Multidimensional exploration of valley-ridge inflection points on potential-energy surfaces.

The ene reaction between singlet oxygen ((1)O(2)) and simple alkenes has been reported as the first experimentally supported example of a potential-energy surface (PES) featuring a valley-ridge inflection that contributes to the selectivity of product formation (J. Am. Chem. Soc. 2003, 125, 1319-1328). That PES, based on gas-phase ab initio calculations and experimental kinetic isotope effects, has shaped the current (1)O(2) ene mechanism by advocating a concerted "two-step no-intermediate" mechanism. Our current investigation of the ene reaction between (1)O(2) and tetramethylethylene in water, DMSO, and cyclohexane using novel 3-dimensional potentials of mean force (3-D PMF) calculations coupled to QM/MM simulations predicts an alternative free-energy surface that supports a traditional stepwise mechanism interpretation featuring a symmetric charge-separated perepoxide intermediate. Solvent polarity dictates the stability of the intermediate and controls the activation barrier for ene product formation. Transformation of the higher order condensed-phase 3-D PMF potential-energy surface, computed following three simultaneous reaction coordinates, into a downgraded 2-D surface results in the "two-step no-intermediate" mechanism. CCSD(T) and MP4(SDQ) free-energy maps in DMSO, constructed from a 3-D grid of B3LYP geometries using the CPCM solvent model, reproduce the QM/MM results and confirm that when solvent is taken into account the gas-phase bifurcation reaction pathway converts into a stepwise mechanism. This manuscript provides new insight into the biologically and synthetically important (1)O(2) ene reaction and highlights a new multidimensional approach for constructing potential-energy surfaces.