the theoretical values for the intramolecular [1,3] hydrogen
and deuterium shifts at various temperatures were calculated
as depicted by broken lines in Fig. 6. Good agreements
between the theoretical (broken lines) and the experimental
(solid lines) values support the fact that the intramolecular
[1,3] hydrogen and deuterium shifts proceed via the tunneling
processes at v \ v (E \ 0) and v \ v [E \ E (3.4 kcal
barrier. Because of the weak exothermicity for step 1, the cal-
culated values of *x, *x@ and E became larger [4.51 ] 10~2
B
nm, 3.50 ] 10~2 nm and 1.64 ] 104 cm~1 (46.8 kcal mol~1),
respectively] compared to those for the direct system,
resulting in a decrease of the tunneling probability. The tun-
neling rate constants for the indirect process were obtained as
k \ 4.23 ] 10~10 and k \ 1.24 ] 10~4 s~1, which were
0
1
v
3
1
2
mol~1 for the 1,3-hydrogen shift and 3.8 kcal mol~1 for the
much smaller than the experimental values listed in Table 3.
From these theoretical considerations, we can conclude that
the intramolecular [1,3] hydrogen shift in the photorear-
ranged intermediate of PTTH proceeds via the direct process
and the intramolecular basic catalysis by the triazine ring is
not involved. According to the WoodwardÈHo†mann
rule,29,30 the 1,3-hydrogen shift in the ground state is for-
bidden. However, in the present system, the ortho-
intermediate has a heteroatom (carbonyl-oxygen atom) as the
reactive position, so that the nonbonding electrons on the cor-
responding carbonylÈoxygen atom may play an important
role in the hydrogen shift. That is, the hydrogen atom directly
shifts to the carbonyl oxygen as in the case of phenyl
acetate.18
0
1,3-deuterium shift)] levels.
Possibility for the indirect intramolecular [1,3] hydrogen shift
Similar theoretical calculations were performed for the indi-
rect intramolecular [1,3] hydrogen shift involving the intra-
molecular basic catalysis of the triazine ring. The indirect
process may proceed through two steps as shown in Scheme 2.
The intermediate (a) and product for the indirect process are
the same as those for the direct process. The structure of the
intermediate (b) corresponds to a proton (or hydrogen) atom
transferred form of 2,4-dimethoxy-6-(2-hydroxy-5-methyl-
phenyl)-s-triazine (product), which can be produced through
excited state intramolecular proton transfer (ESIPT).6,10 The
intermediate (b) can take either a neutral or ionic structure.
According to the PM3 calculations, the CxO bond distance
in the intermediate (b) was almost the same as that in the
intermediate (a). Therefore, it would be reasonable to assume
that the intermediate (b) has the keto form as depicted in
Scheme 2. It has been reported that the excitation of the
product produces the intermediate (b) via the ESIPT. The
intermediate (b), thus, reverts to the product by the ground-
state intramolecular proton transfer (GSIPT) which corre-
sponds to the step 2 in Scheme 2. The reported rate for the
GSIPT is B103 times larger [2.3 ] 103 s~1 in poly(methyl
methacrylate) at 293 K]6 than that for the intramolecular [1,
3] hydrogen shift of PTTH (2.2 s~1 in MCH at 293 K). These
facts imply that step 1 is the rate determining step in the indi-
rect process.
Hence, the TET was applied for step 1: the intermediates (a)
and (b) were used for the reactant and product systems of the
indirect process. Fig. 9 shows the potential energy diagram for
the indirect intramolecuar [1,3] hydrogen shift obtained by
the PM3 calculations. The *H value for step 1 is estimated as
[3.6 kcal mol~1. The exothermicity for the step 1 is much
smaller than that ([20.0 kcal mol~1) for the direct process.
Furthermore, the calculated activation energy (44.3 kcal
mol~1) is very high, so that the thermally activated process
would be difficult to occur. Fig. 10 shows the potential energy
surfaces calculated by the TET for the indirect process in the
same manner as the direct process. It can be seen from eqn.
(19) that the tunneling rate constant strongly depends on the
Conclusions
Kinetic studies were carried out for the 1,3-sigmatropic hydro-
gen shift in the photo-Fries rearranged intermediate of PTTH
(or PTTD) by laser Ñash photolysis and theoretical consider-
ations proposed by Formosinho.20h25 The following conclu-
sions were obtained: (1) The rates for the 1,3-hydrogen and
deuterium shifts in the photo-Fries rearranged intermediate of
PTTH and PTTD were measured in various solvents and the
intrinsic rate constants for the intramolecular [1,3] hydrogen
and deuterium shifts were determined to be 1.7 and
5.7 ] 10~1 s~1 in dehydrated methylcyclohexane at 293 K. (2)
The rate for the 1,3-hydrogen shift was signiÐcantly increased
by the presence of basic catalysts, such as triethylamine. A
similar remarkable increase of the rate for the 1,3-hydrogen
shift was also observed in alcoholic solvents, which was
explained as basic catalysis by solvent molecule(s). (3) It was
found that the intramolecular [1,3] hydrogen and deuterium
shifts in the photo-Fries rearranged intermediate of PTTH
and PTTD proceed via quantum mechanical tunneling at two
vibrational levels [v \ v and v \ v (*EH \ 3.4 kcal mol~1,
0
1
3
*ED \ 3.8 kcal mol~1)] according to the Boltzmann dis-
0
tribution law. (4) The tunneling frequency factor for the
present system took a small value (1 ] 1012 s~1) compared
with that of the system (e.g. 1 ] 1013 s~1 for phenyl acetate).
This could be attributed to the presence of sterically bulky
triazine ring. Because of the di†erence in the tunneling fre-
quency factor, the rates for the intramolecular [1,3] hydrogen
shift in the photo-Fries rearranged intermediate of PTTH
took smaller values at each temperature than those of phenyl
acetate.18 Moreover, theoretical considerations based on the
TET showed that, in the intramolecular [1,3] hydrogen shift,
the hydrogen atom migrated directly to the carbonyl oxygen
without the basic catalysis of the triazine ring (the indirect
process).
width (*x or *x@) and height (E ) of the potential energy
B
This work was supported by a Grant-in-Aid for ScientiÐc
Research from the Ministry of Education, Science and Culture
of Japan (10120204). We thank Professor S. Tajima of Gunma
College of Technology for MS measurements of PTTH and
PTTD, and Dr T. Arai for his support of this work.
References
1
2
3
H. Shizuka, T. Kanai, T. Morita, Y. Ohoto and K. Matsui,
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Y. Ohto, H. Shizuka, S. Sekiguchi and K. Matsui, Bull. Chem.
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Fig. 10 Potential energy surface for the indirect process of the intra-
molecular [1,3] hydrogen shift.
K. Tsutsumi, K. Matsui and H. Shizuka, Mol. Photochem., 1976,
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