Letter Cite This: Org. Lett. 2018, 20, 5799−5802
pubs.acs.org/OrgLett
Interception of Radicals by Molecular Oxygen and Diazo Compounds: Direct Synthesis of Oxalate Esters Using Visible-Light Catalysis Meihua Ma, Weiwei Hao, Liang Ma, Yonggao Zheng, Pengcheng Lian, and Xiaobing Wan* Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China
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ABSTRACT: The synthesis of oxalate esters through a radical process, rather than the traditional ionic reaction, has been well developed in which the radicals induced by visible light are trapped by molecular oxygen and diazo compounds under room temperature. This reaction is operationally simple, mild, and shows broad substrate scopes in α-bromo ketones and diazo compounds.
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Since 2008, visible light-initiated radical reactions contributed by MacMillan and co-workers have grown remarkably in modern organic synthesis,8 and many excellent works have been reported.9,10 Given the abundance and accessibility of oxygen, it is considered to be an ideal oxidant and perfect source of oxygen for the construction of organic molecules.11 In this context, development of a new reaction strategy employing visible light as energy and molecular oxygen as the oxidant or oxygen source under mild conditions seems highly desirable for modern organic synthesis. Recently, significant developments have been made for visible-light-induced aerobic oxidative transformation.12 For instance, in 2011, Jiao’s group13 developed an aerobic oxidation of benzyl halides in which molecular oxygen participated in the photocatalyst cycle and was introduced into the products under visible-light conditions. Very recently, Nicewicz and co-workers14 reported a photoredox-catalyzed direct C−H cyanation of arenes using molecular oxygen as an oxidant to regenerate the photocatalyst. Herein, we envisioned that organic bromide may serve as radical precursor under photoredox catalysis, which could then be trapped by dioxygen and diazo compounds15 to give the oxalate esters (Scheme 1c). Initially a reaction between benzyl bromide and ethyl diazoacetate as the model substrates was attempted using 1.0 mol % eosin Y as photoredox catalyst and CH3CN as solvent under O2 atmosphere under irradiation from 12 W green LED, but the reaction was unsuccessful. Alternatively, however, when the reaction was attempted with α-bromoacetophenone 1a,16 we were delighted to observe the formation of the desired product 3a in 56% yield (Table 1, entry 1). Screening different solvents for the reaction revealed that the reaction in DMF afforded 3a in improved yield of 85% (entries 2−12). Eosin Y proved to be the optimal catalyst for the transformation during investigations of the reaction under different photocatalysts
xalate esters are a prevalent class of compounds that are widely found in natural products1 and pharmaceutical agents2 and can serve as widespread building blocks in organic synthesis.3 The most classical methods for the synthesis of oxalate esters are the coupling reactions of oxalic acid or oxalic acid derivatives with alcohol (Scheme 1a).4 Transition-metalScheme 1. Synthetic Strategies for the Synthesis of Oxalate Esters
catalyzed oxidative coupling of CO with CH3OH also consititutes a good strategy to deliver dimethyl oxalate (Scheme 1b).5 In addition, ozonolysis of alkenes or alkynes in the presence of alcohol has emerged as a powerful method for the synthesis of oxalate esters.6 Other alternative pathways to oxalate esters include dimerization of formates, Oxonemediated oxidative cleavage of β-keto esters, alcoholysis of the trichloroacetyl compounds, and oxidation of ascorbic acid and ascorbic acid derivatives.7 In general, the present approaches for the systhesis of oxalate esters mainly rely on the ionic process. To the best of our knowledge, the radical-promoted formation of unsymmetrical oxalate esters has not been discovered until now. © 2018 American Chemical Society
Received: August 3, 2018 Published: August 29, 2018 5799
DOI: 10.1021/acs.orglett.8b02487 Org. Lett. 2018, 20, 5799−5802
Letter
Organic Letters Table 1. Optimization of the Reaction Conditionsa
Scheme 2. Scope of α-Bromo Ketonesa
entry
photocatalyst
solvent
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19c 20 21d 22e
eosin Y eosin Y eosin Y eosin Y eosin Y eosin Y eosin Y eosin Y eosin Y eosin Y eosin Y eosin Y RhB Ru(bpy)3Cl2.6H2O RhB-6G rose bengal alizarin red Acr+-MeClO4− eosin Y
CH3CN DME 1,1,2-trichloroethane DMF DMSO cyclohexane PE H2O i-PrOH THF Tol NMP DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF
56
