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1. Catalytic asymmetric cycloaddition reactions of enoldiazo compounds

   https://doi.org/10.1039/C9OB00478E  

   Marichev, Kostiantyn O.; Doyle, Michael P. (April 2019, Organic & Biomolecular Chemistry)

   This review describes catalytic asymmetric cycloaddition reactions of silyl-protected enoldiazo compounds for the construction of highly functionalized carbo- and heterocycles which possess one or more chiral center(s). The enoldiazo compound or its derivative, donor–acceptor cyclopropene, form electrophilic vinylogous metal carbene intermediates that combine stepwise with nucleophilic dipolar reactants to form products from [3 + 1]-, [3 + 2]-, [3 + 3]-, [3 + 4]-, and [3 + 5]-cycloaddition, generally in high yield and with exceptional stereocontrol and regioselectivity. 

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2. Site-selective [2 + 2 + n ] cycloadditions for rapid, scalable access to alkynylated polycyclic aromatic hydrocarbons

   https://doi.org/10.1039/C9SC06102A  

   Kiel, Gavin R.; Bergman, Harrison M.; Tilley, T. Don (March 2020, Chemical Science)

   null (Ed.)

   Polycyclic aromatic hydrocarbons (PAHs) are attractive synthetic building blocks for more complex conjugated nanocarbons, but their use for this purpose requires appreciable quantities of a PAH with reactive functional groups. Despite tremendous recent advances, most synthetic methods cannot satisfy these demands. Here we present a general and scalable [2 + 2 + n ] ( n = 1 or 2) cycloaddition strategy to access PAHs that are decorated with synthetically versatile alkynyl groups and its application to seven structurally diverse PAH ring systems (thirteen new alkynylated PAHs in total). The critical discovery is the site-selectivity of an Ir-catalyzed [2 + 2 + 2] cycloaddition, which preferentially cyclizes tethered diyne units with preservation of other (peripheral) alkynyl groups . The potential for generalization of the site-selectivity to other [2 + 2 + n ] reactions is demonstrated by identification of a Cp 2 Zr-mediated [2 + 2 + 1]/metallacycle transfer sequence for synthesis of an alkynylated, selenophene-annulated PAH. The new PAHs are excellent synthons for macrocyclic conjugated nanocarbons. As a proof of concept, four were subjected to alkyne metathesis catalysis to afford large, PAH-containing arylene ethylene macrocycles, which possess a range of cavity sizes reaching well into the nanometer regime. Notably, these high-yielding macrocyclizations establish that synthetically convenient pentynyl groups can be effective for metathesis since the 4-octyne byproduct is sequestered by 5 Å MS. Most importantly, this work is a demonstration of how site-selective reactions can be harnessed to rapidly build up structural complexity in a practical, scalable fashion. 

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3. Catalytic, Enantioselective Diamination of Alkenes

   https://doi.org/10.1055/s-0040-1719822  

   Tao, Zhong-Lin; Denmark, Scott E. (October 2021, Synthesis)

   Abstract Enantioselective diamination of alkenes represents one of the most straightforward methods to access enantioenriched, vicinal diamines, which are not only frequently encountered in biologically active compounds, but also have broad applications in asymmetric synthesis. Although the analogous dihydroxylation of olefins is well-established, the development of enantioselective olefin diamination lags far behind. Nevertheless, several successful methods have been developed that operate by different reaction mechanisms, including a cycloaddition pathway, a two-electron redox pathway, and a radical pathway. This short review summarizes recent advances and identifies limitations, with the aim of inspiring further developments in this area. 1 Introduction 2 Cycloaddition Pathway 3 Two-Electron Redox Pathway 3.1 Pd(0)/Pd(II) Diamination 3.2 Pd(II)/Pd(IV) Diamination 3.3 I(I)/I(III) Diamination 3.4 Se(II)/Se(IV) Diamination 4 One-Electron Radical Pathway 4.1 Cu-Catalyzed Diamination 4.2 Fe-Catalyzed Diamination 5 Summary and Outlook 

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4. Interception and Synthetic Application of Diradical and Diene Forms of Dual‐Nature Azabicyclic o ‐Quinodimethanes Generated by 6π‐Azaelectrocyclization

   https://doi.org/10.1002/anie.202409613  

   Shankar, Majji; Lee, Daniel_J; Inaththappulige, Supuni_I_N_Hewa; Acharya, Ayush; Yennawar, Hemant_P; Giri, Ramesh (September 2024, Angewandte Chemie International Edition)

   Abstract We demonstrate that 2‐alkenylarylaldimines and ketimines undergo thermal 6π‐azaelectrocyclization to generate a wide range of azabicyclico‐quinodimethanes (o‐QDMs). Theseo‐QDMs exist as a hybrid of a diene and a benzylic diradical. The diradical nature was confirmed by their ability to undergo dimerization and react with H‐atom donor, 2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) and O₂. In addition, the interception of the diradicaloido‐QDMs by H‐atom transfer was used to synthesize five tetrahydroisoquinoline alkaloids and related bioactive molecules. The diene form can undergo [4+2] cycloaddition reactions with different dienophiles to generate bridged azabicycles in high endo:exo selectivity. The azabicyclico‐QDMs can be generated for [4+2] cycloaddition from a wide range of electronically and sterically varied 2‐alkenylarylimines, including mono, di, tri and tetrasubstituted alkenes, and imines derived from arylamine, alkylamine (1°, 2°, 3°), benzylamine, benzylsulfonamide andBoc‐amine. 

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5. A sequential cyclization/π-extension strategy for modular construction of nanographenes enabled by stannole cycloadditions

   https://doi.org/10.1039/d2sc00397j  

   Bergman, Harrison M.; Beattie, D. Dawson; Kiel, Gavin R.; Handford, Rex C.; Liu, Yi; Tilley, T. Don (May 2022, Chemical Science)

   The synthesis of polycyclic aromatic hydrocarbons (PAHs) and related nanographenes requires the selective and efficient fusion of multiple aromatic rings. For this purpose, the Diels–Alder cycloaddition has proven especially useful; however, this approach currently faces significant limitations, including the lack of versatile strategies to access annulated dienes, the instability of the most commonly used dienes, and difficulties with aromatization of the [4 + 2] adduct. In this report we address these limitations via the marriage of two powerful cycloaddition strategies. First, a formal Cp 2 Zr-mediated [2 + 2 + 1] cycloaddition is used to generate a stannole-annulated PAH. Secondly, the stannoles are employed as diene components in a [4 + 2] cycloaddition/aromatization cascade with an aryne, enabling π-extension to afford a larger PAH. This discovery of stannoles as highly reactive – yet stable for handling – diene equivalents, and the development of a modular strategy for their synthesis, should significantly extend the structural scope of PAHs accessible by a [4 + 2] cycloaddition approach. 

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