Topic
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.
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TL;DR: The charge-dipole phenomenon responsible for the axial-F conformation in the parent protonated fluoropiperidinium compounds carries over to doubly alkylated salts, and it is shown that it extends to molecular orientation in the packing of the unit cells in the solid state as well.
Abstract: It has previously been shown that the fluorine atom in N-protonated 3-fluoropiperidine salts in water strongly prefers the axial orientation in the six-membered ring chairs. In the present work we examine the proposition that the N-methyl salts are equally disposed to present axial fluorine. Initially, we explored this point by comparing the structures of the corresponding NH 2 + , NHMe + , and NMe 2 + salts by means of density functional theory (DFT), ab initio, and MMFF force field calculations with and without aqueous solvation models. The predictions unambiguously pointed to axial fluorine for all salts investigated, including those with simultaneous axial F and (N)Me. The calculations were followed by synthesis of the corresponding series of 4,4-diphenylpiperidinium salts. These were evaluated by one- and two-dimensional NMR spectroscopy in [D 6 ]DMSO to fully corroborate the axial disposition of the fluorine in each of the compounds. X-ray crystal structure determinations were likewise performed for the diphenyl-3-fluoro NH 2 + and NMe 2 + systems to substantiate axial-F. Comparison of the X-ray structures of the fluorinated and unfluorinated NMe 2 + salts reveals that the fluorine resides axial in spite of substantial steric compression. While the charge-dipole phenomenon responsible for the axial-F conformation in the parent protonated fluoropiperidinium compounds carries over to doubly alkylated salts, we show that it extends to molecular orientation in the packing of the unit cells in the solid state as well. Finally, using the computational methods that successfully motivated our synthesis and structural work, we have made predictions for a number of new structures and re-examined some parallel results reported by the Eliel group in the early 1970s. Although C-F...H-N hydrogen bonds are reported to be weak and few in number, the CF...HN charge-dipole orienting effect is a powerful directing force that matches the hydrogen-bond in both its energetic contribution and conformational consequences.
72 citations
TL;DR: In this article, the reaction of diarylalkylsilanes and Germanes with trityl cation in the presence of a weakly coordinating anion to give the corresponding triarylsilylium or -germylium ions was investigated.
72 citations
TL;DR: The quasi-racemate method for steric correlation of optically active compounds is presented and discussed in this article, with special reference to the correlation of the amino acids and the terpenes to the glyceraldehyde system.
72 citations
72 citations
Abstract: The transmission of polar effects by various groups is assessed by means of the reaction constants ρ or ρ″. The results are critically discussed in terms of the various polar, steric, and solvent e...
72 citations