Direct functionalization of white phosphorus with anionic dicarbenes and mesoionic carbenes: facile access to 1,2,3-triphosphol-2-ides

Rottschäfer D, Blomeyer S, Neumann B, Stammler H-G, Ghadwal R. Direct functionalization of white phosphorus with anionic dicarbenes and mesoionic carbenes: facile access to 1,2,3-triphosphol-2-ides. CHEMICAL SCIENCE. 2019;10(48):11078-11085.A series of unique C2P3-ring compounds [(ADC(Ar))P-3] (ADC(Ar) = ArC{(DippN)C}(2); Dipp = 2,6-iPr(2)C(6)H(3); Ar = Ph 4a, 3-MeC(6)H(4)4b, 4-MeC(6)H(4)4c, and 4-Me(2)NC(6)H(4)4d) are readily accessible in an almost quantitative yield by the direct functionalization of white phosphorus (P-4) with appropriate anionic dicarbenes [Li(ADC(Ar))]. The formation of 1,2,3-triphosphol-2-ides (4a-4d) suggests unprecedented [3 + 1] fragmentation of P-4 into P-3(+) and P-. The P-3(+) cation is trapped by the (ADC(Ar))(-) to give 4, while the putative P- anion reacts with additional P-4 to yield the Li3P7 species, a useful reagent in the synthesis of organophosphorus compounds. Remarkably, the P-4 fragmentation is also viable with the related mesoionic carbenes (iMICs(Ar)) (iMIC(Ar) = ArC{(DippN)(2)CCH}, i stands for imidazole-based) giving rise to 4. DFT calculations reveal that both the C3N2 and C2P3-rings of 4 are 6 pi-electron aromatic systems. The natural bonding orbital (NBO) analyses indicate that compounds 4 are mesoionic species featuring a negatively polarized C2P3-ring. The HOMO-3 of 4 is mainly the lone-pair at the central phosphorus atom that undergoes sigma-bond formation with a variety of metal-electrophiles to yield complexes [{(ADC(Ar))P-3}M(CO)(n)] (M = Fe, n = 4, Ar = Ph 5a or 4-Me-C(6)H(4)5b; M = Mo, n = 5, Ar = Ph 6; M = W, n = 5, Ar = 4-Me(2)NC(6)H(4)7)

Isolation of singlet carbene derived 2-phospha-1,3-butadienes and their sequential one-electron oxidation to radical cations and dications

A synthetic strategy for the 2-phospha-1,3-butadiene derivatives [{(IPr)C(Ph)}P(cAAC$^{Me}$)] (3a) and [{(IPr)C(Ph)}P(cAAC$^{Cy}$)] (3b) (IPr = C{(NDipp)CH}$_{2}$, Dipp = 2,6-iPr$_{2}$C$_{6}$H$_{3}$; cAAC$^{Me}$ = C{(NDipp)CMe$_{2}$CH$_{2}$CMe$_{2}$}; cAAC$^{Cy}$ = C{(NDipp)CMe$_{2}$CH$_{2}$C(Cy)}, Cy = cyclohexyl) containing a C=C–P=C framework has been established. Compounds 3a and 3b have a remarkably small HOMO–LUMO energy gap (3a: 5.09; 3b: 5.05 eV) with a very high-lying HOMO (-4.95 eV for each). Consequently, 3a and 3b readily undergo one-electron oxidation with the mild oxidizing agent GaCl$_{3}$ to afford radical cations [{(IPr)C(Ph)}P(cAAC$^{R}$)]GaCl$_{4}$ (R = Me 4a, Cy 4b) as crystalline solids. The main UV-vis absorption band for 4a and 4b is red-shifted with respect to that of 3a and 3b, which is associated with the SOMO related transitions. The EPR spectra of compounds 4a and 4b each exhibit a doublet due to coupling of the unpaired electron with the $^{31}$P nucleus. Further oneelectron removal from the radical cations 4a and 4b is also feasible with GaCl$_{3}$, affording the dications [{(IPr)C(Ph)}P(cAAC$^{R}$)](GaCl$_{4}$)$_{2}$ (R = Me 5a, Cy 5b) as yellow crystals. The molecular structures of compounds 3–5 have been determined by X-ray diffraction and analyzed by DFT calculations

Expanding the scope of Cu(I) Catalyzed “Click Chemistry” with abnormal NHCs: three-fold click to Tris-Triazoles

Ho NKT, Reichmann SO, Rottschäfer D, Herbst-Irmer R, Ghadwal R. Expanding the scope of Cu(I) Catalyzed “Click Chemistry” with abnormal NHCs: three-fold click to Tris-Triazoles. Catalysts. 2017;7(9): 262.Cationic copper(I) complexes [Cu(aIPrPh)(IPr)]I (3) and [Cu(aIPrPh)2]I (4) featuring an abnormal N-heterocyclic carbene (aNHC) (aIPrPh = 1,3-bis(2,6-diisopropylphenyl)-2-phenyl- imidazol-4-ylidene) and/or an NHC (IPr = 1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene) ligand(s) are reported. Treatment of Cu(aIPrPh)I (2) with IPr affords complex 3. Reaction of (IPrPh)I (1) (IPrPh = 1,3-bis(2,6-diisopropylphenyl)-2-phenyl-imidazolium) with CuI in the presence of K{N(SiMe3)2} leads to the formation of 4. Complexes 3 and 4 represent rare examples of mixed aNHC-NHC and bis-aNHC metal complexes, respectively. They are characterized by elemental analysis, NMR spectroscopic, and mass spectrometric studies. The solid-state molecular structures of 3 and 4 have been determined by single crystal X-ray diffraction analyses. The catalytic activity of 2, 3, and 4 has been investigated in the [3+2] cycloaddition of alkynes and organic azides, affording triazole derivatives in an almost quantitative yield. Notably, complexes 2, 3, and 4 are excellent catalysts for the three-fold cycloaddition of a tris-azide with various alkynes. This catalytic protocol offers a high yield access to tris-triazoles in a shorter reaction time and considerably reduces the experimental work-up compared to the classical synthetic method

1,3‐Imidazol‐basierte mesoionische Carbene und anionische Dicarbene: Erweiterung der Grenzen klassischer N‐heterocyclischer Carbene

Ghadwal R. 1,3‐Imidazol‐basierte mesoionische Carbene und anionische Dicarbene: Erweiterung der Grenzen klassischer N‐heterocyclischer Carbene. Angewandte Chemie. 2023.Klassische N-heterocyclische Carbene (NHCs) mit dem Carbenzentrum an der C2-Position des 1,3-Imidazolgerüsts (d.h. C2-Carbene) sind als vielseitige neutrale Liganden sowohl in der Molekularforschung als auch in den Materialwissenschaften bekannt. Die Effizienz und der Erfolg der NHCs in verschiedenen Bereichen ist im Wesentlichen auf ihre überzeugenden stereoelektronischen Eigenschaften zurückzuführen, insbesondere auf die starke σ-Donoreigenschaft. Die NHCs mit dem Carbenzentrum an der ungewöhnlichen C4- (oder C5-) Position, die sogenannten abnormalen NHCs (aNHCs) oder mesoionischen Carbene (iMICs), sind jedoch deutlich stärkere σ-Donatoren als C2-Carbene. Deshalb haben iMICs ein erhebliches Potenzial als Liganden in der nachhaltigen Synthese und Katalyse. Das Haupthindernis auf diesem Weg ist die anspruchsvolle synthetische Zugänglichkeit zu iMICs. Das Ziel dieses Übersichtsartikels ist es, die aktuellen Fortschritte, insbesondere der Forschungsgruppe des Autors, im Hinblick auf den Zugang zu stabilen iMICs, die Quantifizierung ihrer Eigenschaften sowie die Erforschung ihrer Anwendungen in Synthese und Katalyse aufzuzeigen. Darüber hinaus werden die synthetische Eignung und die Verwendung vicinaler C4,C5-anionische Dicarbene (ADCs), die ebenfalls auf einem 1,3-Imidazolgerüst basieren, vorgestellt. Wie auf den folgenden Seiten deutlich wird, haben iMICs und ADCs das Potenzial, die Grenzen der klassischen NHCs zu erweitern, indem sie den Zugang zu konzeptionell neuartigen Hauptgruppenelement-Heterocyclen, Radikalen, molekularen Katalysatoren, Ligandensätzen und mehr ermöglichen

1,3-Imidazole Based Mesoionic Carbenes and Anionic Dicar-benes: Pushing the Limit of Classical N-Heterocyclic Carbenes

Ghadwal R. 1,3-Imidazole Based Mesoionic Carbenes and Anionic Dicar-benes: Pushing the Limit of Classical N-Heterocyclic Carbenes. Angewandte Chemie International Edition. 2023: e202304665.Classical N-heterocyclic carbenes (NHCs) featuring the carbene center at the C2-position of 1,3-imidazole framework (i.e. C2-carbenes) are well acknowledged as very versatile neutral ligands in molecular as well as in materials sciences. The efficiency and success of NHCs in diverse areas is essentially attributed to their persuasive stereoelectronics, in particular the potent sigma-donor property. The NHCs with the carbene center at the unusual C4 (or C5) position, the so-called abnormal NHCs (aNHCs) or mesoionic carbenes (iMICs), are however superior sigma-donors than C2-carbenes. Hence, iMICs have substantial potential in sustainable synthesis and catalysis. The main obstacle in this direction is rather demanding synthetic accessibility of iMICs. The aim of this review article is to highlight recent advances, particularly by the author's research group, in accessing stable iMICs, quantifying their properties, and exploring their applications in synthesis and catalysis. In addition, the synthetic viability and use of vicinal C4,C5-anionic dicarbenes (ADCs), also based on an 1,3-imidazole framework, are presented. As will be apparent on following pages, iMICs and ADCs hold potentials in pushing the limit of classical NHCs by enabling access to conceptually new main-group heterocycles, radicals, molecular catalysts, ligands sets, and more. © 2023 Wiley-VCH GmbH

Tuning the Electronic Properties of Main-Group Species by N-Heterocyclic Vinyl (NHV) Scaffolds

Ghadwal R. Tuning the Electronic Properties of Main-Group Species by N-Heterocyclic Vinyl (NHV) Scaffolds. Accounts of Chemical Research . 2022.ConspectusMolecules and materials with easily tunable electronic structures and properties are at the forefront of contemporary research. pi-Conjugation is fundamental in organic chemistry and plays a key role in the design of molecular materials. In this Account, we showcase the applicability of N-heterocyclic vinyl (NHV) substituents based on classical N-heterocyclic carbenes (NHCs) for tuning the structure, properties, and stability of main-group species (E) via pi-conjugation and/or pi-donation.NHVs such as [(NHC)═CR] (R = H or aryl) are monoanionic ligands formally derived by the deprotonation of N-heterocyclic olefins (NHOs), (NHC)═CHR. Further deprotonation of [(NHC)═CR] (R = H) is viable, giving rise to N-heterocyclic vinylidene (NHVD) species such as (NHC)═C. NHVs and NHVDs feature a highly polarizable exocyclic CNHC═C bond because of the presence of adjacent pi-donor nitrogen atoms. The nature of the NHC, in particular the pi-acceptor property, has a direct consequence on the polarity of the CNHC═C bond and hence on the magnitude of pi-conjugation in the derived molecules. Thus, the electronic structure, especially the energy and shape of frontier molecular orbitals, HOMO and LUMO, of derived species can be fine-tuned by a judicious choice of the carbene unit. For instance, the HOMO of classical diphosphenes (RP═PR) (R = alkyl or aryl) is invariably the phosphorus lone-pair orbital, while the P═P pi-bond is HOMO - 1 or HOMO - 2. In strong contrast, the HOMO of divinyldiphosphenes (R = NHV) is mainly the P═P pi-bond. This is owing to the pi-conjugation, resulting in the lowering of the LUMO and raising of the HOMO energy. They have a remarkably small HOMO-LUMO energy gap (4.15-4.50 eV) and readily undergo 1e-oxidations, giving rise to stable radical cations and dications.By employing a similar approach, one can access divinyldiarsenes and the corresponding radical cations and dications as crystalline solids. The use of divinyldiphosphenes and divinyldiarsenes as promising ligands in the stabilization of metalloradicals has been shown. By a logical selection of singlet carbenes, stable 2-phosha-1,3-butadiene and 2-arsa-1,3-butadiene compounds, as well as related radical cations and dications, can be prepared as crystalline solids.The relevance of NHV ligands as potent pi-donors has been demonstrated for the stabilization of elusive electrophilic phosphinidene and arsinidene complexes {(NHV)E}Fe(CO)4 (E = P or As). Moreover, stable singlet diradicaloid [(NHC)CP]2 and p-quinodimethane derivatives [(NHC)CP2]2 based on an NHVD framework are accessible as stable solids.In this Account, a special emphasis is given to the contributions from this laboratory. The author hopes that this Account will serve as a useful reference guide for researchers interested in studying and applying NHV and NHVD scaffolds in modern molecular chemistry and materials sciences

Stable Carbon-Centered Radicals Based on N-Heterocyclic Carbenes

Ghadwal R. Stable Carbon-Centered Radicals Based on N-Heterocyclic Carbenes. Synlett. 2019;30(15):1765-1775.Carbon-centered radicals and diradicaloids based on classical N-heterocyclic carbene (NHC) scaffolds are readily accessible as crystalline solids. The presence of an aryl (Ar) substituent at the C2-position is the key to the remarkable stability of these open-shell species as it provides appropriate room for the spin-density delocalization. Two catalytic as well as high-yielding protocols have been developed to install a suitable aryl group at the C2-position of NHCs. The spin-density in mono-radicals (NHCAr)(center dot) is mostly located on the parent carbene carbon (C2) atom. The bridging of two NHCs through a phenylene spacer (C6H4)(n) enables the isolation of various p-quinodimethane (p-QDM) derivatives, which may be considered as open-shell Kekule diradicaloids. The diradical character of these NHC-analogues of Thiele (n = 1), Chichibabin (n = 2), and Muller (n = 3) hydrocarbons [(NHC)(C6H4)(n)(NHC)] can be tuned by a rational choice of the size and/or the topology of spacers. In this account, the synthesis, structure, and properties of this new class of radical hydrocarbons is presented

Synthesis and spectroscopic properties of homo- and heterobimetallic complexes of oxovanadium (V)

Ghadwal R, Singh A. Synthesis and spectroscopic properties of homo- and heterobimetallic complexes of oxovanadium (V). Journal of Chemical Sciences. 2006;118(2):165-170