Subscriber access provided by University of Chicago Library
Outer-Sphere Effects in Visible-Light Photochemical Oxidations with Immobilized and Recyclable Ruthenium Bipyridyl Salts Bryan Tambosco, Kevin Segura, Chloé Seyrig, Damien Cabrera, Marc Port, Clotilde Ferroud, and Zacharias Amara ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00890 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Outer-Sphere Effects in Visible-Light Photochemical Oxidations with Immobilized and Recyclable Ruthenium Bipyridyl Salts Bryan Tambosco, Kevin Segura, Chloé Seyrig, Damien Cabrera, Marc Port, Clotilde Ferroud, Zacharias Amara* Equipe de Chimie Moléculaire, EA 7341, Laboratoire de Chimie Moléculaire, Génie des Procédés Chimiques et Energétiques, Conservatoire National des Arts et Métiers, 2 rue Conté, Paris 75003, France Supporting Information Placeholder ABSTRACT: The ability to accelerate visible-light photochemical reactions in a simple setup and with little photocatalyst modification is an important challenge. We report that adsorption of a widely used organometallic photocatalyst ([Ru(bpy)3]Cl2) on unmodified silica particles provides opportunities in the intensification of photochemical oxidations with an almost ten-fold increase in reactivity. This outstanding performance is attributed to non-covalent outer-sphere interactions between the substrate and the solid particles because higher concentrations of reactive species are produced at the interface. This simple catalytic system is efficiently recycled and shows an up to four-fold increase in stability compared to its homogeneous counterpart. As a proof of concept, we apply this straightforward immobilization strategy to the semi-synthesis of the antimalarial drug artemisinin from dihydroartemisinic acid. Our results demonstrate that the surface has a cooperative and bifunctional role which avoids the use of hazardous acid reagents and can potentially afford a more efficient and lower cost access to this important pharmaceutical compound.
cific interactions. In addition, the support can be used as an orthogonal reagent, here an acid catalyst, to maximize cooperative effects.
KEYWORDS: photochemistry, singlet oxygen, oxidation, ruthenium polypyridine, supported catalysts, outer-sphere, artemisinin The harvesting of visible-light with photocatalysts can promote a wide range of chemical reactions very selectively.1 This approach is becoming increasingly popular in the pharmaceutical and chemical sectors as it avoids using high energy ultraviolet light which can lead to poor selectivities.2 However to date, the chemical industry is still striving to scale up visible-light photocatalytic processes.3 Powerful solutions have been proposed by designing transparent flow reactors generating thin films where light is more efficiently converted into reactive species.4 These enabling technologies have looked at engineering new photoreactors but improvements that can be applied to already existing batch reactors are also highly desirable. In this context, the molecular engineering of photocatalytic systems at the nano-scale level has been found to have a significant impact on productivity.5 This article presents an alternative concept in which an inexpensive and straightforward modification of a readily available photocatalyst can intensify visible-light processes in batch reactors. Our reaction design is based on the development of a “cooperative hybrid organic-inorganic interface”6 using an immobilized photocatalyst ([Ru(bpy)3]2+) on unmodified silica. The immobilization creates an acidic polar microenvironment around the photocatalyst7,8 that can selectively accelerate the photochemical transformation of substrates bearing polar headgroups via an outer-sphere effect (Figure 1).8 The surrounding inorganic framework therefore not only plays a key role in facilitating recovery and recycling but also acts synergistically, by concentrating the substrate closer to the active sites through spe-
Figure 1. A general approach to improve productivity in visiblelight photochemical reactions: adsorption of cationic photocatalyst [Ru(bpy)3]2+ on an oxide. This approach is applied to the synthesis of two commercially important terpenes: artemisinin (1) and rose oxide (2). Immobilization of homogeneous transition metal catalysts has been extensively studied9 but examples in the field of photocatalysis are rather limited.5,10-14 The surprisingly simple adsorption of [Ru(bpy)3]2+ salts on oxides, which is known to produce solar cells and luminescent materials,15 has to the best of our knowledge, been applied to organic chemistry only once in the context of photo-polymerization.16 We speculated that such systems would be ideal to provide simple recycling options for ready to use “off the shelf” [Ru(bpy)3]2+ photocatalysts and we anticipated that surface effects could allow controlling these reactions in a new manner. In the present study, we describe how the adsorption of a [Ru(bpy)3]2+ salt on silica produces an inexpensive hybrid system that is more reactive and more stable than the homogeneous catalyst and that can be easily recycled. We describe here the efficient immobilization by adsorption and photochemical reactivity of [Ru(bpy)3]Cl2 on silica in the context of photochemical oxidations with singlet oxygen (1O2). 1O2 is a key reagent in natural products synthesis, especially in the commercial semi-synthesis of artemisinin (1) and in the production of the fragrance rose oxide (2).17
ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Figure 2. (A) Photo-oxidation of citronellol 3 into two regio-isomers 4 and 5; (B) Conversion rates at different [Ru(bpy)3]2+ densities (mg.g-1) as compared to the homogeneous reaction (S/C = 1000).. Reactions were monitored with GC-MS and 1H NMR with an internal standard (see ESI); (C) Mechanistic hypothesis accounting for the rate acceleration effects with supported [Ru(bpy) 3]2+/SiO2; (D) Substrate scope analysis at fixed S/C ratio and [Ru(bpy)3]2+/SiO2 of 2.5 mg.g-1. In each case, the same silica support was used (Macherey Nagel, silica 60Å, 500 m2/g, 40-63 μm particle size, see Table S2) Our initial studies investigated the retention of [Ru(bpy)3]Cl2 on different commercial oxides (see ESI, table S1). We rapidly identified silica as an efficient adsorbent with a maximum adsorption of 16 mg.g-1 after mixing for 3 days in distilled water or in water/ethanol mixtures (saturation concentration obtained with silica 60Å, 500 m2/g, 40-63 μm particle size Macherey Nagel). Other commercial silica samples gave lower maximum adsorption values (9 mg.g-1 for Silica 150Å, 300 m2/g, 35-70 μm particle size Sigma Aldrich; and 6 mg.g-1 for LiChroprep, 60Å, 480-540 m2/g, 15-25 μm particle size Merck). All these measurements were carried out by UV-Vis absorption spectroscopic analysis of the filtrate by referring to a calibration curve with a sensitivity of 10 6M (see ESI, Figure S2 and S4). These results show no direct relationship between the maximum adsorption of [Ru(bpy)3]2+ and the surface area. As postulated by Furlong, the saturation could instead rely on the topography of the inorganic surface which strongly depends on the method used for silica production.18 This variation in topology could favor different packing of the dyes “at the highest density of silica surface charge attainable”. The adsorption mechanism of [Ru(bpy)3]2+ salts on silica is believed to be mainly driven by electrostatic interactions.19 Adsorption is low when pH falls below the isoelectric point (pH
