Di(hydroxo)porphyrin GeIV complex/silica gel composite as visible light-assisted radical generator

Article abstract of DOI:10.1246/cl.2010.874

A composite of di(hydroxo)germaniumporphyrin with silica gel was used as radical generator which oxidized MeOH to HCHO and hydrocarbons to alcohols in aqueous solution.

Full text of DOI:10.1246/cl.2010.874

doi:10.1246/cl.2010.874
874
Published on the web July 10, 2010
Di(hydroxo)porphyrin GeIV Complex/Silica Gel Composite
as Visible Light-assisted Radical Generator
Tsutomu Shiragami,* Ryuichi Shiraki, Ryuichi Makise, Jin Matsumoto, and Masahide Yasuda
Department of Applied Chemistry, Faculty of Engineering, University of Miyazaki, Gakuen-Kibanadai, Miyazaki 889-2192
(Received May 26, 2010; CL-100502; E-mail: t0g109u@cc.miyazaki-u.ac.jp)
A composite of di(hydroxo)germaniumporphyrin with silica
gel was used as radical generator which oxidized MeOH to
HCHO and hydrocarbons to alcohols in aqueous solution.
1. Fluorescent lamp (22 W)
1
2. Spiral reactor (4 mm
packed with 2
Φ x 2 .5 m)
2
3. Pump
Visible-light driven radical generators have received much
attention from a viewpoint of environmentally conscious
synthetic processes1,2 and P450 models.3 Hydroxometallotetra-
phenylporphyrins, (tpp)(R)M(OH), which have strong visible
light absorption and an HO-axial ligand, are candidates as O-
radical generators to catalyze the hydrogen abstraction from
alkanes (R-H) (Scheme 1). The character of HO-ligands
strongly depends on the central metal atom. Porphyrin com-
plexes of high-valent typical elements which can covalently
bond to axial HO-ligands have unique properties.4 The photo-
generated radical cation (tpp)(R)SbV(OH)+• can easily release
a proton of axial ligand, generating metal-oxo complex,
(tpp)(R)Sb=O or (tpp)(R)Sb-O•, which catalyzes the oxidation
of organic substrates.5 However, Sb complexes have oxidation
potentials too high to be oxidized by usual oxidants. Since
(tpp)GeIV(OH)2 (1a) has lower oxidation potential, we turned
our attentions toward it. The generation of (tpp)(R)Ge-O• was
postulated in the photoreaction of (tpp)(HO)Ge(OOEt) to give
acetaldehyde and ethanol.6 Therefore, the ability of 1a as an
O-radical generator will be examined.
3
4. Sample holder
4
Scheme 2.
Table 1. Photocatalytic oxidation of MeOH in spiral apparatusa
Run Catalyst Oxidant t/hb Product (yields/10¹6 mol) TONc rd
1
2a
2a
2b
O2
O2
O2
180 HCHO (30.6)
60 DCDO (7.8)
36 HCHO (3.4)
3.0 0.027
0.77 0.013
0.03 0.0008
2e
3
4f
2b Fe(NO3)3 132 HCHO (24.6) Fe(II) (139.6) 0.20 0.0018
aIrradiation was applied by fluorescent light to a spiral reactor
(15 cm3) packed with the catalyst 2 (10 g). An aqueous solution
(200 cm3) of MeOH (50 mM) was fed to the reactor from the
sample holder at 30 cm3 min by pump. bIrradiation time.
¹1
cTurnover number (TON) was calculated by the division of the
molar amount of HCHO by amount of 1 on 2 that equals to
¹1
¹1
7.36 © 10¹6 mol g for 1a and 10.3 © 10¹6 mol g for 1b.
dr = (HCHO in mole)/{(irradiation time in h) © (the mole of 1
in 2)}.eUsing MeOH-d3. fUsing Fe(NO3)3 (5.0 mM) as oxidant.
1a7 was insoluble in polar solvents such as MeOH and
MeCN and was poorly soluble even in relatively nonpolar
solvents. Therefore, 1a was used as 1a/SiO2 composite 2a
dispersed in solution. The composite was prepared by dipping
SiO2 (20 g, Fuji Silysia CARiACT Q10, particle size: 1.7-
4.0 mm, surface area: 636 m2 g¹1) into a CH2Cl2-toluene solu-
tion (1:4, 150 cm3) of 1a (147 mg). Most 1a was adsorbed on
SiO2. The particles were filtrated, washed with acetone, and
dried under reduced pressure overnight to yield 2a (content of
1a: 0.53 wt %).
tube (4 mmº © 2.5 m) packed with 2 (12 g) to which an aerated
aqueous solution (200 cm3) containing MeOH (50 mM, 1 mM =
10¹3 mol dm¹3) was fed continuously by pump from the holder
(Scheme 2). The result was shown in Table 1. HCHO was
formed for 180 h with a turnover number (TON) of 3.0,
calculated by division of the molar quantities of HCHO by 1a on
2a.8 No decomposition of 1a was observed in solution, although
1a was eliminated from SiO2. Moreover, the reaction efficiency
was estimated to be r: r = (HCHO in mole)/{(irradiation time
in h) © (the mole of 1 in 2)}. In the photooxidation of CD3OH,
the r values decreased to 48% of that in CH3OH, showing the
isotope effect for hydrogen abstraction by radical reactions (rH/
rD = 2.1),9 as shown in Figure 1.
Photooxidation of MeOH to HCHO was performed at room
temperature by irradiation with fluorescent light in a spiral glass
OH
O2
HO
hν
+
•
•
−
Ph
N
Incident light was absorbed exclusively by 1 on 2. However,
the direct reaction of 1 with MeOH did not occur since the
oxidation potential of MeOH is higher (E1/2ox = 0.56 V vs.
SCE).10 The oxidation of MeOH to HCHO is reasonably
initiated by a hydrogen abstraction from the ¡-carbon of MeOH.
As a reactive species, we suggest (tpp)(HO)Ge-O•/SiO2 radical
(3a) and hydroperoxyl radical (HOO•) which were generated by
an electron transfer from the excited triplet state of 1a (31a*)
M(tpp)
O2
N
SiO2/HO
Ph
OH
M
N
Ph
N
M(tpp)
OH/SiO2
Ph
OH
O•
SiO2
R•
M(tpp)
2
HOO•
3
a: M= Ge(IV)
b: M= Sb(V)+Br
OH/SiO2
to O2. The electron transfer from 1a* to O2 was predicted to
R
H
-
be exoergic by the calculation of the free energy change
by the Rehm-Weller equation11 using E0-0(1a) = 1.63 eV,
E1/2ox(1a) = 1.18 V, and E1/2red(O2) = ¹0.40 V vs. SCE.12 The
3
Scheme 1.
Chem. Lett. 2010, 39, 874-875
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

875
18
16
14
12
10
8
R
Me
OH
n
4f; n=1 (0.43)
4g; n=2 (1.54)
4a; R = CHO (0.25)
4b; R= CH2OH (0.05)
4c; R= COMe (7.95)
4d; R= CH(OH)Me (3.47)
4e; R= C(OH)Me2 (17.5)
6
4
2
Scheme 3. Photooxidation products of hydrocarbons. The
values in parenthesis are the r values.
0
0
12
24
36
48
60
Irradiation time/h
Probes, Eugene, OR, 2002.
2
3
P. Hambright, in The Porphyrin Handbook, ed. by K. M.
Kadish, K. M. Smith, R. Guilard, Academic Press, New
York, 2000, Vol. 3, Chap. 18, pp. 129-210.
J. T. Groves, in Cytochrome P450: Structure, Mechanism,
and Biochemistry, 3th ed., ed. by P. R. Ortiz de Montellano,
Kluwer Academic/Plenum, New York, 2004, Chap. 1,
pp. 1-44.
Figure 1. Time conversion plots of the 2a-photosensitized
oxidation of MeOH (○) and MeOH-d3 (●).
resulting (tpp)Ge(OH)2+•/SiO2 and O2 underwent proton
¹•
transfer to give 3a and HOO•. These O-radicals underwent
•
hydrogen abstraction from MeOH. The resulting CH2OH was
transformed to HCHO through the further oxidation. Although it
4
5
6
7
T. Shiragami, R. Makise, Y. Inokuchi, J. Matsumoto, H.
Inoue, M. Yasuda, Chem. Lett. 2004, 33, 736.
S. Takagi, M. Suzuki, T. Shiragami, H. Inoue, J. Am. Chem.
Soc. 1997, 119, 8712.
A. L. Balch, C. R. Cornman, M. M. Olmstead, J. Am. Chem.
Soc. 1990, 112, 2963.
The preparation of 1a was performed by the reaction of
tetraphenylporphyrin (1.0 g) with GeCl4 (2.0 g) in water-free
quinoline (30 cm3) at 150 °C for 15 h under N2 atmosphere.
The reaction mixture was poured into hexane to give the
precipitate of crude 1a. The CH2Cl2 solution of the resulting
precipitate was washed with aqueous HCl solution and
aqueous NaOH. During the follow-up process, axial chloro
ligands turned to hydroxo ligand. After evaporation, 1a was
isolated by column chromatography on SiO2. Yield 94%.
1H NMR (400 MHz, CDCl3): ¤ ¹2.60 (brs, 2H), 7.77-7.86
(m, 12H), 8.37 (d, J = 5.2 Hz, 8H), 9.04 (s, 8H). 13C NMR
(100 MHz): ¤ 119.3, 128.0, 128.9, 131.3, 134.6, 141.2,
145.7. UV-vis (in MeOH) -max (log ¾) 421 (5.89), 553
(4.34), 591 (4.99) nm.
3
is possible that the reaction of 1a* with O2 generates singlet
1
oxygen (1O2*), abstraction of hydrogen from MeOH by O2*
was ruled out due to insufficient hydrogen affinity.
The photoinduced electron-transfer mechanism was con-
firmed by the use of a composite 2b (content of 1b: 0.87 wt %)
which was prepared from the reaction of (tpp)Sb(OH)2+Br (1b)
¹
with SiO2.13 The photoreaction of 2b in an aqueous MeOH
solution under aerated conditions scarcely gave HCHO. There-
fore, the photoctalytic reaction of 1b was performed using
Fe(NO3)3 as oxidant instead of O2. Irradiation of 2b (50 g)
was performed using an apparatus of Scheme 2 by feeding a
degassed aqueous solution containing MeOH (50 mM) and
Fe(NO3)3 (5.0 mM) to give HCHO along with the formation of
3
FeII ion. In this case, the electron transfer from 1b* to O2 did
not proceed since it was endothermic. However, it is suggested
that electron transfer from the excited triplet state of 1a to FeIII
ion was responsible for initiation since the free energy change
from the triplet state of 1b to FeIII ion was calculated to be
negative using E0-0(1b) = 1.63 eV, E1/2ox(1b) = 1.40 V and
E1/2red(FeIII) = +0.98 V vs. SCE.13 It is suggested that the
hydrogen abstraction from MeOH by (tpp)(HO)Sb-O• radical
8
9
HCHO was quantitatively analyzed by the absorption
spectrophotometry using 4-amino-3-hydrazino-5-sulfanyl-
1,2,4-triazole.
2+
(3b) generated by the deprotonation of (tpp)Sb(OH)2 should
be responsible for the oxidation of MeOH.
Deuterium isotope effect was reported to be 2.03 in
4¹
The radical generator 2a was applied to photooxidation of
hydrocarbons 4. A heterogeneous solution of 4 (4 cm3) and H2O
(10 cm3) in the presence of powdered 2a (2.5 wt %, particle size:
40 ¯m) was irradiated under stirring. The photooxidation of
toluene, ethylbenzene, cumene, methylcyclopentane, and meth-
ylcyclohexane gave a mixture of 4a and 4b, a mixture of 4c and
4d, 4e, 4f, and 4g, respectively (Scheme 3). Especially, their
reactivity was quenched under Ar atmosphere. It was elucidated
that 1a could act as an O-radical generator assisted by visible-
light irradiation. This is the first example of a photocatalytic
radical reaction using a GeIV-porphyrin complex.14
W10O32 -catalyzed oxygenation of 1,1-diphenyl-3,3,3-
trideuterio-2-methylpropene. I. N. Lykakis, G. C.
Vougioukalakis, M. Orfanopoulos, J. Org. Chem. 2006, 71,
8740.
10 H. Kita, Y. Gao, S. Ye, K. Shimazu, Bull. Chem. Soc. Jpn.
1993, 66, 2877.
11 D. Rehm, A. Weller, Isr. J. Chem. 1970, 8, 259.
12 D. T. Sawyer, J. S. Valentine, Acc. Chem. Res. 1981, 14, 393.
13 T. Shiragami, J. Matsumoto, H. Inoue, M. Yasuda, J.
Photochem. Photobiol., C 2005, 6, 227.
14 a) K. M. Kadish, Q. Y. Xu, J.-M. Barbe, J. E. Anderson, E.
Wang, R. Guilard, J. Am. Chem. Soc. 1987, 109, 7705. b) K.
Ishii, S. Abiko, N. Kobayashi, Inorg. Chem. 2000, 39, 468.
c) J.-H. Ha, S. I. Yoo, G. Y. Jung, I. R. Paeng, Y.-R. Kim,
J. Mol. Struct. 2002, 606, 189.
References and Notes
1
R. P. Haugland, in Handbook of Fluorescent Probes and
Research Products, 9th ed., ed. by J. Gregory, Molecular
Chem. Lett. 2010, 39, 874-875
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/