Toward Reversible Dihydrogen Activation by Borole Compounds

Efficient catalytic dihydrogen (H₂) activation is crucial in many fundamental chemical transformations. Detailed B97D/TZVP computational study shows that the H₂ molecule can be cooperatively activated over polar B–C bonds of various borole compounds through a concerted four-center transition structure of partial zwitterionic nature. The remarkable H₂ activation reactivity of borole compounds is attributed to the enhanced Lewis acidity at boron and Lewis basicity at α-carbons within the antiaromatic borole ring, and such theoretical insights are important for the design of new metal-free H₂ activation catalysts. For the first time, new borole compounds are designed as promising catalysts for direct H₂ delivery and even reversible H₂ activation by fusing the central borole ring into extended aromatic rings

Copper-Catalyzed Cross-Coupling of Silicon Pronucleophiles with Unactivated Alkyl Electrophiles Coupled with Radical Cyclization

A copper-catalyzed C­(sp³)–Si cross-coupling of aliphatic C­(sp³)–I electrophiles using a Si–B reagent as the silicon pronucleophile is reported. The reaction involves an alkyl radical intermediate that also engages in 5-exo-trig ring closures onto pendant alkenes prior to the terminating C­(sp³)–Si bond formation. Several Ueno–Stork-type precursors cyclized with excellent diastereocontrol in good yields. The base-mediated release of the silicon nucleophile and the copper-catalyzed radical process are analyzed by quantum-chemical calculations, leading to a full mechanistic picture

Copper-Catalyzed Cross-Coupling of Silicon Pronucleophiles with Unactivated Alkyl Electrophiles Coupled with Radical Cyclization

A copper-catalyzed C­(sp³)–Si cross-coupling of aliphatic C­(sp³)–I electrophiles using a Si–B reagent as the silicon pronucleophile is reported. The reaction involves an alkyl radical intermediate that also engages in 5-exo-trig ring closures onto pendant alkenes prior to the terminating C­(sp³)–Si bond formation. Several Ueno–Stork-type precursors cyclized with excellent diastereocontrol in good yields. The base-mediated release of the silicon nucleophile and the copper-catalyzed radical process are analyzed by quantum-chemical calculations, leading to a full mechanistic picture

Chemistry of Thermally Generated Transient Phosphanoxyl Complexes

Investigations on the reactivity of the transiently formed phosphanoxyl complex [(CO)₅W­(Ph₂PO^•)], thermally generated from [(CO)₅W­(Ph₂PO-TEMP)] in toluene, is presented. Apart from self-reactions, trapping of this radical complex was achieved using group 14 hydrides Ph₃EH (E = Si, Ge, Sn), leading to new phosphane complexes possessing a P–O–EPh₃ bonding motif and the corresponding TEMP-H as byproduct. Reaction pathways, derived from DFT calculations, clearly revealed the intermediacy of various open-shell complexes; EPR measurements showed the presence of radicals, but unfortunately interpretation was not achieved

Chemistry of Thermally Generated Transient Phosphanoxyl Complexes

Investigations on the reactivity of the transiently formed phosphanoxyl complex [(CO)₅W­(Ph₂PO^•)], thermally generated from [(CO)₅W­(Ph₂PO-TEMP)] in toluene, is presented. Apart from self-reactions, trapping of this radical complex was achieved using group 14 hydrides Ph₃EH (E = Si, Ge, Sn), leading to new phosphane complexes possessing a P–O–EPh₃ bonding motif and the corresponding TEMP-H as byproduct. Reaction pathways, derived from DFT calculations, clearly revealed the intermediacy of various open-shell complexes; EPR measurements showed the presence of radicals, but unfortunately interpretation was not achieved

Reaction of a Bridged Frustrated Lewis Pair with Nitric Oxide: A Kinetics Study

Described is a kinetics and computational study of the reaction of NO with the intramolecular bridged P/B frustrated Lewis pair (FLP) <i>endo</i>-2-(dimesitylphosphino)-<i>exo</i>-3-bis­(pentafluorophenyl)­boryl-norbornane to give a persistent FLP-NO aminoxyl radical. This reaction follows a second-order rate law, first-order in [FLP] and first-order in [NO], and is markedly faster in toluene than in dichloromethane. By contrast, the NO oxidation of the phosphine base 2-(dimesitylphosphino)­norbornene to the corresponding phosphine oxide follows a third-order rate law, first-order in [phosphine] and second-order in [NO]. Formation of the FLP-NO radical in toluene occurs with a Δ<i>H</i>^⧧ of 13 kcal mol^–1, a feature that conflicts with the computation-based conclusion that NO addition to a properly oriented B/P pair should be nearly barrierless. Since the calculations show the B/P pair in the most stable solution structure of this FLP to have an unfavorable orientation for concerted reaction, the observed barrier is rationalized in terms of the reversible formation of a [B]-NO complex intermediate followed by a slower isomerization–ring closure step to the cyclic aminoxyl radical. This combined kinetics/theoretical study for the first time provides insight into mechanistic details for the activation of a diatomic molecule by a prototypical FLP

Enantiomerically Pure Trinuclear Helicates via Diastereoselective Self-Assembly and Characterization of Their Redox Chemistry

A tris­(bipyridine) ligand <b>1</b> with two BINOL (BINOL = 2,2′-di­hydroxy-1,1′-binaphthyl) groups has been prepared in two enantiomerically pure forms. This ligand undergoes completely diastereo­selective self-assembly into <i>D</i>₂-symmeteric double-stranded trinuclear helicates upon coordination to copper­(I) and silver­(I) ions and to <i>D</i>₃-symmetric triple-stranded trinuclear helicates upon coordination to copper­(II), zinc­(II), and iron­(II) ions as demonstrated by mass spectrometry, NMR and CD spectroscopy in combination with quantum chemical calculations and X-ray diffraction analysis. According to the calculations, the single diastereomers that are formed during the self-assembly process are strongly preferred compared to the next stable diastereomers. Due to this strong preference, the self-assembly of the helicates from racemic <b>1</b> proceeds in a completely narcissistic self-sorting manner with an extraordinary high degree of self-sorting that proves the power and reliability of this approach to achieve high-fidelity diastereo­selective self-assembly via chiral self-sorting to get access to stereo­chemically well-defined nanoscaled objects. Furthermore, mass spectrometric methods including electron capture dissociation MS^<i>n</i> experiments could be used to elucidate the redox behavior of the copper helicates