Urs Gellrich

ZUSAMMENFASSUNG: Wir berichten, dass Wasser innerhalb von wenigen Minuten bei Raumtemperatur eine schnelle oxidative Addition an leicht zugängliche Dioxaphosphole und ‐phospholane eingeht. Die resultierenden Phenylphosphinatester setzen unter milden Bedingungen (Raumtemperatur bis 60°C) in einem borankatalysierten Prozess Wasserstoff frei. Bemerkenswerterweise reagiert ein Dioxaphospholan mit einem Pinakol‐Rückgrat spontan mit Luftfeuchtigkeit, wobei der zugehörige Phenylphosphinatester entsteht, was die direkte Nutzung von Luftfeuchtigkeit für die katalytische Wasserstoffentwicklung ermöglicht. Der Mechanismus der borankatalysierten Dehydrierung wurde durch eine kinetische Analyse mittels variabler Normierung der Zeitachse ( Variable Time Normalization Analysis , VTNA), die gezielte Synthese von Intermediaten und DFT‐Rechnungen untersucht. Diese Studien deuten darauf hin, dass der katalytische Zyklus mit einer Wasserstofffreisetzung aus dem Boran‐Addukt des Phosphinatesters beginnt, die von einer den Katalysator regenerierenden Retro‐Hydroborierung gefolgt wird.

We report that water undergoes rapid oxidative addition to readily accessible dioxaphospholes and ‐phospholanes within minutes at room temperature. The resulting phenylphosphinate esters release dihydrogen under mild conditions (room temperature to 60°C) in a borane‐catalyzed process. Remarkably, a pinacol‐derived dioxaphospholane reacts spontaneously with atmospheric moisture under ambient conditions to form the corresponding phenylphosphinate ester, thereby enabling the direct utilization of water from air for catalytic hydrogen evolution. The mechanism of the borane‐catalyzed dehydrogenation was investigated by variable time normalization analysis, targeted synthesis of intermediates, and DFT calculations. These studies support a catalytic pathway initiated by hydrogen elimination from the borane adduct of the phosphinate ester, followed by retro‐hydroboration to regenerate the catalyst.

The reaction of silylated alkynes with acid chlorides in the presence of Lewis acids, first described by Birkhofer in 1963, has since emerged as a valuable method for the synthesis of alkynones. Despite its broad synthetic utility, the original protocol suffers from notable drawbacks, including the use of toxic solvents, stoichiometric Lewis acids, and corrosive acylating agents. In light of growing environmental concerns, a more sustainable alternative is developed. Herein, catalytic amounts of iron(III) chloride in combination with biodegradable acetic anhydride enable the efficient synthesis of alkynones under mild conditions.

Herein, we report that sterically encumbered (thio)imidazolones react with simple aliphatic boranes, such as 9‐borabicyclo[3.3.1]nonane (H‐ B ‐9‐BBN) or dicyclohexylborane (HBCy ₂ ), at room temperature to form (thio)imidazolonate boranes,which function as intramolecular frustrated lewis pairs (FLPs). Although direct observation of hydrogen activation by these FLPs is not possible, alkene hydroboration occurs when they are reacted with styrene under hydrogen pressure, suggesting that hydrogen activation is kinetically feasible. In addition, these FLPs rapidly activate NH bonds, as demonstrated by the activation of ammonia and amines under mild conditions. They also cleave the CH bond of terminal alkynes. Density functional theory (DFT) calculations are used to investigate the thermodynamics and kinetics of hydrogen and ammonia activation by these FLPs.

We report the synthesis, isolation, and characterization of a stable donor-acceptor substituted ortho-quinodimethane (oQDM). This system with an imidazolidine scaffold as the donor can also be referred to as acceptor-substituted ortho-N-heterocyclic quinodimethane (oNHQ). We have examined the extent of polarization of the conjugated pi-system using single-crystal X-ray diffraction, NMR and UV/vis spectroscopy, cyclic voltammetry, and DFT computations. The bond lengths in the phenyl linker do not exhibit the alternation typical of oQDMs. In addition, the 13C and 15N NMR shifts suggest significant charge separation, an interpretation supported by the diatropic ring current determined by NICSZZ(r) computations, which is characteristic of aromatic compounds. DFT calculations show that polarization is an electronic effect that is amplified by steric influences. More strikingly, the oxidation and reduction potentials of the push-pull substituted oQDM are virtually identical to those of authenticated anionic and cationic derivatives. The results therefore indicate that an aromatic zwitterionic structure represents the electronic structure more accurately than a neutral quinoidal Lewis structure, which indicates that the acceptor-substituted oNHQ is a rare example of an organic zwitterion in which the centers of charge are in conjugation. The ambiphilic reactivity of the acceptor-substituted oNHQ, which is evidenced by the dehydrogenation of ammonia borane and the addition of phenylacetylene via heterolytic C-H bond cleavage, further supports its notation as an organic zwitterion and is reminiscent of frustrated Lewis pairs (FLPs). Thus, the acceptor-substituted oNHQ can be considered to be an intramolecular carbogenic FLP in terms of its reactivity.

Based on the finding that bis(pentafluorophenyl)borane reacts with dienes at elevated temperatures to alkenyl boranes, contrary to the expected alkyl boranes, we developed a scalable and atom-economic bis(pentafluorophenyl)borane-catalyzed process for the synthesis of alkenyl and (homo)allyl boronates from dienes and boronic esters. This method enables the synthesis of cyclic alkenyl boronates with normal ring size that were previously inaccessible via the catalyzed hydroboration of alkynes.

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.

Wir berichten über die Darstellung eines neuen organischen σ‐Donors mit einem C ₆ H ₄ ‐Linker zwischen einem N ‐heterozyklischen Carben (NHC) und einer exozyklischen Methylidengruppe, den wir als N ‐heterozyklisches Chinodimethan (NHQ) bezeichnen. Die Aromatisierung des C ₆ H ₄ ‐Linkers liefert eine entscheidende Triebkraft für die Reaktion des NHQs mit einem Elektrophil und macht das NHQ deutlich basischer als analoge NHCs oder N ‐heterozyklische Olefine (NHOs), wie DFT‐Berechnungen und Konkurrenzexperimente zeigen. In Lösung findet beim NHQ eine unerwartete Kopf‐Kopf‐Dimerisierung unter formaler Wasserstofffreisetzung durch C−C‐Kopplung der Methylidengruppen statt. DFT‐Berechnungen zeigen, dass diese Reaktion über einen offenschaligen Singulettpfad verläuft, was auf einen diradikalischen Charakter des NHQs hinweist. Das Produkt dieser Dimerisierung kann als konjugiertes N ‐heterozyklisches Bis‐Chinodimethan beschrieben werden, das laut zyklischer Voltammetrie ein starkes organisches Reduktionsmittel ist ( E _1/2 =−1.71 V vs. Fc/Fc ⁺ ) und eine bemerkenswert kleine Singulett–Triplett‐Lücke von Δ E _S→T =4.4 kcal mol ⁻¹ aufweist.

We report the preparation of a new organic σ‐donor with a C ₆ H ₄ ‐linker between an N ‐heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N ‐heterocyclic quinodimethane (NHQ). The aromatization of the C ₆ H ₄ ‐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. DFT computations indicate that this reaction proceeds via an open‐shell singlet pathway revealing the diradical character of the NHQ. The product of this dimerization can be described as conjugated N ‐heterocyclic bis‐quinodimethane, which according to cyclic voltammetry is a strong organic reducing agent ( E _1/2 =−1.71 V vs. Fc/Fc ⁺ ) and exhibits a remarkable small singlet–triplet gap of Δ E _S→T =4.4 kcal mol ⁻¹ .