Enantioselective direct intermolecular aldol reactions with enantiotopic group selectivity and dynamic kinetic resolution

[reaction: see text] Proline-catalyzed aldol reactions of tetrahydro-4H-thipyranone with racemic 1,4-dioxa-8-thia-spiro[4.5]decane-6-carboxaldehyde and with meso/dl 1,4-dioxa-8-thiaspiro[4.5]decane-6,10-dicarboxaldehyde proceed with dynamic kinetic resolution and give single adducts in good yields with excellent ee's. The high enantiotopic group selectivity results from the high intrinsic diastereoface selectivity of the aldehydes. These reactions significantly extend the scope of the enantioselective direct aldol reaction and constitute simple and efficient syntheses of useful tetrapropionate synthons.     

Uncatalysed intermolecular aza-Michael reactions

The catalyst-free reactions of activated alkenes with primary and secondary amines were investigated leading to various mono- and di-hydroamination products, the latter being rare and original. These reactions were shown to depend first on the strength of the nucleophile. Temperature and steric hindrance of the reagents were the other key factors controlling the selectivity of these aza-Michael reactions. In spite of their poor nucleophilicities, some N-heterocyclic amines could react with different activated alkenes affording valuable intermediates. Such results tended to demonstrate the hydrogen-bonding interactions between activated alkenes and poly-nitrogen aromatic cycles may control these concerted or fully conjugate aza-Michael additions.     

Single-molecule fluorescence imaging of nanocatalytic processes

This tutorial review covers recent developments in using single-molecule fluorescence microscopy to study nanoscale catalysis. The single-molecule approach enables following catalytic and electrocatalytic reactions on nanocatalysts, including metal nanoparticles and carbon nanotubes, at single-reaction temporal resolution and nanometer spatial precision. Real-time, in situ, multiplexed measurements are readily achievable under ambient solution conditions. These studies provide unprecedented insights into catalytic mechanism, reactivity, selectivity, and dynamics in spite of the inhomogeneity and temporal variations of catalyst structures. Prospects, generality, and limitations of the single-molecule fluorescence approach for studying nanocatalysis are also discussed.     

Stereoselective intermolecular C-H amination reactions

A novel chiral N-mesyloxycarbamate to perform rhodium-catalyzed stereoselective C-H amination reactions is reported. Chiral benzylic and propargylic amines are produced in good yields and selectivities using ethyl acetate as solvent. The corresponding free amines are easily obtained by cleavage of the chiral reagent, which could also be recovered.     

Intra- and intermolecular reactions of nucleophilic carbenes

General properties of nucleophilic carbenes are addressed briefly. The preparation of oxadiazoline precursors of such carbenes, and some of their chemical reactions, are presented. Intramolecular reactions include rearrangement and attack by the carbene center on a tethered functional group. Intermolecular reactions include nucleophilic attack at the carbonyl carbon of isocyanates and at the triple bond of dimethyl acetylenedicarboxylate.     

Generation and intermolecular reactions of 3-indolylacyl radicals

The generation of 3-indolylacyl radicals from the corresponding phenyl selenoesters and the scope of their participation in intermolecular addition reactions to carbon-carbon double bonds under both reductive and nonreductive conditions have been studied.     

Viscosity dependence of intra- and intermolecular Diels-Alder reactions

The kinetics of the bimolecular Diels-Alder reaction between anthracene-9-carbinol and N-ethyl maleimide have been studied in a series of pyridinium-based ionic liquids with the tetrafluoroborate ([BF(4)](-)) and the bis(trifluorosulfonimide) ([NTf(2)](-)) anions and the viscosity dependence of the rate constants have been compared with the results for the intramolecular Diels-Alder reaction of (E)-1-phenyl-4-[2-(3-methyl-2-butenyloxy)benzylidene]-5-pyrazolone. The comparison leads to the surprising observation that the intermolecular reaction is less susceptible to viscosity variations of the ionic liquids as compared to the intramolecular reaction. The observed similarities and differences emphasize the complicated nature of solvent friction on the kinetics of cycloaddition reactions. The results are explained by considering the bimolecular and intramolecular processes as cases of translational and rotational diffusion, respectively. Plausible indicators of microviscosity consistent with the kinetic data for the Diels-Alder reactions are briefly discussed along with the constraints involved in designing a general microviscosity scale.     

Generation and intermolecular reactions of 2-indolylacyl radicals

The generation of 2-indolylacyl radicals from the corresponding phenyl selenoesters, aldehydes, and alpha-keto carboxylic acids and the scope of their participation in intermolecular addition reactions to carbon-carbon double bonds have been studied. Whereas the phenyl selenoester method has provided easy access to a variety of 1,4-dicarbonyl compounds bearing the 2-acylindole moiety, the glyoxylic acid route has been employed for the preparation of 2-indolyl pyridyl ketones.     

N-heterocyclic carbene-catalysed intermolecular Stetter reactions of acetaldehyde

A facile method for the intermolecular Stetter reaction of various Michael acceptors with acetaldehyde as a biomimetic acylanion source was realized using N-heterocyclic carbene catalysis. This catalytic system has also been applied to the enantioselective Stetter reaction and resulted in moderate to good enantioselectivities for the corresponding Stetter products.     

Time-dependent intermolecular interaction during protein reactions

A recently developed method to monitor reaction kinetics of intermolecular interaction is presented in this perspective. This method is based on time-dependent diffusion coefficient measurements using the pulsed laser induced transient grating technique. Using this method, time dependent biomolecular interactions, such as transient association and dissociation reactions in solution, have been successfully detected in real time. The principles and particular applications are described. In particular, unique features of this time-dependent diffusion coefficient method are emphasized by comparison with other techniques.     

Diastereoselective intermolecular Pauson-Khand reactions of chiral cyclopropenes

In this Letter, it is demonstrated that the unusual reactivity of cyclopropenes can increase the scope and utility of intermolecular Pauson-Khand reactions. The well-defined chiral environment of cyclopropenes has a powerful influence on the diastereoselectivity of the reactions and leads to the production of a single cyclopentenone in each of the described cases. The cyclopropane ring strongly influences the stereochemistry of reactions at the enone, and the three-membered ring can subsequently be cleaved under mild conditions. [reaction: see text]     

New regio and stereoselective intermolecular Pauson-Khand reactions of allenamides

Intermolecular Pauson-Khand reactions are still quite limited in scope and yields. There are also few reports using allenes as the olefinic part in this version. We report herein new regio and stereoselective reactions of allenamides with several alkynes. These reactions give functionalized cyclopentenones bearing an exocyclic enamide, which can be useful synthetic intermediates.     

Competitive intermolecular pericyclic reactions of free and complexed cyclopentyne

Intermolecular competition for cyclopentyne by different alkenes supports the hypothesis that organolithium-promoted decomposition of precursors to cyclopentyne affords one or more lithium-ion-complexed species. Competition reactions with mixtures of 2,3-dihydropyran do not clearly differentiate between complexed and unencumbered (free) forms of cyclopentyne, but those involving spirodiene 2 and cyclohexene do. Remarkably, the cycloaddition reactions of free cyclopentyne are not diffusion-controlled despite its high reactivity.     

Single-molecule analysis using DNA origami

During the last two decades, scientists have developed various methods that allow the detection and manipulation of single molecules, which have also been called "in singulo" approaches. Fundamental understanding of biochemical reactions, folding of biomolecules, and the screening of drugs were achieved by using these methods. Single-molecule analysis was also performed in the field of DNA nanotechnology, mainly by using atomic force microscopy. However, until recently, the approaches used commonly in nanotechnology adopted structures with a dimension of 10-20 nm, which is not suitable for many applications. The recent development of scaffolded DNA origami by Rothemund made it possible for the construction of larger defined assemblies. One of the most salient features of the origami method is the precise addressability of the structures formed: Each staple can serve as an attachment point for different kinds of nanoobjects. Thus, the method is suitable for the precise positioning of various functionalities and for the single-molecule analysis of many chemical and biochemical processes. Here we summarize recent progress in the area of single-molecule analysis using DNA origami and discuss the future directions of this research.     

TiCl4-catalyzed intermolecular hydroamination reactions of norbornene

[reaction: see text] An intermolecular hydroamination reaction of norbornene is presented that uses catalytic amounts of user-friendly TiCl(4) and tolerates a variety of functional groups. In addition, a secondary amine is converted using this methodology.     

Diastereoselectivity in unusual intermolecular tandem Michael reactions

Highly stereoselective consecutive or tandem Michael (MIMI) reactions are reported. A variety of initial nucleophiles react with chiral acylated 1,3-oxazolidin-2-ones to generate a reactive enolate. The enolate then reacts stereoselectively with a second equivalent of the acylated 1,3-oxazolidin-2-one to afford ‘dimeric’ adducts. The adducts have three new contiguous stereogenic centers formed with a high level of control. Single crystal X-ray crystallographic analysis confirmed this controlled formation of novel tandem acyclic conjugate addition or MIMI products in several examples.     

Separation and kinetic analysis of the thermo-oxidative reactions of polyacrylonitrile upon heat treatment

A polyacrylonitrile (PAN) homopolymer and a copolymer with methyl acrylate were analyzed by differential scanning calorimetry (DSC) in order to establish a method for the kinetic analysis of the stabilization reaction steps during heat treatment. The application of a program rarely used for the study of the chemical transformation of PAN and its copolymers resulted in five separate steps, which can be assigned with large probability to various cyclization and oxidation reactions. A more exact identification of the reactions needs further measurements and considerations. Overlapping peaks appearing in the DSC traces were deconvoluted and then quantitatively analyzed to obtain kinetic parameters. The kinetics of the processes were described by the Kissinger model. The model could be fitted well to four of the five processes that may indicate that these belong to individual reactions of the first order. The parameters obtained agree well with the few values of similar investigations published in the literature up to now. The results have large practical relevance, since the kinetic parameters obtained can be used in the preparation of intermediates and for the optimization of the stabilization process.