Stepwise Reversible Oxidation of N-Peralkyl-Substituted NHC–CAAC Derived Triazaalkenes: Isolation of Radical Cations and Dications

"Inverted" Cyclic(Alkyl)(Amino)Carbene (CAAC) Ruthenium Complex Catalyzed Isomerization Metathesis (ISOMET) of Long Chain Olefins to Propylene at Low Ethylene Pressure

Isomerization Metathesis (ISOMET) reaction is an emerging tool for "open loop" chemical recycling of polyethylene to propylene. Novel, latent N-Alkyl substituted Cyclic(Alkyl)(Amino)Carbene (CAAC)-ruthenium catalysts (5a-Ru, 3b-Ru - 6c-Ru) are developed rendering "inverted" chemical structure while showing enhanced ISOMET activity in combination with (RuHCl)(CO)(PPh3)3 (RuH) double bond isomerization co-catalyst. Systematic investigations reveal that the steric hindrance of the substituents on nitrogen and carbon atom adjacent to carbene moiety in the CAAC ligand have significantly improved the catalytic activity and robustness. In contrast to the NHC-Ru and CAAC-Ru catalyst systems known so far, these systems show higher isomerization metathesis (ISOMET) activity (TON: 7400) on the model compound 1-octadecene at as low as 3.0 bar optimized pressure, using technical grade (3.0) ethylene. The propylene content formed in the gas phase can reach up to 20% by volume.

Bis-Olefin Based Crystalline Schlenk Hydrocarbon Diradicals with a Triplet Ground State

A modular approach for the synthesis of isolable crystalline Schlenk hydrocarbon diradicals from m-phenylene bridged electron-rich bis-triazaalkenes as synthons is reported. EPR spectroscopy confirms their diradical nature and triplet electronic structure by revealing a half-field signal. A computational analysis confirms the triplet state to be the ground state. As a proof-of-principle for the modular methodology, the 4,6-dimethyl-m-phenylene was further utilized as a coupling unit between two alkene motifs. The steric conjunction of the 4,6-dimethyl groups substantially twists the substituents at the nonbonding electron bearing centers relative to the central coupling m-phenylene motif. As a result, the spin delocalization is decreased and the exchange coupling between the two unpaired spins, hence, significantly reduced. Notably, 108 years after Schlenk's m-phenylene-bis(diphenylmethyl) synthesis as a diradical, for the first time we were able to isolate its derivative with the same spacer, i.e. m-phenylene, between two radical centers in a crystalline form.

Introducing an orthogonally polarized electron-rich alkene: synthesis of a zwitterionic boron-containing π-conjugated system

The synthesis of an alkene is reported which is concurrently twisted (twist angle = 86.6(8)°), push-pull (dipole moment = 7.48 D), and electron-rich (E1/2 = -1.45 V and -0.52 V vs. Fc/Fc+) in nature, comprising a unique trinity combination for the alkene class of compounds. Subsequently, this newly synthesized alkene-motif was used as a donor for the synthesis of a zwitterionic boron-containing π-conjugated compound (dipole moment = 12.17 D) through an intramolecular charge transfer process exploiting the π-conjugated donor-acceptor system.

Reduction of 2-H-substituted pyrrolinium cations: the carbon-carbon single bond in air stable 2,2'-bipyrrolidines as a two-electron-source

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.

Air and Moisture Stable para- and ortho-Quinodimethane Derivatives Derived from bis-N-Heterocyclic Olefins

Herein we report the development of a new methodology for the synthesis of various quinodimethane derivatives under two-electron oxidation of bis-N-heterocyclic olefins linked by different π-conjugated aromatic spacers. In case of para- and ortho-phenylene bridge, we obtained air and moisture stable diimidazolium para- and 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 π-conjugated aromatic spacer. The formation of the π-conjugated radical-cations, transient redox intermediates between bis-N-heterocyclic olefins and diimidazolium-quinodimethanes, was addressed by an EPR investigation.

A bis-NHC-CAAC dimer derived dicationic diradical

The isolation of carbon-centered diradicals is always challenging due to synthetic difficulties and their limited stability. Herein we report the synthesis of a trans-1,4-cyclohexylene bridged bis-NHC-CAAC dimer derived thermally stable dicationic diradical. The diradical character of this compound was confirmed by EPR spectroscopy. The variable temperature EPR study suggests the singlet state to be marginally more stable than the triplet state (2J = -5.5 cm-1 (ΔE ST = 0.065 kJ mol-1)). The presence of the trans-1,4-cyclohexylene bridge is instrumental for the successful isolation of this dicationic diradical. Notably, in the case of ethylene or propylene bridged bis-NHC-CAAC dimers, the corresponding dicationic diradicals are transient and rearrange to hydrogen abstracted products.

Gauging Radical Stabilization with Carbenes

Carbenes, including N‐heterocyclic carbene (NHC) ligands, are used extensively to stabilize open‐shell transition metal complexes and organic radicals. Yet, it remains unknown, which carbene stabilizes a radical well and, thus, how to design radical‐stabilizing C‐donor ligands. With the large variety of C‐donor ligands experimentally investigated and their electronic properties established, we report herein their radical‐stabilizing effect. We show that radical stabilization can be understood by a captodative frontier orbital description involving π‐donation to‐ and π‐donation from the carbenes. This picture sheds a new perspective on NHC chemistry, where π‐donor effects usually are assumed to be negligible. Further, it allows for the intuitive prediction of the thermodynamic stability of covalent radicals of main group‐ and transition metal carbene complexes, and the quantification of redox non‐innocence.

An Air-Stable Alkene-Derived Organic Radical Cation

Alkenes are known to undergo oxidation to radical cations and dications. The radical cations are often highly reactive and not stable under air. Herein, we report the synthesis, isolation, characterization, and molecular structure of an alkene-derived radical cation A, which is stable in air both in the solid state and in solution. The access to this compound was facilitated from E-diamino tri-substituted alkene B as a synthon for the synthesis of A through one-electron oxidation. The E-diamino tri-substituted alkene B was synthesized by the two-electron reduction of N,N'-1,2-propylene-bridged bis-2-phenyl-pyrrolinium cation C. Under two-electron oxidation, alkene B transforms back to cation C involving a double carbocation rearrangement.

Twisted Push-Pull Alkenes Bearing Geminal Cyclicdiamino and Difluoroaryl Substituents

The systematic combination of N-heterocyclic olefins (NHOs) with fluoroarenes resulted in twisted push-pull alkenes. These alkenes carry electron-donating cyclicdiamino substituents and two electron-withdrawing fluoroaryl substituents in the geminal positions. The synthetic method can be extended to a variety of substituted push-pull alkenes by varying the NHO as well as the fluoroarenes. Solid-state molecular structures of these molecules reveal a notable elongation of the central C-C bond and a twisted geometry in the alkene motif. Absorption properties were investigated with UV-vis spectroscopy. The redox properties of the twisted push-pull alkenes were probed with electrochemistry as well as UV-vis/NIR and EPR spectroelectrochemistry, while the electronic structures were computationally evaluated and validated.

Tethered CAAC-CAAC dimers: oxidation to persistent radical cations and bridging-unit dependent reactivity/stability of the dications

Herein, we report tethered cyclic(alkyl)(amino)carbene (CAAC) dimers in which two CAAC-motifs are connected by an ethylene-, trans-1,2-cyclohexylene- and propylene-spacer through their N-centres. The 1-electron oxidized radical cations are isolable, whereas a significant influence of the bridging unit on the chemical reactivity becomes apparent in and with the 2-electron oxidized products.

Wanzlick's equilibrium in tri- and tetraaminoolefins

How to isolate small carbenes, previously reported to from dimers instantaneously, and how to split triaminoolefins into free carbenes.

Trisubstituted geminal diazaalkene derived transient 1,2-carbodications

The coulombic repulsion between two adjacent cation centres of 1,2-carbodications is known to decrease with π- and/or n-donor substituents by a positive charge delocalization. Here we report the delocalization of the positive charge of transient 1,2-carbodications having one H-substituent by an intramolecular base-coordination. N-heterocyclic olefin (NHO) derived 2-pyrrolidinyl appended trisubstituted geminal diazaalkenes were used for the generation of transient 1,2-carbodications through a 2-e chemical oxidation process. We have also studied the 1-e oxidation reaction of trisubstituted geminal diazaalkenes (electrochemically and chemically) and also studied them using in situ EPR spectroscopy.

Aromaticity and sterics control whether a cationic olefin radical is resistant to disproportionation

We elucidate why some electron rich-olefins such as tetrathiafulvalene (TTF) or paraquat (1,1'-dimethyl-4,4'-bipyridinylidene) form persistent radical cations, whereas others such as the dimer of N,N'-dimethyl benzimidazolin-2-ylidene (benzNHC) do not. Specifically, three heterodimers derived from cyclic (alkyl) (amino) carbenes (CAAC) with N,N'-dimethyl imidazolin-2-ylidene (NHC), N,N'-dimethyl imidazolidin-2-ylidene (saNHC) and N-methyl benzothiazolin-2-ylidene (btNHC) are reported. Whereas the olefin radical cations with the NHC and btNHC are isolable, the NHC compound with a saturated backbone (saNHC) disproportionates instead to the biscation and olefin. Furthermore, the electrochemical properties of the electron-rich olefins derived from the dimerization of the saNHC and btNHC were assessed. Based on the experiments, we propose a general computational method to model the electrochemical potentials and disproportionation equilibrium. This method, which achieves an accuracy of 0.07 V (0.06 V with calibration) in reference to the experimental values, allows for the first time to rationalize and predict the (in)stability of olefin radical cations towards disproportionation. The combined results reveal that the stability of heterodimeric olefin radical cations towards disproportionation is mostly due to aromaticity. In contrast, homodimeric radical cations are in principle isolable, if lacking steric bulk in the 2,2' positions of the heterocyclic monomers. Rigid tethers increase accordingly the stability of homodimeric radical cations, whereas the electronic effects of substituents seem much less important for the disproportionation equilibrium.

N,N'-Ethylene-Bridged Bis-2-Aryl-Pyrrolinium Cations to E-Diaminoalkenes: Non-Identical Stepwise Reversible Double-Redox Coupled Bond Activation Reactions

This work presents a stepwise reversible two‐electron transfer induced hydrogen shift leading to the conversion of a bis‐pyrrolinium cation to an E‐diaminoalkene and vice versa. Remarkably, the forward and the reverse reaction, which are both reversible, follow two completely different reaction pathways. Establishing such unprecedented property in this type of processes was possible by developing a novel synthetic route towards the starting dication. All intermediates involved in both the forward and the backward reactions were comprehensively characterized by a combination of spectroscopic, crystallographic, electrochemical, spectroelectrochemical, and theoretical methods. The presented synthetic route opens up new possibilities for the generation of multi‐pyrrolinium cation scaffold‐based organic redox systems, which constitute decidedly sought‐after molecules in contemporary chemistry.

CAAC-Based Thiele and Schlenk Hydrocarbons

Diradicals have been of tremendous interest for over a century ever since the first reports of p- and m-phenylene-bridged diphenylmethylradicals in 1904 by Thiele and 1915 by Schlenk. Reported here are the first examples of cyclic(alkyl)(amino)carbene (CAAC) analogues of Thiele's hydrocarbon, a Kekulé diradical, and Schlenk's hydrocarbon, a non-Kekulé diradical, without using CAAC as a precursor. The CAAC analogue of Thiele's hydrocarbon has a singlet ground state, whereas the CAAC analogue of Schlenk's hydrocarbon contains two unpaired electrons. The latter forms a dimer, by an intermolecular double head-to-tail dimerization. This straightforward synthetic methodology is modular and can be extended for the generation of redox-active organic compounds.

Nucleophilic Imines and Electrophilic o-Quinone Methides, a Three-Component Assembly of Assorted 3,4-Dihydro-2H-1,3-benzoxazines

A one-pot method for joining three separate components leading to an assortment of N-substituted 3,4-dihydro-2H-1,3-benzoxazines is described. The method involves the addition of a Grignard reagent to an o-OBoc salicylaldehyde in the presence of an imine. With a variety of components, 15 examples are presented, including the diastereoselective incorporation of chiral imines.

Influence of N-Substitution on the Formation and Oxidation of NHC-CAAC-Derived Triazaalkenes

We have studied the effect of N-substitution on the course of the reaction of imidazolium triflate. The reaction of N-heterocyclic carbene with N-tBu-substituted pyrrolinium triflate afforded 2-(pyrrolidin-2-yl)-imidazolium triflate, 3^R. Treatment of 3^R with potassium bis(trimethylsilyl)amide (KHMDS) leads to either the dealkylation product 4 or the deprotonation product, triazaalkene 5, depending on the N-substitution at the imidazolium moiety. Density functional studies using the B3LYP/TZVP setup have been employed to explore various pathways for the dealkylation reaction and the calculated energies support the dealkylation by a large energy margin compared to the deprotonatation process. Theoretical calculations revealed that dealkylation reaction is thermodynamically more favorable than deprotonation. The triazaalkene 5 could be oxidized by AgOTf to the corresponding radical cation 6 and dication 7 in-situ. While 6 and 7 could not be isolated, the formation of the former is inferred by electron paramagnetic resonance spectroscopy and its abstraction of a H-atom to afford 3^Me. Similarly, the formation of the dication 7 is inferred by its ready elimination of isobutylene affording 8.

Neutral Organic Super Electron Donors Made Catalytic

Neutral organic super electron donors (SEDs) display impressive reducing power but, until now, it has not been possible to use them catalytically in radical chain reactions. This is because, following electron transfer, these donors form persistent radical cations that trap substrate-derived radicals. This paper unlocks a conceptually new approach to super electron donors that overcomes this issue, leading to the first catalytic neutral organic super electron donor.

Synthesis of cAAC stabilized biradical of "Me2Si" and "Me2SiCl" monoradical from Me2SiCl2 - an important feedstock material

The cyclic alkyl(amino) carbene (cAAC) coordinated biradical of dimethylsilicon was isolated as (cAAC)2Me2Si (1), (cAAC = C(CH2)(CMe2)2N-2,6-i-Pr2C6H3), synthesized from the reduction of Me2SiCl2 using two equivalents of KC8 in the presence of two equivalents of cAAC. The reduction of Me2SiCl2 by one equivalent of KC8 in the presence of one equivalent of cAAC resulted in the stable dimethylsiliconchloride monoradical (cAAC)Me2SiCl (2).

Stable cyclic (alkyl)(amino)carbene (cAAC) radicals with main group substituents

Recent attempts to isolate cyclic (alkyl)(amino)carbene stabilized radicals of p-block elements have been described here.

Isolation and characterization of stable radicals has been a long-pursued quest. While there has been some progress in this field particularly with respect to carbon, radicals involving heavier p-block elements are still considerably sparse. In this review we describe our recent successful efforts on the isolation of stable p-block element radicals particularly those involving aluminum, silicon, and phosphorus.

Direct access to 2-aryl substituted pyrrolinium salts for carbon centre based radicals without pyrrolidine-2-ylidene alias cyclic(alkyl)(amino)carbene (CAAC) as a precursor

A new strategy to synthesise 2-substituted pyrrolinium salts.

The synthesis of organic radicals is challenging due to their inherent instability. In recent years, cyclic(alkyl)(amino)carbene (CAAC)-derived 2-substituted pyrrolinium salts have been used as synthons for the synthesis of isolable carbon-based radicals. Herein, we report a direct, easy and convenient method for the synthesis of 2-aryl substituted pyrrolinium salts without using CAAC as a precursor. These cations can be reduced to the corresponding radicals. The influence of the aryl substituent at the C-2 position on radical stabilization and dimerization has been investigated. Because of the large scope of our strategy (capability to modulate different substituents at all the C- and N-centres of the pyrrolinium salts), it has the merit to be an extremely effective and productive route for generating carbon-based radicals whose stability as well as reactivity can be varied.

"Abnormal" Addition of NHC to a Conjugate Acid of CAAC: Formation of N-Alkyl-Substituted CAAC

The addition reactions of N-heterocyclic carbenes (NHCs) are mostly known to occur through the carbenic centre (C2), which leads to a "normal" adduct. Herein, we report the "abnormal" addition of NHC^Dip 1 (1,3-(2,6-iPr₂ C₆ H₃ )-imidazole-2-ylidene) to a conjugate acid of cyclic (alkyl)(amino)carbene 2 (CAAC^iPr =1-iPr-3,3,5,5-Me₄ -pyrrolinium triflate). Mechanistic study revealed that this reaction proceeded through the in situ formation of 1,3-(2,6-iPr₂ C₆ H₃ )-imidazolium cation 4 and N-iPr-substituted CAAC 5 followed by the oxidative addition of compound 5 across the C4-H bond (alias backbone C-H) of compound 4. The in situ formation of compound 5 was also proven by the oxidative addition of it to the N-H group of iPrNH₂ . DFT calculations also supported the mechanistic findings. A different methodology for the in situ generation of compound 5 by using TMPLi is also described.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.