Enantiomerization barrier

DPIBF, the resulting 119 was optically active. This suggests that the progenitor of 119 has the allene structure 117 rather than other conceivable constitutions [80, 81]. Even early quantum-chemical calculations on 117 showed a strongly bent, chiral structure, although the enantiomerization barrier was not correctly estimated [19]. 

Trapp, O., Schoetz, G., and Schurig, V. (2001) Determination of enantiomerization barriers by dynamic and stopped-flow chromatographic methods - Review article. Chirality, 13 403-414. 

Maeda, K., Okamoto, Y., Toledano, O., Becker, D., Biali, S. E., and Rappoport, Z. (1994) Multiple buttressing interactions enantiomerization barrier of tetrakis(pentamethylphenyl)ethane, J. Org. Chem. 59, 5473-5475. 

An enantioselective dynamic electrokinetic chromatography technique was used by Trapp et al. for determination of rate constants, enantiomerization barriers (AG (298 K) = 100.9 + 0.5 kJ mol ) and activation parameters [AH (298K) = 89.5+ 2.0 kJ mol AS (298K) = -42+10J mol 1 K ] of 1 at pH 2.2 (02CEJ3629). Introduction of a permanent positive charge in TB 1 significantly decreased the enantiomerization, which is not in conflict with the iminium-based theory. 

The racemate of 1,3,2-benzodithiazole 1-oxide 42 was separated by supercritical fluid chromatography on the (A j )-Whelk-( )l column with supercritical carbon dioxide containing 20% methanol as a mobile phase. Peak areas of enantiomers prior to and after the separation, used for the calculation of the enantiomerization barrier, were detected by computer-assisted peak deconvolution of peak clusters registered on chromatograms using computer software <2002CH1334>. 

Trapp, O., Schurig, V. Novel direct access to enantiomerization barriers from peak profiles in enantioselective dynamic chromatography enantiomerization of dialkyl-1,3-allene-dicarboxylates, Chirality, 2002, 14, 465-470. 

Due to the possible rotation of the P—C bonds, e ht stereoisomers were postulated. In accordance with DFT calculations, one type of the possible structures dominated the others by a 98 2 ratio. The enantiomerization barrier strongly depends on the nature of the R-group. Indeed, phosphine oxide 96 (R=Me) having a 2-methyl substituent showed a half-life of 7 h whereas for the 96 (R= -Pr), which bears a larger isopropyl group, it was estimated to be 10° years. Resolution by chiral preparative-HPLC confirmed the stability of the isopropyl indolylphosphine at temperature up to 60 °C. 

Jung, M. Schurig, V. Determination of enantiomerization barriers by computer simulation of interconversion profiles Enantiomerization of diaziridines during chiral inclusion gas chromatography. JACS 1992, 114 (2), 529-534. 

Table 1 Enantiomerization barrier and activation parameters for representative oxiranyllithiums 8—14...

Figure 1 (a) Barriers to inversion of oxiranyllithium 10/ligand complexes in hexane/ TMEDA at 195 K (b) enantiomerization barrier of oxazolinyl oxiranyllithium 17 in THF/EtjO at 143 K. 

It has also been suggested that such conformation must be very rigid (high enantiomerization barrier for the reversal of propeller helici-ty) due to the intramolecular congestion of the three pairs of voluminous ortho-chlorine atoms. Those clathratogenic features encouraged us to start a systematic study of the inclusion properties of PTM radical, functionalized derivatives and other polychlorinated triarylmethyl radicals. ... 

The structures of l,8-bis(trimethylgermyl and stannyl)naphthalene show the substituents twisted out of the plane of the rings, giving Cg symmetry. The p.m.r. spectrum of both compounds support the structures, with one Me group of each MesM pointing towards the naphthalene plane, and 2 away from it, in contrast to the Bu derivative. Enantiomerization barriers decrease with the size of M in The stabilization of carbanions a to silicon induces 9,9-bis-... 

Another interesting temperature-related phenomenon is the enantiomerization of isomers in compounds showing a low enantiomerization barrier. On these occasions, enantio-selective chromatography can be a useful tool in determining energy barriers (Figure 54.14). 

Trapp O, Trapp G, Schurig V. Direct calculation and computer simulation of the enantiomerization barrier of oxazepam in dynamic HPLC experiments-a comparative study. J. Biochem. Bioph. Afet/icxfs 2002 54 301-313. 

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