A pathway bifurcation in the (4+3)/(5+2) cycloaddition of butadiene and oxidopyrylium ylides: The significance of molecular orbital isosymmetry.

By drawing analogies from the dimerisation of cyclopentadiene, a novel reaction pathway bifurcation is uncovered in the cycloaddition of oxidopyrylium ylides and butadiene. Analysis of the potential energy surface (at the M06-2X/6-311+G(d,p) level of theory) in combination with Born-Oppenheimer Molecular Dynamics simulations (M06-2X/6-31+G(d)) demonstrate that both the (4+3)- and (5+2)-cycloaddition products are accessed from the same transition state. Key indicators of a pathway bifurcation (asynchronous bond formation, and a second transition state for the interconversion of the products) are also observed. The absence of a post-transition state bifurcation in the related oxidopyridinium systems of Krenske and Harmata is rationalised. Finally, the isosymmetry of the oxidopyrylium and cyclopentadiene molecular orbitals, as well as the presence of "secondary orbital interactions" are emphasised as the common source of non-statistical behaviour. Application of these principles will allow for the rapid identification of new reaction pathway bifurcations.