Cocco, Fabio (2008) Sintesi di complessi di Au(III) con leganti azotati e studio della loro reattività. Doctoral Thesis.
In the last twenty years, alkene and alkyne activation by gold catalysts has received considerable attention. Most of the applications of gold catalysts have concerned the addition of nitrogen‐ and oxygen‐nucleophiles to the C‐C multiple bond. Metal‐mediated olefin oxidation is still a subject of great interest for a number of researchers. Various classes of mechanisms have been envisaged for different metal systems. Among these, that one involving metallaoxetanes (1‐metalla‐2‐oxacyclobutanes), as key intermediates in the oxygen‐transfer reaction, has been recently corroborated by a number of experimental evidences. The synthesis of a series of gold(III) oxo complexes, [Au2(N,N)2(μ‐O)2][PF6]2, supported by bidentate 6‐substituted‐2,2’‐bipyridines, has been the main subject of this thesis. The oxygen atom transfer reaction to olefins from these unusual gold(III) complexes has been thoroughly studied. The reaction with styrene, chosen as a model, was studied first and an unprecedented gold(I) olefin complex of the type [Au(N,N)(η2‐CH2=CHPh][PF6] structurally characterized. Styrene oxygenated derivatives, contemporaneously formed, were identified as well. Analogous results have been successively obtained with other terminal olefins, while no reaction takes place with internal olefins. Although not isolated, formation of the olefin complexes and of the oxygenated species was attributed to the decomposition of an oxametallacyclic intermediate. Successively, from the reactions with cyclic alkenes, in particular norbornene, evidence for such intermediates was gained by the isolation and structural characterization of the unprecedented oxaauracyclobutane [Au(bipyMe)(κ2‐O,C‐2‐ oxynorbornyl)][PF6] and the gold(I) alkene complexes [Au(bipyR)(η2‐alkene)][PF6] (R = Me, alkene = norbornene). A plausible mechanism for the formation of these species, involving eliminative reduction reaction, was inferred by the reaction of the auraoxetane with excess norbornene: indeed, the gold(I) olefin complex and exo‐2,3‐epoxynorbornane were obtained. Structural and spectroscopic data of the new alkene complexes [Au(bipyR)(η2‐alkene)][PF6] and [Au2(bipyR)2(μ−η2,η2‐dialkene)][PF6]2 were collected and thoroughly analyzed. On the whole, they suggested an extent π‐back‐donation contribution to the olefin‐gold bond and this was corroborated of the theoretical calculations carried out at hybrid‐DFT level on the model compound [Au(bipy)(η2‐CH2=CH2)]+. A further insight into the structural features and electronic properties of these still rare gold derivatives can be of interest as for the design of new gold catalysts.
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