Conformational Studies by Dynamic NMR. 94.¹ Cogwheel Pathway for the Stereomutations of Durene Derivatives Containing the Mesityl Ring

Abstract

The low-temperature NMR spectra of 1,4-bis(mesitoyl)durene, 1, and of 1,4-bis(mesitylethenyl)durene, 2, reveal the presence of syn and anti rotamers at the equilibrium, their relative proportions depending on the dielectric constant of the solvent. In solution the more stable rotamer of 1 is the anti whereas, in the case of 2, the more stable is the syn. Depending on the crystallization solvent employed the more (anti) and the less stable (syn) rotamers were both observed (X-ray diffraction) in the solid state of 1. On the other hand, only the less stable rotamer (anti) was found to be present in the solid state of 2. As shown by MM calculations, the syn-to-anti interconversion occurs via a correlated process (cogwheel pathway) involving the mesityl-C and durene-C bond rotations: the dynamic NMR technique yields an experimental barrier of 8.2 kcal mol(-)(1) for 1 and 13.1 kcal mol(-)(1) for 2. In the case of derivative 2 a second barrier, due to a second type of correlated rotation process (torsion), was also determined (8.6 kcal mol(-)(1)). As a consequence of the restriction of this second torsional motion the anti rotamer of 2 displays two distinguishable NMR spectra at -133 degrees C, corresponding to a pair of conformers with different symmetry (anti C(i)() and anti C(2)).

Artificial Molecular Rotors

“…Such chains of twofold rotators have been studied. 149,150 For one system of this type, 149 two types of motion are predicted. Correlated rotation occurs with a rotational barrier that increases with increasing chain length.…”

“…As the chain length increases, the likelihood of localized rotation instead of correlated rotation increases due to the increased barrier to rotation, W. A third mode of motion, torsional oscillations with an amplitude less than 180°, has also been observed. 150 The energy of this torsional mode and the barrier for correlated rotation are both altered by changing the linkage between the rotators.…”

1,3-Dipropionyl-2,4,6-trimethylbenzene

“…Casarini & Lunazzi (1996) separated orthogonal syn/anti isomers of 1,4-bis(2,2-dimethyl propanoyl)durene by preparative TLC. In a more recent study Coluccini et al (2003) observed more complicated rotamers in both syn and anti forms of 1,4-bis(mesitoyl)-2,3,5,6-tetramethylbenzene in low temperature NMR spectra. It was of interest to examine the related angles in the molecule of the title compound, (I), for which syn/anti orientations of keto groups are also possible.…”

Structure and conformational dynamics of an aromatic sulfonamide: NMR, X-Ray and computational studies

“…Melting points were determined in a capillary tube using a Büchi 510 melting point apparatus and are uncorrected. 1 N,N'-bis-tosyl-1,4-diamino-2,3-dimethylbenzene. A solution of p-toluensulfonyl chloride (280 mg, 1.47 mmol) in 1 mL of dry DCM was added dropwise at 0°C to a solution of 2,3-dimethyl-1,4-diaminobenzene (100 mg, 0.73 mmol) and dry triethylamine (162 mg, 1.6 mmol) in 0.5 mL of dry DCM. The reaction was stirred at 0 °C under a N 2 atmosphere and monitored by TLC (eluent: DCM/AcOEt 20/1) for 2 hours.…”

“…Since the X-ray structure was determined on crystals grown from a polar solvent (acetonitrile/methanol), it cannot be excluded that crystals containing the anti conformer could be grown from a low polarity solvent. 17 However, when single crystals obtained from a chloroform solution were analyzed, they had the very same crystal cell and structure of the others. Thus the preference of the syn conformation in the solid state must be attributed to a more dense crystal lattice, independently of the conformational preference in solution.…”