UBC Theses and Dissertations
Harnessing radical reactivity with paramagnetic organometallic chromium complexes MacLeod, Kenneth Cory
First-row transition metal complexes are increasingly being investigated as catalysts for organic synthesis. In addition to being more abundant, less expensive, and often less toxic compared to their second and third-row analogues, first-row complexes often exhibit complementary reactivity in the form of single-electron reaction pathways. One way of avoiding this complementary reactivity has been to use redox noninnocent ligands to enforce two-electron chemistry. An alternative is to embrace the radical reactivity of first-row metals. Keeping this in mind, a CpCr β-diketiminate ligand platform, where Cp = cyclopentadienyl, was used to study Cr–C bond homolysis with a series of well-defined paramagnetic Cr(III) hydrocarbyl complexes. The results of that investigation led to the development of the first example of a single-component Cr initiator/mediator for organometallic mediated radical polymerization. Carbon–carbon bond forming reactions for organic synthesis were also targeted. A protocol was established for the Cr-catalyzed radical cyclization of bromo and chloroacetals, in which the first example of chloroacetal cyclization under catalytic conditions is reported. A tertiary phosphine was also prepared by radical C–P bond formation using a Cr catalyst. Like many low-valent transition metal complexes, the Cr(II) compounds reported herein were extremely air-sensitive, leading to the formation of oxidation products containing strong Cr–O bonds. Methods were developed to selectively break the Cr–O bonds to convert the oxidized species back to catalytically relevant compounds and to reduce the need for stringent oxygen-free reaction conditions. The final chapter examines the use of well-defined Cr compounds for C–C bond forming reactions that occur at the metal centre, as opposed to the free radical mechanisms discussed in the other chapters. Replacing the β-diketiminate ligand with a deprotonated benzylamine provided a ligand platform that was structurally similar to the CpCr β-diketiminate system, but incorporated a reactive Cr–C bond that was used to study reductive elimination reactions for C–C bond formation. Initial results indicate that reductive elimination from a Cr(IV) intermediate is achieved by single-electron oxidation of a Cr(III) complex.
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