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. TlIII, PbIV, IIII) that hydroxylate light alkanes. This dissertation reports density-functional theory calculations that reveal the mechanisms, reactivity, and selectivity of TlIII promoted alkane C-H functionalization in trifluoroacetic acid and TlIII-dialkyl functionalization in water. Calculations reveal that TlIII 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 TlIII to similar transition metals reveals that while TlIII and transition metals can have similar activation barriers for C-H activation, TlIII M-R functionalization is significantly faster due to a highly polar Tl-C bond and large TlIII/TlI 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 TlIII promoted alkane C-H oxidation. The proposed TlIII 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 TlIII contrasts the lack of alkane oxidation by HgII. The C-H activation transition state and frontier-orbital interactions provide a straightforward explanation for the higher reactivity of TlIII versus HgII. This frontier-orbital model also provides a rationale for why the electron-withdrawing group in EtTFA provides "protection" against overoxidation. Calculations also reveal that TlIII-dialkyl functionalization by inorganic TlIII in water occurs by alkyl group transfer to form a TlIII-monoalkyl complex that is rapidly functionalized.
College and Department
Physical and Mathematical Sciences; Chemistry and Biochemistry
BYU ScholarsArchive Citation
Gustafson, Samantha Jane, "Computational Studies of Alkane C-H Functionalization by Main-Group Metals" (2016). Theses and Dissertations. 5992.
Main-Group Metal, Alkane, C-H Activation, Thallium, Trifluoroacetic acid