Unusually Large Secondary Deuterium Isotope Effect. Thermal Trans-Cis Isomerization of trans-1-Phenylcyclohexene

摘要

The magnitudes of secondary deuterium isotope effects (SDIE) are generally in the range of 0.9 < kH/Kd < 1.25, and are often satisfactorily rationalized by the zero-point energy (ZPE) change on going from reactant to transition state due to C-H rehybridization. We now report a far larger SDIE for the title reaction. Its rationalization on the basis of transition state theory suggests that it more closely resembles a primary isotope effect. Laser flash photolysis of cis-1-phenylcyclohexene (direct, 266 nm, or sensitized by thioxanthone, 355 nm) affords its trans isomer which in heptane exclusively reverts to 1, k = 2.1 x 10 to the 5th power s at 300 K. Isotopically substituted 2-2-d1 or 2-2,6-6-d3 (generated similarly from the corresponding cis isomers5) both have rates of reversion longer than 2 itself by a factor of 2.0 at room temperature No previously reported SDIE approaches this magnitude.