Probing the Rotational Dynamics of meso‐(2‐Substituted)aryl Substituents in A2B‐Type Subporphyrins

A₂B‐type B‐methoxy subporphyrins 3 a – g and B‐phenyl subporphyrins 7 a – c , e , g bearing meso‐(2‐substituted)aryl substituents are synthesized, and their rotational dynamics are examined through variable‐temperature (VT) ¹H NMR spectroscopy. In these subporphyrins, the rotation of meso‐aryl substituents is hindered by a rationally installed 2‐substituent. The rotational barriers determined are considerably smaller than those reported previously for porphyrins. Comparison of the rotation activation parameters reveals a variable contribution of ΔH^≠ and ΔS^≠ in ΔG^≠. 2‐Methyl and 2‐ethyl groups of the meso‐aryl substituents in subporphyrins 3 e , 3 f , and 7 e induce larger rotational barriers than 2‐alkoxyl substituents. The rotational barriers of 3 g and 7 g are reduced by the presence of the 4‐dibenzylamino group owing to its ability to stabilize the coplanar rotation transition state electronically. The smaller rotational barriers found for B‐phenyl subporphyrins than for B‐methoxy subporphyrins indicate a negligible contribution of S_N1‐type heterolysis in the rotation of meso‐aryl substituents.     

Enhancement of Spin Relaxation in an FeDy2Fe Coordination Cluster by Magnetic Fields

Two [FeLn₂Fe(μ₃‐OH)₂(teg)₂(N₃)₂(C₆H₅COO)₄] compounds (where Ln=Y^III and Dy^III; teg=triethylene glycol anion) have been synthesized and studied using SQUID and Mössbauer spectroscopy. The magnetic measurements on both compounds indicate dominant antiferromagnetic interactions between the metal centers. Analysis of the ⁵⁷Fe Mössbauer spectra complement the ac magnetic susceptibility measurements, which show how a static magnetic field can quench the slow relaxation of magnetization generated by the anisotropic Dy^III ions.     

Conformational Control of Oligo(p‐phenyleneethynylene)s with Intrinsic Substituent Electronic Effects: Origin of the Twist in Pentiptycene‐Containing Systems

Dependence of the backbone planarity of oligo(p‐phenyleneethynylene)s (OPEs) on the intrinsic electronic character of substituents and on the nature of the solvent has been experimentally demonstrated with a series of center‐symmetrical five‐ring systems, pentiptycene‐pentiptycene‐arene‐pentiptycene‐pentiptycene, differing in the substituents on the central arene. In frozen 2‐methyltetrahydrofuran (MTHF), the adjacent pentiptycene units prefer to be in a mutually twisted orientation when the substituents are electron‐withdrawing (F and amido), resulting in a TPPT or TTTT conformation, whereas a planarized PPPP backbone is favored in the case of electron‐donating substituents (alkyl and alkoxy). The propensity to adopt the PPPP form is generally enhanced by replacing MTHF with either methylcyclohexane or mixed ethanol/methanol as solvent. These observations reveal that the twist between adjacent pentiptycene units in OPEs is a consequence of the electronic rather than steric effects of iptycenyl substituents. The electronic effect of iptycenyl substituents is manifested in decreased phenylene π polarizability as the net effect of both electron‐donating hyperconjugation and an electron‐withdrawing inductive effect. Variable‐temperature electronic absorption and emission spectroscopies are the critical tools for this work. Our findings provide important guidelines for conformational and electronic engineering of OPEs and for the design of novel iptycene‐based organic electronic materials.     

Stereoelectronic Basis for the Kinetic Resolution of N‐Heterocycles with Chiral Acylating Reagents

The kinetic resolution of N‐heterocycles with chiral acylating agents reveals a previously unrecognized stereoelectronic effect in amine acylation. Combined with a new achiral hydroxamate, this effect makes possible the resolution of various N‐heterocycles by using easily prepared reagents. A transition‐state model to rationalize the stereochemical outcome of this kinetic resolution is also proposed.     

Synthesis of New Lipid‐Inspired Ionic Liquids by Thiol‐ene Chemistry: Profound Solvent Effect on Reaction Pathway

The synthesis of a series of new lipid‐inspired ionic liquids through thiol‐ene “click” reaction with a single‐step process. This synthesis offers considerable promise as an efficient and orthogonal method to construct structurally diverse imidazolium‐type ionic liquids with linear and branched cationic tails, as well as versatility in the placement of the sulfur heteroatom. Profound solvent effect in this ene reaction regioselectivity has been observed.     

Dispersion Interaction Stabilizes Sterically Hindered Double Fullerenes

By state‐of‐the‐art quantum chemical methods, we show that for bulky functional groups like cyclohexane, [20]fullerene, dodecahedrane, and C₆₀, the attractive dispersion interaction can have a greater impact on stereochemistry than the repulsive steric effect, making the compact isomer the more stable one. In particular, for the double C₆₀ adduct of pentacene 1 , the syn isomer should be the main product instead of the anti one inferred in the original synthesis experiment (Y. Murata et al., J. Org. Chem.­ 1999 , 64, 3483). With and without dispersion interactions taken into account, the Gibbs energy difference ΔG(syn?anti) is ?6.36 and +1.15 kcal mol^?1, respectively. This study reminds us that dispersion interactions as well as electrostatic or hyperconjugation effects, etc. can lead to some unusual stereochemical phenomena.     

Unconventional Assembly of Bimetallic Au–Ni Janus Nanoparticles on Chemically Modified Silica Spheres

This paper reports that Janus Au?Ni nanoparticles (JANNPs) can self‐assemble onto silica spheres in a novel way, which is different from that of single‐component isotropic nanoparticles. JANNPs modified with octadecylamine (ODA) assemble onto catechol‐modified silica spheres (SiO₂?OH) to form a very special core–loop complex structure and finally the core–loop assemblies link each other to form large assemblies through capillary force and the hydrophobic interaction of the alkyl chains of ODA. The nanocomposites disassemble in the presence of vanillin and oleic acid because of the breakage of the catechol–metal link. Vanillin‐induced disassembly enables the JANNPs to reassemble into a core–loop structure upon ODA addition. The assembly of SiO₂?OH and isotropic Ni or Fe₃O₄ particles generates traditional core–satellite structures. This unconventional self‐assembly can be attributed to the synergistic effect of Janus specificity and capillary force, which is also confirmed by the assembly of thiol‐terminated silica spheres (SH?SiO₂) with anisotropic JANNPs, isotropic Au, and Ni nanoparticles. These results can guide the development of novel composite materials using Janus nanoparticles as the primary building blocks.     

Synthesis of Soluble,Alkyne‐Substituted Trideca‐ and Hexadeca‐Starphenes

Two literature‐known TIPS‐ethynyl‐dibromoacenes were prepared and employed to synthesize cyclotrimers by using Yamamoto coupling conditions. Two large, well‐soluble starphenes were isolated in good yields. Crystallographic characterization verifies the triangular shape and shows significant differences in crystal packing.     

Metal‐Catalyzed Reductive Coupling Reactions of Organic Halides with Carbonyl‐Type Compounds

Metal‐catalyzed reductive coupling reactions of aryl halides and (pseudo)halides with carbonyl‐type compounds have undergone an impressive development within the last years. These methodologies have shown to be a powerful alternate strategy, practicality aside, to the use of stoichiometric, well‐defined, and, in some cases, air‐sensitive organometallic species. In this Minireview, the recent findings in this field are summarized, with particular emphasis on the mechanistic interpretation of the results and future aspects of this area of expertise.     

Palladium‐Catalyzed Coupling Reaction of Perfluoroarenes with Diarylzinc Compounds

This report describes the first Pd⁰‐catalyzed cross‐coupling of hexafluorobenzene (C₆F₆) with diarylzinc compounds to give a variety of pentafluorophenyl arenes. This reaction could be applied to other perfluoroarenes, such as octafluorotoluene, pentafluoropyridine, and perfluoronaphthalene, to give the corresponding polyfluorinated coupling products. The optimal ligand in this catalytic reaction was PCy₃, and lithium iodide was indispensable as an additive for the coupling reaction. One of the roles of lithium iodide in this catalytic reaction was to promote the oxidative addition of one C?F bond of C₆F₆ to palladium. Stoichiometric reactions revealed that an expected oxidative‐addition product, trans‐[Pd(C₆F₅)I(PCy₃)₂], generated from the reaction of [Pd(PCy₃)₂] with C₆F₆ in the presence of lithium iodide, was not involved in the catalytic cycle. Instead, a transient three‐coordinate, monophosphine‐ligated species, [Pd(C₆F₅)I(PCy₃)], emerged as a potential intermediate in the catalytic cycle. Therefore, we isolated a novel Pd^II complex, [Pd(C₆F₅)I(PCy₃)(py)], in which pyridine (py) acted as a labile ligand to generate the transient species. In fact, in the presence of lithium iodide, this Pd^II complex was found to react smoothly with diphenylzinc to give the desired pentafluorophenyl benzene, whereas the same reaction conducted in the absence of lithium iodide resulted in a decreased yield of pentafluorophenyl benzene, which indicated that the other role of lithium iodide was to enhance the reactivity of the organozinc species during the transmetalation step.