Experimental and Theoretical Host–Guest Photochemistry; Control of Reactivity with Host Variation and Theoretical Treatment With a Stress Shaped Reaction Cavity; Mechanistic and Exploratory Organic Photochemistry

In our previous studies prediction of the course of photochemical reactions in crystalline media was accomplished using molecular mechanics with imbedded quantum mechanically generated transition structures. However, there was a need for subsequent geometry optimization of the transition structure within the surrounding ‘mini-crystal lattice’. One new approach was devised in which the molecules surrounding the transition structure were replaced by a rigid shell of inert gas atoms with subsequent ab initio geometry optimization of the imbedded transition structure. The present study aimed at providing a more realistic environment surrounding the reacting species. This made use of the recently developed Oniom computations of Gaussian98 which permit ab initio computations on the reacting species while performing molecular mechanical computations on the surrounding molecules. Included was determination of the effect of reaction cavity size. A new analysis, ‘Pairs’, was developed giving the specific important interactions of atom pairs in a large system. Additionally, the present study extended our solid-state photochemistry to inclusion compounds. This study was both experimental and theoretical. Experimentally, five host–guest inclusion compounds having 4-p-cyanophenyl-4-phenylcyclohexenone as the guest were prepared and studied. Three afforded photoproduct resulting from cyanophenyl migration paralleling the regiochemistry in solution; however, the minor products of the solution chemistry were lacking. Strikingly, a fourth inclusion compound gave mainly phenyl migration. The fifth inclusion compound led to 1:1 phenyl versus cyanophenyl migration. The chiral hosts permitted synthesis of enantiomerically pure photoproducts. The Oniom computations required modification but properly predicted migratory behavior in accord with the experimental observations. Several theoretical conclusions in the present study were: (a) the effect of cavity size; (b) the role of crystal relaxation and long range stress effects; (c) the reliability of least motion in predicting reactivity.