Structure, Conformation, and Stereodynamics of the Atropisomers of Highly Hindered Benzyl Ethers

Abstract

Low-temperature and NOE NMR spectra of four of the title compounds indicate that they adopt a synclinal (sc) conformation, in agreement with the prediction of ab initio computations. In the case of the most-hindered derivative (compound 4), the conformation is syn-periplanar (sp), as is also shown by X-ray diffraction. Such stereolabile sp- or sc-atropisomers exist as two conformational enantiomers: the corresponding enantiomerization barriers, covering the range 6.6 to 9.7 kcal mol(-1), could be measured for all the examined compounds. In two cases (compounds 3 and 5), the minor antiperiplanar (ap) atropisomer has been also observed, and the sc to ap interconversion barrier measured (11.7 and 11.9 kcal mol(-1), respectively). In addition, restricted rotation of the isopropyl and tert-butyl substituents has been detected, and the corresponding barriers have been determined.

Formation of Chelated Counteranions Using Lewis Acidic Diboranes: Relevance to Isobutene Polymerization

“…Cumyl azide was prepared using an existing literature method,20c while CumOMe was prepared from cumyl alcohol and MeI as described below. We find the Williamson ether synthesis to be more reliable for the synthesis of CumOMe compared to the more commonly employed acid-catalyzed reactions. In particular, the product is uncontaminated with MeOH, CumOH, or other protic materials, which serve as co-initiators of isobutene polymerization using diborane 1 .…”

Recent Advances in Stereodynamics and Conformational Analysis by Dynamic NMR and Theoretical Calculations

“…When the hydrogen is substituted by a methyl group, [92] the resulting benzyl ethers 24-27 (Scheme 4), are predicted (DFT computations) to adopt mainly a synclinal (or syn-periplanar) conformation in which the OMe group points towards the ortho alkyl substituent. This prediction is confirmed by NOE experiments in the case of compound 24 and by X-ray diffraction in the case of 27.…”

Determination of the Absolute Configuration of CN-Palladacycles by 31P{1H} NMR Spectroscopy Using (1R,2S,5R)-Menthyloxydiphenylphosphine as the Chiral Derivatizing Agent: Efficient Chirality Transfer in Phosphinite Adducts

“…To date, the restricted rotation around the P–C ipso aryl bonds in square planar complexes has been reported only for compounds bearing a very bulky phosphine ligand or in the case of PPh 3 coordinated in a sterically congested environment . The known Δ G ⧧ values for the rotation of the t -Bu group around the C–CMe 3 bond in sterically very crowded organic compounds are varied in the range from 9.4 to 14.5 kcal/mol …”