Silicon‐Containing Formal 4π‐Electron Four‐Membered Ring Systems: Antiaromatic,Aromatic, or Nonaromatic?

Density functional theory calculations (B3LYP) have been carried out to investigate the 4π‐electron systems of 2,4‐disila‐1,3‐diphosphacyclobutadiene (compound 1 ) and the tetrasilacyclobutadiene dication (compound 2 ). The calculated nucleus‐independent chemical shift (NICS) values for these two compounds are negative, which indicates that the core rings of compounds 1 and 2 have a certain amount of aromaticity. However, deep electronic analysis reveals that neither of these two formal 4π‐electron four‐membered ring systems is aromatic. Compound 1 has very weak, almost negligible antiaromaticity, and the amidinate ligands attached to the Si atoms play an important role in stabilizing this conjugated 4π‐electron system. The monoanionic bidentate ligand interacts with the conjugated π system to cause π‐orbital splitting. This ligand‐induced π‐orbital splitting effect provides an opportunity to manipulate the gap between occupied and unoccupied π orbitals in conjugated systems. Conversely, compound 2 is nonaromatic because its core ring does not have a conjugated π ring system and does not fulfill the requirements of a Hückel system.     

C?H/π‐Interaction‐Guided Self‐Assembly in π‐Conjugated Oligomers

We report CH/π hydrogen‐bond‐driven self‐assembly in π‐conjugated skeletons based on oligophenylenevinylenes (OPVs) and trace the origin of interactions at the molecular level by using single‐crystal structures. OPVs were designed with appropriate pendants in the aromatic core and varied by hydrocarbon or fluorocarbon tails along the molecular axis. The roles of aromatic π‐stack, van der Waals forces, fluorophobic effect and CH/π interactions were investigated on the theromotropic liquid crystallinity of OPV molecules. Single‐crystal structures of hydrocarbon OPVs provided direct evidence for the existence of CH/π interactions between the π‐ring (H‐bond acceptor) and alkyl C? H (H‐bond donor). The four important crystallographic parameters, d_c?x=3.79 Å, θ=21.49°, φ=150.25° and d_Hp?x=0.73 Å, matched in accordance with typical CH/π interactions. The CH/π interactions facilitate the close‐packing of mesogens in x–y planes, which were further protruded along the c axis producing a lamellar structure. In the absence of CH/π interactions, van der Waals interactions drove the assembly towards a Schlieren nematic texture. Fluorocarbon OPVs exhibited smectic liquid‐crystalline textures that further underwent Smectic A (SmA) to Smectic C (SmC) phase transitions with shrinkage up to 11 %. The orientation and translational ordering of mesogens in the liquid‐crystalline (LC) phases induced H‐ and J‐type molecular arrangements in fluorocarbon and hydrocarbon OPVs, respectively. Upon photoexcitation, the H‐ and J‐type molecular arrangements were found to emit a blue or yellowish/green colour. Time‐resolved fluorescence decay measurements confirmed longer lifetimes for H‐type smectic OPVs relative to that of loosely packed one‐dimensional nematic hydrocarbon‐tailed OPVs.     

Selective C?F Bond Activation of Tetrafluorobenzenes by Nickel(0) with a Nitrogen Donor Analogous to N‐Heterocyclic Carbenes

N, not NHC : A neutral, basic, strong σ‐donor nitrogen ancillary ligand with properties analogous to those of N‐heterocyclic carbenes (NHCs) was developed to aid in the oxidative additions of challenging substrates to late transition metals. Selective, room‐temperature C? F bond activation was observed with hexa‐, penta‐, and all three isomers of tetrafluorobenzene using a nickel(0) source in the presence of this donor.

     

Orthoamides and Iminium Salts LXXII: N,N,N′,N′,N′′,N′′,N′′′,N′′′‐Octamethyl‐(but‐2‐yne)‐bis(amidinium) Tetrafluoroborate: The First Bis‐amidinium Salt of Acetylenedicarboxyclic Acid. A New Superelectrophile?

A method for the preparation of the first acetylenedicarboxamidinium salt from a bis‐orthoamide derivative of acetylenedicarboxyclic acid has been established. The salt reacted with cyclopentadiene and furan at room temperature to give bicyclic [4+2]‐cycloaddition products. The solid compounds were characterized by solution NMR spectroscopy and by single‐crystal X‐ray diffraction. Quantum‐chemical calculations of the isolated N,N,N′,N′,N′′,N′′,N′′′,N′′′‐octamethyl‐acetylene‐bis(carboxamidinium) ion showed very good agreement with the spectroscopic and diffraction data.     

NMR: A good tool to ascertain σ-silane or σ-borane formulations?

NMR has played a major role in the characterization of dihydrogen complexes and a number of polyhydrides have been reformulated as dihydrogen complexes on the basis of NMR data. If dihydrogen complexes remain the most widely studied class of σ-complexes, silane compounds are also well recognized as an important family of σ-complexes and more recently a few σ-borane compounds have been isolated. One important problem is the discrimination between a σ-formulation and the corresponding hydrido(silyl) or hydrido(boryl) oxidative addition product. In this review we will discuss key literature data on silane and borane complexes to illustrate the benefit gained by using multinuclear NMR spectroscopy to better define the structures and bonding modes. Our goal is also to help the reader to appreciate the limits of the method and to provide valuable insights into the problem of secondary interactions.     

Oligoene‐Based π‐Helicenes or Dispiranes? Winding up Oligoyne Chains by a Multiple Carbopalladation/Stille/(Electrocyclization) Cascade

A domino approach consisting of up to five consecutive steps to access either highly substituted dispiranes or π‐helicenes from oligoyne chains is reported. The domino sequence consists of several carbopalladation reactions, a Stille cross‐coupling to obtain the helicenes, and, depending on the steric demands of the helicene, a final 6π‐electrocyclization to afford the dispiranes. Formally, the latter transformation contravenes the Woodward–Hoffmann rules, as revealed by X‐ray crystallography of the dispirane. Additionally, the racemization barrier of the (Z,Z,Z)‐triene‐based helicene has been determined by a kinetic analysis and compared with results from density functional theory calculations. Characteristic points on the reaction coordinate were further analyzed according to their relaxed force constants (compliance constants).     

Do Carbenes Have a “Hidden” Carbon(0) Character? Revisiting the Electronic Structure of 2,2′‐Bipyridyl Carbene

Density functional calculations predict that 2,2′‐bipyridyl carbenes have some degree of “hidden” carbon(0) character. This is supported by very high second proton affinity values and bond dissociation energies (BDEs) for the dissociation of one AuCl and one Ni(CO)₂ molecule from [(AuCl)₂(2,2′‐bipyridyl carbene)] and [{Ni(CO)₂}₂(2,2′‐bipyridyl carbene)]. An Arduengo type carbene also showed significant BDE values for gem‐dimetallation, thus indicating the possible existence of carbon(0) character. All of the dimetallated derivatives showed metallophilic interactions.     

Rhodium(III)‐Catalyzed ortho Alkenylation of N‐Phenoxyacetamides with N‐Tosylhydrazones or Diazoesters through C?H Activation

A coupling reaction of N‐phenoxyacetamides with N‐tosylhydrazones or diazoesters through Rh^III‐catalyzed C H activation is reported. In this reaction, ortho‐alkenyl phenols were obtained in good yields and with excellent regio‐ and stereoselectivity. Rh–carbene migratory insertion is proposed as the key step in the reaction mechanism.     

Can CH⋅⋅⋅π Interactions Be Used To Design Single‐Chain Magnets?

This theoretical study suggests that CH???π stacking interactions between monomeric units can be used to design novel single‐chain magnets (SCMs), as the sign of coupling is predictable and such chains inherently yield negative axial anisotropy, a condition often difficult to achieve in conventional SCMs.