Transition-metal-catalyst-free electroreductive alkene hydroarylation with aryl halides under visible-light irradiation

• Kosuke Yamamoto,
• Kazuhisa Arita,
• Masami Kuriyama and
• Osamu Onomura

Beilstein J. Org. Chem. 2024, 20, 1327–1333, doi:10.3762/bjoc.20.116

• Kosuke Yamamoto Kazuhisa Arita Masami Kuriyama Osamu Onomura Graduate School of Biomedical Sciences, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan 10.3762/bjoc.20.116 Abstract The radical hydroarylation of alkenes is an efficient strategy for accessing linear alkylarenes with

• high regioselectivity. Herein, we report the electroreductive hydroarylation of electron-deficient alkenes and styrene derivatives using (hetero)aryl halides under mild reaction conditions. Notably, the present hydroarylation proceeded with high efficiency under transition-metal-catalyst-free

• that a reductive radical-polar crossover pathway is likely to be involved in this transformation. Keywords: aryl halides; C–C bond formation; electroreduction; radicals; visible light; Introduction Alkene hydroarylation is an attractive method for the construction of alkylarenes, which serve as

Photoredox catalysis harvesting multiple photon or electrochemical energies

• Mattia Lepori,
• Simon Schmid and
• Joshua P. Barham

Beilstein J. Org. Chem. 2023, 19, 1055–1145, doi:10.3762/bjoc.19.81

• hydroarylation of tert-butyl vinylcarbamate and unactivated alkenes like 1-octene and 3-buten-1-ol although the vinyl carbamate substrate (Epred = −2.2 V vs SCE) is significantly easier to reduce than most aryl chlorides. This selectivity, especially considering the need for an excess of the vinyl carbamate

• ][63][64]. Arylphosphonates 15 were obtained in a photo-Arbuzov reaction by trapping with trimethylphosphite in good to excellent yields (62–88%) (Figure 12B). Additionally, intramolecular trapping via dearomative hydroarylation gave access to spirocyclic cyclohexadienes bearing dihydrobenzofuran and

Pyridine C(sp²)–H bond functionalization under transition-metal and rare earth metal catalysis

• Haritha Sindhe,
• Malladi Mounika Reddy,
• Karthikeyan Rajkumar,
• Akshay Kamble,
• Amardeep Singh,
• Anand Kumar and
• Satyasheel Sharma

Beilstein J. Org. Chem. 2023, 19, 820–863, doi:10.3762/bjoc.19.62

• styrenes 64 using a Ni–Al bimetallic system and NHC ligand 65 through intermolecular hydroarylation with high levels of enantio- and regioselectivity in the alkylated products 66 (Scheme 13). Also, the authors performed DFT studies revealing the reaction mechanism and supported that the interaction of the

• bond of 105 provides a seven-membered rhodacyclic intermediate 106. The protonation at the Rh–C bond of intermediate 106 in the presence of RCOOH furnishes hydroarylation product 104. Nitrogen heterocyclic carbenes (NHCs) are of central importance in organometallic chemistry and in organic synthesis

• 210. In 2012, a hydroarylation of alkenes 211 and alkynes 212 with 2,2′-bipyridines 208 and 2,2′-biquinolines was reported by Chang and co-workers [112] in the presence of Rh(acac)3 as catalyst, IMes·HCl (3 mol %) as ligand and t-BuONa (30 mol %) in toluene for 2 h (Scheme 40). The authors

Strategies in the synthesis of dibenzo[b,f]heteropines

• David I. H. Maier,
• Barend C. B. Bezuidenhoudt and
• Charlene Marais

Beilstein J. Org. Chem. 2023, 19, 700–718, doi:10.3762/bjoc.19.51

• ) chloride (Scheme 26). Alkyne–carbonyl metathesis is proposed to proceed via [2 + 2] cycloaddition and –reversion steps, catalysed by a Brønsted or Lewis acid, with the catalyst proposed to form a σ-complex with the carbonyl group and/or a π-complex with the alkyne [68]. 3.7 Hydroarylation The construction

• of an N-triarylated dibenzo[b,f]azepine scaffold 129 by means of Au(I)-catalysed hydroarylation was reported by Ito et al. [70]. While the attempted synthesis of an N-phenyldibenzazepine derivative 127 was unsuccessful, the authors were able to prepare a fused carbazole-dibenzo[b,f]azepine 129 in 90

• metathesis as key step in the synthesis of dibenzo[b,f]heteropines. Alkyne–aldehyde metathesis in the synthesis of dibenzo[b,f]heteropines. Hydroarylation of 9-(2-alkynylphenyl)-9H-carbazole derivatives. Oxidative coupling of bisphonium ylide intermediate to give pacharin (13). Preparation of 10,11

Enolates ambushed – asymmetric tandem conjugate addition and subsequent enolate trapping with conventional and less traditional electrophiles

• Péter Kisszékelyi and
• Radovan Šebesta

Beilstein J. Org. Chem. 2023, 19, 593–634, doi:10.3762/bjoc.19.44

• was isolated as the main product (Scheme 52B). Thus far, the direct hydroarylation of such enone-dione substrates was unprecedented, presumably, due to the preferred metal-catalyzed aldol cyclization. Their protocol was further verified by a wide substrate scope. Additionally, the reaction showed high

Transition-metal-catalyzed domino reactions of strained bicyclic alkenes

• Austin Pounder,
• Eric Neufeld,
• Peter Myler and
• William Tam

Beilstein J. Org. Chem. 2023, 19, 487–540, doi:10.3762/bjoc.19.38

• previously been a challenging transformation due to the propensity of these systems to produce non-cyclized hydroarylation products due to an unproductive rhodium 1,4-migration on heteroaromatic moieties. The use of benzothiophene, benzofurans, and benzopyrrole boronate esters in this investigation prevented

Friedel–Crafts acylation of benzene derivatives in tunable aryl alkyl ionic liquids (TAAILs)

• Swantje Lerch,
• Stefan Fritsch and
• Thomas Strassner

Beilstein J. Org. Chem. 2023, 19, 212–216, doi:10.3762/bjoc.19.20

• proved to be a potent reaction medium in catalytic hydrosilylation [53], hydroamination and hydroarylation [54], as well as for the synthesis of nanoparticles [55][56]. Results and Discussion Ionic liquids 1–6 were synthesized via a three-step procedure starting from commercially available aniline

Facile and diastereoselective arylation of the privileged 1,4-dihydroisoquinolin-3(2H)-one scaffold

• Dmitry Dar’in,
• Grigory Kantin,
• Alexander Bunev and
• Mikhail Krasavin

Beilstein J. Org. Chem. 2022, 18, 1070–1078, doi:10.3762/bjoc.18.109

• readily available 3(2H)-isoquinolones followed by TfOH-promoted hydroarylation by an arene molecule. Screening of the novel 1,2,4-trisubstituted 1,4-DHIQs against cancer cell lines confirmed high cytotoxicity of selected analogs, which validates this new chemotype for further investigations as anticancer

• cytotoxic agents. Keywords: 1,4-dihydroisoquinol-3-one; heterocyclic diazo compounds; hydroarylation; Regitz diazo transfer; triflic acid; Introduction Besides being derivatives of (or a precursor to) the 1,2,3,4-tetrahydroisoquinoline core which itself bears a special significance from the standpoint of

AlBr₃-Promoted stereoselective anti-hydroarylation of the acetylene bond in 3-arylpropynenitriles by electron-rich arenes: synthesis of 3,3-diarylpropenenitriles

• Yelizaveta Gorbunova,
• Dmitry S. Ryabukhin and
• Aleksander V. Vasilyev

Beilstein J. Org. Chem. 2021, 17, 2663–2667, doi:10.3762/bjoc.17.180

• , benzene and its polymethylated derivatives) under the action of aluminum bromide (AlBr3, 6 equiv) at room temperature for 0.5–2 h result in the stereoselective formation of 3,3-diarylpropenenitriles (Ar(Ar′)C=CHCN) in yields of 20–64%, as products of mainly anti-hydroarylation of the acetylene bond. The

• obtained 3,3-diarylpropenenitriles in triflic acid CF3SO3H (TfOH) at room temperature for 1 h are cyclized into 3-arylindenones in yields of 55–70%. Keywords: aluminum bromide; hydroarylation; indenones; propenenitriles; propynenitriles; Introduction Conjugated acetylene nitriles (propynenitriles, R–C≡C

• ) through the regioselective hydroarylation of the carbon–carbon double bond. In TfOH, the reactions proceed further to 3-arylindanones, as products of the intramolecular aromatic acylation of the 3,3-diarylpropanenitriles by the electrophilically activated nitrile group [16][17]. Based on this study and

Synthesis of 5-arylacetylenyl-1,2,4-oxadiazoles and their transformations under superelectrophilic activation conditions

• Andrey I. Puzanov,
• Dmitry S. Ryabukhin,
• Anna S. Zalivatskaya,
• Dmitriy N. Zakusilo,
• Darya S. Mikson,
• Irina A. Boyarskaya and
• Aleksander V. Vasilyev

Beilstein J. Org. Chem. 2021, 17, 2417–2424, doi:10.3762/bjoc.17.158

• reaction of the acetylenyl-1,2,4-oxadiazoles with arenes in neat triflic acid TfOH (CF3SO3H) at room temperature for 1 h resulted in the formation of E/Z-5-(2,2-diarylethenyl)-3-aryl-1,2,4-oxadiazoles as products of regioselective hydroarylation of the acetylene bond. The addition of TfOH to the acetylene

• bond of these oxadiazoles quantitatively resulted in E/Z-vinyl triflates. The reactions of the cationic intermediates have been studied by DFT calculations and the reaction mechanisms are discussed. Keywords: acetylene-oxadiazoles; Friedel–Crafts reaction; hydroarylation; superelectrophilic activation

• nucleophilic arene molecules, species B should afford substances 5 as products of hydroarylation of the acetylene bond of the starting compounds 3. Indeed, reaction of 5-acetylenyl-1,2,4-oxadiazoles 3a–c with excess of TfOH at room temperature for 1 h resulted in the quantitative preparation of E/Z-isomers of

Transition-metal-free intramolecular Friedel–Crafts reaction by alkene activation: A method for the synthesis of some novel xanthene derivatives

• Tülay Yıldız,
• İrem Baştaş and
• Hatice Başpınar Küçük

Beilstein J. Org. Chem. 2021, 17, 2203–2208, doi:10.3762/bjoc.17.142

• alcohols with diaryliodonium salts [26], the Sc(OTf)3-catalyzed domino reaction [27], and the iodine-catalyzed nucleophilic substitution reaction of xanthen-9-ol [28]. As an example of an intramolecular hydroarylation of an olefin, 9,10-dihydroacridines, which are N derivatives of xanthenes, were

• of xanthenes and thioxanthenes by intramolecular FCA catalyzed with organic Brønsted superacid, which are our works [41][42]. According to the literature, there are also hydroarylation methods with FCA, which are made by unactivated alkenes instead of π-activated alcohols. In these studies, generally

• gold(III) [43], iridium(III) [44][45], iron(III) [46], or bismuth(III) [47][48] were used as catalysts for the intermolecular hydroarylation of unactivated alkenes. Organic Brønsted acids were also used as catalysts in a smaller number of studies [49][50]. In this work, we searched for some organic

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

• activate unsaturated bonds, and examples of olefin hydroalkylation, hydroarylation, hydrofluorination, hydroamination, and aminoheteroarylation, among others, can be found in the literature [36][37]. The first example of an olefin hydroalkylation promoted by gold catalysis appeared in 2004 in the work by

CF₃-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

• Anthony J. Fernandes,
• Armen Panossian,
• Bastien Michelet,
• Agnès Martin-Mingot,
• Frédéric R. Leroux and
• Sébastien Thibaudeau

Beilstein J. Org. Chem. 2021, 17, 343–378, doi:10.3762/bjoc.17.32

• as this α-(trifluoromethyl)allylcarbenium ion could be trapped by some electron rich arenes (i.e., xylene, cumene, etc.). The products 104 further react to afford indanes 105 after hydroarylation. A closely related study on dibrominated allylic α-(trifluoromethyl) alcohols also invoked the transient

• benzylic carbenium ion 122’. Subsequently, 122’ reacts in a Friedel–Crafts reaction with benzene to generate 123. After two successive hydroarylation reactions, the final product 121 is produced via the formation of vinylic and benzylic carbenium ions 124 and 125, respectively (Scheme 32). Moran et al

• a longer reaction time, allenes 127 undergo a subsequent intramolecular hydroarylation reaction leading to indenes 128. The authors suggested the formation of FeCl3–HFIP complexes being involved in a Lewis acid-assisted Brønsted acid catalysis. The CF3-substituted propargyl alcohol 129 was found to

Activation of pentafluoropropane isomers at a nanoscopic aluminum chlorofluoride: hydrodefluorination versus dehydrofluorination

• Maëva-Charlotte Kervarec,
• Thomas Braun,
• Mike Ahrens and
• Erhard Kemnitz

Beilstein J. Org. Chem. 2020, 16, 2623–2635, doi:10.3762/bjoc.16.213

• such as hydrodefluorinations and dehydrofluorinations were observed, followed by hydroarylation and Friedel–Crafts-type reactions under mild conditions. Keywords: aluminum fluoride; C–F bond activation; dehydrofluorination; hydrodefluorination; hydrofluorocarbons; Introduction Hydrofluorocarbons

• transformed by HF addition into 1,1,1,2,2-pentafluoropropane (HFC-245cb, 10b) [33]. When the aromatic solvent C6D6 was used instead of C6D12, 10a was once more transformed into 1 as the main compound, together with traces of the Friedel–Crafts product CF3CFHCH2C6D5 (11) and the hydroarylation product

• surface of ACF can occur, generating carbenium-like species and surface fluorides (Scheme 3). Via HF elimination, the olefin 1 can be produced, followed by a refluorination of the double bond by the released HF, generating 10b (Scheme 3, left). In the presence of C6D6, the hydroarylation product 12 can be

Different reactivity of phosphorylallenes under the action of Brønsted or Lewis acids: a crucial role of involvement of the P=O group in intra- or intermolecular interactions at the formation of cationic intermediates

• Stanislav V. Lozovskiy,
• Alexander Yu. Ivanov and
• Aleksander V. Vasilyev

Beilstein J. Org. Chem. 2019, 15, 1491–1504, doi:10.3762/bjoc.15.151

• such phosphorylallenes under the action of Brønsted or Lewis acids. Apart from that, the reaction of dichlorophosphorylallenes with arenes and AlCl3 led to products of hydroarylation of the allene system, phosphoryl-substituted alkenes and/or indanes. This is the first example of a Lewis acid-promoted

• intermolecular hydroarylation of allenes bearing electron-withdrawing substituents. Plausible reaction mechanisms have been proposed on the basis of the investigated reactions, and NMR analysis and DFT studies of the intermediate cationic species. Keywords: aluminum chloride; cation; intermediate

• ][16]. It should be especially emphasized that intermolecular reactions of phosphorylallenes with arenes have not been yet achieved. In general, intermolecular hydroarylation of allenes has been developed for reactions catalyzed by complexes of various metals [17], such as Pd [18][19][20], Pt [21], Au

Synthesis of the aglycon of scorzodihydrostilbenes B and D

• Katja Weimann and
• Manfred Braun

Beilstein J. Org. Chem. 2019, 15, 610–616, doi:10.3762/bjoc.15.56

• , among them the aglycon of scorzodihydrostilbenes B and D. Keywords: C–H activation; hydroarylation; phenols; regioselectivity; ruthenium; Introduction Among phytochemicals with strong allelopathic effects, various natural products are found that feature the structural motif of dihydrostilbene with

• . Here, a modified version based upon an in situ generation of the ruthenium complex described by Genet and Darses [16] was applied. Thus, O-protected 2,5-dihydroxyacetophenones 6 were submitted to a hydroarylation reaction with two equivalents of styrenes 7 in the presence of [Ru(p-cymene)Cl2]2 and P(4

• substitutions was opened based upon Murai’s ruthenium-catalyzed hydroarylation of olefins. Thus, the aglycon of scorzodihydrostilbene D became accessible in two steps from readily available starting materials. Experimental General: Melting points (uncorrected) were determined with a Büchi 540 melting point

Gold-catalyzed post-Ugi alkyne hydroarylation for the synthesis of 2-quinolones

• Xiaochen Du,
• Jianjun Huang,
• Anton A. Nechaev,
• Ruwei Yao,
• Jing Gong,
• Erik V. Van der Eycken,
• Olga P. Pereshivko and
• Vsevolod A. Peshkov

Beilstein J. Org. Chem. 2018, 14, 2572–2579, doi:10.3762/bjoc.14.234

• . Subjecting these adducts to a gold-catalyzed intramolecular alkyne hydroarylation process allowed to efficiently construct the 2-quinolone core bearing a branched substituent on the nitrogen atom. Keywords: gold catalysis; hydroarylation; 2-quinolones; Ugi reaction; Introduction Quinoline and its oxidized

• derivatives with most recent strategies focusing on the implementation of transition metal-catalyzed C–H activation methods [35][36]. One of the common approaches towards 2-quinolones 2 involves the intramolecular Friedel–Crafts hydroarylation [37][38] of N-arylamides of 3-substituted propynoic acids 1

• nitrogen atom through an intramolecular gold-catalyzed alkyne hydroarylation reaction (Scheme 2). It should be noted, that heterocyclic syntheses through post-Ugi transformations have been under extensive exploration for the last two decades [47][48][49], especially in terms of gold-catalyzed Friedel

Some mechanistic aspects regarding the Suzuki–Miyaura reaction between selected ortho-substituted phenylboronic acids and 3,4,5-tribromo-2,6-dimethylpyridine

• Piotr Pomarański,
• Piotr Roszkowski,
• Jan K. Maurin,
• Armand Budzianowski and
• Zbigniew Czarnocki

Beilstein J. Org. Chem. 2018, 14, 2384–2393, doi:10.3762/bjoc.14.214

• , Tanaka presented an atropselective synthesis of axially chiral all-benzenoid biaryls by gold-catalysed intramolecular hydroarylation of alkynones to give the desired atropisomeric product with a good ee value of 70% [26]. Conventional approaches to the synthesis of biaryl compounds having axial chirality

Hydroarylations by cobalt-catalyzed C–H activation

• Rajagopal Santhoshkumar and
• Chien-Hong Cheng

Beilstein J. Org. Chem. 2018, 14, 2266–2288, doi:10.3762/bjoc.14.202

• the transformations is the addition of C–H to C–X multiple bonds to afford alkylation, alkenation, amidation, and cyclization products using low- or high-valent cobalt catalysts. This hydroarylation is an efficient approach to build new C–C bonds in a 100% atom-economical manner. In this review, the

• recent developments of Co-catalyzed hydroarylation reactions and their mechanistic studies are summarized. Keywords: catalysis; C−C formation; C–H activation; cobalt; hydroarylation; Introduction For the last three decades, atom-economical synthetic approaches have played a substantial role in organic

• (hydroarylation) is a 100% atom economical process to build fundamental alkyls and alkenes (Scheme 2) [42][43][44][45]. It is an efficient alternative to C–H alkylation reactions with alkyl halides where one equivalent of salt waste was released, and dehydrogenative Heck coupling with alkenes for the synthesis of

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

• developed (e.g., reductive coupling [22][23][24][25][26][27][28], formal hydromethylation [29], cycloisomerization [8][30][31], hydrooximation [32], hydroheteroarylation [28][33][34][35], hydroarylation [36][37][38], and cross-coupling [37]). Many of these transformations have found applications in

Cobalt-catalyzed directed C–H alkenylation of pivalophenone N–H imine with alkenyl phosphates

• Wengang Xu and
• Naohiko Yoshikai

Beilstein J. Org. Chem. 2018, 14, 709–715, doi:10.3762/bjoc.14.60

• groups into the ortho position of functionalized arenes has attracted significant attention because of the synthetic versatility of alkenyl groups. The C–H alkenylation has been achieved most extensively by way of the dehydrogenative Heck-type reaction of olefins [7][8][9]. Meanwhile, the hydroarylation

• (e.g., acrylates), and is challenging with unactivated and multisubstituted alkenes [11]. The hydroarylation of alkynes does not allow for the introduction of cycloalkenyl groups because of the unavailability of the corresponding alkynes. In light of such limitations, a coupling between arene

• -directed arene C–H alkenylation reaction with alkenyl phosphates using a different cobalt–NHC catalyst (Scheme 1b) [28]. Meanwhile, we have also demonstrated that pivaloyl N–H imine serves as a powerful directing group for cobalt-catalyzed arene C–H functionalization reactions such as the hydroarylation of

Metal-free hydroarylation of the side chain carbon–carbon double bond of 5-(2-arylethenyl)-3-aryl-1,2,4-oxadiazoles in triflic acid

• Anna S. Zalivatskaya,
• Dmitry S. Ryabukhin,
• Marina V. Tarasenko,
• Alexander Yu. Ivanov,
• Irina A. Boyarskaya,
• Elena V. Grinenko,
• Ludmila V. Osetrova,
• Eugeniy R. Kofanov and
• Aleksander V. Vasilyev

Beilstein J. Org. Chem. 2017, 13, 883–894, doi:10.3762/bjoc.13.89

• ), both under thermal and microwave conditions, leads to 5-(2,2-diarylethyl)-3-aryl-1,2,4-oxadiazoles. The products are formed through the regioselective hydroarylation of the side chain carbon–carbon double bond of the starting oxadiazoles in yields up to 97%. According to NMR data and DFT calculations

• , N4,C-diprotonated forms of oxadiazoles are the electrophilic intermediates in this reaction. Keywords: Friedel–Crafts reaction; hydroarylation; oxadiazoles; superelectrophilic activation; triflic acid; Introduction Oxadiazoles are an important class of heterocyclic compounds and great attention has

• investigation of their properties are of ongoing interest in chemistry and medicine. Based on our previous works on reactions of cinnamides [24] and 5-styryl-2H-tetrazoles [25] with arenes under superelectrophilic activation with Brønsted or Lewis superacids, we turned our attention towards the hydroarylation

Conjugate addition–enantioselective protonation reactions

• James P. Phelan and
• Jonathan A. Ellman

Beilstein J. Org. Chem. 2016, 12, 1203–1228, doi:10.3762/bjoc.12.116

• selective protonation forming the less stabilized terminal chiral alkene 166 over the internal achiral isomer 167, an inherent challenge in the hydroarylation of terminal allenes. Electron-rich, neutral, and electron-poor arylboronic acids 47 added to alkyl (R = Me, Et, n-Bu) diphenylphosphinylallenes 165

• nitroalkenes. Hayashi’s enantioselective hydroarylation of diphenylphosphinylallenes. Hayashi’s enantioselective hydroarylation of diphenylphosphinylallenes. Togni’s enantioselective hydrophosphination of methacrylonitrile. Togni’s enantioselective hydroamination of methacrylonitrile. Acknowledgements We

Catalytic asymmetric synthesis of biologically important 3-hydroxyoxindoles: an update

• Bin Yu,
• Hui Xing,
• De-Quan Yu and
• Hong-Min Liu

Beilstein J. Org. Chem. 2016, 12, 1000–1039, doi:10.3762/bjoc.12.98

• Recently, iridium has emerged as a powerful catalyst for C–H bond functionalization [20][21][22][23]. In 2014, Yamamoto and co-workers reported a cationic iridium complex catalyzed asymmetric intramolecular hydroarylation of α-ketoamides, yielding 3-substituted 3-hydroxy-2-oxindoles with high

• plausible catalytic cycle was proposed as shown in Scheme 8: [Ir(cod)2](BArF4) and the ligand L4 formed the [Ir] precatalyst in situ, which then activated a C–H bond of the substrate to generate aryl–iridium complex A. Subsequent intramolecular asymmetric hydroarylation of intermediate B produced iridium

Synthesis of constrained analogues of tryptophan

• Elisabetta Rossi,
• Valentina Pirovano,
• Marco Negrato,
• Giorgio Abbiati and
• Monica Dell’Acqua

Beilstein J. Org. Chem. 2015, 11, 1997–2006, doi:10.3762/bjoc.11.216

• reactivity upon specific substitution patterns at the indole nucleus were observed by us in competitive Diels–Alder cycloaddition/Michael addition reactions of vinylindoles with classical dienophile and in competitive cycloaddition/hydroarylation reactions with allenes, see [20][21] for a more detailed