Rationally Designed Metal Cocatalyst for Selective Photosynthesis of Bibenzyls via Dehalogenative C–C Homocoupling

Abstract

The construction of C−C bonds by coupling reactions is an important process in synthetic chemistry though expensive catalysts are required. Heterogeneous photocatalysis offers a platform for C−C coupling of aromatic halides via hydro-dehalogenation by employing alcohols as the hydrogen donor; however, a designed photocatalyst with high activity and selectivity is lacking. Here, we show that Cu is a promising candidate to promote the coupling of aromatic halides due to the optimized adsorption energy of the reaction intermediates (benzyl radical and Br atom) over a series of transition metals. The Cu-modified TiO 2 shows a remarkable apparent quantum efficiency (15%) and a great tolerance of harsh reaction conditions for the homocoupling of benzyl bromides into bibenzyl under UV irradiation. The low-cost photocatalyst also shows high performance upon scaling-up and selective coupling of a series of benzyl bromide derivatives, demonstrating the heterogeneous photocatalytic C−C coupling as an attractive process for applications. Additionally, the design strategy can be applied to modify other photocatalysts (i.e., g-C 3 N 4 ) to realize the C−C coupling of benzyl bromide under visible light.

Catalytic Role of Metal Nanoparticles in Selectivity Control over Photodehydrogenative Coupling of Primary Amines to Imines and Secondary Amines

“…Dehydrogenative coupling (i.e., to form N–N, C–C, C–N bonds) is one class of important reaction that has been realized by photocatalysis for the production of value-added fine chemicals. − This process also offers the possibility, though challenging, to manipulate the abstracted H ads , which may be released as molecular H 2 directly or engage in a consecutive hydrogenation reaction with a surface adsorbed hydrogen acceptor. Such selectivity control is highly desirable in the synthesis of azoxy/azoarene via N–N coupling of anilines, α-hydroxy ketones/pinacol via C–C coupling of aldehydes, and imine/secondary amine via C–N coupling of primary amines and alcohols.…”

“…An intuitive strategy for photocatalyst design is to employ metals that are widely used in traditional heterogeneous hydro-/dehydrogenation reactions (i.e., Ni, Pt, and Pd). Several reports show that the adsorption of hydrogen donors and acceptors on metal clusters is species-dependent, − thus offering opportunities for selectivity control in reactions that involve a hydrogen atom transfer process. Unfortunately, the sorption of intermediates and its interplay with the H ads under irradiation conditions are rarely considered.…”

“…Interestingly, as there is no external hydrogen source for the formation of 3a , the additional H is most likely from 1a . Since previous investigations confirm that the adsorption of H ads is stronger on Pd than on Pt surfaces, , the relatively strong adsorption of 2a on Pd/C 3 N 4 appears to be vitally important for the subsequent hydrogenation step.…”

Flat and Stretched Delafossite α-AgGaO2: Manipulating Redox Chemistry under Visible Light

“…Blank tests show that no reaction takes place without irradiation or without a photocatalyst. For the conversion of benzyl bromide into bibenzyl (Figure a), isopropanol serves both as a solvent and as a hydrogen donor . While the F-AGO fully converts benzyl bromide into bibenzyl with high selectivity (>87%) within 3 h of irradiation, the S-AGO only reaches a conversion of ∼10% at identical reaction conditions.…”

“…For the conversion of benzyl bromide into bibenzyl (Figure 3a), isopropanol serves both as a solvent and as a hydrogen donor. 41 While the F-AGO fully converts benzyl bromide into bibenzyl with high selectivity (>87%) within 3 h of irradiation, the S-AGO only reaches a conversion of ∼10% at identical reaction conditions. The rate constants are 1.00 and 0.05 h −1 for the F-AGO and S-AGO, respectively, indicating that the dominant (001) facets in F-AGO are beneficial for photocatalytic reduction reactions.…”