Zum Mechanismus der nucleophilen Addition an 2,3-Dihydrodipyrrin-1(10H)-one

¹H NMR-spectroscopic investigations of the acid catalyzed addition of methanol to dihydrodipyrrinones (Z)-2, (E)-2, and4 show the C-protonation of their enamide parts to be the first and rate determining step forming the key intermediate, the N-acyl-immonium ion N⁺. Its ability to add nucleophiles diastereoselectively can be used to prepare the adductsl-3 andl-5. Exclusive formation of thelike-isomer can be explained by a stereoelectronically favoured approach of the nucleophile and by the thermodynamically favoured arrangement of the bulky ring substituents. Both explanations are based on low temperature X-ray crystal structure determinations: in the first place, the orientation of the added nucleophile could be found to be nearly parallel to the -plane of the lactam unit and quasi-axial with respect to theenvelope-like conformation of the five-ring lactam; in the second place, the relative orientation of the methoxycarbonyl-metyl-group at C-3 and the pyrrolylmethyl-substituent at C-4 could be found to be atrans-quasi-diequatorial one.

     

Studies of reaction heats and structure of complexes formed between strong N-bases and phenols

The heats of reactions between various phenols and two strong N-bases of guanidine-like character in acetonitrile, are determined. The values can be used as a measure of self-assembly abilities of the phenol molecules in the interactions with strong N-bases, playing a very important role in biological systems. In the case of TBD complexes with corresponding nitrophenols, the protonated N-base is hydrogen-bonded to the nitro group excluding the self-assembly process of the phenols. In the case of other phenols, the self-assembly abilities are dependent on pK_a values of phenols. With increasing acidity of phenols their ability to form the hydrogen-bonded chains decreases. The maximum of length of the chains is observed for 4-methylphenol, which has a similar pK_a value to that in the tyrosine residue in biological systems.     

The Elucidation of Peroxyl Radical Reactions in Aqueous Solution with the Help of Radiation-Chemical Methods

Whenever free radicals are formed, independent of whether this occurs thermally, is induced by UV or ionizing irradiation, or takes place in redox reactions, they are converted rapidly into the corresponding peroxyl radicals in the presence of oxygen. Peroxyl radical reactions in aqueous environments are observed not only in aquatic systems (e.g., rivers, lakes and oceans) but also in the living cell and to a considerable degree even in the atmosphere (in water droplets). The peroxyl radical chemistry occurring in this medium is often very different from that observed in the gas phase or in organic solvents. In spite of the great importance of these reactions in medicine (for example in anti-cancer irradiation therapy and ischaemia) there have been comparatively few studies of peroxyl reactions in aqueous media. Radiation-chemical techniques such as pulse radiolysis offer the best means for carrying out such studies, so that it is not surprising that the majority of the information available in this area has been obtained with the help of radiation-chemical methods. The radiation chemistry of water can be con trolled in such a manner that the main products are ^˙OH radicals (90 % yield), which react with substrate molecules to give substrate radicals and in the presence of oxygen to give substrate peroxyl radicals. The experimental conditions can also be varied in such a way that HO/O radicals can be formed in 100 % yield and caused to react with substrates. We therefore have a simple access to these intermediates, which are extremely important in biological systems. A detailed product analysis, supported by kinetic studies carried out with the help of pulse radiolysis, has been used to clarify the chemistry of a series of peroxyl radicals, so that sufficient material is now available to justify a review of the variety of the peroxyl radical reactions studied by means of radiation-chemical methods. A more general survey of the physical properties of the peroxyl radicals and their unimolecular and bimolecular reactions will be followed by a discussion of selected examples of various classes of substance. Because of the great biological importance of radical-induced DNA damage this area will also be treated briefly.     

Methylene-azaphosphirane as a reactive intermediate

Reaction of the transient phosphinidene complexes R-P=W(CO)5 with N-substituted-diphenylketenimines leads unexpectedly to the novel 2-aminophosphindoles, as confirmed by an X-ray crystal structure determined for one of the derivatives. Experimental evidence for a methylene-azaphosphirane intermediate was found by using the iron-complexed phosphinidene iPr2N-P=Fe(CO)4, which affords the 2-aminophosphindole together with the novel methylene-2,3-dihydro-1H-benzo[1,3]azaphosphole. Analysis of the reaction pathways with DFT indicates that the initially formed methylene-azaphosphirane yields both phosphorus heterocycles by way of a [1,5]- or [1,3]-sigmatropic shift, respectively, followed by a H-shift. Strain underlies both rearrangements, which causes these remarkably selective conversions that can be tuned by changing the substituents.     

Markov processes in single molecule fluorescence

This article examines the current status of Markov processes in single molecule fluorescence. For molecular dynamics to be described by a Markov process, the Markov process must include all states involved in the dynamics and the first-passage time (FPT) distributions out of those states must be describable by a simple exponential law. The observation of non-exponential FPT distributions or other evidence of non-Markovian dynamics is common in single molecule studies and offers an opportunity to expand the Markov model to include new dynamics or states that improve understanding of the system.     

A theoretical study of the influence of BF3 on the reaction path of the [4+2] cycloaddition of vinylketene with formaldimine

A comprehensive B3LYP/6-31+G^∗ study of the influence of BF₃ on the [4+2] cycloaddition of vinylketene with formaldimine was conducted. For this purpose, the complete pathway was determined and changes in different magnetic properties (magnetic susceptibility, χ, magnetic susceptibility anisotropy, χ_anis, and the nucleus-independent chemical shifts, NICS) were monitored along the reaction profile with a view to estimate the aromatization associated to the process. We have also applied the ACID (anisotropy of the current-induced density) method with the same intention.     

A study of [Co2(alkyne)(binap)(CO)4] complexes (BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine))

Understanding the interaction of chiral ligands, alkynes, and alkenes with cobaltcarbonyl sources is critical to learning more about the mechanism of the catalytic, asymmetric Pauson-Khand reaction. We have successfully characterized complexes of the type [Co2(alkyne)(binap)(CO)4] (BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine)) and shown that diastereomer interconversion occurs under Pauson-Khand reaction conditions when alkyne=HC[triple bond]CCO2Me. Attempts to isolate [Co2(alkyne)(binap)(CO)x] complexes with coordinated alkenes led to the formation of cobaltacyclopentadiene species.     

Theoretical studies on the metathesis processes, (Tp(PH3)MR(eta 2-H[bond]CH3)]-->[Tp(PH3)M(CH3)(eta 2-H[bond]R)] (M=Fe,Ru, and Os; R=H and CH3)

Theoretical calculations on the metathesis process, [Tp(PH3)MR(eta 2-H[bond]CH3)] --> [Tp(PH3)M(CH3)(eta 2-H[bond]R)] (M=Fe, Ru, and Os; R=H and CH3), have been systematically carried out to study their detailed reaction mechanisms. Other than the one-step mechanism via a four-center transition state and the two-step mechanism through an oxidative addition/reductive elimination pathway, a new one-step mechanism, with a transition state formed under oxidative addition, has been found. Based on the intrinsic reaction coordinate calculations, we found that the trajectories of the transferring hydrogen atom in the metathesis processes studied are similar to each other regardless of the nature of reaction mechanisms.     

聚双环戊二烯反应注射成型

本文介绍了国外有关聚双环戊二烯反应注射成型的研究进展，讨论了国内进行聚双环戊二烯反应注射成型开发研究的现实意义，并对这一领域的发展进行了展望     

Molecular vibrations in a gradient extremal path

Summary The calculation of vibrational states using a gradient extremal path is discussed. Gradient extremal paths are defined by local criteria, which lead to stable solutions. This has certain advantages in comparison with a steepest-descent path, which is often difficult to determine accurately. For cases where a reaction path formalism is applicable, a path based on the gradient extremal concept gives results in close agreement with those obtained using traditional steepest-descent methods. We present algorithms for carrying out such calculations and also give some numerical results.