Computational study of the rearrangement reaction mechanism of phenylcarbene in a molecular container: Cram's hemicarcerand

The rearrangement reaction mechanism of phenylcarbene (PC) to 1,2,4,6-cycloheptatetraene (CHTE) in a molecular container (MC): Cram's hemicarcerand, was studied by means of quantum chemical calculations with the semiempirical AM1 and DFT-B3LYP methods. The free-state rearrangement process of PC was also investigated to make a comparison between the two different states at the same level of theory. The influences of the inner phase of the MC on the PC rearrangement were discussed by analyzing the comparative results. Furthermore, the single point energies of the complexes, interaction energies (Eint) between the host and guest molecule, and deformation energies (Edef) of the MC at the AM1-optimized geometries along the reaction coordinate were evaluated by B3LYP method with 6-31G* basis set. It is found that the inner phase of the MC has some minor impacts on the guest molecule's structure and charge distribution properties, and the conversion process confined in the MC follows about the same mechanism compared with the free-state rearrangement. On the other hand, the activation barrier of the rate-determining step of the overall process is reduced by 5.2 kcal/mol, which is about 20% of that in the free state, indicating that the MC microenvironment facilitates the occurrence of the rearrangement reaction.