Steric Scale of Common Substituents from Rotational Barriers ofN-(o-Substituted aryl)thiazoline-2-thione Atropisomers

Abstract

A steric scale of 20 recurrent groups was established from comparison of rotational barriers on N-(o-substituted aryl)thiazoline-2-thione atropisomers. The resulting energy of activation ΔG reflects the spatial requirement of the ortho substituent borne by the aryl moiety, electronic aspects and external parameters (temperature and solvent) generating negligible contributions. Concerning divergent rankings reported in the literature, the great sensitivity of this model allowed us to show unambiguously that a methyl appears bigger than a chlorine and gave the following order in size: CN > OMe > OH. For the very bulky CF and iPr groups, constraints in the ground state decreased the expected ΔG values resulting in a minimization of their apparent sizes.

Chimera Diimine Ligands in Emissive [Cu(P^P)(N^N)][PF6] Complexes

“…The disorder in [Cu(xantphos)(6-Cl-6'-Mebpy)][PF6], modeled with equal occupancies of Cl and Me groups, indicates that there is no preference for the orientation of the 6-Cl-6'-Mebpy ( Figure 4). This is consistent with the more general observation of similar steric effects for these two substituents [24,25].…”

“…The disorder in [Cu(xantphos)(6-Cl-6 -Mebpy)][PF 6 ], modeled with equal occupancies of Cl and Me groups, indicates that there is no preference for the orientation of the 6-Cl-6 -Mebpy ( Figure 4). This is consistent with the more general observation of similar steric effects for these two substituents [24,25]. (11) In [Cu(xantphos)(N^N)] + complexes which contain asymmetrical 6-substituted 2,2'-bipyridines or 2-(pyridin-2-yl)quinolines, the N^N ligand may favor a conformation in which the 6-substituent or quinoline unit lies over, or is remote from, the xanthene unit [11,12,18,19].…”

Axially chiral Ni(II) complexes of α‐amino acids: Separation of enantiomers and kinetics of racemization

“…66 The relationship between the higher ground state strain and the lower rotational barrier is rather a general, yet poorly recognized phenomenon, which has been overlooked in the properties of many axially chiral compounds. [67][68][69] Summary of the energy barriers for complexes 7a,b and 8a,b is presented in Table 1.…”

Preparative Method for Asymmetric Synthesis of (S)-2-Amino-4,4,4-trifluorobutanoic Acid

“…One of the most rapidly growing areas in the modern drug design is the application of fluorinated residues as bioisosteres of naturally occurring moieties [28]. In particular, based on considerations of the biologically relevant size, as cumulatively defined by van der Waals volume (vdW), A values, Taft Es values, and biphenyl interference values [62][63][64][65][66], trifluoromethyl group has often been considered to be isosteric with an iso-propyl substituent. Although a CF 3 and i-Pr are clearly of a different shape, they are sterically much closer related than to Me, Et, or t-Bu groups.…”