Computational Study on the 1,2-Rearrangement in β-(Nitroxy)vinyl and β-(Acetoxy)vinyl Radicals

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The 1,2-nitroxyl and 1,2-acetoxyl rearrangement in beta-(nitroxy)vinyl and beta-(acetoxy)vinyl radicals 13a and 13b, respectively, has been studied for the gas phase with various ab initio and density functional methods. The energetically most favorable pathway for 13a is calculated to proceed via reversible fragmentation/radical addition through transition state I-19a. In the case of 13b, rearrangement through a five-membered ring transition state III-16b and the fragmentation/radical addition pathway via transition state I-19b are competing processes. Mulliken and natural population analysis reveal a certain degree of charge separation in III-16a/b that may indicate a potential solvent effect on the rearrangement rate. A stepwise group migration through a cyclic radical intermediate V-18a/b or rearrangement through a three-membered ring transition state II-15a/b can be ruled out for both vinyl radicals. A comparison of the results of the calculations with experimental findings provides important insights into the kinetics of "self-terminating radical oxygenations". A significant method dependence on the outcome of the calculations was observed, which revealed the unsuitability of the UHF, MP2, B3LYP, and mPW1PW91 methods for computing these radical rearrangement processes. The results from BHandHLYP/cc-pVDZ calculations showed the best agreement with single-point energy calculations performed at the QCISD and CCSD(T) levels of theory.