorescence could be observed from the Sub-
strates, we conclude that the reaction (2) is very fast and,
consequently, the respective excited state formed in (1) and
reacting in (2) is a singlet.
The Effect of Solvent Viscosity. Analysis of the results
presented in Table 3 and Figure 6, obtained in argon-purged
solutions, shows a direct correlation between the solvent
viscosity30 and the inverse photolysis quantum yield for tetra-
zolones 2a-c. A good linear correlation is observed for the
viscosity variations by 1 order of magnitude, with the photolysis
quantum yields decreasing at higher viscosities. Considering the
quantum yields obtained in the five solvents used in this work
(Table 3), there is no apparent correlation between the photolysis
quantum yields and the solvent dielectric constant. The conjuga-
Considering kd ) b/η, we obtain:
kr
1
φ
1
φpair
)
1 +
(4)
(
)
(b/η) + kST
Note that eq 4 is linear in η at sufficiently small values of η.
The value of φpair for each one of the three tetrazolones 2a-c
is obtained by extrapolating the plot of 1/φ versus viscosity
(Figure 6) to zero viscosity (φpair,2a ) 0.39; φpair,2b ) 0.37; φpair,2c
) 0.33). The reduction of φpair with the size and molecular mass
of the radicals should reflect the increasing probability of their
re-encounters.31
As expected, the cage effect is increasing with viscosity. Thus,
the cage effects can be used to explain the variations of the
photolysis quantum yields as a function of medium for 4-allyl-
tetrazolones 2a-c.
Effect of Oxygen on the Photoreactivity. It is well-known
that, in the absence of low lying 1(π,π*) or charge transfer states,
benzophenones show a high S1 (n,π*) to T1 (n,π*) intersystem
crossing efficiency.32 Considering the structure of tetrazolones
2a-c, the presence of the carbonyl group in the tetrazole ring
is possibly the main structural factor that could make it
(31) Braden, D. A.; Parrack, E. E.; Tyler, D. R. Coord. Chem. ReV. 2001,
211, 279.
(32) Turro, N. J. Photoaddition and photosubstitution reactions. In
Modern Molecular Photochemistry; The Benjamin/Cummings Publishing
Co., Inc.: San Francisco, CA, 1978; pp 362-413.
(30) Lide, D. R., Ed. CRC-Handbook of Chemistry and Physics, version
on CD-ROM; CRC Press: Boca Raton, FL, 2002.
3588 J. Org. Chem., Vol. 71, No. 9, 2006

Photochemistry of 4-Allyl-tetrazolones in Solution
TABLE 4. Conversion of Substratea and Product Distribution in the Photolysisb of 4-Allyl-tetrazolones 2a-c in Acetonitrile, Cyclohexane, and
Methanol
a Relative yields in %, determined by HPLC. b Irradiation time, 20 min.; λ ) 254 nm; concentration of substrate, 1.0 × 10-4 M. c 2a (R ) H), 2b (R )
CH3), 2c (R ) Ph).
susceptible to photodegradation proceeding through the lowest
triplet states. However, in the particular case of photolysis of
compounds 2a-c, we suppose that the reaction proceeds via a
short-lived singlet excited state that quickly leads to a triplet
biradical (Scheme 1, reactions 1 and 2).
Contrary to what should be expected for a triplet-mediated
photoprocess, the presence of O2, an efficient quencher of triplet
states, was found to accelerate the photodegradation of tetra-
zolones in all of the tested solvents (see Table 4 for the
photoconversion of substrates in Ar-purged, air-equilibrated, and
O2-saturated solutions).
One explanation for the increased photodegradation of 2a-c
in oxygen-saturated solutions could be the formation of an oxo-
tetrazolone complex involving tetrazolone and O2 molecules.
This complex could reduce the influence of the solvent cage
and increase photolysis quantum yields. In the oxygen/tetra-
zolone complex, the relevant HOMO and LUMO orbitals might
also have a smaller energy difference, favoring nitrogen
elimination. However, our attempts to detect the formation of
such ground-state complexes in the UV spectra were fruitless.
Another, more plausible explanation for the sensitizing effect
of the molecular oxygen could be acceleration of the T-S
conversion in the triplet biradicals, opening the way for the
formation of the products 6a-c and, consequently, improving
the photolysis quantum yields (see the kinetic Scheme 1). The
quantum yield as a function of oxygen concentration is given
by expression (5), derived from the kinetic Scheme 1:
kd + kST + kq[O2]
pairkd + kST + kr + kq[O2]
φ ) φ
(5)
where kd is the first-order dissociation rate constant of the radical
pair, kr is its first-order recombination rate constant, kST is the
intersystem conversion rate constant for the radical 2-2, and
kq is its second-order reaction rate constant with dissolved
oxygen. Figure 7 shows the product quantum yields in metha-
nolic solutions for substances 2a-c as a function of the oxygen
concentration.
We may use the kq ) 1 × 1010 mol‚L-1‚s-1, equal to the
diffusion-controlled reaction rate in methanol, thus obtaining
an estimate kq[O2] ) 1 × 108 s-1 for the oxygen-induced S-T
conversion rate at the highest oxygen concentrations used. The
significant influence of the dissolved oxygen concentration on
the product yields demonstrates that the rates of the other
reactions in the Scheme 1 should be of comparable magnitude.
However, presently, we are unable to obtain viable estimates
J. Org. Chem, Vol. 71, No. 9, 2006 3589

Frija et al.
HPLC analyses were performed using a chromatograph with a
UV detector and a photodiode array. A column (RP-18, 5 µm) was
used, and the runs were performed using a mixture of water and
acetonitrile (40:60) as the eluent.
GC-MS analyses were carried out using a gas chromatograph
with a series mass selective detector (EI, 70 eV), using a capillary
column with 25 m length and 0.25 mm I.D. The initial temperature
of 50 °C was maintained for 3 min and then a heating rate of 5
°C/min was applied until a final temperature of 250 °C was reached.
Analyses were conducted on irradiated samples and on control
solutions kept in darkness. Controls showed no sign of photodeg-
radation.
Photodegradation studies were conducted in a reactor previously
used to investigate the photochemistry of pesticides,33 employing
a merry-go-round and an immersion-well photochemical reactor
immersed in water for cooling. The water temperature was kept
constant (25 °C) using external circulation through a cooling bath.
A 16-W low-pressure Hg lamp was used as source of the 254-nm
UV radiation. Photolysis of tetrazolones 2a-c was carried out in
acetonitrile, cyclohexane, methanol, 1-propanol, and 1-hexanol,
using 1-cm quartz cells at 10 to 40 cm from the lamp. Generally,
10-4 M starting solutions were used when the analysis was carried
out by HPLC. More concentrated 10-2 M solutions were used for
product identification using GC-MS.
FIGURE 7. Plot of the product quantum yield as a function of oxygen
concentration for 4-allyl-tetrazolones 2a ([), 2b (9), and 2c (2) in
methanolic solutions.
of the rates of the primary reactions without the results from
time-resolved kinetic measurements, which we plan to perform
soon.
Photolysis quantum yields were measured using the ferrioxalate
actinometer.34 A suitable interference filter was used to isolate the
254-nm line of the Hg lamp. Dissolved oxygen concentrations were
varied by bubbling a controlled mixture of N2 and O2 through the
solutions immediately before the photolysis for 10 min.
Conclusion
Photolysis of the 4-allyl-tetrazolones 2a-c in methanol,
1-propanol, and 1-hexanol solutions yields 3,4-dihydro-6-
substituted-3-phenylpyrimidin-2(1H)-ones 6a-c as sole prod-
ucts. These pyrimidinones are easily isolated from the alcoholic
medium as stable compounds in excellent yields, providing an
alternative and attractive synthetic methodology for this class
of compounds. The observed photostability of pyrimidinones
6a-c in alcohols is ascribed to a very efficient solvation, with
formation of hydrogen bonds leading to deactivation of the
excited state by reversible proton transfer, facilitated by stable
solvent cages that enclose the pyrimidinone molecules and
prevent their photodecomposition. The viscosity of the alcohols
is directly related to the cage effects observed for 4-allyl-
tetrazolones 2a-c. Photoexcitation of 4-allyl-tetrazolones should
involve formation of a caged triplet radical pair. This hypothesis
is confirmed by the solvent viscosity effect on the photolysis
quantum yields and the fact that the presence of dissolved
oxygen accelerated photodegradation. The relative yields of all
the identified photoproducts remained similar to those obtained
in oxygenated, air-equilibrated and Ar-purged systems. The
reaction appears to occur via a triplet biradical intermediate 2-2
with a lifetime of about 10-8 s. Pyrimidinones 6a-c form
additional photoproducts in solvents other than alcohols, where
the effect of the solvent hydrogen bonds upon the reactivity of
their excited state is negligible.
Preparation of 5-Allyloxytetrazoles. 1-Phenyl-5-(prop-2-enyl-
oxy)-tetrazole (1a): prop-2-en-1-ol (allyl alcohol; 0.65 g; 11.1
mmol) in dry THF (10 mL) was added to a slurry of sodium hydride
(55-60% in mineral oil; 0.72 g; 16.5 mmol) in dry THF (50 mL).
The mixture was stirred at room temperature under an inert
atmosphere until the effervescence ceased (20 min). 5-Chloro-1-
phenyl-1H-tetrazole (2.0 g; 11.1 mmol) in dry THF (20 mL) was
added, and the mixture was stirred overnight at room temperature.
The reaction was monitored by TLC using a mixture of toluene/
acetone (5:1) as eluent. Ice water (50 mL) was added, and the
organic product was extracted with diethyl ether (3 × 50 mL). The
combined organic extracts were dried (Na2SO4) and evaporated to
dryness to give 1-phenyl-5-(prop-2-enyloxy)-tetrazole as a light
yellow oil (1.8 g; 80% yield). IR νmax: 1592, 1556, 1500, 1456,
1
and 760 cm-1. H NMR (CDCl3): δ 5.10 (2H, d, J ) 5.7 Hz),
5.30-5.60 (2H, m), 6.00-6.20 (1H, m), 7.40-7.60 (3H, m), 7.80
(2H, d, J ) 6.9 Hz). Anal. Calcd for C10H10N4O: C, 59.4; H, 5.0;
N, 27.7%. Found: C, 59.2; H, 5.1; N, 28.2%. MS (EI): m/z 202
[M]+.
Similarly, other allyloxytetrazoles were prepared.
1-Phenyl-5-[(E)-but-2-enyloxy]-tetrazole (1b): obtained from
(E)-but-2-en-1-ol (crotyl alcohol; 1.0 g; 13.9 mmol) as light-yellow
crystals (ethyl acetate, 67% yield), mp 36-37 °C. IR νmax: 1597,
1
1560, 1505, 1448, and 761 cm-1; H NMR (CDCl3): δ 1.75 (3H,
d, J ) 8.7 Hz), 5.05 (2H, d, J ) 7.9 Hz), 5.72-5.90 (1H, m),
5.95-6.10 (1H, m), 7.40-7.65 (3H, m), 7.75 (2H, d, J ) 8.3 Hz).
Anal. Calcd for C11H12N4O: C, 61.1; H, 5.6; N, 25.9%. Found:
C, 61.1; H, 5.6; N, 26.0%. MS (EI): m/z 216 [M]+.
Experimental Section
Equipment. All chemicals were used as purchased. Solvents for
extraction and chromatography were of technical grade. When
required, solvents were freshly distilled from appropriate drying
agents before use. Analytical TLC was performed with silica gel
60 F254 plates. Melting points are uncorrected. UV absorption
spectra were recorded on a UV-visible spectrophotometer, using
1 × 1 cm quartz cells. Mass spectra were obtained on a mass
spectrometer by electron ionization (EI) at 70 eV. 1H NMR spectra
were recorded in CDCl3, CD3OD, and CD3CN (at 300 or 400 MHz)
using TMS as an internal standard (δ ) 0.0 ppm). Elemental
analyses were performed on a standard elemental analyzer. Infrared
spectra were registered in KBr disks.
1-Phenyl-5-[(E)-3-phenylprop-2-enyloxy]-tetrazole (1c): ob-
tained from (E)-3-phenylprop-2-en-1-ol (cinnamyl alcohol; 0.8 g;
6.0 mmol) as colorless needles (ethyl acetate, 72% yield), mp 67-
68 °C. IR νmax: 1596, 1559, 1505, 1368, and 757 cm-1. 1H NMR
(CDCl3): δ 5.25 (2H, d, J ) 7.0 Hz), 6.60 (1H, m), 6.85 (1H, d,
(33) Da Silva, J. P.; Da Silva, A. M.; Khmelinskii, I. V.; Martinho, J.
M. G.; Vieira Ferreira, L. F. J. Photochem. Photobiol., A 2001, 142, 31.
(34) (a) Parker, C. A. Proc. R. Soc. London, Ser. A 1953, 220, 104. (b)
Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956, 235,
518. (c) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; John Wiley and
Sons: New York, 1967, pp 769-804.
3590 J. Org. Chem., Vol. 71, No. 9, 2006

Photochemistry of 4-Allyl-tetrazolones in Solution
J ) 15.6 Hz), 7.24-7.80 (10H, m). Anal. Calcd for C16H14N4O:
C, 69.9; H, 5.1; N, 20.2%. Found: C, 69.8; H, 5.0; N, 20.4%. MS
(EI): m/z 278 [M]+.
(97% isolated yield). IR νmax: 3284 (NH), 3085, 1681 (CdO), 1598,
1548, 1484, 1446, 1378, 1230, 1068, and 755 cm-1 1H NMR
.
(CDCl3): δ 1.58-1.62 (3H, s), 3.75-3.78 (2H, d), 5.22-5.30 (1H,
m), 6.15-6.25 (1H, m), 7.05-7.17 (2H, m), 7.25-7.42 (2H, m),
10.20 (1H, s). MS (EI): m/z 189 [M + H]+. Accurate mass (CI):
Anal. Calcd for C11H13N2O, 189.10280; found, 189.10299.
3,4-Dihydro-3,6-diphenylpyrimidin-2(1H)-one (6c): from 1-phen-
yl-4-(1-phenylprop-2-enyl)-tetrazole-5-one 2c irradiated for 5 h
(90% isolated yield). IR νmax: 3220 (NH), 3066, 1701 (CdO), 1581,
Preparation of 4-Allyl-tetrazolones. 1-Phenyl-4-(prop-2-enyl)-
tetrazol-5-one (2a): 1-phenyl-5-(prop-2-enyloxy)-tetrazole (1.0 g;
4.9 mmol) was heated neat in an oil bath at 150 °C for 3 h to give
the required 1-phenyl-4-(prop-2-enyl)-tetrazole-5-one as the sole
product (quantitative yield). IR νmax: 1729, 1598, 1504, 1388, and
1
757 cm-1. H NMR (CDCl3): δ 4.65 (2H, d, J ) 5.7 Hz), 5.35-
1
5.52 (2H, m), 5.92-6.10 (1H, m), 7.40-7.60 (3H, m), 8.05 (2H,
d, J ) 6.86 Hz). Anal. Calcd for C10H10N4O: C, 59.4; H, 5.0; N,
27.7%. Found: C, 59.8; H, 5.2; N, 28.1%. MS (EI): m/z 202 [M]+.
4-(1-Methylprop-2-enyl)-tetrazol-5-one (2b): 1-phenyl-5-[(E)-
but-2-enyloxy]-tetrazole (1.0 g; 4.6 mmol) was heated neat in an
oil bath at 150 °C for 2 h to give 4-(1-methylprop-2-enyl)-tetrazole-
5-one as the sole product (quantitative yield). IR νmax: 1729, 1599,
1504, 1382, and 757 cm-1. 1H NMR (CDCl3): δ 1.65 (3H, d, J )
5.7 Hz), 4.90-5.10 (1H, m), 5.22-5.40 (2H, m), 6.05-6.20 (1H,
m), 7.30-7.55 (3H, m), 7.95 (2H, d, J ) 8.6 Hz). Anal. Calcd for
C11H12N4O: C, 61.1; H, 5.6; N, 25.9%. Found: C, 61.0; H, 5.6;
N, 26.1%. MS (EI): m/z 216 [M]+.
1543, 1422, 1345, 1226, 1066, and 756 cm-1. H NMR (CDCl3):
δ 3.76-3.78 (2H, d), 5.42-5.48, (1H, m), 6.32-6.45 (1H, m),
6.70-6.75 (1H, m), 6.85-7.00 (2H, m), 7.10-7.35 (6H, m), 9.90
(1H, s). MS (EI): m/z 251 [M + H]+. Accurate mass (CI): Anal.
Calcd for C16H15N2O, 251.11844; found, 251.11809.
Computational Details. The equilibrium geometries for the
4-allyl-tetrazolones 2a-c were fully optimized at the DFT level
of theory with the standard 6-311G(d) basis set, using the Gaussian
98 program.35 DFT calculations were carried out with the three-
parameter density functional, abbreviated as B3LYP, which includes
Becke’s gradient exchange correction36 and the Lee, Yang, and Parr
correlation functional.37 No symmetry restrictions were imposed
on the initial structures. Atom numbering schemes for the molecules
studied are given in Figure 2.
1-Phenyl-4-(1-phenylprop-2-enyl)-tetrazol-5-one (2c): 1-phen-
yl-5-[(E)-3-phenylprop-2-enyloxy]-tetrazole (1.0 g; 3.6 mmol) was
heated neat in an oil bath at 100 °C for 2 h to give 1-phenyl-4-
(1-phenylprop-2-enyl)-tetrazole-5-one as the sole product (quantita-
Acknowledgment. The authors are grateful to Fundac¸a˜o para
a Cieˆncia e a Tecnologia, Portugal, and FEDER for financial
support, Grants POCTI/P/FCB/33580/00 and SFRH/BD/17945/
2004 (L.M.T.F.).
1
tive yield). IR νmax: 1729, 1598, 1504, 1382, and 756 cm-1. H
NMR (CDCl3): δ 5.28-5.52 (2H, m), 6.00 (1H, d, J ) 5.7 Hz),
6.36-6.55 (1H, m), 7.30-7.52 (8H, m), 7.95 (2 H, d, J ) 8.6 Hz).
Anal. Calcd for C16H14N4O: C, 69.1; H, 5.1; N, 20.1%. Found:
C, 68.8; H, 5.0; N, 20.4%. MS (EI): m/z 278 [M]+.
Supporting Information Available: Optimized geometries and
absolute energies for the 4-allyl-tetrazolones are available in
Cartesian coordinates. This material is available free of charge via
the Internet at http://pubs.acs.org.
Preparation of Pyrimidinones. 3,4-Dihydro-3-phenylpyrimi-
din-2(1H)-one (6a): 1-phenyl-4-(prop-2-enyl)-tetrazole-5-one 2a
(0.10 g; 0.49 mmol) in methanol (50 mL) was irradiated at 254
nm in a photochemical reactor using a 16-W low-pressure Hg lamp.
Photolysis was conducted at a distance of 10 cm from the lamp.
Gas formation in the solution was observed, corresponding to the
photoeliminated molecular nitrogen. HPLC analysis indicated total
absence of initial reagent after 3 h. The solvent was evaporated
under reduced pressure at room temperature to give 3,4-dihydro-
3-phenylpyrimidin-2(1H)-one 6a as a yellow oil (0.078 g; 92%
isolated yield). IR νmax: 3212 (NH), 3054, 1693 (CdO), 1590, 1566,
1496, 1368, 1230, 1059, and 756 cm-1. 1H NMR (CDCl3): δ 4.65-
4.68 (2H, d), 5.25-5.50 (1H, m), 7.12-7.20 (1H, m), 7.37-7.43
(3H, m), 7.50-7.57 (2H, t), 10.35 (1H, s). MS (EI): m/z 175 [M
+ H]+. Accurate mass (CI): Anal. Calcd for C10H11N2O, 175.12131;
found, 175.12185.
JO060164J
(35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9; Gaussian,
Inc.: Pittsburgh, PA, 1998.
Similarly, other pyrimidinones were prepared and isolated.
3,4-Dihydro-6-methyl-3-phenylpyrimidin-2(1H)-one (6b): from
4-(1-methylprop-2-enyl)-tetrazole-5-one 2b, irradiated for 3.5 h,
(36) Becke, A. D. Phys. ReV. B: Condens. Matter 1988, 38, 3098.
(37) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B: Condens. Matter 1988,
37, 785.
J. Org. Chem, Vol. 71, No. 9, 2006 3591