Oxygen-Controlled Catalysis by Vitamin B12-TiO2: Formation of Esters and Amides from Trichlorinated Organic Compounds by Photoirradiation

Article abstract of DOI:10.1002/anie.201507782

An oxygen switch in catalysis of the cobalamin derivative (B₁₂)-TiO₂ hybrid catalyst for the dechlorination of trichlorinated organic compounds has been developed. The covalently bound B₁₂ on the TiO₂ surface transformed trichlorinated organic compounds into an ester and amide by UV light irradiation under mild conditions (in air at room temperature), while dichlorostilbenes (E and Z forms) were formed in nitrogen from benzotrichloride. A benzoyl chloride was formed as an intermediate of the ester and amide, which was detected by GC-MS. The substrate scope of the synthetic strategy is demonstrated with a range of various trichlorinated organic compounds. A photo-duet reaction utilizing the hole and conduction band electron of TiO₂ in B₁₂-TiO₂ for the amide formation was also developed.

Full text of DOI:10.1002/anie.201507782

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507782
German Edition: DOI: 10.1002/ange.201507782
Hybrid Catalysts
Oxygen-Controlled Catalysis by Vitamin B12-TiO2: Formation of Esters
and Amides from Trichlorinated Organic Compounds by
Photoirradiation
Hisashi Shimakoshi* and Yoshio Hisaeda*
Abstract: An oxygen switch in catalysis of the cobalamin
derivative (B12)-TiO2 hybrid catalyst for the dechlorination of
trichlorinated organic compounds has been developed. The
covalently bound B12 on the TiO2 surface transformed
trichlorinated organic compounds into an ester and amide by
UV light irradiation under mild conditions (in air at room
temperature), while dichlorostilbenes (E and Z forms) were
formed in nitrogen from benzotrichloride. A benzoyl chloride
was formed as an intermediate of the ester and amide, which
was detected by GC-MS. The substrate scope of the synthetic
strategy is demonstrated with a range of various trichlorinated
organic compounds. A photo-duet reaction utilizing the hole
and conduction band electron of TiO2 in B12-TiO2 for the
amide formation was also developed.
chlorophenyl)-2,2,2-trichloroethane (DDT),[7b] 1,2-migration
of a functional group,[7c] H2 production,[7a] and alkene
reduction.[7a] All of these reactions were mediated by the
supernucleophilic CoI form of B12 formed by electron transfer
from TiO2 during light irradiation. In general, this type of
reaction should be carried out under anaerobic conditions to
avoid auto-oxidation of the CoI species. Furthermore, active
oxygen species are generated by the TiO2, which accelerates
the decomposition of the modified compound on the TiO2.[6]
We now report the development of the distinct oxygen-
controlled catalysis by B12-TiO2 as shown in Figure 1.
Trichlorinated organic compounds were transformed into
different products depending on whether the reaction was
O
rganic–inorganic hybrid materials provide various advan-
tages for the design and preparation of heterogeneous
catalysts that have been developed for separation and
recycling in catalytic chemistry.[1,2] Hybrid materials also
show a synergistic effect on catalysis, which has never been
achieved by a single catalyst. For example, a metal complex
combined with a semiconductor achieves light-driven molec-
ular transformations, such as water splitting,[3] CO2 reduc-
tion,[4] and pollutants decomposition.[5] A band gap excitation
in the semiconductor that occurs by light irradiation initiates
the reduction and oxidation of reactants at the surface of the
semiconductor. The light-driven electron transfer chemistry is
a key-step for the reaction. Among semiconductors, titania
(titanium dioxide, TiO2) is widely used in hybrid catalysts
because of its abundance, high stability, moderate band gap
energy, and usable reduction and oxidation abilities generated
by UV light irradiation.[6]
Figure 1. Oxygen-controlled dechlorination of benzotrichloride by
Recently, we synthesized a cobalamin derivative (B12),
cobyrinic acid, immobilized on TiO2 hybrid catalyst where B12
was coordinated to the TiO2 surface by carboxylic groups.[7]
As naturally-occurring B12-dependent enzymes catalyze var-
ious molecular transformations,[8] this hybrid catalyst medi-
ates various molecular transformations with the B12 function
by UV light irradiation such as the dechlorination of 1,1-bis(4-
B12-TiO2.
carried out under N2 or air (oxygen). Recently, Rueping et al.
reported an oxygen switch during visible-light photoredox
catalysis, and oxygen triggers one of two different pathways to
obtain two different types of products from the same starting
materials.[9] The progress of visible-light photoredox catalysts
has contributed to a wide range of organic synthesis
procedures.[10] In our system using a heterogeneous TiO2
catalyst, valuable esters and amides were obtained from
trichlorinated organic compounds under mild conditions (in
air at room temperature) by a simple light irradiation and
work-up procedure.
[*] Prof. Dr. H. Shimakoshi, Prof. Dr. Y. Hisaeda
Department of Chemistry and Biochemistry
Graduate School of Engineering, Kyushu University
Fukuoka, 819-0395 (Japan)
E-mail: shimakoshi@mail.cstm.kyushu-u.ac.jp
yhisatcm@mail.cstm.kyushu-u.ac.jp
To carry out the aerobic reaction, we newly synthesized
a B12 derivative having two trimethoxysilyl groups and a CoIII
metal center, (CN)2CobIII6C1esterCON{(CH2)3Si(OMe)3}2 (1)
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201507782.
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Table 1: Oxidative dechlorination of benzotrichloride to methyl benzoate
(see the Supporting information), which could be covalently
immobilized on the TiO2 surface (Figure 1). This B12 complex
is easily and stably bound to the TiO2 by mixing with TiO2
(anatase, average surface area: 52 m2 gÀ1) in MeOH at room
temperature. The content of the B12 complex on the surface of
TiO2 was 1.37 ” 10À5 molgÀ1, and the apparent surface cover-
age by the B12 complex was 2.68 ” 10À11 molcmÀ2. This
covalently bound B12-TiO2 hybrid catalyst was characterized
by diffuse reflectance (DR) UV/Vis and IR spectroscopies,
and TEM (Supporting Information, Figures S5–S7). No mass
peak ascribed to B12 was detected by MALDI-TOF MS owing
to its strong binding on the surface of the TiO2, which cannot
be desorbed by laser irradiation of the MALDI MS analysis
(Figure S8). In contrast, B12 coordinated to TiO2 by carboxylic
groups was desorbed by laser irradiation of the MALDI MS
analysis, and was detected by its corresponding mass peak
(Figure S9). Therefore, the covalently bound B12-TiO2 was
more tightly immobilized on TiO2 and was a suitable catalyst
to perform the aerobic photoreaction. In fact, the covalently
bound CoIII form of B12 on the TiO2 surface was reduced to
CoI from CoII by UV light irradiation with absorption maxima
at 390 nm and 470 nm, respectively (Figure S10), which was
similar to a previous B12 (cobyrinic acid)-TiO2 system.[7] Thus,
the CoI form of B12 should reacts with various organic halides
owing to its supernucleophilicity.[11]
by B12-TiO2 or other TiO2s.[a]
Entry
Catalyst
Time [h]
Atmosphere
Yield [%][b]
1
2
3
4
5
6
7
B12-TiO2
B12-TiO2
B12-TiO2
B12-TiO2
TiO2
3
5
3
3
3
3
3
air
O2
air
air
air
air
air
99
91
[c]
[e]
0[d]
trace[d]
3
10
Pt-TiO2[f]
[g]
B12-TiO2
45
[a] [B12-TiO2]=10 mg (B12, 2.28”10À5 m), [substrate]=3.0 mm, solvent
MeOH 6 mL at room temperature by UV light irradiation (black light,
lmax =365 nm, 1.5 mWcmÀ2 at 10 cm distance). [b] Yield was based on
initial concentration of substrate. [c] Dark condition. [d] Most of
substrate was recovered. [e] Solvent CH3CN 6 mL was used instead of
MeOH. [f] Pt-TiO2 =10 mg (0.15 wt% Pt). [g] B12(cobyrinic acid)-
TiO2 =8.7 mg (B12, 2.28”10À5 m).
involvement of reactive oxygen species formed directly by
TiO2 in the reaction is considered negligible.[14,15] The
previous B12(cobyrinic acid)-TiO2 also worked (Table 1,
entry 7), but the yield of ester was 45% and the catalyst
was bleached after the reaction owing to the weak binding of
B12 on TiO2.
As for catalytic reactions, we first examined the anaerobic
dechlorination of benzotrichloride by B12-TiO2 in MeOH
(Supporting information). MeOH was used as a solvent and
a hole (hVB+) consumer of the TiO2 valence band. 1,2-
Dichlorostilbenes (2a and 2b) were quantitatively formed by
UV light irradiation under N2 (each 48%; Scheme 1). The
The reaction was also applied to various benzotrichloride
derivatives (Table 2). The esters were obtained in good to
high yields (75–99%). When the reactions were carried out in
ethanol or n-propanol, ethyl benzoate or n-propyl benzoate
were obtained, respectively. In contrast, in iso-propanol, only
a 10% yield of iso-propyl benzoate was obtained. These
+
results implied that the solvent alcohol works as a hVB
quencher of TiO2, and the quenching efficiency is correlated
with the B12-TiO2 catalytic efficiency.[16] In fact, the oxidized
product, dimethyl acetal, was obtained in moderate yield as
the reaction proceeded.[17]
Scheme 1. Anaerobic dechlorination of benzotrichloride by B12-TiO2.
When the reaction was applied to other trichlorinated
compounds, such as DDT, the yield of the ester (4a) was 56%
in EtOH, but with a high selectivity (Scheme 2).[18] From
1,1,1-trichloroethane and chloroform, ethyl acetate (4b) and
ethyl formate (4c) were obtained in moderate yields,
respectively.[18] As for dihalo benzylic compounds, dichloro-
methyl benzene showed low reactivity for the reaction,[19]
while dibromomethyl benzene afforded moderate yield of the
ester (3a).[20]
The proposed reaction mechanism is shown in Scheme 3.
From the reaction of the CoI species in B12-TiO2, the
dichloromethylbenzene radical (5) could be formed from
the trichloromethylbenzene. Owing to the electron-with-
drawing property of the two chlorine atoms, further reduction
by TiO2 formed the carbanion intermediate (6) under
anaerobic conditions. The carbanion 6 may lead to a carbene
with elimination of the chloride ion. The electrophilic carbene
may react with the carbanion 6 to form 1,2-dichlorostilbene
(2a, 2b). Though coupling of the radical 5 may produce
suspended B12-TiO2 catalyst was easily separated by filtration
after the reaction. The content of the B12 complex on the
surface of TiO2 was 1.30 ” 10À5 molgÀ1 after the reaction, and
ca. 95% of B12 survived during anaerobic reaction. In this way,
the covalently bound B12-TiO2 hybrid catalyst also works in
the dechlorination of organic halides.
When the reaction was carried out under aerobic con-
ditions (Supporting Information),[12] the product was dramat-
ically changed and all of the chlorines were removed from the
substrate. Methyl benzoate was formed in 99% yield (Table 1,
entry 1).[13] The reaction also proceeded under oxygen, with
the yield of ester decreasing to 91% and requiring 5 h to
consume all substrate (Table 1, entry 2). These results suggest
that too much oxygen disturbs the reaction, and the reaction
proceeds efficiently under atmospheric conditions. Without
light irradiation or MeOH (hVB+ consumer), the reaction did
not proceed (Table 1, entries 3 and 4). The B12 was also
essential for the reaction (Table 1, entries 5 and 6), and the
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Table 2: Oxidative dechlorination of benzotrichloride derivatives by B12-
TiO2 under air.[a]
Substrate
Product
Yield
[%]
Substrate
Product
Yield
[%]
99
87
93
99[b]
92[c]
90[d]
96
10[e]
99[f]
75
99[f]
Scheme 2. Scope of the substrate variation.
[a] [B12-TiO2]=10 mg (B12, 2.28”10À5 m), [substrate]=3.0 mm, solvent
MeOH 6 mL at room temperature by 3 h UV light irradiation (black light,
lmax =365 nm, 1.5 mWcmÀ2 at 10 cm distance). [b] Solvent=CD3OH.
[c] Solvent=EtOH. [d] Solvent=n-PrOH. [e] Solvent=iPrOH. [f] [sub-
strate]=1.5 mm, 6 h UV light irradiation.
1,1,2,2-tetrachloro-1,2-diphenylethane[21] and, following de-
chlorination, form 1,2-dichlorostilbenes (2a and 2b),[22] we
could not detect such coupling tetrachloro compounds by
GC-MS during the reaction. Thus we have excluded it from
the proposed mechanism. While under aerobic conditions, the
radical 5 may rapidly react with oxygen to form the peroxy
radical.[23] The coupling and subsequent elimination of oxygen
and disproportionation should form benzoyl chloride as an
intermediate.[24] The benzoyl chloride could react with the
alcohol solvent to form the ester. In fact, formation of the
benzoyl chloride was confirmed by GC-MS during the
photoreaction in CH3CN in which the benzoyl chloride
more stably existed than in the alcohol solvent system
(Figure S12). Oxygen incorporation into the ester product
was also confirmed by using 18O2 [Eq. (1)]. In the IR analysis
Scheme 3. Proposed reaction mechanism by B12-TiO2 for dechlorina-
tion of benzotrichloride.
=
of the ester 4a’, the n(ester C O) peak was shifted to
1701 cmÀ1 from 1733 cmÀ1 of the authentic ester 4a (16O;
Figure S13). In the MS analysis, the parent peak was observed
at m/z = 310 for 4a’, while the parent peak for 4a (16O) was
observed at m/z = 308 (Figure S14). Therefore, it is clear that
one 18O atom was incorporated into the ester as a carbonyl
oxygen.[25]
Detection of the benzoyl chloride under the aerobic
conditions prompted us to develop further applications of the
B12-TiO2 catalysis. When the reactions were carried out in the
presence of amines (30 equiv mole toward substrate; see the
Supporting Information for further details), the correspond-
ing amides (7a–7 f) were efficiently produced in place of the
ester (Table 3). As the amine is more reactive to benzoyl
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Table 3: B12-TiO2 catalyzed amidation of benzotrichloride under air.[a]
reactions reported here occurred under mild conditions, that
is, room temperature and in air, without a precious reagent,
and the catalyst was easy to separate from the products.
Ongoing work in our laboratory is focused on the application
of this synthetic strategy to other organic syntheses, and also
establishing a visible light responsive catalytic system using
various visible light responsive TiO2.[32]
Acknowledgements
This study was partially supported by a Grant-in-Aid for
Scientific Research (C) (No. 26410122) from the Japan
Society for the Promotion of Science (JSPS), and the 2014
Tokuyama Science Foundation.
Keywords: cobalamins · heterogeneous catalysis · oxygen ·
photocatalysis · titanium dioxide
[a] [B12-TiO2]=10 mg (B12, 2.28”10À5 m), [benzotrichloride]=3.0 mm,
[amine]=90 mm, solvent=MeOH 6 mL under air at room temperature
by UV light irradiation (black light, lmax =365 nm, 1.5 mWcmÀ2 at 10 cm
distance).
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15439–15443
Angew. Chem. 2015, 127, 15659–15663
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+
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(Scheme 3).
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[27] D. O. Mµrtire, J. A. Rosso, S. Bertolotti, G. C. L. Roux, A. M.
Braun, M. C. Gonzalez, J. Phys. Chem. A 2001, 105, 5385 – 5392.
[28] Small amounts of methyl benzoate (4–10%) were formed in
each reaction.
[29] B. Probst, A. Rodenberg, M. Guttentag, P. Hamm, R. Alberto,
Inorg. Chem. 2010, 49, 6453 – 6460.
[30] To complete the reaction, 30 equiv mole of TEA was required
À
owing to waste of eCB by reduction of O2 (see the Supporting
information).
[31] M. S. Fradin, J. F. Day, N. Engl. J. Med. 2002, 347, 13 – 18.
[32] See reviews: a) J. Schneider, M. Matsuoka, M. Takeuchi, J.
Zhang, Y. Horiuchi, M. Anpo, D. W. Bahnemann, Chem. Rev.
2014, 114, 9919 – 9986; b) R. Asahi, T. Morikawa, H. Irie, T.
Ohwaki, Chem. Rev. 2014, 114, 9824 – 9852.
[15] Furthermore, 3a formation by the B12-TiO2 was not repressed by
singlet oxygen quenchers such as b-carotene or 1,5-dihydrox-
ynaphthalene (see the Supporting information).
[16] The rates and yields of the photocatalytic oxidation of alcohols
on TiO2 were evaluated, see: Y. Tamaki, A. Furube, M. Murai, K.
Hara, R. Katoh, M. Tachiya, J. Am. Chem. Soc. 2006, 128, 416 –
417.
Received: August 19, 2015
Revised: October 9, 2015
Published online: November 3, 2015
[17] It is considered that oxidation of alcohol by h+ produces
aldehyde and proton. This proton could compensate negative
Angew. Chem. Int. Ed. 2015, 54, 15439 –15443
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