Unexpected complexity in the thermal [(pi)2+(sigma)2+(sigma)2] cycloaddition reactions of quadricyclane: Theory and isotope effects

A combined computational and experimental study has been performed on the t hermal [(pi)2(s) + (sigma)2(s) + (sigma)2(s)] cycloaddition reactions of qu adricyclane with acetylene, ethylene, dicyanoacetylene, and dimethyl acetyl enedicarboxylate (DMAD). The energy surfaces for the reactions were obtaine d using hybrid density functional theory at the B3LYP/6-31G(d) level. All a ttempts to locate concerted pathways for the reactions failed, since the pu tative transition structures for these processes correspond to second-order saddle points. Nonconcerted pathways, proceeding via biradical intermediat es, were found for these reactions using unrestricted B3LYP/6-31G(d) theory . Intramolecular secondary kinetic isotope effects (SKIEs) were determined experimentally for the reaction of quadricyclane-1-d(1) and quadricyclane-1 ,5-d(2) with DMAD in benzene at 95 degrees C. The results were compared wit h the intermolecular SKIE for the reaction of quadricyclane-1,5,6,7-d(4) wi th DMAD under the same conditions. The k(H)/k(D1) and k(H)/k(D2) values are in quits good agreement with calculated values for the nonconcerted biradi cal mechanism, The experimental intermolecular SKIE for the addition of DMA D to quadricyclane-d(0) or -d(4) is negligible. This observation conflicts with either concerted or biradical mechanisms. Alternatives are discussed.