The aza-Morita-Baylis-Hillman reaction of electronically and sterically deactivated substrates.

The aza-Morita–Baylis–Hillman (azaMBH) reaction has been studied for electronically and sterically deactivated Michael acceptors. It is found that electronically deactivated systems can be converted with electron-rich phosphanes and pyridines as catalysts equally well. For sterically deactivated systems clearly better catalytic turnover can be achieved with pyridine catalysts. This is in accordance with the calculated affinities of the catalysts towards different Michael-acceptors

Gas-liquid mass transfer with parallel reversible reactions—I. Absorption of CO2 into solutions of sterically hindered amines

A numerical method developed by Verteeg (1989, Chem. Engng Sci.44, 2295–2310; 1990, Chem Engng Sci.45, in press) is applied to some specific problems in gas—liquid mass transfer. The experimental results of Chakraborty (1986, Chem. Engng Sci.41, 997–1003) and Zioudas and Dadach (1986, Chem. Engng Sci.41, 405–408) on the absorption of Co2 into aqueous solutions of sterically hindered amines are evaluated with the numerical model. It is shown that studying the absorption of CO2 into aqueous solutions of sterically hindered amines requires a rigorous numerical solution of the differential equations describing the mass transfer instead of analytical and numerical approximations based on a reduction of the number of reactions by neglecting or lumping reactions. It is demonstrated that the absorption rates of CO2 into sterically hindered amine solutions can be explained in terms of the established reactions rates of CO2 in amine solutions alone, and no new reaction paths are necessary to explain the observed behaviour

Isomeric triazines exhibit unique profiles of bioorthogonal reactivity.

Expanding the scope of bioorthogonal reactivity requires access to new and mutually compatible reagents. We report here that 1,2,4-triazines can be tuned to exhibit unique reaction profiles with biocompatible strained alkenes and alkynes. Computational analyses were used to identify candidate orthogonal reactions, and the predictions were experimentally verified. Notably, 5-substituted triazines, unlike their 6-substituted counterparts, undergo rapid [4 + 2] cycloadditions with a sterically encumbered strained alkyne. This unique, sterically controlled reactivity was exploited for dual bioorthogonal labeling. Mutually orthogonal triazines and cycloaddition chemistries will enable new multi-component imaging applications

Syntheses of Sterically Shielded Stable Carbenes of the 1,2,4-Triazole Series and their Corresponding Palladium Complexes: Efficient Catalysts for Chloroarene Hydrodechlorination

The new sterically shielded 1,3,4-trisubstituted 1,2,4-triazol-5-ylidenes 8b-d were synthesized by a three step method starting from 2-phenyl-1,3,4-oxadiazole. The syntheses of palladium complexes 9a-d and 10a-d (including the sterically shielded derivatives 9c,d and 10a-d) were carried out via the reactions of the stable carbenes 8a-d with palladium halogenide salts in THF or toluene solution. Complexes 9c,d and 10a-d were found to be excellent catalysts for the reductive dechlorination (hydrodechlorination) of p-dichlorobenzene. The structures of 8c, 9a,b, and 10a were determined by single-crystal X-ray diffraction.Ukrainian Academy of Sciences 118Robert A. Welch Foundation F-0003Chemistr

N-Triflylphosphorimidoyl Trichloride: A Versatile Reagent for the Synthesis of Strong Chiral Brønsted Acids

A series of strong Brønsted acids has been synthesized in high yields using N-triflylphosphorimidoyl trichloride as reagent. The syntheses proceed efficiently with electron-rich, electron-deficient, and sterically hindered substrates

The Role of Torsional Dynamics on Hole and Exciton Stabilization in π‐Stacked Assemblies: Design of Rigid Torsionomers of a Cofacial Bifluorene

Exciton and charge delocalization across π‐stacked assemblies is of importance in biological systems and functional polymeric materials. To examine the requirements for exciton and hole stabilization, cofacial bifluorene (F2) torsionomers were designed, synthesized, and characterized: unhindered (model) MeF2, sterically hindered tBuF2, and cyclophane‐like CF2, where fluorenes are locked in a perfect sandwich orientation via two methylene linkers. This set of bichromophores with varied torsional rigidity and orbital overlap shows that exciton stabilization requires a perfect sandwich‐like arrangement, as seen by strong excimeric‐like emission only in CF2 and MeF2. In contrast, hole delocalization is less geometrically restrictive and occurs even in sterically hindered tBuF2, as judged by 160 mV hole stabilization and a near‐IR band in the spectrum of its cation radical. These findings underscore the diverse requirements for charge and energy delocalization across π‐stacked assemblies

Sterically stabilized lock and key colloids: A self-consistent field theory study

A self-consistent field theory study of lock and key type interactions between sterically stabilized colloids in polymer solution is performed. Both the key particle and the lock cavity are assumed to have cylindrical shape, and their surfaces are uniformly grafted with polymer chains. The lock-key potential of mean force is computed for various model parameters, such as length of free and grafted chains, lock and key size matching, free chain volume fraction, grafting density, and various enthalpic interactions present in the system. The lock-key interaction is found to be highly tunable, which is important in the rapidly developing field of particle self-assembly

Experimental rheology of model colloidal dispersions

Experimental rheological aspects of colloidal dispersions are now being investigated with improved skills and with better defined model systems. New methods allow more detailed investigations. The relationship between the rheological macroproperties, microproperties and processes is the focus of the most recent investigations. Better insight has been gained concerning this relationship

Conformations of N-Heterocyclic Carbene Ligands in Ruthenium Complexes Relevant to Olefin Metathesis

The structure of ruthenium-based olefin metathesis catalyst 3 and model π-complex 5 in solution and in the solid state are reported. The N-tolyl ligands, due to their lower symmetry than the traditional N-mesityl substituents, complicate this analysis, but ultimately provide explanation for the enhanced reactivity of 3 relative to standard catalyst 2. The tilt of the N-tolyl ring provides additional space near the ruthenium center, which is consistent with the enhanced reactivity of 3 toward sterically demanding substrates. Due to this tilt, the more sterically accessible face bears the two methyl substituents of the N-aryl rings. These experimental studies are supported by computational studies of these complexes by DFT. The experimental data provides a means to validate the accuracy of the B3LYP and M06 functionals. B3LYP provides geometries that match X-ray crystal structural data more closely, though it leads to slightly less (0.5 kcal mol^(−1)) accuracy than M06 most likely because it underestimates attractive noncovalent interactions