Chemistry of Unique Chiral Olefins. 2. Unexpected Thermal Racemization of cis -1,1‘,2,2‘,3,3‘,4,4‘-Octahydro-4,4‘- biphenanthrylidene
Year: 1997
Type: article
Abstract: During the studies of the unique chiral olefins, ( E )-1,1‘,2,2‘,3,3‘,4,4‘-octahydro-4,4‘-biphenanthrylidene ( 1 ) and its ( Z )-isomer ( 2 ), we found the unexpected thermal racemization of cis -olefin 2 at room temperature. The CD spectrum of optically active 2 shows rapid decrease of intensity with the half-life time t 1/2 = 1.2 h at room temperature. After the CD Cotton effects vanish, the sample of cis -olefin 2 was checked by HPLC with a chiral stationary phase, and two enantiomers of cis -olefin 2 were detected, but trans -olefin 1 was never detected. The racemization of sterically more-hindered cis -olefin 2 occurs by the direct interconversion between the two enantiomers ( M, M )-( Z )- 2 and ( P, P )-( Z )- 2 without formation of trans -olefin 1 as an intermediate. The enantiomeric interconversion between the two enantiomers of 2 was also studied by the 1 H NMR magnetization transfer technique. The kinetic values of the enantiomeric interconversion of 2 were obtained by CD and NMR spectroscopic studies and lead to an activation energy E a = 21.5 kcal mol - 1, activation enthalpy Δ H ⧧ = 20.8 kcal mol - 1, and activation entropy Δ S ⧧ = −8.5 cal K - 1 mol - 1 . The stereostructure of cis -olefin 2 was established by the X-ray crystallographic analysis. On the other hand, trans -olefin 1 does not racemize at room temperature, but can be observed to racemize at 55−95 °C. The kinetic values of the enantiomeric interconversion of 1 were obtained by polarimetric studies: activation energy E a = 25.2 kcal mol - 1, activation enthalpy Δ H ⧧ = 24.6 kcal mol - 1, and activation entropy Δ S ⧧ = −9.1 cal K - 1 mol - 1 .
DOI: 10.1021/ja970668m
Institutions: University of Groningen, Tohoku University
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2165961791