Gold complexes based on (N,C) and (N,C,C) chelating ligands: ligand design and reactivity studies in cross-coupling catalysis

Homogeneous reactions catalyzed by gold have mainly been developed in the past two decades, stemming from the proneness of Au(I) and Au(III) complexes to coordinate and activate carbon-carbon multiple bonds towards a wide range of transformations. However, and in contrast to classical cross-coupling methodologies based on catalytic cycles mediated by other late transition metals, two-electron Au(I)/Au(III) catalytic cycles have long remained elusively attainable. The rationale behind this fact is the high redox potential of the Au(I)/Au(III) pair that makes Au(I) especially reluctant to being oxidized to Au(III). Therefore, cycling between +1 and +3 oxidation states in gold-catalyzed cross-coupling reactions is far from trivial. Besides, compared to other transition metals, with emphasis on the metals of groups 9 and 10, there is still limited knowledge of the main elementary steps of organometallic chemistry in the case of gold complexes. Achieving and studying these fundamental steps is crucial to understand gold’s reactivity and to provide mechanistic insight that would help in designing new gold-catalyzed transformations. In this regard, the projects presented in this dissertation aim to synthesize novel gold complexes using pre-designed ligands, as well as to carry out reactivity studies with them. The first part of the thesis deals with Au(I) complexes bearing (N,C) hemilabile ligands. On one hand, gold complexes with mesoionic carbene (MIC) ligands were subjected to oxidative conditions and tested in oxidative addition to obtain (N,C)-cyclometalated gold(III) complexes, thereby confirming the suitability of the (N,C) ligands for chelating Au(III) centers. Then, we employed these Au(I) complexes as catalysts for the arylation-lactonization of γ-alkenoic acids, the mechanism of which was proposed to operate through an oxidantfree Au(I)/Au(III) catalytic cycle based on an oxidative addition/reductive elimination pathway. On the other hand, gold complexes bearing NHC ligands were subjected to oxidative conditions and, conversely, pure Au(0) nuggets were formed along with the oxidation of the ligand. Thus, these systems served to study the decomposition reaction of Au(I) complexes into Au(0), instead of obtaining (N,C)-cyclometalated gold(III) complexes. The second part of the thesis focuses on developing a synthetic strategy to access nonsymmetric borylated (N,C,C) ligands, which might be utilized for the synthesis of nonsymmetric (N,C,C)-biscyclometalated Au(III) pincer complexes through boron-to-gold(III) transmetalation. With this metalation approach we overcome the limitations of other current methods that generate toxic waste or are limited to symmetric (N,C,C) platforms. ​
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