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    Air and moisture stable para- and ortho-quinodimethane derivatives derived from bis-N-Heterocyclic olefins
    (American Chemical Society, 2023) Jana, Subhadip; Elvers, Benedict J.; Patsch, Sebastian; Sarkar, Pallavi; Krummenacher, Ivo; Nayak, Mithilesh Kumar; Maiti, Avijit; Chrysochos, Nicolas; Pati, Swapan K.; Schulzke, Carola; Braunschweig, Holger; Yıldız, Cem Bural
    Herein we report the development of a new methodology for the synthesis of various quinodimethane derivatives under two-electron oxidation of bis-Nheterocyclic olefins linked by different pi-conjugated aromatic spacers. In case of paraand ortho-phenylene bridge, we obtained air and moisture stable diimidazolium paraand ortho-quinodimethane derivatives. Analogues of the para-phenylene spacer such as tetrafluoro-p-phenylene and p-anthracene also led to the corresponding air and moisture stable quinodimethane derivatives. This emphasizes the influence of imidazolium substituents which facilitate the air and moisture stability of the quinodimethane derivatives. Differences were observed for the electron transfer processes: two one-electron vs one two-electron redox transitions between bis-N-heterocyclic olefins and diimidazolium-quinodimethanes depending on the employed pi-conjugated aromatic spacer. The formation of the pi-conjugated radical-cations, transient redox intermediates between bis-N-heterocyclic olefins and diimidazolium-quinodimethanes, was addressed by an EPR investigation.
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    Carbodicarbenes and striking redox transitions of their conjugate acids: influence of NHC versus CAAC as donor substituents
    (John Wiley and Sons Inc, 2023) Dolai, Ramapada; Kumar, Rahul; Elvers, Benedict J.; Pal, Pradeep Kumar; Joseph, Benson; Sikari, Rina; Nayak, Mithilesh Kumar; Yıldız, Cem Burak
    Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF6] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC8 results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon–carbon sigma bond formation. The resulting relatively elongated carbon–carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.
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    Comment on "Disclosing Cyclic(Alkyl)(Amino)Carbenes as a One-Electron Reductant: Synthesis of Acyclic(Amino)(Aryl)Carbene-Based Kekulé Diradicaloids"
    (Wiley-VCH - Home, 2023) Maiti, Avijit; Elvers, Benedict J.; Bera, Sachinath; Lindl, Felix; Krummenacher, Ivo; Ghosh, Prasanta; Braunschweig, Holger; Yıldız, Cem B.; Schulzke, Carola
    The authors would like to correct a misassignment of an EPR spectrum in the title paper (https://doi.org/10.1002/chem. 202104567). The authors of the original paper reported the one-electron oxidation of the cyclic(alkyl)(amino)carbene (CAAC) 1 to produce the intermediate iminium radical cation 2, as deduced from the high isolated yield of the hydrogen abstraction product 3 (Figure 1). By low-temperature (200 K) EPR spectroscopy, we observed a weak transient, three-line EPR spectrum with a g-factor of 2.0043 and a 14N hyperfine splitting (hfs) of a( 14N)=5.9 G, which we attributed to the radical cation 2 (the spectrum was depicted as Figure 1b, in the TOC cover image, in the Cover Feature (https://doi.org/10.1002/chem. 202201219), and as Figure S46 of the original paper). However, as one of the present authors (H.G.K.) later pointed out, the foregoing EPR parameters, in particular the g-factor, would not be in agreement with the proposed vinyl-type ?-radical cation.
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    Dianionic and Neutral Diboron-Centered Classical Diradicaloids
    (American Chemical Society, 2024) Elvers, Benedict J.; Das, Ayan; Chrysochos, Nicolas; Uddin, Sk Imraj; Gangber, Tejaswinee; Gangber, Tejaswinee
    Herein, we report the syntheses and electronic structures of crystalline dianionic as well as neutral diboron-centered classical diradicaloids as boron analogues of classical Thiele, Chichibabin, and Müller (this only for dianionic diradicaloids!) hydrocarbons. These are based on borane radical anion and NHC-stabilized boryl radical spin carriers, respectively. All these dianionic diboron-centered diradicaloids exhibit triplet population at room temperature regardless of the ?-conjugated spacer: p-phenylene, p,p?-biphenylene, or p,p?-terphenylene. In the case of neutral diboron-centered diradicaloids, the employed ?-conjugated spacer plays a crucial role for the triplet population at room temperature: EPR inactive for p-phenylene vs EPR active for p,p?-biphenylene. The findings emphasize the importance of the spin carriers for the resulting ground-state: borane radical anion vs NHC-stabilized boryl radical along with the pivotal role of the ?-conjugated spacer as spin-coupler between two spins. Notably, 100 years (a century) after the first report by Krause of the triphenyl borane radical-anion, being isoelectronic to the triphenylmethyl radical, we convey borane radical anion-based diradicaloids. Furthermore, while donor-stabilized boryl radicals were introduced in the 1980s by Giles and Roberts, said concept is herewith being extended to NHC-stabilized boryl radical-based diradicaloids.
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    Realizing 1,1-Dehydration of secondary alcohols to carbenes: pyrrolidin-2-ols as a source of cyclic (Alkyl)(Amino)carbenes
    (Wiley-VCH Verlag, 2022) Daş, Ayan; Elvers, Benedict J.; Nayak, Mithilesh Kumar; Chrysochos, Nicolas; Anga, Srinivas; Kumar, Amar; Rao, D. Krishna; Narayanan, Tharangattu N.; Schulzke, Carola; Yıldız, Cem Burak; Jana, Anukul
    Herein we report secondary pyrrolidin-2-ols as a source of cyclic (alkyl)(amino)carbenes (CAAC) for the synthesis of CAAC-Cu-I-complexes and cyclic thiones when reacted with Cu-I-salts and elemental sulfur, respectively, under reductive elimination of water from the carbon(IV)-center. This result demonstrates a convenient and facile access to CAAC-based Cu-I-salts, which are well known catalysts for different organic transformations. It further establishes secondary alcohols to be a viable source of carbenes-realizing after 185 years Dumas' dream who tried to prepare the parent carbene (CH2) by 1,1-dehydration of methanol. Addressed is also the reactivity of water towards CAACs, which proceeds through an oxidative addition of the O-H bond to the carbon(II)-center. This emphasizes the ability of carbon-compounds to mimic the reactivity of transition-metal complexes: reversible oxidative addition and reductive elimination of the O-H bond to/from the C(II)/C(IV)-centre.
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    Reduction of 2-H-substituted pyrrolinium cations: the carbon-carbon single bond in air stable 2,2?-bipyrrolidines as a two-electron-source
    (Royal Society of Chemistry, 2023) Nayak, Mithilesh Kumar; Elvers, Benedict J.; Mandal, Debdeep; Das, Ayan; Ramakrishnan, Raghunathan; Mote, Kaustubh R.; Schulzke, Carola; Yıldız, Cem Burak; Jana, Anukul
    Reduction of 2-H-substituted pyrrolinium cations via initially formed secondary radicals results in either dimerisation or H-abstracted products, while the outcome depends on the N-substituents. The resultant central carbon-carbon single bond in the dimerised 2,2?-bipyrrolidine derivatives can be oxidised chemically and electrochemically. The notably air and moisture-stable dimers were subsequently utilised as a source of two electrons in various chemical transformations.