Switchable molecular tweezers: design and applications

• Pablo Msellem,
• Maksym Dekthiarenko,
• Nihal Hadj Seyd and
• Guillaume Vives

Beilstein J. Org. Chem. 2024, 20, 504–539, doi:10.3762/bjoc.20.45

• reported tweezers 32 (Figure 17) incorporating chiral Cr(III)–salen arms for the allosteric regulation of a Henry reaction between benzaldehyde derivatives and nitromethane. In the closed form induced by K+ binding, high enantiomeric excesses around 90% and yields (<70%) were obtained due to the

• allosteric catalytic regulation (Figure 21). Mirkin and co-workers reported the symmetric tweezers 40 based on a Rh(I) complex with a phosphine and a labile thioether site and Cr(III)–salen arms as catalytic sites for the asymmetric ring opening of cyclohexene oxide by TMSN3 [80]. The closed tweezers showed

• higher activity and enantioselectivity due to the preorganization of the Cr(III) catalytic centers enabling bimetallic catalysis compared to the more flexible arrangement of the salen in the open form. The tweezers could be opened by breaking the thioether–rhodium bond with a combination of Cl− and CO

Green and sustainable approaches for the Friedel–Crafts reaction between aldehydes and indoles

• Periklis X. Kolagkis,
• Eirini M. Galathri and
• Christoforos G. Kokotos

Beilstein J. Org. Chem. 2024, 20, 379–426, doi:10.3762/bjoc.20.36

• well as Lewis acids, such as FeCl3 [23][24], RuCl3·3H2O [25], AgNO3 [26], glycerol and [Fe(III)-(salen)]Cl [27], Fe(DS)3·nH2O [28], Sc(OTf)3 [29], B(C6F5)3 or PhSiCl3 [30] and Cp2TiCl2 [31]. However, most of these reactions face some serious drawbacks, such as the requirement of large quantities of the

Radical ligand transfer: a general strategy for radical functionalization

• David T. Nemoto Jr,
• Kang-Jie Bian,
• Shih-Chieh Kao and
• Julian G. West

Beilstein J. Org. Chem. 2023, 19, 1225–1233, doi:10.3762/bjoc.19.90

• simplified manganese salen complex I, allowing for the identity of the carbon–heteroatom bond to be controlled based on added nucleophile and enabling C–Cl, C–N, and C–S bonds to be formed directly while completely suppressing traditional ATRA products [9]. In mechanistic studies, rearrangement products

Dissecting Mechanochemistry III

• Lars Borchardt and
• José G. Hernández

Beilstein J. Org. Chem. 2022, 18, 1454–1456, doi:10.3762/bjoc.18.150

• the use of halogenated compounds and include the mechanochemical preparation of isocyanides [10], formylated and acetylated amines [11], and the mechanosynthesis of unsymmetrical salens ligands for preparing metal–salen catalysts [12]. This illustrates the broad applicability of mechanochemical

Mechanochemical synthesis of unsymmetrical salens for the preparation of Co–salen complexes and their evaluation as catalysts for the synthesis of α-aryloxy alcohols via asymmetric phenolic kinetic resolution of terminal epoxides

• Shengli Zuo,
• Shuxiang Zheng,
• Jianjun Liu and
• Ang Zuo

Beilstein J. Org. Chem. 2022, 18, 1416–1423, doi:10.3762/bjoc.18.147

• the unsymmetrical salens with zinc, copper, and cobalt was studied and the chiral Co–salen complex 2f was obtained in 98% yield. Hydrolytic kinetic resolution (HKR) of epichlorohydrin with water catalyzed by complex 2f (0.5 mol %) was explored and resulted in 98% ee, suggesting complex 2f could serve

• as an enantioselective catalyst for the asymmetric ring opening of terminal epoxides by phenols. A library of α-aryloxy alcohols 3 was thereafter synthesized in good yield and high ee using 2f via the phenolic KR of epichlorohydrin. Keywords: α-aryloxy alcohols; chiral Co–salen; HKR

• key organic intermediates in the drug discovery and process chemistry [4][5][6]. Chiral metal–salen complexes were designed for catalyzing reaction processes that resulted in good yield, high regioselective and enantioselective control for the asymmetric ring opening of terminal epoxides. Various

Heteroleptic metallosupramolecular aggregates/complexation for supramolecular catalysis

• Prodip Howlader and
• Michael Schmittel

Beilstein J. Org. Chem. 2022, 18, 597–630, doi:10.3762/bjoc.18.62

• showed a two-fold activity in the catalytic opening of epoxide 121 to 122 [119]. In another example by Mirkin, a well-known aluminum(III) salen catalyst was hidden in switch 1252+ [118] between sterically demanding biphenyl rings preventing ε-caprolactone from accessing the catalytic site (Figure 28). As

• a result, the polymerization of ε-caprolactone (123) was switched OFF. Addition of chloride anions from n-Bu4NCl led to the substitution of the tertiary amine ligand at both rhodium centers generating the semi-open form [125(Cl)2]. Since the aluminum(III) salen center became now exposed, the

Facile and innovative catalytic protocol for intramolecular Friedel–Crafts cyclization of Morita–Baylis–Hillman adducts: Synergistic combination of chiral (salen)chromium(III)/BF₃·OEt₂ catalysis

• Karthikeyan Soundararajan,
• Helen Ratna Monica Jeyarajan,
• Raju Subimol Kamarajapurathu and
• Karthik Krishna Kumar Ayyanoth

Beilstein J. Org. Chem. 2021, 17, 2186–2193, doi:10.3762/bjoc.17.140

• .17.140 Abstract The chiral (salen)Cr(III)/BF3·OEt2 catalytic combination was found to be an effective catalyst for intramolecular Friedel–Crafts cyclization of electron-deficient Morita–Baylis–Hillman adducts. In presence of mild reaction conditions the chiral (salen)Cr(III)/BF3·OEt2 complex affords 2

• -substituted-1H-indenes from unique substrates of Morita–Baylis–Hillman adducts via an easy operating practical procedure. Keywords: boron trifluoride etherate; chiral (salen)chromium(III); intramolecular Friedel–Crafts cyclization; Morita–Baylis–Hillman adducts; substituted-1H-indenes; Introduction

• M(salen) complexes. Intramolecular Friedel–Crafts cyclizations in Morita–Baylis–Hillman adducts are known to undergo annulations generating cyclic frameworks of indene. To the best of our knowledge, five reports are cited for the intramolecular FC reaction in MBH adducts. The first report was by

Asymmetric organocatalyzed synthesis of coumarin derivatives

• Natália M. Moreira,
• Lorena S. R. Martelli and
• Arlene G. Corrêa

Beilstein J. Org. Chem. 2021, 17, 1952–1980, doi:10.3762/bjoc.17.128

• together with tetraphenylphosphonium monoperoxysulfate (TPPP) as the stoichiometric oxidant [76]. The authors mentioned that this epoxidation had been previously reported using Jacoben’s (S,S)-(+)-salen-Mn(III) catalyst with 78–83% yield and 95% ee, and via organocatalysis they obtained 98% yield and ≥99

Sustainable manganese catalysis for late-stage C–H functionalization of bioactive structural motifs

• Jongwoo Son

Beilstein J. Org. Chem. 2021, 17, 1733–1751, doi:10.3762/bjoc.17.122

• solution obtained by the nuclear reaction using a cyclotron, and manganese–salen complex 7 was used as a fluoride transfer catalyst, which facilitated late-stage C–H radiofluorination, affording the corresponding radiofluorinated bioactive molecules 8a–h. In general, the regioselectivity of fluorination

• conjugative transformations, such as azide–alkyne [3 + 2]-cycloaddition [30][31][32][33][34][35][36][37]. Based on their previous late-stage fluorination studies [22][25], Groves et al. further showcased a manganese(III)–salen-catalyzed azidation process using an aqueous azide solution as a convenient azide

• the tertiary position using the manganese–salen catalyst. Furthermore, this Mn-catalyzed azidation protocol is highly robust in air, highlighting the practical simplicity of late-stage C–H azidation of bioactive molecules. A plausible reaction pathway was proposed, as illustrated in Figure 3. Similar

Methodologies for the synthesis of quaternary carbon centers via hydroalkylation of unactivated olefins: twenty years of advances

• Thiago S. Silva and
• Fernando Coelho

Beilstein J. Org. Chem. 2021, 17, 1565–1590, doi:10.3762/bjoc.17.112

• isomerization protocol triggered by an MHAT process using salen cobalt complexes as catalysts to furnish isomerized and cycloisomerized products (Scheme 19) [72]. Five and six-membered carbocycles containing quaternary carbon centers were synthetized in good to excellent yields and with excellent

• comparison with the five-membered ones, led to the observation of a mixture of linear and cycloisomerized products when diolefin 36l was subjected to the optimized conditions. The selectivity was improved when a more electron-rich Salen ligand was employed, highlighting the importance of the ligand structure

• oxidant (Scheme 30) [99]. Bicycle units 82 founded in natural products could be synthetized under Co(II) salen complex 80 catalysis at room temperature with excellent stereoselectivity. Only trans-decalins 82 were observed in the presence of secondary OTBS groups in the substrate backbone; this was not

Manganese/bipyridine-catalyzed non-directed C(sp³)–H bromination using NBS and TMSN₃

• Kumar Sneh,
• Takeru Torigoe and
• Yoichiro Kuninobu

Beilstein J. Org. Chem. 2021, 17, 885–890, doi:10.3762/bjoc.17.74

• reported the manganese-porphyrin-catalyzed chlorination and bromination of C(sp3)−H bonds, respectively (Scheme 1d). Groves et al. also reported the manganese-salen-catalyzed fluorination of benzylic C(sp3)−H bonds [49]. Although these methods are efficient, they have a limited substrate scope

Valorisation of plastic waste via metal-catalysed depolymerisation

• Francesca Liguori,
• Carmen Moreno-Marrodán and
• Pierluigi Barbaro

Beilstein J. Org. Chem. 2021, 17, 589–621, doi:10.3762/bjoc.17.53

A systematic review on silica-, carbon-, and magnetic materials-supported copper species as efficient heterogeneous nanocatalysts in “click” reactions

• Pezhman Shiri and
• Jasem Aboonajmi

Beilstein J. Org. Chem. 2020, 16, 551–586, doi:10.3762/bjoc.16.52

• –salen Cu(II) complex, 11, was synthesized in several steps [26]. Initially, 5‐(chloromethyl)‐2‐hydroxybenzaldehyde (6) was prepared by the reaction of 2‐hydroxybenzaldehyde (5) and paraformaldehyde in concentrated HCl at a temperature between 5 and 10 °C. At the same time, benzimidazole-substituted

• -functionalized nanosilica. Finally, the complexation reaction of Cu(II) with a supported ligand on propylamine-functionalized nanosilica, 10, was performed to produce a Cu(II) benzimidazole–salen complex supported by imine-functionalized silica (BS–Cu(II)@SiO2 (11), Scheme 2). The nanocatalyst 11 was used to

A review of asymmetric synthetic organic electrochemistry and electrocatalysis: concepts, applications, recent developments and future directions

• Munmun Ghosh,
• Valmik S. Shinde and
• Magnus Rueping

Beilstein J. Org. Chem. 2019, 15, 2710–2746, doi:10.3762/bjoc.15.264

• improvements of these methods have recently been achieved by Moeller et al. who designed a photovoltaic apparatus for the efficient Sharpless dihydroxylation of styrene in up to 94% enantiomeric excess [65]. Tanaka and co-workers explored the catalytic applicability of optically active Mn-salen complex 72

• and regenerate the Cu–L* catalyst (Scheme 30). In 2014, Wang and Lu reported a method for asymmetric fixation of CO2 via electrocarboxylation of 82 catalyzed by an electrogenerated chiral [CoI(salen)]− complex [68]. The potentiostatic electrolysis of 82 in an undivided cell in presence of catalytic

• chiral Co(II)-(R,R)salen complex 83 in DMF saturated with CO2 resulted in the formation of 84 in a low yield but with good enantioselectivity (Scheme 31). Very recently, Meggers research group described an application of an asymmetric Lewis acid catalyst using electricity to provide a synthetic route

Recent advances in transition-metal-catalyzed incorporation of fluorine-containing groups

• Xiaowei Li,
• Xiaolin Shi,
• Xiangqian Li and
• Dayong Shi

Beilstein J. Org. Chem. 2019, 15, 2213–2270, doi:10.3762/bjoc.15.218

• hydrocarbons, substituted cyclic molecules, terpenoids, and steroid derivatives, were selectively fluorinated at some otherwise inaccessible sites, however, in low to moderate yields. On the other hand, the same group [83] developed Mn(salen)Cl as a catalyst for the direct C–H fluorination at benzylic

• positions with a nucleophilic fluorine source. Notably, Groves adapted the method for the 18F-radiofluorination of benzylic and aliphatic C–H bonds using no-carrier-added [18F]-fluoride with Mn(salen)OTs [84]. In 2013, Lectka’s group [85][86] reported an iron-catalyzed C(sp3)–H fluorination of benzylic

• stereocontrolled acrylic allylic fluorides. Fluorination of acidic carbonyl compounds: In 2010, Itoh and co-workers [96] demonstrated the asymmetric fluorination of cyclic and acyclic β-ketoesters by using a catalytic amount of Co(acac)2 with (R,R)-Jacobsen’s salen ligand (Scheme 48). The α-fluorinated products

Mechanochemistry of supramolecules

• Anima Bose and
• Prasenjit Mal

Beilstein J. Org. Chem. 2019, 15, 881–900, doi:10.3762/bjoc.15.86

• ]. Subsequently, compound 2 underwent substitution with α,α’-dibromo-o-xylene to create the nickel(II) complex 3 [54]. In 2014, James and co-workers reported a one-pot two-step mechanochemical synthesis of metal complexes 7 (Figure 3). First, the salen-type ligand 6 was synthesized from o-hydroxybenzaldehyde (4

• salen-type ligand followed by metal-complex formation in the same pot [55]. Otera’s solvent-free approach by which the formation of self-assembled supramolecules could be accelerated [56]. Synthesis of a Pd-based metalla-supramolecular assembly through mechanochemical activation for C–H-bond activation

Synthesis of acylglycerol derivatives by mechanochemistry

• Karen J. Ardila-Fierro,
• Andrij Pich,
• Marc Spehr,
• José G. Hernández and
• Carsten Bolm

Beilstein J. Org. Chem. 2019, 15, 811–817, doi:10.3762/bjoc.15.78

• -opening reaction of 2 with stearic acid (3a) was evaluated (Scheme 3a). Specifically, we focused on the use of Jacobsen cobalt(II)-salen complex (S,S)-cat (Scheme 3b), since similar salen complexes had originally been reported to facilitate epoxide ring-opening reactions with carboxylic acids as

• nucleophiles [36]. Moreover, salen complexes endure mechanochemical conditions, as proven during their preparation in ball mills [37]. In addition, various related Jacobsen salen complexes have shown catalytic activity under solvent-free conditions [38]. Collectively, these precedents made this synthetic route

Copolymerization of epoxides with cyclic anhydrides catalyzed by dinuclear cobalt complexes

• Yo Hiranoi and
• Koji Nakano

Beilstein J. Org. Chem. 2018, 14, 2779–2788, doi:10.3762/bjoc.14.255

• synthetic method for polyesters as the polymers’ architectures and properties can be easily controlled depending on the combination of two monomers. Thus, a variety of catalyst designs has been reported to prepare the desired copolymers efficiently. We herein report dinuclear cobalt–salen complexes with a

• benzene ring as a linker and their activities in copolymerization reactions. The dinuclear cobalt complexes showed a higher catalytic activity for the copolymerization of propylene oxide with phthalic anhydride than the corresponding mononuclear cobalt–salen complex and achieved one of the highest

• developed based on well-defined metal complexes such as metalloporphyrins and metal–salen complexes [15][16][17][18][19][20][21]. In parallel to the development of catalysts, new polyester materials also were prepared by employing unprecedented monomers or by the combination with other polymerization

Cobalt bis(acetylacetonate)–tert-butyl hydroperoxide–triethylsilane: a general reagent combination for the Markovnikov-selective hydrofunctionalization of alkenes by hydrogen atom transfer

• Xiaoshen Ma and
• Seth B. Herzon

Beilstein J. Org. Chem. 2018, 14, 2259–2265, doi:10.3762/bjoc.14.201

• , the iron oxalate–sodium borohydride system, introduced by Boger and co-workers [8], is the only reagent combination shown to accommodate a broad range of SOMOphiles. However, the cobalt–salen complexes that are commonly employed [10][11][13][15][16][30][31][32][36][37][48] contain many different

• ] to afford the corresponding products 4g and 4h in 96% and 89% yields, respectively. Carreira and co-workers reported the hydroazidation of alkenes using cobalt–salen complexes as hydrogen atom transfer agents and para-toluenesulfonyl azide as an azide source [16][48][55]. After careful optimization

Chiral bisoxazoline ligands designed to stabilize bimetallic complexes

• Deepankar Das,
• Rudrajit Mal,
• Nisha Mittal,
• Zhengbo Zhu,
• Thomas J. Emge and
• Daniel Seidel

Beilstein J. Org. Chem. 2018, 14, 2002–2011, doi:10.3762/bjoc.14.175

• chiral multi-metallic complexes such as the hetero-bimetallic complex 1, in which the two different metals play distinct roles [14][15]. Jacobsen and co-workers reported dimeric salen complexes 2 which show cooperative reactivity between the two metal centers in the asymmetric ring opening of meso

β-Hydroxy sulfides and their syntheses

• Mokgethwa B. Marakalala,
• Edwin M. Mmutlane and
• Henok H. Kinfe

Beilstein J. Org. Chem. 2018, 14, 1668–1692, doi:10.3762/bjoc.14.143

• molecular sieves [38]. Whilst yields and enantioselectivities were impressive, only t-BuSH was used as the nucleophile and reaction times were quite variable, ranging from 9 to 137 hours (Scheme 12). A year later (1998), Wu and Jacobsen reported the use of (salen)Cr(III) complex 45 as a catalyst for the

• time (Scheme 13). In the same year, Hou and co-workers reported the use of (salen)Ti(IV) in the ring opening of meso-epoxides with various thiophenols and benzylmercaptan, in yields of up to 93% over 2–24 hours, albeit with lower enantioselectivities [40]. The use of salen complexes inspired others

• , with Zhou et al. in 2006 reporting the same (salen)Ti(IV) complexes as Wue et al. and variations thereof, prepared in situ, to be useful in thiolysis of epoxides with dithiophosphorus acid as the nucleophile, in toluene at room temperature, under an inert atmosphere for 20 minutes. Product yields

Recent applications of chiral calixarenes in asymmetric catalysis

• Mustafa Durmaz,
• Erkan Halay and
• Selahattin Bozkurt

Beilstein J. Org. Chem. 2018, 14, 1389–1412, doi:10.3762/bjoc.14.117

• six-membered-ring transition state similar to that described earlier by Feng et al. [69]. Epoxidation In 2014 Sciotto et al. reported the synthesis of two novel calix[4]arene–salen ligands 105a,b in 1,3-alternate conformation. Reaction of the salen ligands with appropriate metal acetate salt according

• coligand, their uranyl-(salen) derivatives 106a,b have been employed as models of the oxo-Mn(V)–(salen) oxidant species. The reactions were highly efficient in terms of productivity (up to 96% yield) and enantioselectivity (up to 93% ee) when rigid bicyclic alkenes such as 1,2-dihydronaphthalene and

• calix[4]arene–(salen) complexes. Enantioselective epoxidation of 108 catalyzed by 107a/107b. Synthesis of inherently chiral calix[4]arene catalysts 111 and 112. Enantioselective MPV reduction. Synthesis of chiral calix[4]arene ligands 116a–c. Asymmetric MPV reduction with chiral calix[4]arene ligands

Recent developments in the asymmetric Reformatsky-type reaction

• Hélène Pellissier

Beilstein J. Org. Chem. 2018, 14, 325–344, doi:10.3762/bjoc.14.21

• ketones with ethyl iodoacetate performed with enantioselectivities of up to 86% ee in the presence of a chiral manganese salen complex [36]. The same year, this author also described the first catalytic enantioselective aza-Reformatsky reaction between in situ generated imines and ethyl iodoacetate [37

Solvent-free sonochemistry: Sonochemical organic synthesis in the absence of a liquid medium

• Deborah E. Crawford

Beilstein J. Org. Chem. 2017, 13, 1850–1856, doi:10.3762/bjoc.13.179

• sonochemical reactions being carried out in the absence of solvent, or a liquid reagent [8][9]. Herein, we report two condensation reactions (investigated extensively by ball milling), one to form salen ligand 1 by sonicating o-vanillin and 1,2-phenylenediamine, and the second to form 1,3-indandione 2 from

Exploring endoperoxides as a new entry for the synthesis of branched azasugars

• Svenja Domeyer,
• Mark Bjerregaard,
• Henrik Johansson and
• Daniel Sejer Pedersen

Beilstein J. Org. Chem. 2017, 13, 644–647, doi:10.3762/bjoc.13.63

• anticipated that the reaction conditions could be optimised to favour the desired diol product. Ring contraction of acetonide-protected endoperoxide 23 by treatment with triphenylphosphine provided ready access to tetrahydrofuran 24 in good yield [21]. Finally, treatment of endoperoxide 23 with Co(salen) gave