An Acceptor-Substituted -Heterocyclic -Quinodimethane: Pushing the Boundaries of Polarization in Donor-Acceptor-Substituted Polyenes.

We report the synthesis, isolation, and characterization of a stable donor-acceptor substituted -quinodimethane (QDM). This system with an imidazolidine scaffold as the donor can also be referred to as acceptor-substituted --heterocyclic quinodimethane (NHQ). We have examined the extent of polarization of the conjugated π-system using single-crystal X-ray diffraction, NMR and UV/vis spectroscopy, cyclic voltammetry, and DFT computations.

Utilization of C NMR Carbon Shifts for the Attribution of Diastereomers in Methyl-Substituted Cyclohexanes.

In this report, we address the challenge of assigning diastereomers for methyl cyclohexanes, particularly those with quaternary centers, which remains nontrivial despite modern NMR techniques. By utilizing a HSQC NMR experiment to identify methyl-carbons coupled with a simple conformational analysis, we identified an effective and quite general method for assigning stereochemistry, even in cases where diastereomeric mixtures are inseparable.

An N-Heterocyclic Quinodimethane: A Strong Organic Lewis Base Exhibiting Diradical Reactivity.

We report the preparation of a new organic σ-donor with a CH-linker between an N-heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N-heterocyclic quinodimethane (NHQ). The aromatization of the CH-linker provides a decisive driving force for the reaction of the NHQ with an electrophile and renders the NHQ significantly more basic than analogous NHCs or N-heterocyclic olefins (NHOs), as shown by DFT computations and competition experiments. In solution, the NHQ undergoes an unprecedented dehydrogenative head-to-head dimerization by C-C coupling of the methylidene groups.

Synthesis and characterization of a formal 21-electron cobaltocene derivative.

Metallocenes are highly versatile organometallic compounds. The versatility of the metallocenes stems from their ability to stabilize a wide range of formal electron counts. To date, d-block metallocenes with an electron count of up to 20 have been synthesized and utilized in catalysis, sensing, and other fields.

The entropic penalty for associative reactions and their physical treatment during routine computations.

A systematic study of the entropic penalty for associative reactions is presented. It is shown that computed solution-phase Gibbs free energies typically overestimate entropic contributions. This entropic penalty for associative reactions in solution, , if the number of particles decreases along the reaction coordinate (sum of stoichiometric numbers ), originates from the insufficient treatment of entropic effects by implicit solvent models.

1,2-Carboboration of Arylallenes by In Situ Generated Alkenylboranes for the Synthesis of 1,4-Dienes.

We herein report a novel method for the coupling of unactivated alkynes and arylallenes, which relies on an unprecedented and regioselective 1,2-carboboration of the allene by an alkenylborane. The alkenylborane is conveniently prepared in situ by hydroboration of an alkyne with Piers' borane, i. e.

Piers' Borane-Induced Tetramerization of Arylacetylenes.

We herein report that the reaction of Piers' borane, i. e. HB(C F ) , with an excess of arylacetylenes at room temperature leads to tetramerization of the acetylene and the diastereoselective formation of boryl-substituted tetra-aryl-tetrahydropentalenes.

Indene formation upon borane-induced cyclization of arylallenes, 1,1-carboboration, and retro-hydroboration.

We herein report the reaction of arylallenes with tris(pentafluorophenyl)borane that yields pentafluorophenyl substituted indenes. The tris(pentafluorophenyl)borane induces the cyclization of the allene and transfers a pentafluorophenyl ring in the course of this reaction. A Hammett plot analysis and DFT computations indicate a 1,1-carboboration to be the C-C bond-forming step.

Boron-Ligand Cooperation: The Concept and Applications.

The term boron-ligand cooperation was introduced to describe a specific mode of action by which certain metal-free systems activate chemical bonds. The main characteristic of this mode of action is that one covalently bound substituent at the boron is actively involved in the bond activation process and changes to a datively bound ligand in the course of the bond activation. Within this review, how the term boron-ligand cooperation evolved is reflected on and examples of bond activation by boron-ligand cooperation are discussed.

Formation of Nucleophilic Allylboranes from Molecular Hydrogen and Allenes Catalyzed by a Pyridonate Borane that Displays Frustrated Lewis Pair Reactivity.

Here we report the in situ generation of nucleophilic allylboranes from H and allenes mediated by a pyridonate borane that displays frustrated-Lewis-pair reactivity. Experimental and computational mechanistic investigations reveal that upon H activation, the covalently bound pyridonate substituent becomes a datively bound pyridone ligand. Dissociation of the formed pyridone borane complex liberates Piers borane and enables a hydroboration of the allene.

Semihydrogenation of Alkynes Catalyzed by a Pyridone Borane Complex: Frustrated Lewis Pair Reactivity and Boron-Ligand Cooperation in Concert.

The metal-free cis selective hydrogenation of alkynes catalyzed by a boroxypyridine is reported. A variety of internal alkynes are hydrogenated at 80 °C under 5 bar H with good yields and stereoselectivity. Furthermore, the catalyst described herein enables the first metal-free semihydrogenation of terminal alkynes.

Efficient Organocatalytic Dehydrogenation of Ammonia Borane.

Dehydrogenation of ammonia borane by sterically encumbered pyridones as organocatalysts is reported. With 6-tert-butyl-2-thiopyridone as the catalyst, a turnover frequency (TOF) of 88 h was achieved. Experimental mechanistic investigations, substantiated by DLPNO-CCSD(T) computations, indicate a mechanistic scenario that commences with the protonation of a B-H bond by the mercaptopyridine form of the catalyst.

Reversible Hydrogen Activation by a Pyridonate Borane Complex: Combining Frustrated Lewis Pair Reactivity with Boron-Ligand Cooperation.

A pyridone borane complex that liberates dihydrogen under mild conditions is described. The reverse reaction, dihydrogen activation by the formed pyridonate borane complex, is achieved under moderate H pressure (2 bar) at room temperature. DFT and DLPNO-CCSD(T) computations reveal that the active form of the pyridonate borane complex is a boroxypyridine that can be described as a single component frustrated Lewis pair (FLP).

Manganese-Catalyzed N-Formylation of Amines by Methanol Liberating H : A Catalytic and Mechanistic Study.

The first example of a base metal (manganese) catalyzed acceptorless dehydrogenative coupling of methanol and amines to form formamides is reported herein. The novel pincer complex (iPr-PN P)Mn(H)(CO) catalyzes the reaction under mild conditions in the absence of any additives, bases, or hydrogen acceptors. Mechanistic insight based on the observation of an intermediate and DFT calculations is also provided.

The Ferraquinone-Ferrahydroquinone Couple: Combining Quinonic and Metal-Based Reactivity.

A ferraquinone-ferrahydroquinone organometallic redox couple was prepared and characterized. Intricate cooperativity of the metal was observed with different positions on the ligand. This allowed cooperative activation of small molecules like molecular hydrogen, oxygen, and bromine.

Reversible Aromaticity Transfer in a Bora-Cycle: Boron-Ligand Cooperation.

Aromaticity is a central concept in chemistry. Reaction pathways involving reversible ligand dearomatization sequences emerged as a powerful tool for bond activation by metal complexes. Exploring this concept with a metal-free system, we have synthesized a pyridine-coordinated aminoborane which undergoes a temperature-induced formal dearomatization of the pyridine ring.

Template Catalysis by Metal-Ligand Cooperation. C-C Bond Formation via Conjugate Addition of Non-activated Nitriles under Mild, Base-free Conditions Catalyzed by a Manganese Pincer Complex.

The first example of a catalytic Michael addition reaction of non-activated aliphatic nitriles to α,β-unsaturated carbonyl compounds under mild, neutral conditions is reported. A new de-aromatized pyridine-based PNP pincer complex of the Earth-abundant, first-row transition metal manganese serves as the catalyst. The reaction tolerates a variety of nitriles and Michael acceptors with different steric features and acceptor strengths.

Reductive Cleavage of CO2 by Metal-Ligand-Cooperation Mediated by an Iridium Pincer Complex.

A unique mode of stoichiometric CO2 activation and reductive splitting based on metal-ligand-cooperation is described. The novel Ir hydride complexes [((t)Bu-PNP*)Ir(H)2] (2) ((t)Bu-PNP*, deprotonated (t)Bu-PNP ligand) and [((t)Bu-PNP)Ir(H)] (3) react with CO2 to give the dearomatized complex [((t)Bu-PNP*)Ir(CO)] (4) and water. Mechanistic studies have identified an adduct in which CO2 is bound to the ligand and metal, [((t)Bu-PNP-COO)Ir(H)2] (5), and a di-CO2 iridacycle [((t)Bu-PNP)Ir(H)(C2O4-κC,O)] (6).

Mechanistic investigations of the catalytic formation of lactams from amines and water with liberation of H2.

The mechanism of the unique lactam formation from amines and water with concomitant H2 liberation with no added oxidant, catalyzed by a well-defined acridine-based ruthenium pincer complex was investigated in detail by both experiment and DFT calculations. The results show that a dearomatized form of the initial complex is the active catalyst. Furthermore, reversible imine formation was shown to be part of the catalytic cycle.

Mechanistic insight into the ruthenium-catalysed anti-Markovnikov hydration of alkynes using a self-assembled complex: a crucial role for ligand-assisted proton shuttle processes.

A combined computational and experimental study is presented that investigates the mechanism of the anti-Markovnikov hydration of phenylacetylene by [Ru(η(5)-C5H5)(6-DPPAP)(3-DPICon)](+) (where 6-DPPAP = 6-(diphenylphosphino)-N-pivaloyl-2-aminopyridine) and 3-DPICon = 3-diphenylphosphinoisoquinolone). The proposed mechanism, modelled using density functional calculations, involves an initial alkyne-vinylidene tautomerism, which occurs via a ligand-assisted proton shuttle (LAPS) mechanism. Intramolecular ligand assistance from the 6-DPPAP and 3-DPICon ligands, particularly the basic nitrogen of 6-DPPAP, is also involved in subsequent stages of the mechanism and three LAPS processes in total are observed.

Mechanistic investigations of the rhodium catalyzed propargylic CH activation.

Previously we reported the redox-neutral atom economic rhodium catalyzed coupling of terminal alkynes with carboxylic acids using the DPEphos ligand. We herein present a thorough mechanistic investigation applying various spectroscopic and spectrometric methods (NMR, in situ-IR, ESI-MS) in combination with DFT calculations. Our findings show that in contrast to the originally proposed mechanism, the catalytic cycle involves an intramolecular protonation and not an oxidative insertion of rhodium in the OH bond of the carboxylic acid.

Realistic energy surfaces for real-world systems: an IMOMO CCSD(T):DFT scheme for rhodium-catalyzed hydroformylation with the 6-DPPon ligand.

The hydroformylation of terminal alkenes is one of the most important homogeneously catalyzed processes in industry, and the atomistic understanding of this reaction has attracted enormous interest in the past. Herein, the whole catalytic cycle for rhodium-catalyzed hydroformylation with the 6-diphenylphosphinopyridine-(2H)-1-one (6-DPPon) ligand 1 was studied. This catalytic transformation is challenging to describe computationally, since two requirements must be met: 1) changes in the hydrogen-bond network must be modeled accurately and 2) bond-formation/bond-breaking processes in the coordination sphere of the rhodium center must be calculated accurately.

Tandem decarboxylative hydroformylation-hydrogenation reaction of α,β-unsaturated carboxylic acids toward aliphatic alcohols under mild conditions employing a supramolecular catalyst system.

A new atom economic catalytic method for a highly chemoselective reduction of α,β-unsaturated carboxylic acids to the corresponding saturated alcohols under mild reaction conditions, compatible with a wide range reactive functional groups, is reported. The new methodology consists of a novel tandem decarboxylative hydroformylation/aldehyde reduction sequence employing a unique supramolecular catalyst system.

A systematic investigation of coinage metal carbonyl complexes stabilized by fluorinated alkoxy aluminates.

The reaction of Cu(I), Ag(I), and Au(I) salts with carbon monoxide in the presence of weakly coordinating anions led to known and structurally unknown non-classical coinage metal carbonyl complexes [M(CO)n][A] (A = fluorinated alkoxy aluminates). The coinage metal carbonyl complexes [Cu(CO)n(CH2Cl2)m](+)[A](-) (n = 1, 3; m = 4-n), [Au2(CO)2Cl](+)[A](-), [(OC)nM(A)] (M = Cu: n = 2; Ag: n = 1, 2) as well as [(OC)3Cu⋅⋅⋅ClAl(OR(F))3] and [(OC)Au⋅⋅⋅ClAl(OR(F))3] were analyzed with X-ray diffraction and partially IR and Raman spectroscopy. In addition to these structures, crystallographic and spectroscopic evidence for the existence of the tetracarbonyl complex [Cu(CO)4](+)[Al(OR(F))4](-) (R(F) = C(CF3)3) is presented; its formation was analyzed with the help of theoretical investigations and Born-Fajans-Haber cycles.

Hydrogen-bond and solvent dynamics in transition metal complexes: a combined simulation and NMR-investigation.

Self-assembling ligands through complementary hydrogen-bonding in the coordination sphere of a transition metal provides catalysts with unique properties for carbon-carbon and carbon-heteroatom formation. Their most distinguishing chemical bonding pattern is a double-hydrogen-bonded motif, which determines much of the chemical functionality. Here, we discuss the possibility of double proton transfer (DPT) along this motif using computational and experimental methods.