Multinuclear Beryllium Chloro Carboxylates.

Reaction of 1 equiv of BeCl2 with mesityl (Mes) or o-tolyl (o-Tol) carboxylic acid in benzene gives hexanuclear heterocyles [BeCl(MesCO2)]6 and [BeCl(o-TolCO2)]6, respectively. Small amounts of the oxocarboxylates [Be4O(MesCO2)6] and [Be4O(o-TolCO2)6] are also formed. If chloroform is used as the solvent, a mixture of these complexes together with the unprecedented tertranuclear cage compounds [Be4Cl2(MesCO2)6] and [Be4Cl2(o-TolCO2)6] is obtained.

ß3-Tryptophans by Iron-Catalyzed Enantioselective Amination of 3-Indolepropionic Acids.

A straightforward and general strategy for the catalytic asymmetric synthesis of ß3-tryptophans by carboxylic-acid-directed intermolecular C-H amination has been developed. The iron-catalyzed C-H amination of 3-indolepropionic acids with BocNHOMs (Boc, tert-butyloxycarbonyl; OMs, methylsulfonate) in the presence of the base piperidine provides N-Boc-protected ß3-tryptophans in a single step with high enantiomeric excess (ee) of up to >99%. Mechanistic experiments and density functional theory calculations support a mechanism through carboxylate-directed iron-mediated C(sp3)-H nitrene insertion. The method incorporates two key sustainability criteria: the use of iron as an abundant, non-toxic, and environmentally benign metal, along with the achievement of streamlined enantioselective C-H functionalization.

Multinuclear beryllium amide and imide complexes: structure, properties and bonding.

The beryllium amide and imide complexes [Be(HNMes)2]3, [(py)2Be(HNMes)2], [Be(HNDipp)2]2, [Be(NPh2)(µ2-HNDipp)]2 and [Be(NCPh2)2]3 have been prepared and characterised with NMR and IR spectroscopy as well as single crystal X-ray diffraction. Analysis of the localised molecular orbitals (LMOs) and intrinsic atomic orbital (IAO) atomic charges in the framework of the intrinsic bond orbital (IBO) localization method revealed a covalent bonding network consisting of 2-electron-2-centre and 2-electron-3-centre σ bonds, in which one electron pair of the anionic N-donor ligands is involved. The electron deficiency at the beryllium atoms is partially compensated through additional electron donation from the lone pair at the nitrogen atoms.

Stereogenic-at-iron mesoionic carbene complex for enantioselective C-H amidation.

Electronically tuned C 2-symmetric stereogenic-at-iron complexes, featuring strongly σ-donating 1,2,3-triazolin-5-ylidene mesoionic carbene (MIC) ligands, exhibit enhanced catalytic efficiency compared to conventional imidazol-2-ylidene analogs, as demonstrated in nitrene-mediated ring-closing C(sp3)-H amidation reactions. Furthermore, a chiral pinene-derived pyridyl triazole ligand enables a highly diastereoselective synthesis of a non-racemic chiral iron catalyst, thereby controlling the absolute configuration at the metal center, as confirmed by NMR and X-ray crystallography. This pinene-modified stereogenic-at-iron MIC complex demonstrates high catalytic activity and a respectable asymmetric induction in the ring-closing C(sp3)-H amination of N-benzoyloxyurea, yielding 2-imidazolidinones with enantiomeric ratios of up to 92 : 8. These findings reflect the profound potential of this new class of mesoionic carbene iron complexes in further understanding and tuning the reactivity of iron-based catalysts.

Mono-Ortho-Beryllated Carbodiphosphoranes: Synthesis, Structure, Bonding and Reactivity.

The reaction of organoberyllium compounds with hexaphenylcarbodiphosphorane yields mono-ortho-beryllated complexes, which feature a double dative Be=C bond. The bonding situation in these compounds together with a simple carbodiphosphorane and an N-heterocyclic carbene adduct was analysed with energy decomposition analysis in combination with natural orbital for chemical valence as well as with quantum theory of atoms-in-molecules. Furthermore, the driving forces accountable for mono-ortho-beryllation were elucidated along with the reactivity of the Be=C bond.

Synthesis and Derivatives of Beryllium Triflate.

Various pathways for the synthesis of beryllium triflate were investigated. The reaction of triflic acid or trimethylsilyl triflate with beryllium metal in liquid ammonia led to the formation of mono-, di-, and tetra-nuclear beryllium ammine complexes. Utilization of SMe2 as a solvent gave homoleptic Be(OTf)2. Various beryllium triflate complexes with N- and O-donor ligands as well as the complex anions [Be(OTf)4]2- and [Be2(OTf)6]2- were synthesized to evaluate the reactivity and solution properties of beryllium triflate. This showed that OTf- is not a weakly coordinating anion for Be2+ cations and that it exhibits good bridging properties.

Nitrene-Mediated Enantioselective Intramolecular Olefin Oxyamination to Access Chiral γ-Aminomethyl-γ-Lactones.

Attaching a nitrene precursor to an intramolecular nucleophile allows for a catalytic asymmetric intramolecular oxyamination of alkenes in which the nucleophile adds in an endocyclic position and the amine in an exocyclic fashion. Using chiral-at-ruthenium catalysts, chiral γ-aminomethyl-γ-lactones containing a quaternary carbon in γ-position are provided in high yields (up to 99 %) and with excellent enantioselectivities (up to 99 % ee). DFT calculations support the possibility of both a singlet (concerted oxyamination of the alkene) and triplet pathway (stepwise oxyamination) for the formation of the predominant stereoisomer. γ-Aminomethyl-γ-lactones are versatile chiral building blocks and can be converted to other heterocycles such as δ-lactams, 2-oxazolidinones, and tetrahydrofurans.

Experimental and Computational Studies on Cobalt(I)-Catalyzed Regioselective Allylic Alkylation Reactions.

Here, we report the development of cobalt(I)-catalyzed regioselective allylic alkylation reactions of tertiary allyl carbonates with 1,3-dicarbonyl compounds. A family of well-defined tetrahedral cobalt(I) complexes bearing commercially available bidentate bis(phosphine) ligands [(P,P)Co(PPh3 )Cl] are synthesized and explored as catalysts in allylic alkylation reactions. The catalyst [(dppp)Co(PPh3 )Cl] (dppp=1,3-Bis(diphenylphosphino)propane) enables the alkylation of a large variety of tertiary allyl carbonates with high yields and excellent regioselectivity for the branched product. Remarkably, this methodology is selective for the activation of tertiary allyl carbonates even in the presence of secondary allyl carbonates. This contrasts with the selectivity observed in cobalt-catalyzed allylic alkylations enabled by visible light photocatalysis. Mechanistic insights by means of experimental and computational investigations support a Co(I)/Co(III) catalytic cycle.

Gauging ambiphilicity of pseudo-halides via beryllium-trispyrazolylborato compounds.

The ambiphilicity of pseudo-halides has been the object of extensive debate. Herein, we use a series of trispyrazolylborato beryllium pseudo-halido complexes [TpBe(X')] with X' = CN-, N3-, NCO- and NCS- to explore the origins of the preferred isomers. Thus, we have synthesised and characterised through NMR and IR spectroscopy as well as single crystal X-ray diffraction these complexes. A combination with quantum chemical calculations within the DFT framework enabled an in-depth understanding of the bonding modes and preferences of the investigated pseudo-halido ligands.

A Preference for Heterolepticity - Schlenk Type Equilibria in Organometallic Beryllium Systems.

The reaction of homoleptic beryllium halide with diphenyl beryllium complexes leads to the clean formation of heteroleptic beryllium Grignard compounds [(L)1-2 BePhX]1-2 (X=Cl, Br, I; L=C-, N-, O-donor ligand). The influence of ligands and solvent on these compounds, their formation and exchange equilibria in solution were investigated, together with the factors determining the complex constitution.

Trading Symmetry for Stereoinduction in Tetradentate, non-C2 -Symmetric Fe(II)-Complexes for Asymmetric Catalysis.

A series of stereogenic-at-metal iron complexes comprising a non-C2 -symmetric chiral topology is introduced and applied to asymmetric 3d-transition metal catalysis. The chiral iron(II) complexes are built from chiral tetradentate N4-ligands containing a proline-derived amino pyrrolidinyl backbone which controls the relative (cis-α coordination) and absolute metal-centered configuration (Λ vs. Δ). Two chloride ligands complement the octahedral coordination sphere. The modular composition of the tetradentate ligands facilitates the straightforward incorporation of different terminal coordinating heteroaromatic groups into the scaffold. The influence of various combinations was evaluated in an asymmetric ring contraction of isoxazoles to 2H-azirines revealing that a decrease of symmetry is beneficial for the stereoinduction to obtain chiral products in up to 99 % yield and with up to 92 % ee. Conveniently, iron catalysis is feasible under open flask conditions with the bench-stable dichloro complexes exhibiting high robustness towards oxidative or hydrolytic decomposition. The versatility of non-racemic 2H-azirines was subsequently showcased with the conversion into a variety of quaternary α-amino acid derivatives.

Exchange Interactions and Magnetic Properties of a Molecular Mn18 -Ring Complex.

[Mn3 O(OAc)7 (HOAc)]6 ⋅ x AcOH (x=6-9) represents a rare example of a compound containing molecular Mn18 -rings. These are formed by Mn3 (µ3 -O) subunits in which the high-spin Mn(III) centers are bridged by three pairs of acetate anions (AcO- ). An AcOH molecule coordinates to one of the Mn atoms leading to [Mn3 (µ3 -O)(µ2 -OAc)6 (AcOH)]-units, designated in short as Mn3 -units, that are interconnected by acetate anions via the other two Mn atoms to form Mn18 -rings. Magnetic measurements show weak ferromagnetic interactions between them that are suppressed in strong magnetic field. Quantum-chemical calculations on Mn3 model complexes using independently DFT and ab-initio multi reference methods (CASSCF/NEVPT2) show a correlation between the orientation of the pseudo-Jahn-Teller axes of pairs of Mn(III) magnetic centers and corresponding exchange coupling energies. Weak coupling between Mn3 -units within the Mn18 -ring allowed to simulate the magnetic susceptibility versus temperature dependence in terms of basically uncoupled magnetic moments of each Mn3 -unit within the ring.

Ligand exchange at tetra-coordinated beryllium centres.

Mono and dinuclear phosphine complexes of beryllium halides [(PMe3)2BeX2], [(PMe3)BeX2]2 and [(PCy3)BeX2]2 (X = Cl, Br, I) were synthesised and characterised via NMR and IR spectroscopy as well as single crystal X-ray diffraction experiments. Dissociation and ligand exchange processes at these complexes were investigated through variable temperature NMR experiments in combination with line shape analysis and complemented by quantum chemical calculations. The PMe3 dissociation energy is smallest in [(PMe3)2BeCl2], while PMe3 exchange is similar in energy in all mononuclear [(PMe3)2BeX2] complexes and follows an interchange mechanism. While [(PMe3)BeX2]2 dissociates homolytically, [(PCy3)BeX2]2 cleaves one phosphine ligand. These distinctive dissociation processes account for the different chemical behaviour of these complexes.

Nitrene-Mediated C-H Oxygenation: Catalytic Enantioselective Formation of Five-Membered Cyclic Organic Carbonates.

The synthesis of non-racemic 5-membered cyclic carbonates from abundant alcohols is reported. Conversion of the alcohol into an azanyl carbonate is followed by a chiral-at-ruthenium catalyzed cyclization to provide chiral cyclic carbonates in yields of up to 95 % and with up to 99 % ee. This new synthetic method is proposed to proceed through a nitrene-mediated intramolecular C(sp3 )-H oxygenation which includes an unusual 1,7-hydrogen atom transfer within a ruthenium nitrene intermediate. The method is applicable to the synthesis of non-racemic chiral mono-, di- and trisubstituted cyclic alkylene carbonates.

Surprises in the Solvent-Induced Self-Ionization in the Uranium Tetrahalide UX4 (X = Cl, Br, I)/Ethyl Acetate System.

The reaction of the uranium(IV) halides UCl4, UBr4, or UI4 with ethyl acetate (EtOAc) leads to the formation of the complexes [UX3(EtOAc)4][UX5(EtOAc)] (X = Cl, Br) or [UI4(EtOAc)3]. Thus, both UCl4 and UBr4 show self-ionization in ethyl acetate to a distorted pentagonal bipyramidal [UX3(EtOAc)4]+ cation and a distorted octahedral [UX5(EtOAc)]- anion. Surprisingly, the chloride and bromide compounds are not isotypic. While [UCl3(EtOAc)4][UCl5(EtOAc)] crystallizes in the orthorhombic crystal system, space group P212121 at 250 K, the bromide compound crystallizes in the monoclinic crystal system, P121/n1 at 100 K. Unexpectedly, UI4 does not show self-ionization but forms [UI4(EtOAc)3] molecules, which crystallize in the monoclinic crystal system, P21/c, at 100 K. The compounds were characterized by single-crystal X-ray diffraction, IR, Raman, and NMR spectroscopy, as well as molecular quantum chemical calculations using solvent models.

Reactivity of Diphenylberyllium as a Brønsted Base and Its Synthetic Application.

Diphenylberyllium [Be3 Ph6 ] is shown here to react cleanly as a Brønsted base with a vast variety of protic compounds. Through the addition of the simple molecules tBuOH, HNPh2 and HPPh2 , as well as the more complex 1,3-bis-(2,6-diisopropylphenyl)imidazolinium chloride, one or two phenyl groups in diphenylberyllium were protonated. As a result, the long-postulated structures of [Be3 (OtBu)6 ] and [Be(µ-NPh2 )Ph]2 have finally been verified and shown to be static in solution. Additionally [Be(µ-PPh2 )(HPPh2 )Ph]2 was generated, which is only the second beryllium-phospanide to be prepared; the stark differences between its behaviour and that of the analogous amide were also examined. The first crystalline example of a beryllium Grignard reagent with a non-bulky aryl group has also been prepared; it is stabilised with an N-heterocyclic carbene.

Electrochemical Enantioselective Nucleophilic α-C(sp3)-H Alkenylation of 2-Acyl Imidazoles.

Merging electrochemistry with asymmetric catalysis promises to provide an environmentally friendly and efficient strategy for the construction of nonracemic chiral molecules. However, in practice, significant challenges arise from the instability or incompatibility of the chiral catalysts under the electrochemical conditions at the interface of electrode and solution. Herein, we report a catalytic asymmetric indirect electrolysis employing the combination of a redox mediator and a chiral-at-rhodium Lewis acid, which achieves a previously elusive enantioselective nucleophilic α-C(sp3)-H alkenylation of ketones. Specifically, 2-acyl imidazoles react with potassium alkenyl trifluoroborates in high yields (up to 94%) and with exceptional enantioselectivities (27 examples with ≥99% ee) without the need for any additional stoichiometric oxidants (overall 40 examples). The new indirect electrosynthesis can be scaled to gram quantities and was applied to the straightforward synthesis of intermediates of the natural product cryptophycin A and a cathepsin K inhibitor.

Synthesis of FeIII and FeIV Cyanide Complexes Using Hypervalent Iodine Reagents as Cyano-Transfer One-Electron Oxidants.

We disclose a new reactivity mode for electrophilic cyano λ3 -iodanes as group transfer one-electron oxidants to synthesize FeIII and FeIV cyanide complexes. The inherent thermal instability of high-valent FeIV compounds without π-donor ligands (such as oxido (O2- ), imido (RN2- ) or nitrido (N3- )) makes their isolation and structural characterization a very challenging task. We report the synthesis of an FeIV cyanide complex [(N3 N')FeCN] (4) by two consecutive single electron transfer (SET) processes from FeII precursor [(N3 N')FeLi(THF)] (1) with cyanobenziodoxolone (CBX). The FeIV complex can also be prepared by reaction of [(N3 N')FeIII ] (3) with CBX. In contrast, the oxidation of FeII with 1-cyano-3,3-dimethyl-3-(1H)-1,2-benziodoxole (CDBX) enables the preparation of FeIII cyanide complex [(N3 N')FeIII (CN)(Li)(THF)3 ] (2-LiTHF ). Complexes 4 and 2-LiTHF have been structurally characterized by single crystal X-ray diffraction and their electronic structure has been examined by Mössbauer, EPR spectroscopy, and computational analyses.

Preussochromone Puzzle: Structural Revision of Preussochromones E and F by Total Synthesis.

A stereoselective synthesis of the proposed and actual structures of the natural products preussochromones E and F is reported. The key step is a ring-closing metathesis to close the five-membered ring and install the trans configuration of the annulated five-six ring system. The analysis of the 3J NMR couplings of the isolated natural product with the synthesized compound revealed its real structure with a cis annulation, which could also be synthesized using an intramolecular aldol reaction of a vic-tricarbonyl compound.

Improving the Configurational Stability of Chiral-at-Iron Catalysts Containing Two N-(2-Pyridyl)-Substituted N-Heterocyclic Carbene Ligands.

Recently, we introduced the first example of chiral-at-iron catalysts in which two achiral N-(2-pyridyl)-substituted N-heterocyclic carbene (NHC) ligands in addition to two labile acetonitriles are coordinated around a central iron, to generate a stereogenic metal center [Hong Y.Chiral-at-Iron Catalyst: Expanding the Chemical Space for Asymmetric Earth-Abundant Metal Catalysis. J. Am. Chem. Soc.2019, 141, 4569-4572]. A more facile synthesis of such chiral-at-iron catalysts was developed, which omits the use of expensive silver salts and an elaborate electrochemical setup. Configurational robustness was improved by replacing the imidazol-2-ylidene carbene moieties with benzimidazol-2-ylidenes. The π-acceptor properties of the altered NHCs were investigated by Ganter's 77Se NMR method. The obtained benzimidazol-2-ylidene chiral-at-iron complex is an excellent catalyst for an asymmetric hetero-Diels-Alder reaction under open-flask conditions.