Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.

Combined Experimental and Theoretical Study of the Mechanism and Enantioselectivity of Palladium- Catalyzed Intermolecular Heck Coupling

Abstract: The asymmetric Heck reaction using P,N-ligands has been studied by a combination of theoretical and experimental methods The reaction follows Halpern-style selectivity; that is, the major isomer is produced from the least favored form of the pre-insertion intermediate The initially formed Ph-Pd(P,N) species prefers a geometry with the phenyl trans to N However, the alternative form, with Ph trans to P, is much less stable but much more reactive In the preferred transition state, the phenyl moiety is trans to P, but significant electron density has been transferred to the alkene carbon trans to N The steric interactions in this transition state fully account for the enantioselectivity observed with the ligands studied The calculations also predict relative reactivity and nonlinear mixing effects for the investigated ligands; these predictions are fully validated by experimental testing Finally, the low conversion observed with some catalysts was found to be caused by inactivation due to weak binding of the ligand to Pd(0) Adding monodentate PPh3 alleviated the precipitation problem without deteriorating the enantioselectivity and led to one of the most effective catalytic systems to date

End of Frustration: Catalytic Precision Polymerization with Highly Interacting Lewis Pairs

Abstract: Herein we report on the catalytic polymerization of diverse Michael-type monomers with high precision by using simple but highly active combinations of phosphorus-containing Lewis bases and organoaluminum compounds. The interacting Lewis pair catalysts enable the control of molecular weight and microstructure of the produced polymers. The reactions show a linear Mn vs consumption plot thus proving a living type polymerization. The initiation has been investigated by end-group analysis with ESI mass spectrometric analysis. With these main-group element Lewis acid base pairs, it is not only possible to polymerize sterically demanding, functionalized as well as heteroatom containing monomers but also, for the first time, to catalytically polymerize extended Michael systems, like 4-vinylpyridine.

Study of Catalytic Sites on Ruthenium For Hydrogenation of N-ethylcarbazole: Implications of Hydrogen Storage via Reversible Catalytic Hydrogenation

Abstract: Hydrogen storage is a significant challenge for the development and viability of hydrogen-powered vehicles. Storage of molecular hydrogen in nitrogen-substituted polyunsaturated aromatic organic molecules through reversible catalytic hydrogenation and dehydrogenation is a promising approach. The success of developing a catalytic hydrogen storage concept is highly dependent on finding an efficient catalyst; however, understanding how molecules interact with metal catalytic sites is, at present, rather limited. In this work, a combined experimental and theoretical study is conducted to identify efficient catalytic sites on metallic surfaces and to understand the reaction mechanism for the forward hydrogenation reaction. It is clearly revealed from experimentation that hydrogenation of N-ethylcarbazole, a typical nitrogen-substituted polyunsaturated aromatic organic molecule, is taking place in a stepwise manner over metal catalysts. Because of steric constraints at terrace sites, the kinetically stable pyrrole intermediate, formed by partial hydrogenation of N-ethylcarbazole, cannot be readsorbed once desorbed into solution. Therefore further hydrogenation occurs at the low coordinated sites where no similar steric hindrance is encountered. Thus, the mechanism for hydrogenation involves an unusual shuttling of partially hydrogenated intermediates from terrace sites to higher indexed sites via solution. First-principles calculations confirm that the pyrrole intermediate can strongly adsorb to various low coordination sites, typically steps on the vicinal (109) surface, while the adsorption is extremely weak on flat (001) terraces. This work is the first example of catalytic site analysis to account for observed activity, selectivity and recyclability of a typical metal catalyst for catalytic hydrogen storage, which could lead to rational design of superior materials. © 2010 American Chemical Society.