Intermolecular Formyloxyarylation of Alkenes by Photoredox Meerwein Reaction Chang-Jiang Yao, †,‡ Qiu Sun, † Namrata Rastogi, † and Burkhard Kö nig* ,† † Department of Chemistry and Pharmacy, Universitä t Regensburg, Universitä tsstrasse 31, Regensburg 93040, Germany ‡ Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China * S Supporting Information ABSTRACT: The intermolecular formyloxylation-arylation of stilbenes occurs in the presence of diazonium salts, a photocatalyst, visible light, and DMF. The photo-Meerwein addition products are obtained in good yields up to 76%. We propose the formation of an iminium ion intermediate, which is hydrolyzed to the product. KEYWORDS: formyloxylation-arylation, Meerwein reaction, photocatalysis, diazonium salts, N,N-dimethylformamide 1. INTRODUCTION The arylation of alkenes is an important transformation in organic synthesis. First reports of the reaction using aryl radicals generated from diazonium salts by the Meerwein protocol date back more than 100 years; 1 however, the classic reaction conditions of the Meerwein arylation require metal salts, such as copper oxides, and aqueous media, leading to side reactions and only moderate product yields. 2 Therefore, many improved protocols for arylations with diazonium salts have been reported over the years. 3 The recently introduced photo- Meerwein arylation uses visible-light-induced electron transfer from a photoredox catalyst, for example, eosin Y or Ru(bpy) 3 Cl 2 , to reduce the diazonium salt to aryl radicals, providing a clean conversion under mild conditions. 4 Most of the reported applications of the photo-Meerwein arylation apply an arylation-elimination sequence, regenerating the alkene double bond. The equally important Meerwein arylation addition reaction, in which a nucleophile adds to the carbenium ion intermediate obtained from aryl radical addition and oxidation of the benzyl radical, is less explored in its photocatalytic variant. We recently reported the intermolecular amino arylation of alkenes by photoredox Meerwein addition using nitriles as nucleophile and subsequent Ritter reaction, 5 but there are still few reports about photoredox oxy arylations employing diaryliodonium. 6 Here, we describe the related formyloxy arylation of alkenes by photo-Meerwein addition in the presence of dimethylformamide (Scheme 1). The three- component reaction allows the facile functionalization of styrene derivatives. 2. RESULTS AND DISCUSSION Diazonium salts are labile toward base. The selection of a nonbasic nucleophile to trap the carbenium ion intermediate arising from aryl radical addition to the alkene and oxidation by back electron transfer to the photocatalysts (see below for the mechanistic proposal) is therefore essential. N,N-Dimethylfor- mamide (DMF) is widely used as a dipolar aprotic solvent. DMF can act as an electrophile 7 in reactions with organo- metallic reagents and as a nucleophile or ligand in coordination chemistry. 8 In visible light photoredox catalysis, DMF was recently employed to generate the Vilsmeier reagent. 9 Irradiating 4-chlorophenyl diazonium tetrafluoroborate (1a) and styrene (2a) in the presence of eosin Y (2 mol %) in 0.5 mL of DMF with 530 nm green light gave the alkene formyloxylation arylation product 3aa in 41% yield (Table 1, entry 10). The molecular structure of the compound was unambiguously confirmed by X-ray single crystal analysis Received: February 13, 2015 Revised: March 30, 2015 Scheme 1. Types of Photo-Meerwein Arylation Reactions a a (a) Photo-Meerwein arylation-elimination, (b) photo-Meerwein arylation-addition with nitriles and with dimethylformamide. Research Article pubs.acs.org/acscatalysis © XXXX American Chemical Society 2935 DOI: 10.1021/acscatal.5b00314 ACS Catal. 2015, 5, 2935-2938