UBC Theses and Dissertations
Harnessing the reactivity of late transition metals for the making and breaking of C-X (X = O, S, N) bonds Addison, Desnoyer N.
This thesis explores the fundamental reactivity of π-complexes of rhodium and nickel, their reactivity to form well-defined 2-metallaoxetanes, as well as subsequent functionalization chemistry of these rare metallacycles. More generally, we also examine C-O and C-S bond cleavage processes. In Chapter 1, we discuss the history of 2-metallaoxetanes, as well as outline some of the fundamental organometallic chemistry of group 10 transition metal complexes. In Chapter 2, the chemistry of a well-defined 2-rhodaoxetane with unsaturated electrophiles is explored. In all cases, insertion into the Rh-O bond is observed. When electron-deficient alkynes are used as substrates, rhodadihydropyrans are formed. Reactivity studies have found these complexes to be robust. In contrast, when aldehydes are used, the product rhodaacetals are much less stable. Curiously, the aldehyde insertions were found to be reversible. Chapter 3 outlines the reactivity of low-valent nickel complexes with three-membered oxacycles. When epoxides were used, isomerization to the corresponding aldehyde was observed as the primary reaction pathway. Experiments with tetrasubstituted epoxides indicate that these reactions occur via 2-nickela(II)oxetane intermediates. Further, catalytic functionalization was achieved using HBpin or B2pin2. When using oxaziridines as the three-membered heterocycle, N-O oxidative addition was found to rapidly generate oxazanickela(II)cyclobutanes. Fragmentation of these metallacycles resulted in the formation of a mixture of imine and aldehyde products. Chapter 4 discusses the synthesis, mechanism of formation and reactivity of a family of well-defined 2-nickela(II)oxetanes. These nickelacycles are formed with retention of configuration, which had not been observed previously. Computational calculations were performed, which support an unexpected bimetallic mechanism of oxidative addition that would allow for the observed stereochemistry. Reactivity studies indicate that the nickelaoxetanes are susceptible to protonolysis, insertion and oxidatively-induced reductive elimination reactions. Chapter 5 describes the chemistry of nickel with esters and thioesters. For thioesters, Cacyl-S bond cleavage followed by decarbonylation was observed to form methyl-thiolate complexes of nickel(II). In contrast, aryl esters were found to undergo Caryl-O oxidative addition, producing aryl-acetate nickel(II) complexes. Both of these classes of compounds were found to be competent in stoichiometric Suzuki-type cross-coupling reactions, but attempts to render the reactions catalytic have so far been unsuccessful.
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