Solvent/base effects in the selective domino synthesis of phenanthridinones that involves high-valent palladium species: experimental and theoretical studies

The domino reaction of o-bromobenzamides 1a-m in the presence of K(2)CO(3) and the [PdCl(2)(PPh(3))(2)] catalyst granted a selective access to phenanthridinones 2 or to the new 1-carboxamide phenanthridinones 3 depending on the solvent, DMF or 1,4-dioxane, respectively. Investigations of the reaction parameters provided the first example of a direct correlation between the base dissociation and the solvent polarity on the selectivity observed. Moreover, mechanistic studies (NMR spectroscopy and ESI-MS monitoring) allowed us to characterize Pd(II) palladacycle 4 and biaryl species as common intermediates for these two domino processes. On that basis, C(sp(2))-C(sp(2)) bond formation is envisaged by generation of a Pd(IV) complex after oxidative addition of 1 into Pd(II) palladacycle 4, a rationale that is supported by DFT calculations. A general catalytic cycle is proposed to account for these observations.     

An easily accessible Re-based catalyst for the selective conversion of methanol: evidence for an unprecedented active site structure through combined operando techniques

Heterogeneous Re/SiO(2) catalysts prepared using a one pot sol-gel synthesis were found to display high activity in the direct, selective methanol conversion to methylal, which is correlated to an unprecedented rhenium oxide structure.     

Unraveling gold(I)-specific action towards peptidic disulfide cleavage: a DFT investigation

Ground-state disulfide dissociation is a target of prime importance in structural biochemistry. A main difficulty consists in avoiding competition with carbon–sulfur and backbone scission pathways. In tandem mass spectrometry, such selectivity is afforded using transition elements or coinage-metal ions as catalyst. Yet, the underlying gas-phase mechanistic details remain poorly understood. Gold(I)-assisted disulfide cleavage is investigated by means of DFT calculations, to elucidate the highly selective and specific catalytic action of this transition-metal cation, a most promising one in tandem mass spectrometry. The preferential cleavage of sulfur–sulfur versus carbon–sulfur linkages on dimethyldisulfide, taken as a prototypical aliphatic compound, is rationalized on the basis of molecular orbital arguments. Secondly, it is revealed that the disulfide dissociation profile is dramatically impacted by a peptidic environment. Calculations on L,L-cystine derivatives show two main factors: the topological frustration for an embedded -CH(2)-S-S-CH(2)- motif induces a 5 kcal mol(-1) penalty, whereas electrophilic assistance via complexation of nitrogen and oxygen atoms lowers activation barriers by a factor of 3. S-S weakening is both thermodynamically and kinetically driven by the versatile coordination mode of gold(I). The influence of amine-terminus group protonation is finally sketched: it gives rise to an intermediate reactivity. This study sheds lights on the key action of the peptidic environment in tuning the dissociation profile in the presence of this transition-metal monocation.     

Über Gesamtschwankungen von Funktionen mehrerer Veränderlichen

Ohne Zusammenfassung     

Controlled chemical functionalization of multiwalled carbon nanotubes by kiloelectronvolt argon ion treatment and air exposure

The chemical and morphological modifications of multiwalled carbon nanotubes (MWCNTs), by 2 keV Ar(+) treatment, have been followed by field emission scanning (FESEM) and high-resolution transmission (HRTEM) electron microscopies and by X-ray photoelectron (XPS) and Raman spectroscopies. Morphological changes were followed, both in situ and on subsequent air exposure, and the data indicate that free radical defects, initially produced under low Ar(+) treatment doses ( approximately 10(13) ions/cm(2)), act as the nuclei for the formation of localized asperities that form along the walls of the CNTs. Continued treatment results in their stublike elongation that continues with further treatment, forming extensions under heavy treatment doses. The chemical changes that occur, on reaction with air, reveal that the defects initially created are secondary C atoms, formed when a single bond breaks; further treatment breaks an additional bond to form primary C atoms; free radical fragments, lost when the third bond breaks, condense on the free radical defects to form the asperities. The extent of primary and secondary C atoms, and thus their functionalization on air exposure, may be controlled by the extent of treatment, offering a method for the controlled surface functionalization of CNTs by low-energy Ar(+) treatment.     

All-optical polarization-mode dispersion monitor for return-to-zero optical signals at 40 Gbits/s and beyond

The operation of a polarization-mode dispersion monitor insensitive to chromatic dispersion is demonstrated at 40 Gbits/s. The high-speed processing device is based on the Kerr effect and provides an optical power output as a reading of differential group delay. The monitor is compatible with return-to-zero modulation formats at data rates in excess of 40 Gbits/s and does not require the use of high-data-rate electronics.     

Chemical Consequences of the Mechanical Bond: A Tandem Active Template‐Rearrangement Reaction

We report the unexpected discovery of a tandem active template CuAAC‐rearrangement process, in which N₂ is extruded on the way to the 1,2,3‐triazole product to give instead acrylamide rotaxanes. Mechanistic investigations suggest this process is dictated by the mechanical bond, which stabilizes the Cu^I‐triazolide intermediate of the CuAAC reaction and diverts it down the rearrangement pathway; when no mechanical bond is formed, the CuAAC product is isolated.     

Synthetic and computational assessment of a chiral metal–organic framework catalyst for predictive asymmetric transformation

Understanding and controlling molecular recognition mechanisms at a chiral solid interface is a continuously addressed challenge in heterogeneous catalysis. Here, the molecular recognition of a chiral peptide-functionalized metal–organic framework (MOF) catalyst towards a pro-chiral substrate is evaluated experimentally and in silico. The MIL-101 metal–organic framework is used as a macroligand for hosting a Noyori-type chiral ruthenium molecular catalyst, namely (benzene)Ru@MIL-101-NH-Gly-Pro. Its catalytic perfomance toward the asymmetric transfer hydrogenation (ATH) of acetophenone into R- and S-phenylethanol are assessed. The excellent match between the experimentally obtained enantiomeric excesses and the computational outcomes provides a robust atomic-level rationale for the observed product selectivities. The unprecedented role of the MOF in confining the molecular Ru-catalyst and in determining the access of the prochiral substrate to the active site is revealed in terms of highly face-specific host–guest interactions. The predicted surface-specific face differentiation of the prochiral substrate is experimentally corroborated since a three-fold increase in enantiomeric excess is obtained with the heterogeneous MOF-based catalyst when compared to its homogeneous molecular counterpart.

Understanding and controlling molecular recognition mechanisms at a chiral solid interface has been addressed in metal–organic framework catalysts for the asymmetric transfer hydrogenation reaction.     

Factors governing electron capture by small disulfide loops in two-cysteine peptides

Integrated molecular orbital-molecular orbital (IMOMO) calculations on 17 short disulfide-bridged peptides (up to 16 residues, with at most five intraloop residues) were performed to elucidate some factors controlling their electron capture. These illustrative systems display contrasted behaviors, shedding light on several criteria of differentiation: size, shape, and rigidity of the disulfide-linking loop, intramolecular hydrogen bonds, etc. The geometrical malleability of disulfide radical anions, whose existence and role as intermediate have been evidenced, is discussed. The disulfide elongation (by ca. 0.7 A) upon electron capture induces "soft" structural damages for these turn structures, with a weakening or cleavage of vicinal hydrogen bond(s). On the basis of a series of six Cys-Alan-Cys peptides, it is proposed that electron affinity reflects the topological frustration of these short and highly constrained structures. Results for a series of amino acid mutations are analyzed for the Cys-Xxx-Yyy-Cys motif, common to redox enzymes of the thioredoxin superfamily.