How bulky ligands control the chemoselectivity of Pd-catalyzed N-arylation of ammonia

Steric bulk has been recognized as a central design principle for supporting ligands in the widely utilized Buchwald–Hartwig amination. In a recent example, it was shown that a Pd-catalyst carrying a phosphine ligand can successfully aminate aryl halides using ammonia as the nitrogen source. Interestingly, the chemoselectivity of this reaction was found to depend on the steric demand of the phosphine ligand. Whereas a sterically less demanding phosphine affords diphenylamine as the major product, it was shown that the amination reaction can be stopped after the first amination to give aniline if a sterically more encumbering phosphine ligand is used. Density functional theory calculations were carried out to examine the relationship between the steric demand of the phosphine ligand and the chemoselectivity. It was found that the key feature that leads to the chemoselectivity is the ability of the phosphine ligand to rotate the biaryl moiety of the ligand away from the Pd-center upon amine addition to release some of the steric crowding from the Pd-coordination site.

Steric bulk has been recognized as a central design principle for ligands in the widely utilized Buchwald–Hartwig amination. This mechanistic study reveals how this steric effect manipulates the reaction pathway and determines the chemoselectivity.     

cis,cis-[(bpy)2RuVO]2O4+ catalyzes water oxidation formally via in situ generation of radicaloid RuIV-O*

The mechanism of the catalytic oxidation of water by cis,cis-[(bpy)(2)Ru(OH(2))](2)O(4+) to give molecular dioxygen was investigated using Density Functional Theory (DFT). A series of four oxidation and four deprotonation events generate the catalytically competent species cis,cis-[(bpy)(2)Ru(V)O](2)O(4+), which breaks the H-OH bond homolytically at the rate determining transition state to give a hydroperoxo intermediate. Our calculations predict a rate determining activation barrier of 25.9 kcal/mol in solution phase, which is in reasonable agreement with the previously reported experimental estimate of 18.7-23.3 kcal/mol. A number of plausible coupling schemes of the two metal sites including strong coupling, weak ferromagnetic and weak antiferromagnetic coupling have been considered. In addition, both high-spin and low-spin states at each of the Ru(V)-d(3) centers were explored and we found that the high-spin states play an important mechanistic role. Our calculations suggest that cis,cis-[(bpy)(2)Ru(V)O](2)O(4+) performs formally an intramolecular ligand-to-metal charge transfer when reacting with water to formally give a cis,cis-[(bpy)(2)Ru(IV)O*](2)O(4+) complex. We propose that the key characteristic of the diruthenium catalyst that allows it to accomplish the most difficult first two oxidations of the overall four-electron redox reaction is directly associated with this in situ generation of two radicaloid oxo moieties that promote the water splitting reaction. A proton coupled metal-to-metal charge transfer follows to yield a Ru(V)/Ru(III) peroxo/aqua mixed valence complex, which performs the third redox reaction to give the superoxo/aqua complex. Finally, intersystem crossing to a ferromagnetically coupled Ru(IV)/Ru(III) superoxo/aqua species is predicted, which will then promote the last redox event to release triplet dioxygen as the final product. A number of key features of the computed mechanism are explored in detail to derive a conceptual understanding of the catalytic mechanism.     

Studies of the generation and pericyclic behavior of cyclic pentadienyl carbanions. Alkylation reactions as an efficient route to functionalized cis-bicyclo[3.3.0]octenes

Carbolithiation has been studied with alkyllithium reagents in a series of six- through nine-membered 3-methylene-1,4-cycloalkadienes, efficiently producing the corresponding cyclic pentadienyl carbanions. These pentadienyl anions display unique reactivity, depending on ring size. Cyclooctadienyl anions readily undergo disrotatory electrocyclization to cis-bicyclo[3.3.0]octenyl systems, which are trapped with a variety of electrophiles to stereoselectively provide functionalized cis-bicyclo[3.3.0]octenes. The carbolithiation and electrocyclization processes are examined using low-temperature (1)H NMR experiments. An expedient synthesis of a linear triquinane illustrates this methodology. Electrocyclization of the corresponding cyclononadienyl anion requires unusually high temperatures (120 degrees C), and computational studies provide insights into this change in reactivity. Cycloheptadienyl and cyclohexadienyl anions, generated via carbolithiation, provide functionalized cycloheptadienes and cyclohexadienes upon electrophilic capture. Trapping experiments reveal that the cycloheptadienyl anions are transformed to heptatrienyl anions. A series of experiments have been designed to explore evidence for the feasibility of equilibration of open and closed anionic systems, and these studies report the first isolation of a cis-bicyclo[3.1.0]hexene derived from electrocyclization of a cyclohexadienyl carbanion.     

Coating on a Cold Substrate Largely Enhances Power Conversion Efficiency of the Bulk Heterojunction Solar Cell

Spin‐coating a mixture solution of P3HT and PCBM on a cold substrate largely enhanced the power conversion efficiency (PCE) of the bulk heterojunction (BHJ) solar cells. This concept was based on the abrupt decrease in the solubility of P3HT as solution temperature decreased. The selective precipitation of P3HT on the PEDOT:PSS‐coated cold substrate facilitated a desirable rich composition of P3HT at the interface with the PEDOT:PSS layer. The high crystallinity of P3HT suppressed the movement of PCBM during thermal annealing, preventing aggregation of PCBM. The morphological excellence of the pristine film gave a comparable PCE to that made by the conventional fabrication process. After thermal annealing, the device made via coating on a cold substrate showed above 30% increase in PCE from the BHJ solar cells made by the conventional method.

     

An Hetero‐Epitaxially Grown Zeolite Membrane

The secondary growth methodology to form zeolite membranes has stringent requirements for homogeneous epitaxial intergrowth of the seed layer and limits the number of accessible high‐quality zeolite membranes. Despite previous reports on hetero‐epitaxial growth, high‐performance zeolite membranes have yet to be reported using this approach. Here, the successful hetero‐epitaxial growth of highly siliceous ZSM‐58 (DDR‐type zeolite) films from a SSZ‐13 (CHA‐type zeolite) seed layer is reported. The resulting membranes show excellent CO₂ perm‐selectivities, having maximum CO₂ /N₂ and CO₂ /CH₄ separation factors (SFs) as high as about 17 and 279, respectively, at 30 °C. Furthermore, the hybrid membrane maintains the CO₂ perm‐selectivity in the presence of water vapor (the third main component in both cases), that is, CO₂ /N₂ SF of about 14 and CO₂ /CH₄ SF of about 78, respectively, at 50 °C (a representative temperature of both CO₂‐containing streams).     

Identifying the major proteome components of Helicobacter pylori strain 26695

The whole genome sequences of Helicobacter pylori strain 26695 have been reported. Whole cell proteins of H. pylori strain 26695 cells were obtained and analyzed by two-dimensional electrophoresis, using immobilized pH gradient strips. The most abundant proteins were shown in the region of pI 4.0-9.5 with molecular masses from 10 to 100 kDa. Soluble proteins were precipitated by the use of 0-80% saturated solutions of ammonium sulfate. Soluble proteins precipitated by the 0-40% saturations of ammonium sulfate produced similar spot profiles and their abundant protein spots had acidic pI regions. However, a number of soluble proteins precipitated by more than 60% saturation of ammonium sulfate were placed in the alkaline pI regions, compared to those precipitated by 40% saturation. In addition, we have performed an extensive proteome analysis of the strain utilizing peptide MALDI-TOF-MS. Among the 345 protein spots processed, 175 proteins were identified. The identified spots represented 137 genes. One-hundred and fifteen proteins were newly identified in this study, including DNA polymerase III beta-subunit. These results might provide guidance for the enrichment of H. pylori proteins and contribute to construct a master protein map of H. pylori.     

Effect of acetic acid on the performance of solution-processed gallium doped indium oxide thin film transistors

We investigated a role of acetic acid for solution processed gallium doped indium oxide thin film transistors (TFTs). By adding acetic acid in solution instead of commonly used ethanolamine, electrical performance of GIO TFTs is significantly enhanced. We demonstrated that acetic acid plays a role in enhancing crystallinity, lowering decomposition temperature and reducing hydroxyl groups in the film. The GIO TFTs formed from acetic acid added solution have mobility of 12.68 cm² V^?1 s^?1, threshold voltage of ?7.4 V, on/off current ratio of 1.07 × 10⁸ and subthreshold slope of 0.78 V/decade.     

Random vicious walks and random matrices

A lock step walk is a one‐dimensional integer lattice walk in discrete time. Suppose that initially there are infinitely many walkers on the nonnegative even integer sites. At each moment of time, every walker moves either to its left or to its right with equal probability. The only constraint is that no two walkers can occupy the same site at the same time. Hence we describe the walk as vicious. It is proved that as time tends to infinity, a certain limiting conditional distribution of the displacement of the leftmost walker is identical to the limiting distribution of the (scaled) largest eigenvalue of a random GOE matrix (GOE Tracy‐Widom distribution). The proof is based on the bijection between path configurations and semistandard Young tableaux established recently by Guttmann, Owczarek, and Viennot. The distribution of semistandard Young tableaux is analyzed using the Hankel determinant expression for the probability obtained from the work of Rains and the author. The asymptotics of the Hankel determinant are then obtained by applying the Deift‐Zhou steepest‐descent method to the Riemann‐Hilbert problem for the related orthogonal polynomials. © 2000 John Wiley & Sons, Inc.     

Reactions of p-nitrobenzyl halides with dialkyl phosphite anions in dimethyl sulfoxide

The reactions of p-O₂NC₆H₄CH₂Cl with (RO)₂PO⁻ in Me₂SO with R = Me, Et, Pr, Bu, CF₃CH₂, i-Pr or Ph involve the formation of p-O₂NC₆H₄CH₂P(O)(OR)₂ by S_N2 substitution followed by a further S_RN1 p-nitrobenzylation of p-O₂NC₆H₄CH[P(O)(OR)₂]⁻ and p-O₂NC₆H₄C(CH₂C₆H₄NO₂-p)[P(O)(OR)₂]⁻. With p-O₂NC₆H₄CH₂Br, the reactions proceed mainly to form p-O₂NC₆H₄CH, which undergoes reaction with p-O₂NC₆H₄CH₂Br to form p-O₂NC₆H₄CH₂CH₂C₆H₄NO₂-p. Halophilic reaction of (RO)₂PO⁻ with p-O₂NC₆H₄CH(CH₃)X (X = Cl, Br) leading to the bibenzyl is the preferred reaction course. Reactions of (RO)₂PO⁻ or p-O₂NC₆H₄CH[P(O)(OR)₂]⁻ with p-O₂NC₆H₄CH₂X in Me₂SO do not form significantamounts of p-O₂NC₆H₄CHX⁻ that would yieldp-O₂NC₆H₄CH=CHC₆H₄NO₂-p. However, p-Cl-C₆H₄CH[P(O)(OEt)₂]⁻ readily abstracts the benzylic proton from p-O₂NC₆H₄CH₂X to form the stilbene, although p-O₂NC₆H₄CH₂Br reacts with p-O₂NC₆H₄-CH[P(O)(OR)₂]⁻ to form p-O₂NC₆H₄CH(CH₂C₆-H₄NO₂-p)P(O)(OR)₂ in a reaction mixture not inhibited by (t-Bu)₂NO•. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:201–208, 1998