Chiral free energy barriers

Although planar structures for 111 and 112 were not attained, it is still likely that these novel diazabiaryls can serve as chiral ligands with C2 symmetry. In support of this likelihood, the resolution of 112 was accomplished recently on swollen, microcrystalline triacetylcellulose by Jan Sandstrom. The free energy barrier to ring inversion of 112 was found to be about 101 kJ/mol, through a thermal racemization process using chiral 112. ... 

Configurational stability (or persistence) is one of the important properties of a chiral material. The definition of the lower limit for the free energy barrier for racemization may depend on the specific application. For optoelectronic applications, accelerated aging tests may provide very approximate guidelines [114]. For the purpose of estimating the free energy barrier for racemization, we will assume that the less than 1 % conversion of the major enantiomer to the minor enantiomer in such aging tests is tolerable, i.e. Aa / a < 0.02, where a (in units °mm 1) denotes rotatory power of thin-film material. With these assumptions,... 

Barriers for inversion of configuration in chiral, overcrowded PAHs are relatively low. In hexabenzotriphenylene, a free energy barrier of 26.2 kcal mol-1 was found for isomerization of 53-C2 to the more thermodynamically stable 53 -D3. However, the free energy barrier for racemization in 53-C2, as determined from the 1H NMR coalescence temperature (AG rac = 11.7 kcal mol"1, Tc = 247 K, Av = 102 Hz at 500 MHz), was relatively low [95]. Pentacene 55 (Fig. 15.21), with a screw-type end-to-end twist of 144°, racemized slowly at room temperature (AG raCi25°c = 23.8 kcal mol-1) [97], that is, the barrier for racemization is similar to that in [5]helicene (AG rac = 24.1 kcal mol"1). 

However, non-bridged calix[6]arene derivatives may also be chiral. It was possible to prepare two diastereomeric 1,2-dibenzyl ethers from /-butyl-calix[6]arenes with a syn and anti orientation of the ether residues. The free energy barrier for their mutual interconversion was found to be about 27 kcal mol". 160 This should be high enough to separate the enantiomers of the a/ift -isomer, which has effective C2 symmetry. The same should be true for another example of this type, a 1,2-p-nitrobenzoate with an anti-configuration161 and for 1,2,4-tri-O-alkyl or acyl derivatives.162... 

The recent rapid development of selectively functionalized calix[6]arenes also has a large, yet unexplored potential for obtaining inherently chiral derivatives. Two different diastereomeric 1,2-dibenzylethers of t-butylcalix[6]arene (syn and anti) have been obtained, for instance, from which the anti isomer (17) is chiral (C2 symmetry) [22]. The free energy barrier for their mutual interconversion (about 27 kcal mol ) should also be sufficient to isolate the enantiomers of 17. 1,2,4-Tri-O-alkyl or -acylderivatives [23] should be separable into the enantiomers as long as neither the O-acyl nor the p-substituent can penetrate the annulus. The macrobicyclic 2,5-esters 18 obtained from 1,4-di-p-methylbenzylethers are also chiral, having Ca-symmetry in the all-syn isomer [24]. 

Free energies of activation for the enantiomerization of a series of iV-aryl-1,3,2-benzodithiazole 1-oxides 41 have been determined by dynamic high-performance liquid chromatography (DHPLC) on a chiral stationary phase <1999JOC1483>. From a comparison of experimental and computer-simulated chromatograms, the barriers to stereoinversion at sulfur were found to be around 80 kj mol 1 and relatively insensitive to effects from substituents in the iV-aryl group. 

H- and 13C-NMR spectroscopy have helped to solve several configurational and conformational problems in hydantoinsj71,79,184-186 and the energy barrier for free rotation about the N—C bond in 3-aryl-2-thiohydantoins has been calculated by H-NMR experiments.187 Diaste-reomeric solute-solvent interactions of 1- and 3-arylhydantoins in chiral solvents have been studied.188... 

Radicals with very polar substituents e.g. trifluoromethyl radical 2), and radicals that arc part of strained ring systems (e.g. cydopropyl radical 3) arc ct-radicals. They have a pyramidal structure and are depicted with the free spin resident in an spJ hybrid orbital. nr-Radicals with appropriate substitution are potentially chiral, however, barriers to inversion are typically low with respect to the activation energy for reaction. 

---