Correlation of Calculated Halonium Ion Structures with Experimental Product Distributions from Terminal Alkenes: The Effect of Electron-Withdrawing Fluorine Substituents on the Structure and Charge Localization of Halonium Ions (PREPRINT)

摘要

The structures and charge densities of halonium ions, derived from addition of halogen electrophiles to fluoro-substituted terminal alkenes, were computed using quantum chemical methods. Geometry optimizations were performed at the second-order perturbation theory level (MP2, also known as MBPT(2)), using the Spartan02 program and also at the density functional theory level, using the GAMESS quantum chemistry code with the B3LYP hybrid functional. An additional set of B3LYP calculations incorporating the Polarizable Continuum Model (PCM) to probe methanol solvent effects was also performed. The halonium ions from terminal fluoroalkenes were found to be symmetrical (C), unsymmetrical (B or D), or open-ions described by A or E. These calculated data support experimental product distributions and they can predict the product regiochemistry from halonium ions opened by anions in aprotic solvents or when opened by protic solvents like methanol. Electron-withdrawing alkyl groups tend to decrease the SN1-character of the product-determining ring-opening transition state, and an SN-2-like process occurs which is more susceptible to steric effects.