Solid angle as steric parameter of acyclic saturated groups

Abstract

With the early aim of quantifying steric consequences of chirality, efforts to define a nonempirical steric parameter of chemical groups are reported. Steric hindrance of a reacting center by any acyclic saturated R group has been characterized by a geometric "axial steric parameter": the solid angle of R. When the group is a "symmetric top substituent" (i.e., when all the terminal atoms are equivalent), the solid angle matches the solid angle of a cone envelope of R. The definition of this cone is compared with Tolman's definition of a ligand cone in organometallic complexes. The chemical significance of this parameter is shown by an excellent correlation with the Dubois' experimental steric parameter Els. Modeling steric repulsion by the cone of R, and correcting solid angles for conformational effects, only 3 empirical coefficients are needed to calculate 33 values of E', with less than 10% error. The cone model is suggested to be relevant within the limits of random and independent free rotations about all the bonds in the C-R group, A separation between "axial cone steric hindrance" and other steric effects is proposed. The basic model and the corrections proposed allow the conformational features of esters to be discussed.

Syntheses and crystallographic studies of [Ir(η5-C5Me5)(L)(RF)I] (L = CO, PMe3; RF = CF2CF3, CF2CF2CF3, CF2C6F5, CF(CF3)2) complexes. Cone and solid angle steric parameters for perfluoroalkyl ligands

“…Very little solid angle data have been published for alkyl groups, precluding a meaningful comparison with the perfluoroalkyl data. The solid angle of the perfluoroethyl group (Ω = 2.64-4.27 sr) is greater than for the ethyl group (Ω = 1.638 sr), 43 while the perfluoro-n-propyl ligand at 3.37 sr is larger than the n-propyl group (Ω = 2.017 sr). 43 The perfluorobenzyl and perfluoroisopropyl groups are both larger than the tert-butyl group (Ω = 3.317 sr), 43 and comparable to the CBr 3 group (Ω = 4.376 sr).…”

“…43 The perfluorobenzyl and perfluoroisopropyl groups are both larger than the tert-butyl group (Ω = 3.317 sr), 43 and comparable to the CBr 3 group (Ω = 4.376 sr). 43…”

1,4‐Dialkynylbutatrienes: Synthesis, Stability, and Perspectives in the Chemistry of carbo‐Benzenes

“…The perfectly planar conformation of the representatives 6 and 18 in the crystal state ( Figures 3 and 4) thus suggests that the phenyl rings may significantly contribute to their chemical stability. To evaluate the cross-conjugation effect, replacement of the phenyl groups by trifluoromethyl groups of similar conical size [27] but without a p system was envisaged. The s-withdrawing ability of the CF 3 groups was, however, expected to induce concentration (and thus polarization) of the p-electron cloud above the electron-deficient s system of the butatriene unit.…”

Quantitative Structure−Metabolism Relationships: Steric and Nonsteric Effects in the Enzymatic Hydrolysis of Noncongener Carboxylic Esters

“…For sufficiently flexible molecules, the value of Ω h as defined here may vary considerably; therefore, some energetic considerations have to be included in any reasonable treatment. Nevertheless, until now such measures of steric hindrance have been applied only in a few chemical studies − , and, to our knowledge, not at all in pharmaceutical studies.…”