Since first postulated by Woodward and Hoffman circa 1965 the concept of secondary orbital interactions (SOI) has been used extensively to explain facile steroselectivity in Diels-Alder type reactions. Generally, the concept involves accounting for the energetic contributions to a sterodefining transition state brought about by bonding and antibonding overlap between the frontier orbitals of each reacting moiety.  While the concept of SOI is now deeply embedded in organic chemistry it has never been invoked to explain steroselectivity within a heterogeneous reaction.  Here, we consider the reaction of 1,3-cyclohexadiene (1,3-CHD) on Si(100) and show that the observed reactivity and stereoselectivity cannot be explained on the basis of thermodynamics. We make use of our recently published DFT and Brillouin zone folding technique  that generates the full set of high symmetry crystal wavefunctions (CWs) appropriate to the ground state Si(100)-c (4x2) surface. Using these results we introduce a simple algorithm based on the conservation of orbital symmetry that identifies possible frontier orbital interactions on the endo and exo reaction trajectories. We demonstrate with the aid of DFT that SOIs stabilize the transition state en route to exo product formation, and that this assignment is consistent with a careful re-examination of the original STM data.

Stereoselective cycloaddition of 1,3-cyclohexadiene on Si(100): A simple algorithm for product identification based on secondary orbital interactions

243rd National Meeting (2012)

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