The effect of a complexed lithium cation on a norcarane-based radical clock

Density-functional theory (DFT) and ab initio calculations have been used to investigate the effect of a complexed lithium cation on the radical-clock rearrangement of the 2-norcaranyl radical to the 3-cyclohexenylmethyl radical. As found earlier for ring-closing radical clocks, complexation with a metal ion leads to a significant lowering of the barrier to rearrangement. DFT calculations on a model for the norcaranyl clock in cytochrome P450 confirm the two-state reactivity proposal of Shaik et al. and indicate that the porphyrin exerts little or no electrostatic effect on the rearrangement barrier.