Green molecular transformation by a B12-TiO2 hybrid catalyst as an alternative to tributyltin hydride

Article abstract of DOI:10.1039/b913255d

The B₁₂-TiO₂ hybrid catalyst was effectively used for the radical reactions, such as 1,2-migrations of phenyl groups and acyl groups under irradiation by UV light, and the selectivity of the reaction was controlled by the solvent system.

Full text of DOI:10.1039/b913255d

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Green molecular transformation by a B12–TiO2 hybrid catalyst
as an alternative to tributyltin hydridew
Hisashi Shimakoshi, Makoto Abiru, Shin-ichiro Izumi and Yoshio Hisaeda*
Received (in Cambridge, UK) 3rd July 2009, Accepted 18th August 2009
First published as an Advance Article on the web 9th September 2009
DOI: 10.1039/b913255d
The B12–TiO2 hybrid catalyst was effectively used for the
radical reactions, such as 1,2-migrations of phenyl groups and
acyl groups under irradiation by UV light, and the selectivity of
the reaction was controlled by the solvent system.
A compound with a cobalt–carbon bond, which was found as
the key intermediate in the B12 (cobalamin derivative)-
dependent enzymic reaction, is a useful reagent for the genera-
tion of an organic radical species.1 The cobalt–carbon bond can
be homolytically cleaved by photolysis, electrolysis or thermo-
lysis to form Co(II) and the corresponding radical species.2
Therefore, application of the alkylated complex for the
radical-mediated organic synthesis is quite interesting from the
viewpoint of organometallic and catalytic chemistries.3 Based on
Fig. 1 Structure of B12 derivative.
this idea, we have been investigating the cobalamin derivative,
(Fig. 1) (ESIw) and TiO2 powder (P-25; a mixture of rutile
heptamethyl cobyrinate perchlorate, which has ester groups in
place of the peripheral amide moieties of the naturally occurring
vitamin B12, and succeeded in performing various molecular
transformations, such as the 1,2-migration of functional groups,
asymmetric reductions, ring expansion reactions, the addition of
alkyl radicals to olefins and the reductive dechlorination of
organic halides by an electrochemical technique.4–8 To achieve
these reactions, one of the most significant properties of
cobalamin is the high nucleophilicity toward various alkyl
halides in its Co(I) state.9 Recently we reported the new catalytic
system of the B12–TiO2 hybrid catalyst where B12 was immobi-
lized on the surface of TiO2.10 As band gap excitation of the
TiO2 produced a conduction band electron under irradiation by
UV light,11 it was strong enough to reduce the Co(II) state of B12
to form the reactive Co(I) species by TiO2.10 This hybrid
catalyst was used for the dechlorination of various chlorinated
organic compounds, such as 1,1-bis(4-chlorophenyl)-2,2,2-
trichloroethane (DDT), under irradiation by UV light. Therefore,
the B12–TiO2 hybrid catalyst was considered to be an efficient
and green catalyst while using a simpler and more facile system.
We now report the further application of this green catalyst for
molecular transformation. Utilizing the property of the B12–TiO2
hybrid catalyst to form a radical species, the 1,2-migration of the
functional group with a radical intermediate was successfully
mediated by the B12–TiO2 hybrid catalyst. The mechanism of
the reaction was also investigated.
(20%) and anatase (80%) with a BET surface area of
50 m2 gꢀ1, Japan Aerosil) as the reported method.10
A diethyl 2-bromomethyl-2-phenylmalonate (1) substrate
and authentic sample of diethyl 2-methyl-2-phenylmalonate
(3) were synthesized from diethyl phenylmalonate (ESIw). The
catalytic reaction was carried out using the following procedure.
To a solution of 1 (29.5 mg, 9.0 ꢁ 10ꢀ5 mol) in 30 mL of
ethanol, 20 mg of the hybrid catalyst (B12 content, 5.6 ꢁ 10ꢀ7 mol)
was added, and nitrogen gas was bubbled through to degas the
oxygen. The solution was stirred for 10 h at room temperature
under irradiation by 365 nm UV light (1.76 mW cmꢀ2 at 12 cm
distance). After the reaction, the hybrid catalyst was readily
separated from the products by centrifugation or filtration.
Table 1 1,2-Migration of a phenyl group catalyzed by B12–TiO2 systema
Products (yieldb (%))
2
3
Entry
Catalyst
Conversionb (%)
1
2
3
4
5
6
B12–TiO2c
B12–TiO2
B12,TiO2
95
28
—
12
14
—
—
65
—
29
31
—
—
Trace
43
46
Trace
Trace
d
e
B12,TiO2
TiO2
B12
f
g
a
Condition: [substrate] = 3.0 ꢁ 10ꢀ3 M, catalyst (B12–TiO2) = 20 mg
(B12,1.86 ꢁ 10ꢀ5 M), solvent C2H5OH 30 mL under N2 at room
b
temperature, reaction time = 10 h. Conversions were estimated by
The B12–TiO2 hybrid catalyst was prepared by the reaction
of cyanoaquacobyrinic acid [(CN)(H2O)Cob(III)7COOH]Cl
the recovery of the substrate. Yields were based on initial concentration
of the substrate. In the dark. Condition: [substrate] = 3.0 ꢁ 10ꢀ3 M,
TiO2 (P-25, 20 mg), [(CN)(H2O)Cob(III)7COOH]Cl, 0.6 mg, solvent
C2H5OH 30 mL under N2 at room temperature, reaction time = 10 h.
c
d
e
Condition: [substrate] = 3.0 ꢁ 10ꢀ3 M, TiO2 (P-25, 20 mg), hepta-
Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, Motooka, Fukuoka 819-0395, Japan.
E-mail: yhisatcm@mail.cstm.kyushu-u.ac.jp; Fax: +81-92-802-2827
w Electronic supplementary information (ESI) available: Details of
experimental procedures. See DOI: 10.1039/b913255d
methyl cobyrinate perchlorate, 0.6 mg, solvent C2H5OH 30 mL under N2
f
at room temperature, reaction time = 10 h. Unmodified TiO2 (P-25,
g
20 mg) was used as the catalyst. [(CN)(H2O)Cob(III)7COOH]Cl, 0.6 mg.
ꢂc
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The products were analyzed by NMR and GC-mass spectro-
scopies. Formation of the phenyl migrated product, diethyl
benzylmalonate (2), was confirmed by spectroscopic compar-
ison to a commercially available authentic sample. The results
are summarized in Table 1. Almost all of the substrate was
converted to products, and the phenyl migrated product (2)
was formed along with the formation of the simply reduced
product (3) as expressed by eqn (1). When TiO2 and the B12
derivative were separately added, conversions were decreased
to half as shown by entries 3 and 4 in Table 1, respectively. The
reaction did not proceed when we used the unmodified TiO2 or
Table 3 Solvent dependence of product distributiona
Products (yieldc (%))
Solventb
Conversionc (%)
2
3
DMF
CH3CN
PhCN
95
85
97
21
39
61
70
44
19
a
Condition: [substrate]
=
3.0
ꢁ
10ꢀ3 M, catalyst
= 20 mg
(B12, 1.86 ꢁ 10ꢀ5 M), solvent 30 mL under N2 at room temperature,
b
reaction time = 10 h. Triethanolamine (200 mg, 1.35 mmol) was
c
used as a sacrificial reagent. Conversions were estimated by the
recovery of the substrate. Yields were based on initial concentration
of the substrate.
B
12 alone as shown by entries 5 and 6 in Table 1, respectively.
Therefore, the B12–TiO2 system is expected to be an efficient
and green alternative for the n-Bu3SnH–AIBN system.12z
The reaction mechanism was also examined by the
spin-trapping technique with a-phenyl N-(tert-butyl)nitrone
(PBN). An ESR spectrum was observed for the PBN spin
adduct formed during the photo-reaction: g = 2.006, AN
=
14.6 G, AH = 3.7 G (104 G = 1 Tesla). Upon the addition of
PBN, the formations of both products were inhibited. These
results indicated that both 2 and 3 are formed from the same
radical intermediate (4) under the present conditions.
ð1Þ
In order to determine the source of the hydrogen in the
products, the reactions shown in eqn (2) were carried out in
C2D5OD, C2D5OH and C2H5OD. The distributions of the
products were analyzed by GC-MS, and the results are summar-
ized in Table 2. Both 20 and 30 gained hydrogen from the ethanol
solvent as shown by entry 1 in Table 2. The incorporation of
deuterium into 20 did not occur during the reaction in C2D5OH as
shown by entry 2 in Table 2. Eighty eight percent of the deuterium
ion was incorporated into 20 in C2H5OD as shown by entry 3 in
Table 2. Therefore, 2 was mainly produced from the anionic
intermediate and was not directly formed from the radical inter-
mediate. In contrast, 92% of the deuterium was incorporated into
the simply reduced product 30 in C2D5OH as shown by entry 2 in
Table 2. Thus, the selectivity of the reaction should be controlled
by the solvent. Based on these results, the reaction was carried out
in a less hydrogen radical donating solvent to improve the product
selectivity. When we used DMF as the reaction solvent, the yield
of the phenyl migrated product 2 decreased to 21% as shown in
Table 3 since DMF is a good hydrogen radical donor.13 In
contrast, the yield of 2 was increased in CH3CN and significantly
increased in PhCN, since PhCN has a strong C–H bond.
The proposed reaction mechanism is shown in Fig. 2. The
Co(II) complex is reduced to Co(I) by electron transfer from
TiO2 under irradiation by UV light.y The hole was quenched
by the solvent, ethanol (or triethanolamine).z The corresponding
alkylated complex is generated by the reaction of the
supernucleophilic Co(I) with the substrate. The cobalt–carbon
bond of the alkylated complex is subsequently cleaved by
photolysis to form the substrate radical 4, which was detected
by an ESR spin-trapping experiment. The hydrogen radical
abstraction from the solvent simply afforded the reduced
product 3. In PhCN, the primary formed substrate radical 4
was effectively converted to the phenyl migrated radical (5).
The radical 5 should be reduced by TiO2 or other reductant
under irradiation by UV light to form the anionic intermediate
(6) due to the two-electron-withdrawing group at the a
position. The protonation of the thus formed 6 probably
afforded 2. To confirm this step (B12-independent step),
we synthesized the probe compound, diethyl 2-benzyl-2-
bromomalonate. Diethyl 2-benzyl-2-bromomalonate was
transformed to the reduced product with an 88% deuterium
incorporation in C2H5OD by the reaction of TiO2 under
irradiation with UV light as shown in Scheme 1 (ESIw).
ð2Þ
Table 2 Deuterium atom incorporation ratio from solventa
D-atom incorporation ratiob (%)
20
30
Entry
Solvent
1
2
3
C2D5OD
C2D5OH
C2H5OD
95
4
88
94
92
8
a
Condition: [substrate] = 3.0 ꢁ 10ꢀ3 M, catalyst (B12–TiO2) = 20 mg
(B12, 1.86 ꢁ 10ꢀ5 M), solvent 30 mL under N2 at room temperature,
b
reaction time = 10 h. Deuterium atom incorporation ratios were
determined by GC-MS.
Fig. 2 Proposed reaction mechanism.
ꢂc
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Notes and references
z The same reaction was carried out using the conventional
n-Bu3SnH–AIBN system as follows. A solution of n-Bu3SnH (10 mL,
3.3 ꢁ 10ꢀ5 mol) and AIBN (0.5 mg, 3.0 ꢁ 10ꢀ6 mol) in benzene (10 mL)
was added dropwise over 3 h to a refluxing solution of 1 (10 mg,
3.0 ꢁ 10ꢀ5 mol) in 10 mL of benzene. The products were analyzed and
quantified by GC-MS with diphenyl as the internal standard. Yields: 2,
50%; 3, 47%.
y Formation of Co(I) species of B12 on TiO2 was confirmed by
diffusion UV-vis spectroscopy with a reflectance maxima at 390 nm
(Fig. S2 in ESIw).
z As the oxidized product, acetoaldehyde diethyl acetal was detected
by GC-MS in C2H5OH. GC-MS (EI, m/z): [M]+, 118; [M ꢀ CH3]+
,
Scheme 1
113; [M ꢀ CH3CH2 + 1]+, 90. The decomposition of oxidized
product for triethanolamine proceeds via a well-known sequence to
diethanolamine and glycolaldehyde.15
This catalytic system was applied for ring-expansion
reactions of alicyclic ketones (5, 6 and 7-membered rings) with
a carboxylic ester and a bromomethyl group6,14 as shown in
eqn (3). The reactions largely proceeded for all the substrates
as shown in Table 4.
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ð3Þ
Table 4 Ring expansion reaction catalyzed by the B12–TiO2 systema
n
Time/h
Conversionb (%)
Products (yieldb (%))
1
2
3
10
12
13
B99
B99
B99
80
77
80
6 Y. Hisaeda, J. Takenaka and Y. Murakami, Electrochim. Acta,
1997, 42, 2165.
a
Condition: [substrate] = 3.0 ꢁ 10ꢀ3 M, catalyst (B12–TiO2) = 35 mg
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(B12, 1.21 ꢁ 10ꢀ5 M), solvent C2H5OH 30 mL under N2 at room
b
temperature, reaction time = 10–13 h. Conversions were estimated
by the recovery of the substrate. Yields were based on initial concen-
tration of the substrate.
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In summary, the B12–TiO2 hybrid catalyst inspired from the
natural coenzyme B12 showed a high reactivity for molecular
transformation under mild conditions. The present system was
clean and economical in contrast to the conventional radical
reagent from the viewpoint of green chemistry. Therefore, the
present system would be readily applicable for the design of an
eco-friendly catalyst.
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This work was partially supported by a Grant-in-Aid for
Scientific Research on Priority Areas (452 and 460) and
Global COE Program ‘‘Science for Future Molecular
Systems’’ from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of Japan and a Grant-
in-Aid for Scientific Research (A) (No. 21245016) from the
Japan Society for the Promotion of Science (JSPS).
ꢂc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6427–6429 | 6429