Jonathan O. Bauer

Hydrogen release from silicon–hydrogen systems spans a field ranging from extended silicon materials to discrete molecular compounds. Across these regimes, Si─H activation and H ₂ evolution arise from the interplay of structure, coordination environment, and access to reactive geometries. In extended silicon systems, hydrogen release is governed by structure‐dependent recombinative desorption processes. Zintl phases bridge solid‐state and molecular silicon chemistry by enabling transfer of polyanionic silicon units into solution, thereby providing access to discrete Si─H reactivity. In molecular systems, metal‐mediated and metal‐free pathways contribute to hydrogen release, including oxidative addition, σ‐bond metathesis, electrophilic activation, cooperative heterolysis, and donor‐assisted Si─H activation. These mechanistic manifolds form a continuum in which variations in coordination environment and electronic structure determine whether dehydrocoupling, hydrogen transfer, or bond‐forming processes dominate. A recurring mechanistic feature is the involvement of higher‐coordinate silicon species. However, productive H ₂ evolution depends on their formation, but also on the ability of the silicon framework to access reactive geometries through fluxional behavior and dynamic permutational isomerism. Overall, silicon‐based hydrogen‐release chemistry illustrates how structure, coordination, and molecular dynamics collectively govern bond activation and hydrogen evolution, providing a conceptual basis for the design of silicon‐based hydrogen storage and release systems.

A systematic study of four‐membered cationic SiCPN cycles derived from sterically tuned hydrosilyl‐functionalized phosphinimines is presented. Hydride abstraction of the methyl‐ and iso ‐propyl‐substituted precursors successfully affords the corresponding cyclic cations, while the tert ‐butyl analogue resists cyclization, yielding a protonated intermediate instead. Multinuclear NMR spectroscopy reveals significant downfield shifts in both ³¹ P and ²⁹ Si NMR signals, reflecting enhanced cationic character at silicon and pronounced modulation of hyperconjugative n _N → σ* _(P–C) interactions. Correlated shifts at phosphorus and silicon highlight efficient electronic communication across the R ₃ P–N–SiMe ₃ framework, consistent with its isoelectronic analogy to disiloxane linkages. Structural data from single‐crystal X‐ray diffraction analysis, thermochemical density functional theory investigations, and natural bond orbital analyses show that increasing steric bulk at silicon weakens intramolecular N–Si bonding, in line with systematically reduced ring‐opening Gibbs energies. These findings provide a clear picture of the interplay between steric and electronic effects in SiCPN cations and offer design principles for strained donor–acceptor silicon heterocycles.

Propofol (2,6-di-iso-propylphenol) (1) is one of the most widely used intravenous anesthetics, yet its high lipophilicity, formulation challenges, and incompletely understood binding mode motivate the exploration of structural analogues. Here, we report the synthesis and comprehensive characterization of the first silicon analogues of propofol, monosilapropofol (2) and disilapropofol (3), in which one or both iso-propyl groups are replaced by dimethylsilyl substituents. Key steps involve optimized [1,3]-retro-Brook rearrangements, with tert-butyllithium-mediated Li/Br exchange enabling efficient access to both targets. Crystalline potassium phenolate 2-K provided the first X-ray diffraction analysis of a silapropofol derivative, and complementary quantum chemical analysis based on orbital, topological, and localizability descriptors revealed pronounced polarization effects and bond umpolung in this pharmacologically relevant scaffold arising from carbon–silicon isosterism. Stability studies under physiological conditions uncovered a strong divergence between the two analogues: while 2 undergoes gradual hydrolysis to 2-iso-propylphenol and dimethylsilanol, 3 proved remarkably robust in neutral saline solution. These findings demonstrate that silicon substitution offers a powerful strategy to modulate both electronic properties and aqueous stability in propofol derivatives, highlighting carbon–silicon isosterism as a valuable concept for anesthetic drug design.

Boron-ligand cooperation (BLC) has emerged as a powerful principle of bond activation with main-group elements, yet pyridine-based systems have so far eluded experimental evidence of CO₂ activation. We show here that four-membered pyridyl-boracycles activate CO₂ through a dearomatizing boron-carbon bond cleavage, unambiguously proceeding by a BLC rather than a B/N-FLP-type mechanism, as confirmed by density functional theory (DFT) studies, in contrast to previously predicted computational pathways. This process furnishes boryl silyl ketene acetals, a hitherto unknown class of enolate equivalents in which the two oxygen atoms are differentiated by boryl and silyl substituents. These intermediates exhibit remarkable follow-up reactivity, eventually leading to a mild, one-step C═C double-bond cleavage that delivers fulvene derivatives under additive-free conditions, thereby constituting an unprecedented form of metathesis. Overall, our findings establish boryl silyl ketene acetals derived from CO₂ as a novel class of main-group systems that unlock a reactivity platform with far-reaching synthetic implications.

Kurzzusammenfassung: Die dehydrierende Kupplung (DHC) von Hydridosilanen mit Silanolen unter metallfreien Bedingungen bietet einen nachhaltigen Zugang zur Si─O‐Bindungsbildung. Der mechanistische Ursprung der Wasserstofffreisetzung in solchen Systemen blieb jedoch bislang unklar. Hier zeigen wir, dass die dynamische Permutationsisomerie pentakoordinierter Siliciumintermediate eine entscheidende Voraussetzung für die Si─H‐Aktivierung und H ₂ ‐Freisetzung darstellt. Unter Verwendung sterisch maßgeschneiderter Diaminohydridosilane demonstrieren wir, dass nur Liganden, die den Zugang zu axialen Hydridkonfigurationen ermöglichen, eine Si─O‐Kupplung mit produktiver H ₂ ‐Eliminierung vollziehen. Im Gegensatz dazu fixiert eine N‐ tert ‐Butylsubstitution das Hydrid in der äquatorialen Position und lenkt die Reaktivität in Richtung Si─N‐Bindungsspaltung. Multinukleare variable‐Temperatur‐NMR‐Spektroskopie, kombiniert mit quantenchemischen Berechnungen, offenbart ein Gleichgewicht zwischen äquatorialen und axialen Hydridkonfigurationen, das Berry‐Pseudorotation und Wasserstoffentwicklung ermöglicht. Diese Ergebnisse liefern eine mechanistische Grundlage für die H ₂ ‐Freisetzung in Hydridosilicaten und etablieren die ligandengesteuerte Isomerie als allgemeines Konstruktionsprinzip für die selektive, metallfreie Si─H‐Aktivierung.

Dehydrogenative coupling (DHC) of hydridosilanes with silanols under metal‐free conditions provides a sustainable route to Si─O bond formation. Yet, the mechanistic origin of hydrogen release in such systems has remained unclear. Here, we show that dynamic permutational isomerism of pentacoordinate silicon intermediates is a key prerequisite for Si─H activation and H ₂ release. Using sterically tailored diaminohydridosilanes, we demonstrate that only ligands enabling access to axial hydride configurations facilitate Si─O coupling with productive H ₂ elimination. In contrast, N– tert ‐butyl substitution locks the hydride in the equatorial position and diverts reactivity toward Si─N bond cleavage. Multinuclear variable‐temperature NMR spectroscopy, combined with quantum chemical calculations, reveals an equilibrium between equatorial and axial hydride configurations, enabling Berry pseudorotation and hydrogen evolution. These findings provide a mechanistic rationale for H ₂ release in hydridosilicates and establish ligand‐directed isomerism as a general design principle for selective, metal‐free Si─H activation.

Aminophosphate stehen im Mittelpunkt der Forschung zur präbiotischen Phosphorylierungschemie. Ihre bifunktionelle Beschaffenheit macht sie auch zu einer leistungsstarken Klasse von Organokatalysatoren. Allerdings sind die Strukturchemie und Dynamik der Protonenbindung in Phosphorylierungs‐ und organokatalytischen Mechanismen noch immer nicht vollständig verstanden. Aminophosphinchalkogenide, die das zentrale H₂N−P⁺−Ch⁻‐Strukturmotiv bewahren, stellen geeignete molekulare Modelle dar, welche die Protonenbindung, das Schalten von Wasserstoffbrückenbindungen und das supramolekulare Selbstorganisationsverhalten katalytisch und präbiotisch relevanter Moleküle nachahmen. Durch spektroskopische (IR, ¹H‐DOSY, ¹⁵N‐NMR), moleküldynamische und computergestützte Untersuchungen wurde die dynamische Protonenschaltfähigkeit von aminophosphatanalogen Verbindungen nachgewiesen. Es wurde gezeigt, unter welchen Bedingungen die Amino‐ (NH₂) oder Chalkogen‐ (Ch) Funktionen in H₂N−P⁺−Ch⁻‐Struktureinheiten protoniert werden. In der Tat wurden alle denkbaren Arten der Wasserstoffbrückenbindung identifiziert, was erhebliche Unterschiede zwischen dem Sauerstoffderivat und den schwereren Congeneren offenbarte. Bei Verwendung koordinierender Anionen wurden supramolekulare Zickzack‐ und würfelförmige Anordnungen im Festkörper und in Lösung gefunden. Nach dem Aufbrechen der Würfelstruktur gehen die Sulfide und Selenide keine stabilen Wechselwirkungen mehr mit HCl‐Molekülen ein. In Abwesenheit koordinierender Anionen findet hingegen bevorzugt die Protonierung der Chalkogenfunktion statt. Im Gegensatz zum Sauerstoffderivat zeigen die schwereren protonierten Congenere ein dynamisches intramolekulares Protonenhüpfen zwischen der Chalkogen‐ und der Aminofunktion.

Aminophosphates are the focus of research on prebiotic phosphorylation chemistry. Their bifunctional nature also makes them a powerful class of organocatalysts. However, the structural chemistry and dynamics of proton‐binding in phosphorylation and organocatalytic mechanisms are still not fully understood. Aminophosphine chalcogenides, preserving the central H₂N−P⁺−Ch⁻ structural motif, represent well‐suited molecular models that mimic proton‐binding, hydrogen‐bond switching and supramolecular self‐assembling behavior of catalytically and prebiotically relevant molecules. Through spectroscopic (IR, ¹H DOSY, ¹⁵N NMR), molecular dynamics, and computational investigations, the dynamic proton switching capability of aminophosphate analogs was demonstrated. It was shown under which conditions the amino (NH₂) or chalcogen (Ch) functions in H₂N−P⁺−Ch⁻ structural units are protonated. In fact, all conceivable modes of hydrogen‐bonding were identified, revealing substantial differences between the oxygen derivative and the heavier congeners. Using coordinating anions, supramolecular zigzag‐ and cube‐shaped arrangements were found in the solid‐state and in solution. After break‐up of the cube structure, the sulfides and selenides no longer form stable interactions with HCl molecules. In the absence of coordinating anions, however, protonation of the chalcogen function is preferred. In contrast to the oxygen derivative, the heavier protonated congeners show dynamic intramolecular proton‐hopping between the chalcogen and the amino function.

A sequence involving photocatalytic hydrogen atom transfer (HAT), reductive radical-polar crossover (RRPCO), and protonation/deuteration for stereochemical editing at benzylic positions is described. A systematic screening of substrates with benzylic C–H bonds provides trends in reactivity for C–H activation by silane thiols. A cis/trans isomerization of dihydrobenzofurans proceeding under kinetic control in the HAT step is presented, and the concept is transferred to a deracemization by chiral silane thiols as HAT reagents in a proof of concept study.

For more than five decades, P‐stereogenic aminophosphine chalcogenides and boranes have attracted scientific attention and are still in the focus of ongoing research. In the last years, novel transition metal‐based synthesis methods have been discovered, in addition to the long‐known use of chiral auxiliaries. Enantiomerically pure compounds with N−P⁺−X⁻ (X=O, S, BH₃) motifs served as valuable reactive building blocks to provide new classes of organophosphorus derivatives, thereby preserving the stereochemical information at the phosphorus atom. Over the years, intriguing applications in organocatalysis and transition metal catalysis have been reported for some representatives. Asymmetric reductions of C=C, C=N, and C=O double bonds were feasible with selected P‐stereogenic aminophosphine oxides in the presence of hydrogen transfer reagents. P‐stereogenic aminophosphine boranes could be easily deprotected and used as ligands for various transition metals to enable catalytic asymmetric hydrogenations of olefins and imines. This review traces the emergence of a synthetically and catalytically powerful functional compound class with phosphorus‐centered chirality in its main lines, starting from classical approaches to modern synthesis methods to current applications.