RISM-SCF study of the free-energy profile of the Menshutkin-type reaction NH3+CH3Cl→NH3CH3++Cl− in aqueous solution

The free-energy profile for the Menshutkin-type reaction NH₃ + CH₃Cl → NH₃CH₃ ⁺ + Cl⁻ in aqueous solution is studied using the RISM-SCF method. The effect of electron correlation on the free-energy profile is estimated by the RISM-MP2 method at the HF optimized geometries along the reaction coordinate. Solvation was found to have a large influence on the vibrational frequencies at the reactant, transition state and product; these vibrational frequencies are utilized to calculate the zero-point energy correction of the free-energy profile. The computed barrier height and reaction exothermicity are in reasonable agreement with those of experiment and previous calculations. The change of solvation structure along the reaction path is represented by radial distribution functions between solute-solvent atomic sites. The mechanisms of the reaction are discussed from the view points of solute electronic and solvation structures. Received: 26 June 1998/Accepted: 28 August 1998 / Published online: 2 November 1998     

Distance dependence of noncontact-AFM image contrast on Si(111) × –Ag structure

Atomic resolution imaging of the Si(111) × R30°–Ag surface was investigated using a noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum. NC-AFM images showed three types of contrasts depending on the distance between an AFM tip and a sample surface. When the tip–sample distance was about 1–3 Å, the images showed the honeycomb arrangement with weak contrast. When the tip–sample distance was about 0–0.5 Å, the images showed the periodic structure composed of three bright spots with relatively strong contrast. On the other hand, the contrasts of images measured at the distance of 0.5–1 Å seemed to be composed of the above-mentioned two types of contrasts. By comparing the site of bright spots in the AFM images with honeycomb-chained trimer (HCT) model, we suggested the following models: when the tip is far from the sample surface, tip–sample interaction force contributing to imaging is dominated by physical bonding interaction such as Coulomb force and/or van der Waals (vdW) force between the tip apex Si atoms and Ag trimer on the sample surface. On the other hand, just before the contact, tip–sample interaction force contributing to imaging is dominated by chemical bonding such as the force due to hybridization between the dangling bond out of the tip apex Si atom and the orbit of Si–Ag covalent bond on the sample surface.     

Anionic polymerization of o‐substituted styrene derivatives: Control of reactivity and stereochemistry by aminomethyl group

Ortho‐substituted styrenes, such as 2‐(N,N‐dimethylaminomethyl)styrene ( 1 ), 2‐(1‐pyrrolidinylmethyl)styrene ( 2 ), and 2‐[(S)‐2‐(1‐pyrrolidinylmethyl)‐1‐pyrrolidinylmethyl]styrene ( 3 ), were synthesized, and the effects of the ortho‐substituents on the polymerizability and stereoregularity of the obtained polymers using the anionic method were examined. The bulkiness and coordination of the ortho‐substituted amino groups to the counter cation significantly affected the polymerizability and stereochemistry of the obtained polymers. The anionic and radical polymerizations of 2 with a less hindered ortho‐substituent afforded polymers in good yields, whereas those of 1 and 3 resulted in lower yields. The anionic polymerization of 3 bearing an optically active diamine derivative at the ortho‐position with n‐butyllithium in toluene at 0 °C gave a polymer with a high stereoregularity and stable regular conformation based on the stereoregular backbone structure. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4088–4094, 2000     

Enantioselective Ring Opening of Epoxides with 4-Methoxyphenol Catalyzed by Gallium Heterobimetallic Complexes: An Efficient Method for the Synthesis of Optically Active 1,2-Diol Monoethers

Useful chiral building blocks such as 1,2-diols can be obtained by the enantioselective ring opening of achiral epoxides with oxygen nucleophiles. The ring opening is carried out effectively (up to 94 % ee) with 4-methoxyphenol and catalytic amounts of gallium complexes. The novel complex GaSO 1 displays a particularly high catalytic activity.     

Stereocontrol using Lewis acids in radical polymerization

The radical polymerization of various (meth)acrylamides in the presence of Lewis acids such as Yb(OTf)₃ and Y(OTf)₃ was carried out. The polymerization with Lewis acids led to highly isotactic polymers, while the polymers synthesized without Lewis acids were atactic or syndiotactic. The dependence of the polymer properties on the tacticity was also demonstrated.     

Magnetism in Nonplanar Zigzag Edge Termini of Graphene Nanoribbons

Graphene nanoribbons (GNRs) and nanographenes synthesized by on-surface reactions using tailor-made molecular precursors offer an ideal playground for a study of magnetism towards nano-spintronics. Although the zigzag edge of GNRs has been known to host magnetism, the underlying metal substrates usually veil the edge-induced Kondo effect. Here, we report the on-surface synthesis of unprecedented, π-extended 7-armchair GNRs using 7-bromo-12-(10-bromoanthracen-9-yl)tetraphene as the precursor. Characterization by scanning tunneling microscopy/spectroscopy revealed unique rearrangement reactions leading to pentagon- or pentagon/heptagon-incorporated, nonplanar zigzag termini, which demonstrated Kondo resonances even on bare Au(111). Density functional theory calculations indicate that the nonplanar structure significantly reduces the interaction between the zigzag terminus and the Au(111) surface, leading to a recovery of the spin localization of the zigzag edge. Such a distortion of planar GNR structures offers a degree of freedom to control the magnetism on metal substrates.     

Oxidation of 5,15-Dioxaporphyrin: Its Generality and Novelty as an Oxaporphyrin Analogue

5,15-Dioxaporphyrin ( DOP ) is a novel meso-oxaporphyrin analogue and exhibits unique 20π-antiaromaticity, unlike its mother congener of 18π-aromatic 5-oxaporphyrin, commonly known as its cationic iron complex called verdohem, which is a key intermediate of heme catabolism. To reveal its reactivities and properties as an oxaporphyrin analogue, the oxidation of tetra-β-arylated DOP ( DOP-Ar₄ ) was explored in this study. Stepwise oxidation from the 20π-electron neutral state was achieved, and the corresponding 19π-electron radical cation and 18π-electron dication were characterized. Further oxidation of the 18π-aromatic dication resulted in the formation of a ring-opened dipyrrindione product by hydrolysis. Considering a similar reaction of verdoheme to ring-opened biliverdin in the heme degradation in nature, the current result consolidates the ring-opening reactivity of oxaporphyrinium cation species.     

Enantioselective Synthesis of 1,2-Benzothiazine 1-Imines via RuII/Chiral Carboxylic Acid-Catalyzed C−H Alkylation/Cyclization

Sulfondiimines are diaza-analogues of sulfones with a chiral sulfur center. Compared to sulfones and sulfoximines, their synthesis and transformations have so far been studied to a lesser extent. Here, we report the enantioselective synthesis of 1,2-benzothiazine 1-imines, i.e., cyclic sulfondiimine derivatives from sulfondiimines and sulfoxonium ylides via C−H alkylation/cyclization reactions. The combination of [Ru(p-cymene)Cl₂]₂ and a newly developed chiral spiro carboxylic acid is key to achieving high enantioselectivity.     

Photoinduced Nickel-Catalyzed Carbon–Heteroatom Coupling**

Herein, we report visible light-promoted single nickel catalysis for diverse carbon–heteroatom couplings under mild conditions. This mild, general, and robust method to couple diverse nitrogen, oxygen, and sulfur nucleophiles with aryl(heteroaryl)/alkenyl iodides/bromides exhibits a wide functional group tolerance and is applicable to late-stage modification of pharmaceuticals and natural products. On the base of preliminary mechanistic studies, a Ni^I/Ni^III cycle via the generation of active Ni^I complexes that appear from homolysis of Ni^II−I rather than Ni^II−aryl bond was tentatively proposed.