Photocatalytic reduction of acetylpyridine to pinacol using [fac-Re(bpy)(CO)3{4-(MeCO)py}]+ (bpy = 2,2′-bipyridine, py = pyridine)

Article abstract of DOI:10.1246/cl.2000.376

[fac-Re(bpy)(CO)₃{4-(MeCO)py}]⁺ can act as an efficient photocatalyst for the reduction of 4-acetylpyridine to a pinacol (py-CMeOHCMeOH-py) in the presence of triethanolamine. The reaction proceeds via formation of a pinacol complex [fac-Re(bpy)(CO)₃(py-CMeOHCMeOH-py)]⁺.

Full text of DOI:10.1246/cl.2000.376

3
76
Chemistry Letters 2000
+
Photocatalytic Reduction of Acetylpyridine to Pinacol Using [fac-Re(bpy)(CO) {4-(MeCO)py}]
3
(bpy = 2,2'-bipyridine, py = pyridine)
Hisao Hori*, Kazuhide Koike, Koji Takeuchi, and Osamu Ishitani†
National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Urawa, Saitama 338-8570
†
(Received November 30, 1999; CL-991016)
+
is 2, by comparing with those of the commercial species.8
Figure 1 shows the formation of 2 with 405 nm irradiation.
[
fac-Re(bpy)(CO) {4-(MeCO)py}] can act as an efficient
3
9
photocatalyst for the reduction of 4-acetylpyridine to a pinacol
py-CMeOHCMeOH-py) in the presence of triethanolamine.
(
The pinacol 2 increased linearly until 4 h, and the quantum
1
0
The reaction proceeds via formation of a pinacol complex [fac-
yield for the formation of 2 was 0.25. The amount of 2 was
almost constant by prolonged irradiation, e.g., 51% yield (2 ×
+
Re(bpy)(CO) (py-CMeOHCMeOH-py)] .
3
[
2] / [4-MeCOpy] used) at 20 h irradiation with a turnover
number of 5.5. The 4-(MeCO)py has very weak absorption at
405 nm. Actually, the yield of 2 for 4 h irradiation in the
absence of 1 was 0.4%, which is much less than that in the
presence of 1 by a factor of 128. This finding indicates that the
formation of 2 is photocatalyzed by 1.
Rhenium bipyridine complexes [fac-Re(bpy)(CO) X]n+ (n
0 or 1; bpy = 2,2'-bipyridine; X = neutral or anionic mono-
3
=
dentate ligand) have received a great deal of attention with
1
regard to their photochemical properties. Irradiation of these
complexes with an amine causes an electron transfer from the
amine to the excited rhenium complexes to produce the one-
To elucidate the changes of the structure of the complex
by irradiation, electrospray (ES) mass spectra of the reaction
•
–
(n-1)+ 2
11
electron reduced complexes [fac-Re(bpy )(CO) X]
.
The
solutions were measured. Before irradiation, the spectrum
3
reduced complexes bring about interesting photochemical reac-
consisted of a large peak of 1 (m/z = 548). After irradiation
started, the peak of 1 became small and an outstanding peak
appeared at m/z of 671, corresponding to a complex with 2 as
3
4
tions such as CO reduction, ligand substitution, ethylation of
2
5
the bpy ligand, and rearrangement of the ligand from CN-pyri-
-
6
dine to CN .
a ligand, i.e., the pinacol complex [fac-Re(bpy)(CO) (py-
3
+
+
We report here that [fac-Re(bpy)(CO) {4-(MeCO)py}] (1)
CMeOHCMeOH-py)] (3) (Figure 2). The peak of 3 was
3
acts as an efficient photocatalyst for the reduction of 4-
acetylpyridine [4-(MeCO)py] to a pinacol type compound 2,3-
di(4-pyridyl)-2,3-butanediol (py-CMeOHCMeOH-py) (2) in
7
the presence of triethanolamine (TEOA). This photocatalytic
reaction does not proceed via simple photosensitization of 1 for
the electron transfer, but proceeds via the reaction on the ligand
of the Re-complex.
3
Irradiation of a TEOA / DMF (1/5, v/v, 4 cm ) solution of
-
3
-3
1
(2.60 mmoldm ) and excess 4-(MeCO)py (56.8 mmol dm )
1
at 405 nm caused white precipitate in the reaction cell. The H-
and 13C-NMR spectral measurements indicated that the product
detected until the formation of 2 stopped. This observation
indicates that 3 acts as an important intermediate in the photo-
catalytic formation of 2.
To clarify the role of 4-(MeCO)py, 1 was irradiated in
TEOA / DMF without addition of free 4-(MeCO)py. Despite
the absence of free 4-(MeCO)py in the initial solution, free 4-
(
(
MeCO)py was detected after the short period of irradiation
Figure 3). The yield of 4-(MeCO)py reached a maximum
around 5 min irradiation (9%, based on [1] used) and then
decreased. The appearance of free 4-(MeCO)py can be
•
−
explained by the formation of one-electron reduced species 1 ,
which should be produced by photochemical electron transfer
from TEOA [equation (1)] and subsequent ligand substitution
4
,7
with solvent molecules (S : TEOA or DMF) [equation (2)].
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
377
References and Notes
1
K. Kalyanasundaram, in “Photochemistry of Polypyridine
and Porphyrin Complexes,” Academic Press, London
(1992).
2
3
C. Kutal, M. A. Weber, G. Ferraudi, and D. Geiger,
Organometallics, 4, 2161 (1985).
It is noteworthy that the formation of 2 has induction period
and appears after the decrease of free 4-(MeCO)py.
a) J. Hawecker, J.- M. Lehn, and R. Ziessel, Helv. Chim.
Acta, 69, 1990 (1986). b) H. Hori, F. P. A. Johnson, K.
Koike, O. Ishitani, and T. Ibusuki, J. Photochem.
Photobiol. A: Chem., 96, 171 (1996). c) K. Koike, H. Hori,
M. Ishizuka, J. R. Westwell, K. Takeuchi, T. Ibusuki, K.
Enjoji, H. Konno, K. Sakamoto, and O. Ishitani,
Organometalliics, 16, 5724 (1997).
The ES-mass spectrum of the irradiated solution in the
absence of free 4-(MeCO)py showed the formation of pinacol
complex 3 and the solvent complexes, i.e., [fac-Re(bpy)-
+
+
(
CO) DMF] (m/z = 500) and [fac-Re(bpy)(CO) TEOA] (m/z =
3
3
576). When the formation of 2 was increased (10 to 20 min in
Figure 3), the peak height of 3 in the ES-mass spectra decreased
while those of the solvent complexes increased, indicating that
4
H. Hori, F. P. A. Johnson, K. Koike, K. Takeuchi, T.
3
releases 2 to form solvent complexes.
Ibusuki, and O. Ishitani, J. Chem. Soc., Dalton Trans.,
1997, 1019.
5
6
7
O. Ishitani, I. Namura, S. Yanagida, and C. Pac, J. Chem.
Soc., Chem. Commun., 1987, 1153.
H. Hori, J. Ishihara, K. Koike, K. Takeuchi, T. Ibusuki, and
O. Ishitani, Chem. Lett., 1997, 1249.
The complex [1][SbF ] was prepared according to a litera-
6
ture method, see H. Hori, J. Ishihara, K. Koike, K.
Takeuchi, T. Ibusuki, and O. Ishitani, J. Photochem.
Photobiol. A: Chem., 120, 119 (1999).
8
9
The decomposition temperature of 2 (215 – 217 °C) was
also in good agreement with the reported value (219 - 220
°C) [M. J. Allen and H. Cohen, J. Electrochem. Soc., 106,
451 (1959)].
The amount of 2 was measured by a HPLC system.
Column: Develosil-ODS-UG-5 (Nomura Chemical),
mobile phase: a mixture (30 : 70, v/v) MeOH / KH PO -
2
4
-
3
NaOH buffer (0.05 mol dm , pH 5.9), detection wave-
length: 270 nm. When the sample included solid, all the
content in the reaction cell was diluted by EtOH to make a
homogeneous solution, then subjected to the measure-
ments. Other alcohol species such as 4-(MeCHOH)py
were not observed.
Although the formation of 3 from 1 is still unclear, two
mechanisms would be possible. One is the direct reaction of
•
−
the one-electron reduced species 1 and/or its protonated form
1
10 The quantum yield was calculated by the equation (forma-
•
+
-H• with free 4-(MeCO)py to produce 3 , followed by the
one-electron injection, and another is the coupling reaction of
-H• to form [Re(bpy)(CO) (py-CMeOHCMeOHpy)-
tion rate of 2) / (light intensity). The light intensity was
-8
-1
23
1.49 × 10 einstein s (1 einstein = 6.022 × 10 photons).
11 The irradiated sample solution was introduced to the elec-
trospray probe without any pretreatment. Details of the
system, see H. Hori, J. Ishihara, K. Koike, K. Takeuchi, T.
Ibusuki, and O. Ishitani, Anal. Sci., 14, 287 (1998).
1
(
[
3
2
+
CO) (bpy)Re] , followed by its cleavage to 3 and
Re(bpy)(CO) ] . It should be noted that, in both cases, 2 is
3
+
3
produced via the reaction on the ligand of the Re-complex and
these mechanisms are different from the reported photosensi-
tized pinacol formation explained by simple photoelectron
transfer processes.12
Further application of this method to other pyridyl ketones
and details of the reaction mechanisms are being investigated in
our laboratory.
12 a) C. Pac, S. Kaseda, K. Ishii, S. Yanagida, and O. Ishitani,
in “Photochemical Processes in Organized Molecular
Systems,” ed by K. Honda, Elsevier, Amsterdam (1991), p.
177. b) O. Ishitani, S. Yanagida, S. Takamuku, and C. Pac,
J. Org. Chem., 52, 2790 (1987).