Relative steric size of SCH3, OCH3, and CH3 groups from circular dichroism measurements

Abstract

The relative steric size of methyl, methoxy, and methylthio groups was determined from circular dichroism (CD) spectroscopy using a sensitive system based on the bilirubin model. In the cyclohexane model, equatorial vs. axial orientation and conformational analysis led to quantitative measurements of orientation preference or steric demand: conformational A-values CH 3 > SCH 3 > OCH 3 . A more sterically demanding model for assessing group size has been found in bilirubin analogs, which are yellow pigments that adopt a ridge-tile shape stabilized by a matrix of intramolecular hydrogen bonds. Optically active bilirubins have been shown to exhibit intense bisignate CD Cotton effects from exciton coupling of their two dipyrrinone chromophores held in either of two enantiomeric ridge-tile conformations. Interconversion of these M and P conformational enantiomers of helical chirality is rapid at room temperature but may be displaced toward either enantiomer by intramolecular nonbonded steric interactions that arise when substituents are introduced at equivalent sterically demanding sites, viz., the ␣ or ␤ carbons of the pigment's propionic acid chains. Such substituents shift the conformational equilibrium toward the M or the P-chirality conformer, depending only on the S or R stereochemistry at the ␣ and ␤ sites, and the resulting exciton CD for the ∼430 nm transition(s) was used to evaluate the relative steric size, SCH 3 > CH 3 > OCH 3 .

Steric Size in Conformational Analysis. Steric Compression Analyzed by Circular Dichroism Spectroscopy

“…This chirognostic interplay between nonbonded steric interaction and pigment conformation is possible only in the intramolecularly hydrogen-bonded ridge-tile conformation, and it has been used recently to compare the relative effectiveness of steric buttressing from R and β methyls 9d and to evaluate the relative size of OCH 3 and SCH 3 groups. 12 In the following, we report on a novel determination of the relative steric size of methyl, ethyl, isopropyl, tert-butyl, phenyl, and benzyl groups from CD spectroscopy of new synthetic bilirubins 2-6, each with an R-substituent in one propionic acid chain and a β′(S) methyl group in the other propionic group. The templates are the interconverting, intramolecularly hydrogen- bonded ridge-tile conformational enantiomers of Figure 3, and the intrinsic, nonbonded face-to-face steric compression.…”

“…9d Even when the R-substituents are electron-withdrawing methoxyl or methylthio groups and the β′-substituent is methyl, the (RR,β′S) diastereomer is more polar than the (RS,β′S). 12 In all examples, the configurational assignment was based on knowledge of the absolute configuration of the right half component 9a and nuclear Overhauser effect (NOE) experiments. As will be shown later, the assignment of configuration of 1-12 (Scheme 2) is based initially on the absolute configuration of dipyrrinone precursors (36 and 37), and the relative polarity of the diastereomeric rubins was confirmed by 1 H{ 1 H}-NOE measurements.…”

“…In contrast, the carbon resonances in the acid chain at C(8) are scarcely affected, as are the methyls at C(2)/C(18). In 7-12, where the R and β′ stereogenic centers have the same chiral sense (for which we use the term: syn-chiral), 25 intramolecular nonbonded steric interactions are minimized when the (25) A reviewer has suggested that a term should be used to indicate the series (7)(8)(9)(10)(11)(12) where the R and β′ stereocenters have the same helical sense and a term for when R and β′ stereocenters of the series (1-6) have the opposite helical sense. For the same helical sense, we use the term synchiral; for the opposite helical sense, we use the term anti-chiral.…”

Regulation of Supramolecular Chirality and Morphology of the LB Film of Achiral Barbituric Acid by Amphiphilic Matrix Molecules

“…Supramolecular chirality, which describes the chirality in the supramolecular level or the chirality of the supramolecular assemblies, has been attracting great interest in recent years. − The research on the supramolecular chirality can not only help us deeply understand and mimic various important molecular processes via a precise molecular and supramolecular chirality recognitions in the biological systems but also expand our research versions on the functionality of supramolecular systems including the electronic, photomagnetic, and sensing properties . Supramolecular chirality can be achieved through various noncovalent interactions such as hydrogen bonding (H-bonding), electrostatic interaction, hydrophobic interaction, π−π stacking, and coordination. − Interestingly, achiral molecules can also contribute to the supramolecular chirality through interaction with a chiral matrix. − In a particular case, even complete achiral molecules can lead to the molecular assemblies showing supramolecular chirality through the cooperative stereo arrangement or spontaneous symmetry breaking. , For example, some achiral porphyrin derivatives and cyanine dyes were reported to show chirality under stirring, where the chirality followed the direction of the stirring . Some inorganic salt such as NaClO 3 could spontaneously form the chiral crystals without any chiral factor .…”

Interfacial Organization-Induced Supramolecular Chirality of the Langmuir−Schaefer Films of a Series of PPV Derivatives

“…It is also a fundamental issue in sterochemistry with crucial importance in crystal growth, asymmetric catalysis, and molecular recognition . Chirality can be observed in various levels from a single molecule to supramoelcualr systems, in which noncovalent interactions such as hydrogen-bonding, electrostatic interactions, hydrophobic interactions, π−π stacking, and coordination play important roles. − Besides the chiral components, achiral components can also contribute to the supramolecular chirality through interactions with chiral substrate or matrix. In some particular cases, even complete achiral molecules can form chiral assemblies.…”