Naturally-occurring B₁₂(cobalamin)-dependent enzymes catalyze various molecular transformations that are of particular interest from the viewpoint of biological chemistry as well as synthetic organic chemistry and catalytic chemistry.¹  For example, the B₁₂-dependent enzyme catalyzes the rearrangement reactions and the methylation reaction as in the synthesis of methionine.  All of these reactions are mediated by the cobalt alkylated complex which is generally formed by the reaction of the Co(I) state of the B₁₂ with various electrophiles in vitro.  Recently, we have reported the unique catalysis of the B₁₂-titanium oxide (TiO₂) hybrid catalyst in which the B₁₂ complex, cyanoaquacobyrnic acid, is immobilized on the surface of TiO₂ and the B₁₂ complex is reductively activated to form the Co(I) species by electron transfer from TiO₂ under UV-light irradiation.²  The hybrid catalyst mediated the dehalogenation of various organic halides and was applied to the radical-mediated organic reaction via an alkylated complex as a catalytic intermediate.  The great advantage of the catalyst is the facile and efficent formation of Co(I) species by only UV light irradiation.  This property prompted us to investigate further applications of the B₁₂-TiO₂ catalyst utilizing the high reactivity of the Co(I) species of the B₁₂ complex.  We now report the new catalysis of the B₁₂-TiO₂ for H₂O reduction to form hydrogen.  Cobalt complexes have been studied as excellent catalysts for hydrogen production, and the cobalt hydride complex is thought to be an intermediate for the reaction which could be formed by the reaction of Co(I) and a proton.  As the metal hydride complexes are widely used for the reduction of unsaturated compounds, such as alkenes, an application of the B₁₂-TiO₂catalyst for the hydrogenation of C-C mutiple bonds was also investigated.

[1] a) K. ó Proinsias, M. Giedyk, D. Gryko, Chem. Soc. Rev. 2013, 42, 6605-6619; b) K. Gruber, B. Puffer, B. Kräutler, Chem. Soc. Rev. 2011, 40, 4346-4363; c) W. Buckel, B. T. Golding, Annun. Rev. Microbiol. 2006, 60, 27-49; d) K. L. Brown, Chem. Rev. 2005, 105, 2075-2149; e) T. Toraya, Chem. Rev. 2003, 103, 2095-2127, f) R. Banerjee, S. W. Ragsdale, Annu. Rev. Biochem. 2003, 72, 209-247.

[2] a) H. Shimakoshi, Y. Hisaeda, Angew. Chem. Int. Ed. 2015, in press; b) H. Shimakoshi, Y. Hisaeda, ChemPlusChem 2014, 79, 1250-1253; c) H. Shimakoshi, M. Abiru, S. Izumi, Y. Hisaeda, Chem. Commun. 2011, 6427-6429; d) S. Izumi, H. Shimakoshi, M. Abe, Y. Hisaeda, Dalton Trans. 2010, 39, 3302-3307; e) H. Shimakoshi, M. Abiru, K. Kuroiwa, N. Kimizuka, M. Watanabe, Y. Hisaeda, Bull. Chem. Soc. Jpn. 2010, 83, 170-172; f) H. Shimakoshi, E. Sakumori, K. Kaneko, Y. Hisaeda, Chem. Lett. 2009, 38, 468-469.