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[October 30, 2014]

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(ChemPlusChem Via Acquire Media NewsEdge) The B12 -TiO2 hybrid catalyst mediates H2O reduction to form hydrogen under UV irradiation (turnover number of one per hour). The catalyst also mediates reductions of alkenes such as styrene derivatives and alkylacrylates (maximum turnover number of 100 per hour) under mild conditions of room temperature, ordinary pressure, and water or alcohol as solvent.



Keywords : cobalamin · heterogeneous catalysis · hydrogen · hydrogenation · titanium Naturally occurring B12 (cobalamin)-dependent enzymes cata- lyze various molecular transformations that are of particular in- terest from the viewpoint of biological chemistry as well as synthetic organic chemistry and catalytic chemistry.[1a-f] For ex- ample, B12-dependent enzymes catalyze rearrangement reac- tions as typified by the conversion of methylmalonyl-CoA to succinyl-CoA, the reversible interconversion of l-glutamate and threo-b-methyl-l-aspartate, and the methylation reaction as in the synthesis of methionine. All these reactions are mediated by the cobalt alkylated complex, which is generally formed by the reaction of the CoI state of B12 with various electrophiles in vitro.[2a, b] Therefore, the investigation of the reactivity of the CoI species of B12 is important to elucidate the catalytic ability of the enzyme. During the course of these studies, a variety of molecular transformations were achieved by cobalamin deriva- tives, including not only B12-mimic reactions,[2a] but also bioin- spired reactions such as cyclopropane ring cleavage,[3] the re- duction of a variety of substrates such as nitrite,[4] nitrate,[4, 5a] hy- droxylamine,[5b] oxyhalogens,[5c] or- ganic disulfides,[5d] unsaturated es- ters,[5e, 6] thiosulfate,[7] and sulfite,[7] reductive radical cyclization,[8a-c] the photoreduction of CO2,[9] and the reductive dehalogenation of organic halides.[10-15] Recently, we reported the unique catalysis of a B12-titanium oxide (TiO2) hybrid catalyst, in which the B12 complex, cyanoa- quacobyrinic acid (CoIII oxidation state), is immobilized on the sur- face of TiO2 and the B12 complex is reductively activated to form the CoI species by electron transfer from TiO2 under UV ir- radiation.[16a-d] The hybrid catalyst mediated the dehalogena- tion of various organic halides, and was applied to the radical- mediated organic reaction[16a] via an alkylated complex as a cat- alytic intermediate. The great advantage of the catalyst is the facile and efficient formation of the CoI species simply by UV ir- radiation. This prompted us to investigate further applications of the B12-TiO2 catalyst utilizing the high reactivity of the CoI species of the B12 complex. We now report the new catalysis of B12-TiO2 for H2O reduction to form hydrogen, as shown in Figure 1. Cobalt complexes have been studied as excellent cat- alysts for hydrogen production, and the cobalt hydride com- plex is thought to be an intermediate in the reaction, which could be formed by the reaction of CoI and a proton.[17a-f] As metal hydride complexes are used widely for the reduction of unsaturated compounds such as alkenes,[18] the application of the B12-TiO2 catalyst for the hydrogenation of C^C multiple bonds was also investigated.

Hydrogen evolution by the B12-TiO2 was investigated in an aqueous ethylenediamine-N,N,N',N'-tetraacetic acid disodium salt (EDTA·2 Na, sacrificial reagent) (0.1 m) solution, in which the B12-TiO2 (10 mg, anatase, B12 content = 3.43 ^ 10^7 mol) was sus- pended under anaerobic conditions. The results for the hydro- gen production are summarized in Table 1. During UV irradia- tion (365 nm, 1.76 mW cm^2 at a distance of 12 cm), hydrogen gas evolved gradually, as shown in Figure 2. Without the B12 complex (i.e. , using bare TiO2) or UV irradiation, only a little hy- drogen gas evolved (entries 4 and 5 in Table 1). If the B12 com- plex was not immobilized on TiO2 but dissolved in the solution, in which heptamethyl cobyrinate perchlorate (see Supporting Information, Figure S1) was used as the B12 complex, the hy- drogen evolution efficiency decreased (entry 3 in Table 1). The immobilization of the B12 complex on the surface of TiO2 should enhance the reaction, probably owing to the short dis- tance between the B12 complex and TiO2. The hydrogen evolu- tion efficiency was also dependent on the TiO2 crystal type. Anatase-type TiO2 was superior to rutile TiO2 for this reaction, as shown in Figure 2 (entries 1 and 2 in Table 1), because the conduction band electron is more negative in anatase (Ered = ^0.5 V vs. NHE in pH 7 aqueous solution) than in rutile TiO2 (Ered= ^0.3 V vs. NHE in pH 7 aqueous solution).[19] This is an advantage for B12 reduction to form reactive CoI species, be- cause the redox potential for the CoII/CoI couple of the cobala- min derivatives is observed at ^0.3 to ^0.4 V versus NHE in various media.[2a] The hydrogen evolution is inhibited in the presence of a spin-trapping reagent, 5,5-dimethyl-1-pyrroline- N-oxide (DMPO) (entry 6 in Table 1), which suggests the exis- tence of an intermediate having radical character during the reaction.


From the time course of the hydrogen evolution, the turn- over number for the B12 complex was estimated to be about one per hour for the B12-TiO2 (anatase) system. The hydrogen evolution by a cobalamin derivative, cobinamide, combining the use of Eosin Y as a photosensitizer, was reported recently, and the turnover number of the cobalt complex for hydrogen evolution was only around 4.4 ^10^3 per hour.[20] The turnover number frequency for hydrogen evolution by the B12-TiO2 system was superior to that of the reported system, probably because of the efficient production of the CoI species.

The B12-TiO2 catalyst also showed a high activity for the re- duction of alkenes. Upon dissolution of an atropic acid (1)in the solution, hydrogen evolution by the B12-TiO2 (anatase) was completely inhibited and 2-phenyl propionic acid (2) was ob- tained quantitatively within 1 h, as shown by Equation (1).

The B12-TiO2 was separated from the solution easily after the reaction by filtration or centrifugation (Figure S2). The turnover number of the B12 complex was 100 per hour. The hydrogen evolution started after the consumption of all the alkene (Fig- ure S5). This suggests that the same intermediate is involved during hydrogen production and alkene reduction. Notably, the alkene reduction did not proceed with the platinum- loaded TiO2 as well as the bare TiO2. The B12 complex effective- ly mediates the reaction. The source of the hydrogen in the re- duced product was determined by using D2O as the solvent. Two deuterium atoms were incorporated into the 2-phenyl propionic acid, PhCD(CH2D)COOH, as confirmed by 1H NMR and MS analyses (Figures S3b and S4 b). Most hydrogenations by transition-metal systems are conducted under H2, and sometimes under high pressure, whereas the reaction by the B12-TiO2 was achieved without H2,withH2O used as the H source under mild conditions (room temperature, ordinary pressure, and water as the solvent).

The generality of the reaction was then examined by using a variety of substrates. The C^C double bonds of unsaturated esters, that is, alkylacrylates (3a-5a), were selectively reduced to form alkyl propionates (3b-5b) in MeOH with moderate yields, but required a long reaction time [Eq. (2)] . Here, MeOH could work as a solvent and sacrificial reagent to consume the hole of TiO2.[16b] In contrast, nonconjugated alkenes such as al- lylbenzene or vinylcyclohexane were not reduced.

With styrene (6a) as the substrate, 2,3-diphenylbutane (6b) (racemi and meso) was obtained as the main product, and a small amount of ethylbenzene (6c) was also produced [Eq. (3)] . A similar product distribution was also seen for a- methylstyrene (7a), with 2,3-dimethyl-2,3-diphenylbutane (7b) and isopropylbenzene (7c) obtained as the main and minor products, respectively. Note that the reductive dimerization of arylalkenes catalyzed by B12/reducing agents such as Zn or TiIII citrate was reported by Donk and co-workers.[21] The reported photosensitizing system has further advantages in terms of its use of clean light energy and the fact that no additional chem- ical reagents are required.

Interestingly, the B12-TiO2 catalyst showed dual reactions of dehalogenation and hydrogenation. A b-bromostyrene (8)was converted to 2,3-diphenylbutane (6b) (racemi and meso) and ethylbenzene (6c), though substitution at the a position of styrene lowered the reactivity so a long reaction time was re- quired [Eq. (4)] . As the product distribution was similar to that of styrene, the debromination of b-bromostyrene afforded the styrene, and the subsequent reaction of styrene formed the same products according to Equation (3). In contrast, the reac- tion with a-bromostyrene (9) afforded a different product, and 2,3-diphenylbutane (10)(cis/trans ratio 1:1) was produced effi- ciently within 2 h [Eq. (5)] . The position of the Br substituent should change the reaction mechanism dramatically.

On the basis of these results, the mechanism for alkene re- duction by the B12-TiO2 is proposed as shown in Figure 3. The CoI species of B12 first reacts with a proton to form the cobalt- hydride intermediate, CoIII-H. It is likely that the CoIII-H complex has radical character, and hydrogen radical attack occurs at the b-position of the alkene avoiding the steric hindrance of the phenyl group to form the a-radical intermediate (11). If R is H or a methyl group, radical coupling occurs significantly to form the dimerized product. If R is an electron-withdrawing (EWD) group, further reduction by TiO2 to form the carbanion inter- mediate (12) and subsequent protonation to form the simply reduced product as 2-phenylpropionic acid (2) with R =COOH occurs. If R =Br, the carbanion may lead to a carbene (13) with elimination of the bromide ion.[12b, 22] The electrophilic carbene may react with the carbanion (12) to form 2,3-diphenylbutane (10).

As for the hydrogen evolution, if the cobalt-hydride complex has a radical character, a bimetallic mechanism for the CoIII-H complex occurs significantly, as shown in Figure 4. However, the redox potential of the CoIII-H/CoII-H couple is thought to be more positive than that of the CoII/CoI couple in the cobalt complex,[23a,b] so the CoIII-H complex may be reduced to the CoII-H complex under the applied conditions. The more elec- tron-rich CoII-H complex might be nucleophilic and react with a proton to form hydrogen through a hydride mechanism (monometallic mechanism).[24] Determination of the detailed mechanism of hydrogen evolution by B12, that is, the cobala- min derivative, is now in progress in our group.

In conclusion, we have described the catalysis of the B12-TiO2 hybrid catalyst for hydrogen evolution and hydrogenation. The synergistic effect of the naturally occurring cobalamin deriva- tive, cobyrinic acid, and inorganic TiO2 provides the unique cat- alysis of the metal complex-semiconductor hybrid system. Co- byrinic acid and TiO2 are both nontoxic, and the reactions re- ported here all proceeded under mild conditions, with water used as the hydrogen source for the hydrogenation. The heter- ogeneous catalyst, B12-TiO2, is easy to separate from the prod- uct through a simple procedure. Therefore, the B12-TiO2 catalyst would be readily applicable for the design of an ecofriendly catalyst, and the development of such bioinspired catalysis will play an important role in next-generation science and technol- ogy. Research is in progress on further applications of the metal complex-semiconductor system including the B12 com- plex and other metal complexes inspired by natural metal en- zymes.

Experimental Section The B12-TiO2 hybrid catalyst was prepared according to a method reported previously.[16d] The content of the B12 complex on the sur- face of anatase TiO2 was 3.43^ 10^5 mol g^1, and the apparent sur- face coverage by the B12 complex was 7.0 ^10^11 mol cm^2; the B12 complex content on the surface of rutile TiO2 was 3.84 ^ 10^5 mol g^1, and the apparent surface coverage by the B12 com- plex was 7.8^ 10^11 molcm^2. For the alkene reduction, 100 equiva- lents (mol) of alkenes (5.7 ^10^3m) versus the B12 complex on the TiO2 were dissolved in the solvent. The reactions were performed in MeOH except that for atropic acid (1) owing to their poor solu- bilities in H2O. After the photoreaction, the products were identi- fied by GC-MS or NMR spectroscopy.

Acknowledgements This study was supported by a Grant-in-Aid for Scientific Research on Priority Areas (No. 20031021) and Innovative Areas (No. 25105744) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, a Grant-in-Aid for Sci- entific Research (C) (No. 23550125) from the Japan Society for the Promotion of Science (JSPS), an Industrial Technology Re- search Grant Program in 2005 from New Energy and Industrial Technology Development Organization (NEDO) of Japan, and the General Sekiyu Foundation.

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Received : March 24, 2014 Published online on July 10, 2014 Hisashi Shimakoshi* and Yoshio Hisaeda*[a] [a] Prof. Dr. H. Shimakoshi, Prof. Dr. Y. Hisaeda Department of Chemistry and Biochemistry, Graduate Schoolof Engineering Kyushu University, 744 Motooka, Nishi-ku, Fukuoka (Japan) Fax: (+ 81) 92-802-2830 E-mail : [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402081.

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