Visible-Light Photocatalytic E to Z Isomerization of Activated Olefins
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catalysts Article Visible-Light Photocatalytic E to Z Isomerization of Activated Olefins and Its Application for the Syntheses of Coumarins Kun Zhan and Yi Li * Department of Chemistry, Xi’an Jiaotong-Liverpool University, Suzhou 215123, China; [email protected] * Correspondence: [email protected]; Tel.: +86-0512-8816-1431 Received: 17 October 2017; Accepted: 7 November 2017; Published: 9 November 2017 Abstract: Photocatalytic isomerization of thermodynamically stable E-alkene to less stable Z-alkene has been the subject of numerous studies, being successfully achieved mainly under UV irradiation. Recent development of visible light photoredox catalysis has witnessed it emerging as a powerful tool for the access of new structural complexity and many challenging targets. Herein, we report a visible light-promoted E to Z isomerization of cinnamates. When E-isomer of cinnamates was irradiated with blue light in the presence of an organo-photocatalyst, fac-Ir(ppy)3, Z-isomer was exclusively obtained in high yields and with good selectivity. The mild, convenient reaction condition has made this protocol an effective synthetic methodology, which was subsequently implemented in an efficient synthesis of coumarins. Keywords: E to Z isomerization; alkenes; visible-light; photoredox catalyst; coumarin 1. Introduction Photocatalysis under UV activation have been studied and developed for many decades and show capability of promoting the construction of a wide variety of nontraditional bonds in organic synthesis [1–3]. Recently, visible light photocatalysis emerged as an attractive strategy for activation of small molecules, when the chemical reaction is initiated mainly by the light-absorbing photocatalyst, providing mild conditions and high functional group tolerance [4–8]. Alkene has been widely used in organic synthesis as one of the most important building blocks and the essential intermediates in numerous organic transformations. The synthesis of alkene, especially alkene with specific geometry, had been the area of many efforts since the early era of organic chemistry, and is still the field of invention for modern organic synthesis. Over the decades, many methods have been established to obtain the thermodynamically more stable E-alkenes, however the reliable approaches to access the less stable Z-alkene are far less common [9–12]. For example, traditional Wittig reaction with unstable yield [13], alkyne hydrogenation, silicon-based stereospecific elimination and contemporary cross-coupling reaction [14], and cross-metathesis [15] are among a few methods which have so far enjoyed widespread applications, due to their simplicity, convenience, and relatively high geometrical control. The direct photocatalytic E to Z isomerization of alkene, still considered as a fancy reaction in textbooks, has not yet won recognition as an orthodox method for the synthesis of Z-alkene and is a reaction of last resort. However, it has been well established that photochemical activation via a sensitizer, traditionally iodine [16], dyes; and currently photocatalyst [17], riboflavin [18], etc. can convert light into chemical energy and assist to circumvent the ‘uphill’ energy barrier of E to Z isomerization of alkene. The rotational barrier, which restricts the rotation about the double bond on the ground state, mitigates when the olefin is subjected to the photochemical excitation at an appropriate wavelength. Promotion of an electron by excitation from a π orbital to a π* orbital decreases the bond Catalysts 2017, 7, 337; doi:10.3390/catal7110337 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 337 2 of 9 appropriate wavelength. Promotion of an electron by excitation from a π orbital to a π* orbital decreases the bond order and permits the rotation and subsequent relaxation to the ground state [19]. The isomerization of alkenes starts with triplet and singlet excited states, and a change in geometry leads toCatalysts this 2017twisted, 7, 337 intermediate in form of zwitterionic or diradical. The sensitizers with2 of a 9 triplet state have higher energy than that of the excited state of the substrates, proving to be efficient for isomerizationorder and [20]. permits the rotation and subsequent relaxation to the ground state [19]. The isomerization Numerousof alkenes starts studies with on triplet theoretical and singlet perspectives excited states, and and synthetic a change inapplications geometry leads have to thisbeen twisted conducted. In 2014,intermediate Weaver discovered in form of zwitterionic an isomerization or diradical. of E The‐alkene sensitizers to Z‐isomer with a triplet using state blue have LED higher (light energy‐emitting diode)than source, that via of the a photochemical excited state of the pumping substrates, mechanism, proving to be mediated efficient for by isomerization transition‐metal [20]. photocatalyst Numerous studies on theoretical perspectives and synthetic applications have been conducted. [21]; however, the application of this method was limited to an allylamine system (Scheme 1a). In In 2014, Weaver discovered an isomerization of E-alkene to Z-isomer using blue LED (light-emitting 2015, Gilmourdiode) source, reported via a photochemical a bio‐inspired, pumping catalytic mechanism, E to Z mediatedisomerization by transition-metal of activated photocatalyst olefin, (Scheme [21]; 1b), translatinghowever, Nature the’ applications falvin‐mediated of this method photo was‐isomerization limited to an into allylamine the prototype system (Scheme of small1a). molecule In 2015, [18]. The transformationGilmour reported was a bio-inspired,successfully catalytic achievedE toviaZ aisomerization selective UV of‐light activated irradiation olefin, (Scheme at 402 1nm.b), More recently,translating Wang andNature’s Zhaifalvin-mediated disclosed a blue photo-isomerization‐light‐promoted into carbon–carbon the prototype double of small bond molecule isomerization [18]. and itsThe application transformation in the was syntheses successfully of achieved quinolines via a selectivein the absence UV-light of irradiation photocatalyst at 402 nm. [22]. More recently, DuringWang and our Zhai investigation disclosed a of blue-light-promoted synthetic utilities carbon–carbon of visible‐lightdouble photoredox bond isomerization catalysts and for its carbon– application in the syntheses of quinolines in the absence of photocatalyst [22]. carbon coupling reaction, it was noticed that the (E)‐ethyl cinnamates could undergo an isomerization During our investigation of synthetic utilities of visible-light photoredox catalysts for to formcarbon–carbon its Z‐isomer couplingin the presence reaction, of it photoredox was noticed catalyst, that the this (E)-ethyl observation cinnamates prompted could undergo us to explore the probabilityan isomerization and potential to form its ofZ-isomer this transformation. in the presence of Herein, photoredox we catalyst,report the this observationoutcomes of prompted this work, in which usE to Z explore isomerization the probability occurred and potential when E of‐activated this transformation. olefins were Herein, irradiated we report with the blue outcomes light in the presenceof this of organic work, in photocatalyst, which E to Z isomerization fac‐Ir(ppy)3 occurred, to form when its Z‐Eisomer-activated in high olefins yields were irradiatedand good with selectivity (Schemeblue 1c), light and in the presencesynthetic of applications organic photocatalyst, of this transformationfac-Ir(ppy)3, to form was its perfectlyZ-isomer demonstrated in high yields by a facile synthesisand good selectivityof coumarins (Scheme (Scheme1c), and 1d). the syntheticWhile the applications alkene isomerization of this transformation has been was achieved perfectly mostly demonstrated by a facile synthesis of coumarins (Scheme1d). While the alkene isomerization has been with UV activation, this transformation could easily progress via the activation of visible‐light with achieved mostly with UV activation, this transformation could easily progress via the activation of a photoredoxvisible-light catalyst. with a photoredox catalyst. SchemeScheme 1. Photocatalytic 1. Photocatalytic E to Z isomerizationisomerization of olefins.of olefins. Catalysts 2017, 7, 337 3 of 9 2. ResultsCatalysts and2017, Discussion7, 337 3 of 9 Starting from the first observation, we further investigated the photo‐isomerization of (E)‐ethyl cinnamates2. Results under and Discussionvarious reaction conditions. It was verified that this transformation occurred in the presence Startingof fac‐Ir(ppy) from the3 first(5 mol observation, %) under we visible further‐light investigated irradiation the photo-isomerization with blue light, ofleading (E)-ethyl to the formationcinnamates of its under Z‐isomer various with reaction a modest conditions. selectivity It was of verifiedZ/E 65:35 that (Table this transformation 1, entry 1). The occurred reaction in was carriedthe out presence at room of fac temperature-Ir(ppy)3 (5 moland %)finished under in visible-light a quantitative irradiation yield, withthe products blue light, were leading purified to the after a simpleformation filtration of its to Z-isomer remove with the a catalyst modest selectivityfrom the ofreactionZ/E 65:35 mixture. (Table1 ,Further entry 1). experiments The reaction wasrevealed that carriedthe photocatalyst out at room temperaturewas essential and for finished the success in a quantitative of this reaction, yield, the no products isomerization were purified product after could be detecteda simple in filtration