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.

Interplay between steric and electronic factors in determining the strength of intramolecular resonance-assisted NH···O hydrogen bond in a series of β-ketoarylhydrazones

Abstract: The crystal structures of six β-ketoarylhydrazones are reported: 1,(Z)-2-(2-bromophenylhydrazono)-3-oxobutanenitrile; 2, (Z)-2-(2-methylphenylhydrazono)-3-oxobutanenitrile; 3, (E)-methyl-2-(2-methoxyphenylhydrazono)-3-oxobutanoate; 4, E, methyl-2-(2-cyanophenylhydrazono)-3-oxobutanoate; 5, (Z)-methyl-2-(4-cyanophenylhydrazono)-3-oxobutanoate; 6, pentane-2,3,4-trione-3-(2-carboxyphenylhydrazone). All of them form intramolecular hydrogen bonds assisted by resonance (RAHB), with N···O distances in the range 2.541(5)–2.615(3) A. These hydrogen bonds are differently affected by the substituents at the heterodienic fragment, being strengthened by electronwithdrawing substituents in position 2 (more by 2-COMe than 2-CN substitution), and weakened in β-esterhydrazones and when the N–H forms a bifurcated hydrogen bond. The role played by the different steric and electronic properties of the substituents in strengthening the H-bond is investigated, besides X-ray crystallography, by IR and 1H NMR characterization of the NH proton, and quantum mechanical DFT calculations at the B3LYP/6-31+G(d,p) level of theory on test molecules.

Hydrogen-bond basicity pKHB scale of secondary amines

Abstract: Using 4-fluorophenol as a reference hydrogen-bond donor, equilibrium constants K for the formation of 1 ∶ 1 hydrogen-bonded complexes have been obtained by FTIR spectrometry for 33 secondary amines in CCl4 and/or C2Cl4 at 298 K. A spectroscopic scale of hydrogen-bond basicity is constructed from the IR frequency shift Δν(OH) of methanol hydrogen-bonded to secondary amines. The comparison of the pKHB (log K), Δν(OH), and pKa scales points to the sensitivity of pKHB to steric effects, and of Δν(OH) to the p character of the nitrogen lone pair. The pKHB scale of secondary amines extends from 2.59 for pyrrolidine to −0.45 for (Me3Si)2NH. The main effects explaining the pKHB variations are (i) the opposite polarizability and steric effects in alkylamines, (ii) field-inductive effects (e.g.NCCH2NHMe), (iii) intramolecular hydrogen bonding, e.g. in (MeOCH2CH2)2NH, and (iv) the ring size giving the order: pyrrolidine = azetidine > piperidine > 2-methylaziridine > azepane. IR spectra show the attachment of 4-fluorophenol to the nitrile nitrogen of NCCH2NHMe and NCCH2CH2NHMe, to the oxygen of morpholine and (MeOCH2CH2)2NH, and to the sulfur of thiomorpholine and thiazolidine, in addition to attachment to the amino nitrogen. The correlation of pKHB with the minimum electrostatic potential on the nitrogen lone pair is used for unravelling the basicity of each nitrogen of 1-methyl-1,4-diazepane.

The influence of chain-branching on the steric outcome of some olefin-forming reactions

Abstract: When applied to the construction of internal disubstituted double bonds in compounds such as pentadec-7-ene and 2,6,11,15-tetramethylhexadeca-2,6,8,10,14-pentaene (5), the reductive elimination of vicinal benzoyloxy-sulphones gives products with a trans : cis ratio of ca. 4 : 1. When applied to the construction of disubstituted olefins in which branching occurs adjacent to the newly formed double bond, much higher trans-stereoselectivity is observed, e.g, for 5-ethyl-2-methylhept-3-ene (21) and 3,6-dimethylocta-2,4,6-triene (25) it is at least 98%. A similar effect of chain-branching is also observed in some Wittig and Homer reactions using primary allylic phosphorus derivatives. When a Z-allylic sulphone is used as the starting material in the benzoyloxy-sulphone method, the original Z-geometry is preserved in the product; (Z)-2-methylbut-2-enyl p-tolyl sulphone and tiglic aldehyde give almost exclusively (2E,4E,6Z)-3,6-dimethylocta-2,4,6-triene (27).

Resonance in Sterically Hindered Aromatic Molecules

Abstract: Many aromatic molecules, typical of which are the triphenylmethyl radical and cis‐stilbene, cannot have the planar structure which would maximize the resonance energy, because of steric hindrance. A semiempirical method of treating such molecules has been developed. It is assumed that the primary effect of the steric strain is to rotate the phenyl groups out of the plane of the molecule. The resulting reduction in resonance energy is estimated using the valence‐bond method. The steric interaction energies are calculated using a method of Eyring, based on the ``perfect pairing approximation'' of valence bond theory. A plot of the sum of these two energies vs twist angle gives the equilibrium configuration, and the effective resonance energy of the molecule.The triphenylmethyl radical has a predicted twist angle of 32°, and is stabilized partly by its resonance energy and partly by steric strain in hexaphenylethane. The theoretical twist angle for the free radical di‐p‐anisyl nitric oxide is 25°, as compare...