Cyclic Alkyl Amino Carbene Research Articles

Experimentelle Beobachtung eines terminalen Borylen‐Distickstoff‐Addukts durch Spaltung eines 1,2,3,4,5‐Diboratriazolins

AbstractWährend Azide mit einfachen Alkenen nur unter harschen Bedingungen reagieren, reagiert ein Diboranalogon eines Alkens, das zweifach durch cyclische Alkyl(amino)carbene (CAAC) stabilisierte Dicyanodiboren 1, spontan mit organischen Aziden (7–10 Äq.) bei Raumtemperatur, um zwei Äquivalente stabiler CAAC‐stabilisierter Imino(cyano)borane (2‐R) zu ergeben. NMR‐spektroskopische Kontrollen der Reaktionsmischungen zeigen die anfängliche Bildung einer 1 : 1‐Mischung aus 2‐R und einem relativ langlebigen Zwischenprodukt (Int), welches in Gegenwart von überschüssigem Azid in ein zweites Äquivalent 2‐R umgewandelt wird. In Abwesenheit von überschüssigem Azid zersetzt sich Int jedoch zu 3, dem Produkt einer intramolekularen C−H‐Bindungsaktivierung durch eine mutmaßlich zweifach koordinierte Borylen‐Zwischenstufe, „(CAAC)B(CN)“. Mechanistische Erkenntnisse aus Abfangexperimenten, NMR‐spektroskopischen und hochauflösenden Massenspektrometrie‐Daten sowie DFT‐Berechnungen zeigen, dass Int das endständige Borylen‐Stickstoff‐Addukt [(CAAC)B(CN)(η1‐N2)] ist. Die Bildung der Iminoborane 2‐R aus Diboren 1 und RN3 erfolgt über eine Azid‐Diboren‐Huisgen‐[3+2]‐Cycloaddition, gefolgt von einer [3+2]‐Cycloreversion, wobei 2‐R und [(CAAC)B(CN)(η1‐N2)] entstehen. Letzteres durchläuft dann entweder eine N2‐Abspaltung und intramolekulare C−H‐Bindungsaktivierung zur Bildung von 3 oder eine Staudinger‐artige Reaktion mit einem zweiten Äquivalent Azid zur Bildung eines zweiten Äquivalents des Iminoborans 2‐R.

Experimental Observation of a Terminal Borylene-Dinitrogen Adduct via Cleavage of a 1,2,3,4,5-Diboratriazoline.

While azides do not react with simple alkenes except under harsh conditions, a diboron alkene analogue, the doubly cyclic alkyl(amino)carbene (CAAC)-stabilized dicyanodiborene 1, reacts spontaneously with organic azides (7-10 equiv.) at room temperature to yield two equivalents of stable CAAC-imino(cyano)boranes (2-R). NMR-spectroscopic monitoring of the reaction mixtures shows the initial formation of a 1 : 1 mixture of 2-R and a relatively long-lived intermediate (Int), which in the presence of excess azide is converted into a second equivalent of 2-R. In the absence of excess azide, however, Int decomposes to 3, the product of an intramolecular C-H activation by a putative dicoordinate borylene intermediate "(CAAC)B(CN)". Mechanistic insights from trapping experiments, NMR-spectroscopic and high-resolution mass spectrometry data, as well as DFT computations reveal that Int is the terminal borylene end-on-dinitrogen adduct [(CAAC)B(CN)(η1-N2)]. The formation of the iminoboranes 2-R from diborene 1 and RN3 proceeds via an azide-diborene Huisgen-type [3+2] cycloaddition reaction, followed by a retro-[3+2] cycloaddition, yielding 2-R and [(CAAC)B(CN)(η1-N2)]. The latter then undergoes either N2 extrusion and intramolecular C-H activation to generate 3, or a Staudinger-type reaction with a second equivalent of azide to generate a second equivalent of the iminoborane 2-R.

Synthesis of Thiourea and Thioamide S-Oxides via SO Transfer from a Thiirane S-Oxide onto N-Heterocyclic Carbenes.

Thiourea S-oxides are labile compounds that suffer from a quick overoxidation when directly prepared from thioureas. Therefore, we have developed an alternative route on the basis of the SO transfer from a thiirane S-oxide onto sterically encumbered N-heterocyclic carbenes, including imidazole- and imidazoline-ylidenes, and a cyclic alkyl amino carbene. Reaction conditions and DFT computations suggest a concerted SO transfer mechanism. Thiourea S-oxides are nucleophiles adding to dimethyl acetylenedicarboxylate under fission of the S-O bond.

A Digermanium(III) 1,2-Dication Stabilized by Amidinate and cAAC-Phosphinidenide Ligands.

A digermanium(III) 1,2-dication comprises two cationic centers located at two interconnected Ge atoms. The strong Coulombic repulsion between two positively charged germanium cations hinders their bond formation. Balancing these two oppositions was achieved by using amidinate and cyclic (alkyl)amino carbene (cAAC)-phosphinidenide ligands, where an amidinato cAAC-phosphinidenidogermylene complex, [LGeP(cAACMe)] (2, where L = PhC(NtBu)2, cAACMe = :C{C(Me)2CH2C(Me)2NAr}, and Ar = 2,6-iPr2C6H3), underwent one-electron oxidation with a bis(phosphinidene) radical cation, [(cAACMe)P]2•+, to form a digermanium(III) 1,2-dication, [LGeP(cAACMe)]22+, in compound 4.

Streamlined synthetic assembly of α-chiral CAAC ligands and catalytic performance of their copper and ruthenium complexes.

The unique electronic and steric parameters of chiral cyclic alkyl amino carbene (CAAC) ligands render them appealing steering ligands for enantioselective transition-metal catalyzed transformations. Due to the lack of efficient synthetic strategies to access particularly attractive α-chiral CAACs assessment and exploitation of their full synthetic potential remain difficult. Herein, we report a streamlined strategy to assemble a library of diastereo- and enantiomerically pure CAAC ligands featuring the notoriously difficult to access α-quaternary stereogenic centers. A tailored Julia-Kocienski olefination reagent allows the Claisen-rearrangement to be leveraged as an expedient route to form the synthetically pivotal racemic α-chiral methallyl aldehydes. Subsequent condensation with chiral amines and further cyclization provided a library of diastereomeric mixtures of the targeted ligand precursors. The CAAC salts as well as their corresponding metal complexes are conveniently separable by standard silica gel flash chromatography closing a long-standing accessibility gap in chiral CAAC ligands with proximal α-chirality. The rapid availability of both diastereomers enables testing of the relevance and synergistic effects of two chiral centers on the ligand in catalytic applications. A broad range of metal complexes with copper, gold, rhodium and ruthenium were obtained and structurally analyzed. The catalytic performances of the corresponding chiral CAAC copper and ruthenium complexes were assessed in enantioselective conjugate borylations and asymmetric ring closing metathesis, displaying selectivities of up 95 : 5 er.

Isolation of a New Silepin with an Imine Ligand Based on Cyclic Alkyl Amino Carbene

AbstractTo date, there are solely a handful of silacycloheptatriene structures (silepins) based on acyclic silylenes described in the literature. The unique property of such compounds is the interconversion between their silepin and silylene forms, which has raised great interest due to their potential catalytic applications. Herein, we want to report a new silyl substituted silepin with an implemented Cy‐cAAC moiety, presumably enhancing the stability of the respective low‐valent species. In this work, we will demonstrate the synthesis and facile purification of a newly isolated silepin with an imine ligand based on a cyclic alkyl amino carbene as well as its typical silylene‐like behavior towards small molecules.

Synthesis and Reactivity of Highly Electron-Rich Zerovalent Group 10 Diborabenzene Pogo-Stick Complexes.

A cyclic alkyl(amino)carbene (CAAC)-stabilized 1,4-diborabenzene (DBB, 1) reacts with the group 10 precursor [Ni(CO)4] to yield the DBB pogo-stick complex [(η6-DBB)Ni(CO)] (2) as a dark-green crystalline solid. The IR-spectroscopic and X-ray crystallographic data of 2 highlight the strong π-donor properties of the DBB ligand. The reaction of 1 with [M(nbe)2] (M = Pd, Pt; nbe = norbornene) yields the unique zerovalent heavier group 10 arene pogo-stick complexes [(η6-DBB)M(nbe)] (3-M), isolated as dark-purple and black crystalline solids, respectively. 3-Pd and 3-Pt show strong near-IR (NIR) absorptions at 835 and 904 nm, respectively. Time-dependent density functional theory (TD-DFT) calculations show that these result from the S0→S1 excitation, which corresponds to a transfer of electron density from a metal d orbital aligned with the z direction (dxz or dyz) to a d orbital located in the xy plane (dxy or dx2-y2), with the redshift for 3-Pt resulting from the higher spin-orbit coupling (SOC). In complex 2, electron donation from the nickel center into the carbonyl 2π* orbital destabilizes the DBB···Ni interaction, resulting in an absorption at a higher energy. Complexes 2 and 3-M react with [Fe(CO)5] to yield the doubly CO-bridged M(0)→Fe(0) (M = Ni, Pd, Pt) metal-only Lewis pairs (MOLPs) 4-M as black (M = Ni, Pt) and dark-turquoise (M = Pd) crystalline solids. Furthermore, 3-Pt undergoes oxidative Sn-H addition with Ph3SnH to yield the corresponding Pt(II) stannyl hydride, [(η6-DBB)PtH(SnPh3)] (5).

Neutral and Cationic Molybdenum Imido Alkylidene Cyclic Alkyl Amino Carbene (CAAC) Complexes for Olefin Metathesis.

The first neutral and cationic Mo imido alkylidene cyclic alkyl amino carbene (CAAC) complexes of the general formulae [Mo(N-Ar)(CHCMe2Ph)(X)2(CAAC)] and [Mo(N-Ar)(CHCMe2Ph)(X)(CAAC)][B(ArF)4] (X = Br, Cl, OTf, OC6F5; CAAC = 1-(2,6-iPr2-C6H3)-3,3,5,5-tetramethyltetrahydropyrrol-2-ylidene) have been synthesized from molybdenum imido bishalide alkylidene DME precursors. Different combinations of the imido and "X" ligands have been employed to understand synthetic peculiarities. Selected complexes have been characterized by single crystal X-ray analysis. Due to the pronounced σ-donor/π-acceptor characteristics of CAACs, the corresponding neutral and cationic molybdenum imido alkylidene CAAC complexes do not require the presence of stabilizing donor ligands such as nitriles. Calculations on the PBE0-D3BJ/def2-TZVP level for PBE0-D3BJ/def2-SVP optimized geometries revealed partial charges at molybdenum similar to the corresponding molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes with a slightly higher polarization of the molybdenum alkylidene bond in the CAAC complexes. All cationic complexes have been tested in olefin metathesis reactions and showed improved activity compared to the analogous NHC complexes for hydrocarbon-based substrates, allowing for turnover numbers (TONs) up to 9500 even at room temperature. Some Mo imido alkylidene CAAC complexes are tolerant towards functional groups like thioethers and sulfonamides.

Synthesis, Structural Characterization, and Bonding of Molecular Heavier Beryllium Chalcogenides.

The reactions of a cyclic (alkyl)amino carbene (CAAC)-stabilized beryllium radical with E2Ph2 (E = S, Se, Te) and of a beryllole with HEPh (E = S, Se) yield the corresponding beryllium phenylchalcogenides, including the first structurally authenticated beryllium selenide and telluride complexes. Calculations show that their Be-E bonds are best described by the interaction between the Be+ and E- fragments, with Coulombic forces accounting for ca. 55% of the attraction and orbital interactions dominated by the σ component.

Structure and Electronics of a Series of CAAC-Stabilized Diboron-Doped Acenes from 1,4-Diboranaphthalene to 6,13-Diborapentacene.

This combined experimental and theoretical study examines the influence of acene elongation, boron atom position, and acene substitution pattern on the structure and electronics of cyclic alkyl(amino)carbene (CAAC)-stabilized diboraacenes and presents the first syntheses of neutral diboranaphthalene (DBN) and diborapentacene (DBP). Whereas 2,3-diethyl-substituted 1,4-(CAAC)2-Et2DBN is isolated as a mixture of a planar (structurally characterized) NMR-active conformer and a presumably bent EPR-active conformer, 6,13-(CAAC)2-DBP resembles 9,10-(CAAC)2-DBA (DBA = diboraanthracene), with a highly puckered 6,13-DBP core and a typical biradical EPR signal. Both species are easily reduced to their puckered dianions. DFT calculations confirm that 6,13-(CAAC)2-DBP is only stable in its bent conformation, whereas 1,4-(CAAC)2-Et2DBN exists as both flat closed-shell and bent open-shell biradical conformers, which interchange by thermally activated ethyl and CAAC rotation/diboraacene bending processes. An in-depth computational study of the series of unsubstituted, CAAC-stabilized, symmetrically diboron-doped acenes from 1,4-(CAAC)2-DBN to 6,13-(CAAC)2-DBP was carried out. The results show interesting trends dependent on the position of the boron atoms within the acene framework as well as on the relative orientation of the CAAC ligands, which enable fine-tuning of the electronic and structural features.

C-C and C-N Bond Activation, Lewis-Base Coordination and One- and Two-Electron Oxidation at a Linear Aminoborylene.

A cyclic alkyl(amino)carbene (CAAC)-stabilized dicoordinate aminoborylene is synthesized by the twofold reduction of a [(CAAC)BCl2 (TMP)] (TMP=2,6-tetramethylpiperidyl) precursor. NMR-spectroscopic, X-ray crystallographic and computational analyses confirm the cumulenic nature of the central C=B=N moiety. Irradiation of [(CAAC)B(TMP)] (2) resulted in an intramolecular C-C bond activation, leading to a doubly-fused C10 BN heterocycle, while the reaction with acetonitrile resulted in an aryl migration from the CAAC to the acetonitrile nitrogen atom, concomitant with tautomerization of the latter to a boron-bound allylamino ligand. One-electron oxidation of 2 with CuX (X=Cl, Br) afforded the corresponding amino(halo)boryl radicals, which were characterized by EPR spectroscopy and DFT calculations. Placing 2 under an atmosphere of CO afforded the tricoordinate (CAAC,CO)-stabilized aminoborylene. Finally, the twofold oxidation of 2 with chalcogens led, in the case of N2 O and sulfur, to the splitting of the B-CCAAC bond and formation of the 2,4-diamino-1,3,2,4-dichalcogenadiboretanes and CAAC-chalcogen adducts, whereas with selenium a monomeric boraselenone was isolated, which showed some degree of B-Se multiple bonding.

Dibenzoberylloles: antiaromatic s-block fluorene analogues.

Among a series of Lewis-base-stabilised antiaromatic dibenzoberylloles (DBBes) the cyclic alkyl(amino)carbene (CAAC) analogue undergoes a complex but highly selective thermal decomposition, involving the breaking and making of four bonds each, which yields a rare beryllium η2-alkene complex. Two-electron reduction of the CAAC-stabilised DBBe analogue yields an aromatic dianion.

Redox Active cAAC-Fluorene/Indene Systems Displaying Solvatochromism, Green Luminescence and pH Sensing: Functionalization of Fluorenyl/Indenyl Rings with Radical Carbene.

Two new series of air stable compounds of cAACX = fluorene/indene (X = Me2 , Et2 , Cy) [cAAC = cyclic (alkyl) amino carbene] have been isolated and well characterized by X-ray single crystal diffraction, photoluminescence, cyclic voltammogram (CV) and electron paramagnetic resonance (EPR) studies. Fluorescence studies reveals green light emission of cAAC bonded fluorene, whereas free fluorene generally displays a violet emission. Interestingly, the sterically crowded cAAC-fluorene analogue display solvatochromism and CF3 CO2 H sensing in solution. CV of the these compounds show a quasi-reversible electron transfer process, indicating the functionalization of fluorene/indene with radical anionic form of carbene, confirmed by CV/EPR measurements. DFT/TDDFT calculations and energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) have been carried out to study different aspects of bonding and electronic transitions. Such a class of redox active and thermally stable organic molecules may be suitable for molecule based spin memory devices in future.

Metal-Free Intermolecular C-H Borylation of N-Heterocycles at B-B Multiple Bonds.

Carbene-stabilized diborynes of the form LBBL (L=N-heterocyclic carbene (NHC) or cyclic alkyl(amino)carbene (CAAC)) induce rapid, high yielding, intermolecular ortho-C-H borylation at N-heterocycles at room temperature. A simple pyridyldiborene is formed when an NHC-stabilized diboryne is combined with pyridine, while a CAAC-stabilized diboryne leads to activation of two pyridine molecules to give a tricyclic alkylideneborane, which can be forced to undergo a further H-shift resulting in a zwitterionic, doubly benzo-fused 1,3,2,5-diazadiborinine by heating. Use of the extended N-heteroaromatic quinoline leads to a borylmethyleneborane under mild conditions via an unprecedented boron-carbon exchange process.

Ambient Temperature Carbene-Mediated Depolymerization: Stoichiometric and Catalytic Reactions of N-Heterocyclic- and Cyclic(Alkyl)Amino Carbenes with Poly(N-Methylaminoborane) [MeNH-BH2]n.

The reactions of the N-heterocyclic carbenes (NHCs) IDipp and ItBu and the cyclic(alkyl)amino carbene (CAAC) CAACMe with polyaminoborane [MeNH-BH2]n were investigated. Stoichiometric quantities of each carbene were found to cause rapid and complete depolymerization, with the major B-N-containing product identified as the NHC-aminoborane adduct, IDipp-BH2NMeH (1), cyclic borazane [MeNH-BH2]3, or borazine [MeNBH]3 with IDipp, ItBu, and CAACMe, respectively. With substoichiometric quantities of IDipp and ItBu (down to 10 and 2.5 mol %, respectively), complete loss of high molar mass material was also detected, indicating that the depolymerization is catalytic. The main products of the reaction with substoichiometric IDipp were IDipp-BH2NMeH (1) and [MeNH-BH2]3 and with substoichiometric ItBu, [MeNH-BH2]3, and [MeNBH]3 with product ratios dependent on the quantity of NHC used. Under analogous conditions with CAACMe, high molar mass material persisted alongside the formation of [MeNBH]3. Further reactivity studies with cyclic borazane [MeNH-BH2]3 and MeNH2·BH3 provided insights into depolymerization pathways. IDipp showed no reactivity toward [MeNH-BH2]3, whereas with 3 equiv of ItBu and CAACMe, the dehydrogenation product [MeNBH]3, was formed. With MeNH2·BH3, 2 equiv of carbene were used as the first acts to accept dihydrogen; the major products with IDipp, ItBu, and CAACMe were IDipp-BH2NMeH (1), [MeNBH]3, and (CAACMeH)HB═NMeH (2), respectively. The double E-H (E = B, N) bond activation product (CAACMeH)HB═NMe(HCAACMe) (3) was isolated from the reaction between 3 equiv of CAACMe and MeNH2·BH3. A unified mechanism for donor-mediated depolymerization of [MeHN-BH2]n is proposed.

Highly Strained Arene-Fused 1,2-Diborete Biradicaloid.

The stepwise reduction of a doubly cyclic alkyl(amino)carbene (CAAC)-stabilized 2,3-bis(dibromoboryl)naphthalene enables the isolation of the corresponding mono- and bis(boryl) radicals (one- and two-electron reduction), a 2π-aromatic 1,2-diborete (four-electron reduction), which shows biradical character in the solid-state EPR spectrum, and its cyclic bis(alkylidene)diboron dianion (six-electron reduction). The X-ray crystallographic analysis of the diborete shows a highly strained and twisted four-membered ring with a formal cis-diborene motif featuring a very elongated B-B double bond. Calculations based on DFT and multireference approaches reveal that the diborete possesses an open-shell singlet biradicaloid ground state, which is slightly energetically preferred to its EPR-active triplet-state congener. The addition of CO to the diborete resulted in B-B bond splitting and the formation of the corresponding closed-shell singlet, doubly Lewis base-stabilized bis(borylene), whereas a twofold γ insertion of phenyl azide generates a 1,3-bis(diazenyl)-1,3,2,4-diazadiboretidine.

Synthesis and Photophysical Studies of Copper(I) CAAC Half-Sandwich Complexes as a Highly Modifiable Class of Emitters.

Cyclopentadienyls are well-known strong donor ligands and have been successfully employed in catalysis as they tolerate a variety of substituents to adjust their steric and electronic properties. Although such highly modifiable ligands are of great interest for luminescence and photocatalytic applications, studies of CpR-containing photoactive transition-metal complexes are quite rare. In this work, we present a structural, electrochemical, and first elaborated photophysical investigation of a series of copper(I) half-sandwich complexes bearing cyclic alkyl(amino)carbenes (CAACs) as chromophore ligands and compare them with [Cu(Cp)(IDipp)] and [Cu(Cp*)(IDipp)] bearing a traditional N-heterocyclic carbene. Furthermore, we present the first molecular structure derived from single-crystal X-ray diffraction of a copper(I) indenyl complex, which can be described as an η2 (σ, π)-coordination. The CuI half-sandwich complexes show blue-green to orange phosphorescence with a photoluminescence quantum yield of up to 59% and radiative rate constants kr of up to 4 × 104 s-1 in the solid state, depending on the substitution pattern of the CpR ligand. Our TD/DFT calculations suggest that the emitting excited states are of 3MLCT/LLCT character. We determined the excited-state lifetime of the CuI half-sandwich complexes in solution to be as long as 600 ns, which in combination with the large π-surface of the CpR ligands allows for Dexter energy transfer for photocatalytic applications. In addition, the chiroptical properties of chiral [Cu(Cp/Cp*)(CAACMenthone)] were studied and compared to [CuCl(CAACMenthone)], of which we demonstrate that its circular polarized luminescence is the result of excimer formation and not, as previously reported, attributed to the monomeric C1-symmetric structure.

Preparation of Dienyl Boronates by Tandem Ene-Yne Metathesis/Dienyl Isomerization: Ready Access to Diene Building Blocks for the Synthesis of Polyenes.

The ene-yne metathesis of alkenyl boronates with terminal alkynes is reported. These challenging metatheses were accomplished using a Grubbs catalyst bearing the cyclic alkyl amino carbene (CAAC) ligand, whereas N-heterocyclic carbene (NHC) derived catalysts gave lower yields. Subsequent dienyl isomerization via a cobalt-catalyzed hydrogen atom transfer (HAT) furnished the more substituted dienyl boronate with high EE/EZ ratios. Finally, the resulting dienyl boronate products were successfully used in Suzuki-Miyaura cross-coupling reactions and in a Diels-Alder cycloaddition.

Chiral Cyclic Alkyl Amino Carbene (CAAC) Transition-Metal Complexes: Synthesis, Structural Analysis, and Evaluation in Asymmetric Catalysis

Despite recent advances in the field of cyclic alkyl amino carbenes (CAACs) including a few complementary synthetic strategies affording CAACs with various substitution patterns, the application potential of chiral CAACs to efficiently catalyze asymmetric organometallic transformations remains largely underdeveloped. Herein, we describe a convenient and robust route that incorporates common chiral primary amine allowing access of a broad range of chiral CAACs precursors. The corresponding transition-metal complexes with Cu, Au, Ru, Rh, Ir, and Pd were obtained in a straightforward manner. The steric parameters of the complexes were comprehensively collected by X-ray single-crystal analysis to serve as a source of information for further ligand design. The preliminary application potential of the copper CAAC complexes was tested in asymmetric conjugate borylation of an α,β-unsaturated ester providing 89:11 er, thus illustrating the potential of chiral CAACs in asymmetric catalysis.

Synthesis and catalytic olefin metathesis activity of amberlyst-15 supported cyclic and bicyclic alkyl amino carbene ruthenium complexes

Amberlyst-15 supported cyclic alkyl amino carbene and bicyclic alkyl amino carbene ruthenium olefin metathesis catalysts for sustainable catalytic applications have been synthesized by the well-known wet impregnation method utilizing ionic complex/support interaction. Surface coverages are as high as 4 and 7 wt% were achieved in the case of the significantly higher pore volume Amberlyst-15, compared to Amberlyst-36. These phase separable catalysts show high activity in cross metathesis, ring closing metathesis and ethenolysis reactions compared to the reported heterogenized olefin metathesis catalysts. Leeching tests revealed no more than 1.5 ppm ruthenium content for the investigated metathesis reactions, which is well below the accepted 10 ppm limit in case of consumer products.