Chemistry of Unique Chiral Olefins. 2. Unexpected Thermal Racemization of <i>cis</i>-1,1‘,2,2‘,3,3‘,4,4‘-Octahydro-4,4‘- biphenanthrylidene

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 t1/2 = 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 1H 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 Ea = 21.5 kcal mol-1, activation enthalpy ΔH⧧ = 20.8 kcal mol-1, and activation entropy ΔS⧧ = −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 Ea = 25.2 kcal mol-1, activation enthalpy ΔH⧧ = 24.6 kcal mol-1, and activation entropy ΔS⧧ = −9.1 cal K-1 mol-1.