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.

Metalloradical activation of methane

Abstract: The authors approach to extending the range of C-H bond reactions for rhodium(II) porphyrin complexes has been to introduce ligand steric requirements that reduce the Rh{sup II}-Rh{sup II} bond energy without seriously weakening the Rh-C bonding (Tetramesitylporphyrinato)rhodium(II), (TMP) Rh{center dot}, constitutes a limiting case where ligand steric requirements completely exclude metal-metal bonding. This article reports on the selective reaction of (TMP)Rh{center dot} with methane in benzene solution and further describes aspects of the thermodynamic and kinetic-mechanistic features for this reaction.

Theoretical studies of stereoselectivities of intramolecular aldol cyclizations catalyzed by amino acids.

Abstract: The effects of different amino acid catalysts and substrate substituents on the stereoselectivity of the title reactions have been studied with the aid of density functional theory methods. Experimental data available in the literature have been compiled. B3LYP/6-31G(d) calculations match the general experimental trends and provide useful insights into the origins of the variations in stereoselectivities. Acyclic primary amino acids allow a greater conformational flexibility in the aldol transition states compared with proline. This makes them poorer enantioselective catalysts with triketone substrates with a methyl ketone side chain. The steric repulsion upon substitution at the terminal methyl group increases the energy difference between anti- and syn-chairs with primary amino acid catalysts and, consequently, the stereoselectivities. Proline, in contrast, is a poor catalyst for the latter reactions because the substituent's steric bulkiness raises the activation energy of the favored C−C bond-forming ...

Ethene−Norbornene Copolymerization with Homogeneous Metallocene and Half-Sandwich Catalysts: Kinetics and Relationships between Catalyst Structure and Polymer Structure. 4. Development of Molecular Weights

Abstract: The molecular weights of ethene−norbornene copolymers, produced with various metallocene− and amidocyclopentadienyl−methylaluminoxane (MAO) catalysts, have been determined by high-temperature gel-permeation chromatography: with one exception, increasing molecular weights were found with an increasing norbornene content in the copolymer. This observation is due to the fact that the β-hydride elimination process is not possible in the case of norbornene, because of special steric conditions of the cis-2,3-exo inserted norbornene; other possible termination reactions are discussed, considering the specialties of the norbornene. The effect of the catalyst structure on the molecular weights is discussed in terms of steric and electronic factors of the different ligands: sterically less hindered ligands produce ethene−norbornene copolymers with relatively low molecular weights; ligands with a larger extension and electron-pushing effect such as the fluorenyl ligand, induced the formation of high molecular wei...