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.

Terthiophene Radical Cations End-Capped by Bicyclo[2.2.2]octene Units: Formation of Bent π-Dimers Mutually Attracted at the Central Position

Abstract: A terthiophene fused with bicyclo[2.2.2]octene units only at both ends was newly synthesized. Since there is no steric hindrance at the central position, this terthiophene has a possibility to interact only at the central position. One-electron oxidation of this terthiophene afforded a highly stable radical-cation salt as deep blue crystals. The result of X-ray crystal structural analysis demonstrated a characteristically bent π-dimereric structure, which is formed by mutual attraction of single radical-cation species at the central position to minimize the steric repulsion. Remarkably short intermolecular distances between the central thiophene rings of each unit of the dimeric pair, that is, 2.976(10) A for Cβ−Cβ, 3.091(10) A for Cα−Cα, and 3.779(3) A for S−S, are good indication of the existence of attracting interaction, which was confirmed by theoretical calculations. This interaction was experimentally demonstrated by the reversible formation of the π-dimer in CH2Cl2 solution using ESR and UV−vis−NI...

Electronic Control of the Regiochemistry in the Heck Reaction

Abstract: The migratory insertion of propene into the Pd-phenyl bond of a set of neutral palladium(II) complexes with N-N, P-O, and N-O chelating ligands has been investigated by using density functional methods. In contrast to cationic palladium(II)-diimine complexes, which favor 1,2-insertion, the more electron-rich neutral complexes lead to 2,1-insertion. Trans influences are substantial in the unsymmetrical ligand systems, primarily affecting the insertion barrier. Introducing steric effects leads toward improving 1,2-selectivity, due to destabilization of the 2,1-transition state. Electronic and steric influences can be systematically combined to control the regiochemistry in the Heck reaction.

Synthesis and characterization of a series of sterically-hindered amidines and their lithium and magnesium complexes

Abstract: A series of sterically hindered amidinate ligands containing terphenyl substituents at the carbon atom of the amidinate backbone have been synthesized in high yields; their lithium derivatives were prepared, including the first two examples of monodentate lithium amidinates Additionally, two independent routes to magnesium amidinates have been developed, yielding both mono- and bis-amidinate magnesium complexes The presence of substituents at the 2′, 6′, 2″, and 6″ positions on the terphenyl moieties attached to the amidinate backbone induces a strong steric effect In the free-base amidines, equilibria between E-syn and Z-syn isomers were dependent upon the size of these groups, while in the lithium amidinates, the steric bulk of these ligands led to the formation of the first monodentate lithium amidinates Finally, in the magnesium amidinates, short C–N bond lengths and reduced N–C–N bond angles were observed within the amidinate backbone

Readily Accessible Bulky Iron Catalysts exhibiting Site Selectivity in the Oxidation of Steroidal Substrates.

Abstract: Bulky iron complexes are described that catalyze the site-selective oxidation of alkyl C-H bonds with hydrogen peroxide under mild conditions. Steric bulk at the iron center is introduced by appending trialkylsilyl groups at the meta-position of the pyridines in tetradentate aminopyridine ligands, and this effect translates into high product yields, an enhanced preferential oxidation of secondary over tertiary C-H bonds, and the ability to perform site-selective oxidation of methylenic sites in terpenoid and steroidal substrates. Unprecedented site selective oxidation at C6 and C12 methylenic sites in steroidal substrates is shown to be governed by the chirality of the catalysts.