Low‐lying stepwise paths for ethylene 1,3‐dipolar cycloadditions: A DFT study

Abstract Ethylene reacts with 1,3‐dipoles such as diazomethane, nitrile oxide, and nitrone to give a single adduct and the potential energy surfaces of these reactions were completely surveyed with Density Functional Theory at the B3LYP/6‐31G(d) level; B3LYP/6‐311+G(d,p), QCISD/6‐31G(d) level calculations were performed for comparison. These reactions were found to have one concerted and four stepwise paths and all of them were thoroughly examined. Calculations show that anti and syn approaches in the stepwise paths merge at one point in the potential energy surface and the stepwise processes (i.e., through syn transition states) are low‐lying and concerted paths that are in close competition with them. A closer examination of the computed barriers of the reactions of ethylene with the above dipoles, cyclopentadiene, 1,3‐butadiene, and allyl anion reveals that there is a mechanistic cross‐over from concerted to stepwise path. While the neutral cycloaddition partners prefer a concerted path, the charged partners strongly favor a stepwise path. The dipoles have both concerted and stepwise (syn) paths in close competition. Such a mechanistic cross‐over has been induced by the polar influence of the charged species and this change‐over in mechanism could not be observed with allene cycloadditions with the same set of partners because allene is strongly biased towards the stepwise mechanism. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005