P?C‐Activated Bimetallic Rhodium Xantphos Complexes: Formation and Catalytic Dehydrocoupling of Amine–Boranes

{Rh(xantphos)}‐based phosphido dimers form by P C activation of xantphos (4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene) in the presence of amine–boranes. These dimers are active dehydrocoupling catalysts, forming polymeric [H₂BNMeH]_n from H₃B⋅NMeH₂ and dimeric [H₂BNMe₂]₂ from H₃B⋅NMe₂H at low catalyst loadings (0.1 mol %). Mechanistic investigations support a dimeric active species, suggesting that bimetallic catalysis may be possible in amine–borane dehydropolymerization.     

Which nido:nido‐Macropolyhedral Boranes Are Most Stable?

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.     

How good is a Broyden–Fletcher–Goldfarb–Shanno-like update Hessian formula to locate transition structures? Specific reformulation of Broyden–Fletcher–Goldfarb–Shanno for optimizing saddle points

Based on a study of the Broyden–Fletcher–Goldfarb–Shanno (BFGS) update Hessian formula to locate minima on a hypersurface potential energy, we present an updated Hessian formula to locate and optimize saddle points of any order that in some sense preserves the initial structure of the BFGS update formula. The performance and efficiency of this new formula is compared with the previous updated Hessian formulae such as the Powell and MSP ones. We conclude that the proposed update is quite competitive but no more efficient than the normal updates normally used in any optimization of saddle points. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 349–362, 1998     

Scavenging Effects of Aliphatic Alcohols and Acetone on H<Superscript>•</Superscript> Radicals in Anodic Contact Glow Discharge Electrolysis: Determination of the Primary Yield of H<Superscript>•</Superscript> Radicals

A study of scavenging effects of iso-propanol, n-propanol, n-butanol and acetone on H^• radicals, precursors for nonfaradaic products of anodic contact glow discharge electrolysis (CGDE) in aqueous media, has been carried out. A kinetic analysis of the competing reactions of H^• with different species in the system leads to a primary yield of 9.8 mol (mol electron)⁻¹ each of H^• and OH^• radicals in the liquid-phase reaction zone of anodic CGDE. The method is considerably more convenient to apply than those reported earlier based on the use of OH^• scavengers.     

Intervalent Bis(μ‐aziridinato)MII?MI Complexes (M=Rh,Ir): Delocalized Metallo‐Radicals or Delocalized Aminyl Radicals?

Reactions of the methoxo complexes [{M(mu-OMe)(cod)}(2)] (cod=1,5-cyclooctadiene, M=Rh, Ir) with 2,2-dimethylaziridine (Haz) give the mixed-bridged complexes [{M(2)(mu-az)(mu-OMe)(cod)(2)}] [(M=Rh, 1; M=Ir, 2). These compounds are isolated intermediates in the stereospecific synthesis of the amido-bridged complexes [{M(mu-az)(cod)}(2)] (M=Rh, 3; M=Ir, 4). The electrochemical behavior of 3 and 4 in CH(2)Cl(2) and CH(3)CN is greatly influenced by the solvent. On a preparative scale, the chemical oxidation of 3 and 4 with [FeCp(2)](+) gives the paramagnetic cationic species [{M(mu-az)(cod)}(2)](+) (M=Rh, [3](+); M=Ir, [4](+)). The Rh complex [3](+) is stable in dichloromethane, whereas the Ir complex [4](+) transforms slowly, but quantitatively, into a 1:1 mixture of the allyl compound [(eta(3),eta(2)-C(8)H(11))Ir(mu-az)(2)Ir(cod)] ([5](+)) and the hydride compound [(cod)(H)Ir(mu-az)(2)Ir(cod)] ([6](+)). Addition of small amounts of acetonitrile to dichloromethane solutions of [3](+) and [4](+) triggers a fast disproportionation reaction in both cases to produce equimolecular amounts of the starting materials 3 and 4 and metal--metal bonded M(II)--M(II) species. These new compounds are isolated by oxidation of 3 and 4 with [FeCp(2)](+) in acetonitrile as the mixed-ligand complexes [(MeCN)(3)M(mu-az)(2)M(NCMe)(cod)](PF(6))(2) (M=Rh, [8](2+); M=Ir, [9](2+)). The electronic structures of [3](+) and [4](+) have been elucidated through EPR measurements and DFT calculations showing that their unpaired electron is primarily delocalized over the two metal centers, with minor spin densities at the two bridging amido nitrogen groups. The HOMO of 3 and 4 and the SOMO of [3](+) and [4](+) are essentially M--M d-d sigma*-antibonding orbitals, explaining the formation of a net bonding interaction between the metals upon oxidation of 3 and 4. Mechanisms for the observed allylic H-atom abstraction reactions from the paramagnetic (radical) complexes are proposed.     

Catalytic Diastereoselective Tandem Conjugate Addition–Elimination Reaction of Morita–Baylis–Hillman C Adducts by C?C Bond Cleavage

Through the cleavage of the C? C bond, the first catalytic tandem conjugate addition–elimination reaction of Morita–Baylis–Hillman C adducts has been presented. Various S_N2′‐like C‐, S‐, and P‐allylic compounds could be obtained with exclusive E configuration in good to excellent yields. The Michael product could also be easily prepared by tuning the β‐C‐substituent group of the α‐methylene ester under the same reaction conditions. Calculated relative energies of various transition states by DFT methods strongly support the observed chemoselectivity and diastereoselectivity.     

The Stability of Cα Peptide Radicals: Why Glycyl Radical Enzymes?

The conformational space of dipeptide models derived from glycine, alanine, phenylalanine, proline, tyrosine, and cysteine has been searched extensively and compared with the corresponding C(α) dipeptide radicals at the G3(MP2)-RAD level of theory. The results indicate that the (least-substituted) glycine dipeptide radical is the thermochemically most stable of these species. Analysis of the structural parameters indicates that this is due to repulsive interactions between the C(α) substituents and peptide units in the radical. A comparison of the conformational preferences of dipeptide radicals and their closed-shell parents also indicates that radical stability is a strongly conformation-dependent property.     

Resonance‐Assisted Intramolecular Chalcogen–Chalcogen Interactions?

High-level B3LYP/6-311+G(3df,2p) density functional calculations have been carried out for a series of saturated chalcogenoaldehydes: CH(X)-CH(2)-CH(2)YH (X, Y=O, S, Se, Te). Our results indicate that in CH(X)-CH(2)-CH(2)YH (X=Y=O, S, Se) the X-H...X intramolecular hydrogen bond (IHB) competes in strength with the X...XH chalcogen-chalcogen interaction, while the opposite is found for the corresponding tellurium-containing analogues. For those derivatives in which X does not equal Y, X being the more electronegative atom, the situation is more complicated due to the existence of two non-equivalent X-H and Y-H tautomers. The Y-H tautomer is found to be lower in energy than the X-H tautomer, independently of the nature of X and Y. For X=O, S, Se and Y=S, Se the most stable conformer b is the one exhibiting a Y-H...X IHB. Conversely when Y=Te, the chelated conformer d, stabilized through a X...YH chalcogen-chalcogen interaction is the global minimum of the potential energy surface. Systematically the IHB and the chalcogen-chalcogen interactions observed for saturated compounds are much weaker than those found for their unsaturated analogues. This result implies that the nonbonding interactions involving chalcogen atoms, mainly Se and Te, are not always strongly stabilizing. This conclusion is in agreement with the fact that intermolecular interactions between Se and Te containing systems with bases bearing dative groups are very weak. We have also shown that these interactions are enhanced for unsaturated compounds, through an increase of the charge delocalization within the system, in a mechanism rather similar to the so call Resonance Assisted Hydrogen Bonds (RAHB). The chalcogen-chalcogen interactions will be also large, due to the enhancement of the X-->Y dative bond, if the molecular environment forces the interacting atoms X and Y to be close each other.     

“What is KRI?” – R&D based on excellent tradition and innovation –

As far as KRI's clients were concerned, KRI's appeal was not only in having researchers who had a high level of expert knowledge, but it was also in the efficiency with which high risk jobs were conducted, and the speed of response time, especially when corporations made comparisons with their own in‐house labs. KRI, while being a facility for contract research, is also capable of becoming a profitable corporation, mostly thanks to the adoption of this system. KRI sincerely hopes to continue serving as a group made up of quality professionals who embrace a dream, and to contribute to society through its achievements. At the same time, the company strives to be sensitive to changing needs, and, of course, to conduct R&D and consulting activities which will ensure success for its clients.     

Diketopyrrolopyrrole?Thiophene‐Based Acceptor–Donor–Acceptor Conjugated Materials for High‐Performance Field‐Effect Transistors

We report the synthesis, morphology, and field‐effect‐transistor (FET) characteristics of new acceptor–donor–acceptor conjugated materials that consist of diketopyrrolopyrrole (DPP) acceptor groups and one of four different thiophene moieties, that is, dithiophene (2T), thieno[3,2‐b]‐thiophene (TT), dithieno[3,2‐b:2′,3′‐d]‐thiophene (DTT), and 5,5′′′‐di‐(2‐ethylhexyl)‐[2,3′;5′,2′′;4′′,2′′′]quaterthiophene (4T). The optical band gaps of the as‐prepared materials are smaller than 1.7 eV, which is attributed to the strong intramolecular charge transfer and the backbone coplanarity of the thiophene moieties. The order of both crystallinity and FET mobility (×10^?2–×10^?4 cm² V^?1 s^?1) is TT2DPP > 4T2DPP > 2T2DPP >DTT2DP, which differ in the structure of the π‐conjugated cores and core symmetry. Well‐ordered intermolecular chain packing was confirmed by the GIXD and AFM results. In particular, the FET hole mobility of TT2DPP was further improved to 0.1 cm² V^?1 s^?1, which was attributed to the well‐interconnected structure through solution‐shearing. These experimental results suggest the potential applications of the new DPP? thiophene? DPP conjugated materials for organic electronic devices.     

Reaction of <Emphasis Type="Italic">o</Emphasis>-Quinones in Cyclohexane Solutions with Alkyl Radicals Generated by Solution γ-Irradiation

The structures of products formed in reactions of tert-butylated o-quinones with alkyl radicals were determined by ¹H and ¹³C NMR and two-dimensional ¹H-¹H NOESY spectroscopy, and also by gas chromatography-mass spectrometry. The major products formed upon γ-irradiation of deaerated solutions of 4-tert-butyl-1,2-benzoquinone and 3,5-di-tert-butyl-1,2-benzoquinone in cyclohexane are monoalkyl ethers and products of addition to the C=C bond. In the case of 4-tert-butyl-1,2-benzoquinone, these are products of mixed O-C and C-C alkylation; the adduct formed by addition of the cyclohexyl radical to the C=C bond in 3,5-di-tert-butyl-1,2-benzoquinone gives an unsymmetrical dimer whose structure was proved by single-crystal X-ray diffraction.