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Heterogeneous Photocatalytic Click Chemistry Bowen Wang, Javier Esteban Durantini, Jun Nie, Anabel E. Lanterna, and Juan C. Scaiano J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b06922 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016
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Journal of the American Chemical Society
Heterogeneous Photocatalytic Click Chemistry Bowen Wang‡a,b, Javier Durantini‡a, Jun Nie*b, Anabel E. Lanterna*a and Juan C. Scaiano*a a Department 4
of Chemistry and Biomolecular Sciences and Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5. b State Key Laboratory of Chemical Resource Engineering, Faculty of Science, Beijing University of Chemical Technology. Supporting Information Placeholder ABSTRACT: Copper-doped semiconductors are designed to photo-assist the alkyne-azide cycloaddition catalysis by Cu(I). Upon irradiation, injection of electrons from the semiconductor into copper oxide nanostructures produces the catalytic Cu(I) species. The new catalysts are air and moisture tolerant and can be readily recovered after use and reused several times.
Cu on TiO2 for water splitting,11 CO2 photoreduction or for biomedical research,12 but to the best of our knowledge, not to photoinitiated catalytic organic reactions. In a related article Bowman et al.9 explored the use of suspended TiO2 to reduce Cu(II) in solution; while the system worked, it was dismissed as impractical for the imaging applications under study. In a recent review, Pale et al. covered numerous methods involving heterogeneous catalysis of click chemistry that have been, or are currently being developed;13 surprisingly, none of them involves photo-activated catalysis.
Click chemistry (as in Scheme 1) is typically a thermal process efficiently catalyzed by Cu(I)1 or by copper nanostructures.2 In a recent publication we demonstrated that the process involves ‘truly’ heterogeneous catalysis3 as opposed to examples where heterogeneous systems act as suppliers for soluble catalysts; in such cases the advantages of heterogeneous chemistry are defeated by the leaching of active materials.4 A commercial copper-on-charcoal catalyst is also available for click chemistry,5 which in spite of its usefulness in organic chemistry, contains less than 0.003% active surface.6 We report here on a heterogeneous, photoactivated catalyst for the click reaction, specifically for the Huisgen cycloaddition of azides and terminal alkynes. Further, this makes catalyst separation and re-use straightforward. In fact, there have been a few interesting reports of “photo-click” chemistry. Some of them involve photoactivation of an organic reagent,7 while others use organic photoreducing agents to convert soluble Cu(II) to Cu(I), the active catalyst.8;9 The latter approach can afford more flexibility, as it could be largely independent of the detailed structure of the substrates.
This work was undertaken with the goal of developing a hybrid catalyst as it is illustrated in Figure 1 for TiO2 (it is similar for Nb2O5) decorated with copper nanoparticles, mainly present as oxides (vide infra). When supported on TiO2, the conduction band of CuO is about 0.3 eV lower than for TiO2 (note that exact gap will depend on particle size), making the electron transfer shown quite favorable.14 For both semiconductors, excitation in the band gap region (normally in the UVA region, see Figure S1) promotes a valence band electron to the conduction band. Under normal circumstances electron-hole recombination is a relatively fast process unless either electron or hole is trapped; in our case CuO on the surface can trap the electron, yielding Cu(I), Figure 1.12;14-16 Electron donors can trap the hole and in the process hinders the recombination; amines and alcohols can perform this role (vide infra). In other systems the electron can be trapped by O2 and the hole by water in well understood processes, particularly in the case of TiO2,12;17 although there are also a few examples involving Nb2O5.16,18 There are examples12 where taking advantage of the reduced recombination rates, the electron, the hole, or both are trapped by species in solution, including the solvent itself.14 The quantum yields of these processes are limited to
