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Letter
Merging Catalysis in Single Electron Steps with Photoredox Catalysis – Efficient and Sustainable Radical Chemistry Zhenhua Zhang, Ruben Richrath, and Andreas Gansaeuer ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00787 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 9, 2019
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ACS Catalysis
Merging Catalysis in Single Electron Steps with Photoredox Catalysis – Efficient and Sustainable Radical Chemistry Zhenhua Zhang, Ruben B. Richrath, and Andreas Gansäuer* Kekulé-Institut für Organische Chemie und Biochemie Rheinische Friedrich Wilhelms-Universität Bonn Gerhard DomagkStraße 1, 53121 Bonn, Germany KEYWORDS: electron transfer, photoredox catalysis, radical chemistry, sustainable chemistry, titanocenes ABSTRACT: We describe a combination of catalysts that allows the coupling of titanocene(III) catalysis with photoredox catalysis. Oxidation of radical intermediates by a photoredox catalyst opens novel catalytic mechanisms for reductive epoxide ring opening and redox-neutral epoxide radical arylation. In the former case, the requirement of metallic reductants and stoichiometric acidic additives is bypassed.
‘Catalysis in single electron steps’1 or ‘metalloradical catalysis’2 is a concept merging the advantages of radical chemistry with those of transition metal catalysis. The realization of this idea critically depends on the generation of radicals and their transformation to the closed-shell products through oxidative additions and reductive eliminations in single electron steps.3 For both steps, the metal catalyst is in principle, able to control the typical selectivities exactly as in traditional transition metal catalysis. However, the applicability of these catalytic reactions is limited by the redox properties of the catalyst that determine which oxidative additions and reductive eliminations are possible. Therefore, it is highly desirable to widen the ‘redox potential window’ of such catalytic reactions in order to obtain more broadly applicable processes. Here, we demonstrate that this idea can be realized by merging Cp2TiX-catalyzed reactions with photoredox catalysis. Despite the utility of titanocenecatalyzed radical reactions, 4 an inherent limitation of this methodology is arising from the reducing power of Cp2TiCl, the most commonly employed catalyst.5 The titanocene(IV) intermediates generated during radical generation are often unable to undergo a reductive elimination because the Ti(IV) species are not strong enough oxidants. We will show how this limitation can be overcome with the aid of photoredox catalysis (PRC) for two examples. The first reaction examined is the reduction of epoxide 1 with Hantzsch-ester 2 to give alcohol 3 (Scheme 1). The reaction should have a high thermodynamic driving force due to the aromatization of 2 and the release of ring strain in 1. However, Cp2TiCl is not able to catalyze the reduction of 1 by 2 without additives. In the presence of 10 mol% Cp2TiCl less than 5% of 3 are formed. We postulate that this result is due to the inability of titanocene(IV) derivatives to oxidize the radical σ–complex C to the cationic σ–complex D. This highlights the problem associated with the low oxidizing power of titanocene(IV) complexes.5
OH
O
Bn
In the absence of PRC: 10 mol% Cp2TiCl:
