7 Reaction mechanisms Part ( ii ) Pericyclic reactions

Research into the mechanisms of pericyclic reactions continues at a rapid pace. Many of the studies described below address the factors which influence whether a reaction will proceed via concerted rearrangement or will follow a stepwise alternative. Numerous papers published this year report high-level computational studies undertaken to explain unusual regiochemistry or stereochemistry determined by experiment. Heteroatomic versions of pericyclic rearrangements also have attracted much attention. The existence of secondary orbital interactions in pericyclic reactions was questioned by García, Mayoral and Salvatella. Several examples were reinterpreted, using other factors such as solvent effects, steric interactions, hydrogen bonds and electrostatic forces to account for stereoselectivity previously explained by secondary orbital interactions. The authors center their discussion on the endo+exo selectivity in the Diels–Alder reaction. Klärner and Wurche reviewed the effects of pressure on organic reactions, including pericyclic reactions. Pressure effects are thought to arise from differences in packing coefficients between reactants and transition states, and pericyclic activation volumes tend to be negative due to the highly organized transition states. Swiss and Firestone question whether accurate activation volumes are obtained by measuring the pressure dependence of reaction rates. They point to a second factor, solvent viscosity, which can also affect reaction rates. For example, Diels–Alder reactions, 1,3-dipolar cycloadditions and Claisen rearrangements are accelerated in solvents with increasing viscosity; thus, the activation volumes previously determined must be corrected for the pressure-induced change in solvent viscosity, and as such the changes in volumes become less negative. This correction can be as great as 61%. Two books which include discussions of pericyclic reactions were published. Ansari, Qureshi and Qureshi authored a book titled Electrocyclic Reactions: From