Reactivity and Characterization of Gold(I) Carbenes: Key Intermediates in Gold(I) Catalysis

Identifier:  TDX:2931

Handle: https://hdl.handle.net/20.500.11797/TDX2931

Authors:  García Morales, Cristina

Abstract:
Gold(I) carbenes have been proposed as key intermediates in gold(I)-catalyzed transformations and their structure have attracted great interest from the outset of the homogeneous gold catalysis era. Through these intermediates, gold(I) catalysts exert such exquisite control over the intramolecular cyclosiomerization of 1,n-enynes. However, the corresponding intermolecular transformations are more challenging and the complexity increased when, moreover, high levels of enantioselectivity must be induced in these processes. In this doctoral thesis, we explored two top subjects of homogeneous gold(I) catalysis, namely the enantioselective synthesis of cyclobutenes and the characterization of gold(I) carbene complexes. Our group implemented the intermolecular gold(I)-catalyzed [2+2] cycloaddition of alkynes with alkenes to access racemic cyclobutenes. In this context, we developed the enantioselective gold(I)-catalyzed synthesis of cyclobutenes using non C2-chiral Josiphos digold(I) complexes as catalysts. Our mechanistic studies indicate that only one of the gold(I) centers is directly involved in the activation of the alkyne, although the second one is required to induce the enantioselectivity. Our work also reveals that both ligand exchange and electrophilic addition can be turnover-limiting steps in this catalytic cycloaddition. We next focused our attention on the role of carbenoid complexes as carbenes precursors. To this end, we developed a convenient approach to access chloromethylgold(I) carbenoids bearing bulky ligands which, upon dehalogenation, display the typical reactivity of gold(I) carbenes in solution, namely homocoupling, cyclopropanation and Buchner reaction. Finally, we have characterized aryl gold(I) carbenes in solution from substituted gold(I) carbenoids. These aryl gold(I) carbene complexes promote cyclopropanation, C-H insertion and oxidation. In the absence of nucleophiles, a bimolecular reaction, similar to that observed for other metal carbenes, leads to a symmetrical alkene. Furthermore, they correspond to the reactive intermediates generated in the gold(I)-catalyzed decarbenation of cycloheptatrienes. Computationally studies have been carried out to gain more insights into the gold-carbon bond situation in these complexes.