Computational Studies of Alkane C-H Functionalization by Main-Group Metals Computational Studies of Alkane C-H Functionalization by Main-Group Metals Computational Studies of Alkane C-H Functionalization by Main-Group Metals

Samantha Gustafson, Samantha Gustafson, Daniel Ess, Chair Merritt, B Andrus, Roger Harrison, Matt Peterson, Kara Stowers, Samantha Gustafson
2016 unpublished
The most efficient homogeneous catalysts for hydroxylation of light alkanes utilize transition metals in superacid solvent and operate by tandem electrophilic C-H activation/metal-alkyl (M-R) functionalization. An emerging alternative strategy to transition metals is the use of high-oxidation state main-group metals (e.g. Tl III , Pb IV , I III) that hydroxylate light alkanes. This dissertation reports density-functional theory calculations that reveal the mechanisms, reactivity, and
more » ... of Tl III promoted alkane C-H functionalization in trifluoroacetic acid and Tl III-dialkyl functionalization in water. Calculations reveal that Tl III oxidizes alkanes via a closed-shell C-H activation and M-R functionalization mechanism that is similar to transition-metal C-H functionalization mechanisms. Comparison of Tl III to similar transition metals reveals that while Tl III and transition metals can have similar activation barriers for C-H activation, Tl III M-R functionalization is significantly faster due to a highly polar Tl-C bond and large Tl III /Tl I reduction potential. The combination of a moderate C-H activation barrier combined with a low M-R functionalization barrier is critical to the success for Tl III promoted alkane C-H oxidation. The proposed Tl III C-H activation/M-R functionalization mechanism also provides an explanation for ethane conversion to a mixture of ethyl trifluoroacetate and ethane-1,2-diyl bis(2,2,2-trifluoroacetate). The reactivity of Tl III contrasts the lack of alkane oxidation by Hg II. The C-H activation transition state and frontier-orbital interactions provide a straightforward explanation for the higher reactivity of Tl III versus Hg II. This frontier-orbital model also provides a rationale for why the electron-withdrawing group in EtTFA provides "protection" against overoxidation. Calculations also reveal that Tl III-dialkyl functionalization by inorganic Tl III in water occurs by alkyl group transfer to form a Tl III-monoalkyl complex that is rapidly functionalized.