Enantiomerization process

Buser H-R, MD Muller (1997) Conversion reactions of various phenoxyaUcanoic acid herbicides in soil 2 elucidation of the enantiomerization process of chiral phenoxy acids from incubation in a DjO/soil system. Environ Sci Technol 31 1960-1967. 

The somewhat unusual aminomethylene compound 22 has been described by Senning and Kelly (40). Its H NMR spectrum shows a hindered rotation of the NEt2 group with a barrier of ca. 14 kcal/mol, and an enantiomerization process... 

The thermal rearrangement of vinylcyclopropane to cyclopentene was uncovered in I96090 91. That vinylcyclopropanes, like other cyclopropanes, may undergo cis, trans iso-merizations was inferred in 1964 when trans-l-vinyl-2-methylcyclopropane was thermally converted to mostly (4Z)-1,4-hexadiene, a product formed at much lower temperatures from cw-1-vinyl-2-methylcyclopropane92. The reversible interconversion of the cis and trans isomers of l-vinyl-2-d-cyclopropane (equation 2) was reported soon thereafter, in 196793"96. Additional examples, including cases showing both geometrical isomerization and enantiomerization processes, soon followed. 

Temperature-dependent FI NMR spectra showed that the octaanionic dimer of 57 undergoes a dynamic process, rationalized as a fast in-plane rotation motion of each layer against the other, which corresponds to an enantiomerization process. This dynamic motion is inhibited at low temperatures. In 58 and the trimer (dodecaanion of 57) this enantiomerization process is severely hindered by the interlocked, gear-meshed structure90. 

Biphenyls containing four large groups in the ortho positions cannot freely rotate about the central bond because of steric hindrance. For example, the activation energy (rotational barrier) for the enantiomerization process was determined, AG = 21.8 0.1 kcal mol , for the chiral 2-carboxy-2 -methoxy-6-nitrobiphenyl. In such compounds, the two rings are in perpendicular planes. If either ring is symmetrically substituted, the molecule has a plane of symmetry. For example, consider the biaryls ... 

The determination of the rate constant at different temperatures allows the derivation of the AH and AS of the enantiomerization process (67AK115, 74CS226, 80JCS(P1)1599). 

The diastereoisomerization and enantiomerization processes in hexanes at 71 °C were monitored by HPLC and NMR. Starting from one pure anti enantiomer of 561a, one observes a fast initial increase of the syn form followed by a slow increase to its final concentration indicating... 

Scheme 6.2 Enantiomerization process of ( )-5 and half-lives for two enantiomers...

A rather similar enantiomerization process for [HM3(CO)9(ju.j-MeC=C=CMe2)] is faster for M = Ru than for M = Os (Scheme 19). A feature in common between Schemes 18 and 19 is that an approximately sp -hybridized carbon atom is turned over so that its opposite face is bound as part of an 17 linkage. A hydride shift must accompany the movement of the allenyl ligand. 

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