Mechanistic Studies of pi-Activation Catalysis by Cationic Gold(I) and Brønsted-acid

Abstract

<p>Soluble gold(I) complexes are highly efficient catalysts for the functionalization of C-C multiple bonds through the addition of carbon- or heteroatom-nucleophiles across &pi;-bonds or cycloisomerizations of enynes and related &pi;-systems. Mechanisms involving outer-sphere attack of a nucleophile on the electrophilic &pi;-ligand of a cationic gold &pi;-complex are typically invoked for gold(I)-catalyzed hydrofunctionalization and cycloisomerization processes, but direct experimental evidence for this mechanism is limited. </p><p>As an extension of the pioneering research in the Widenhoefer lab on the synthesis and characterization of gold(I) &pi;-complexes, a diverse family of 15 gold(I) &pi;-complexes in three distinct series are reported herein. First, the synthesis, characterization, and solution behavior of a series of seven gold(I) &pi;-diene complexes is reported. In each case, gold binds preferentially to the less substituted C&#9552;C bond of the diene, but intermolecular exchange of the complexed and uncomplexed C&#9552;C is facile. The gold-alkene interaction is stabilized via substitution-dependant donation of electron density from the uncomplexed C&#9552;C bond to the complexed C&#9552;C bond of the diene.</p><p>In addition, a pair of axially chiral dicationic, bis(gold) &pi;-alkene complexes that contain a 2,2&#8242;-bis(phosphino)biphenyl ligand are reported. The complexes show no intramolecular Au-Au interactions or facial selectivity for complexation, but solution analyses suggest that the environment about one gold center affects the behavior of the proximal gold center through a yet unknown mechanism. Gold(I) &pi;-alkene, alkyne, diene, and allene complexes that bear a triphenylphosphine supporting ligand have also been synthesized and characterized in situ. The &pi;-ligands in the triphenylphosphine gold complexes were considerably more labile than those bearing bulky, electron rich phosphine or N-heterocyclic carbene ligands, and the complexes decomposed in solution above -20 °C.</p><p>Mechanistic investigation of the gold-catalyzed cycloisomerization of a 7-aryl-1,6-enyne led to characterization of the first organometallic complex directly observed in the course of an enyne cycloisomerization. The complex is best described as a gold &pi;-(bicyclo[3.2.0]heptane) complex with a domination metallacyclopropane binding interaction and undergoes an acid-catalyzed rearrangement to yield a stable bicyclo[3.2.0]heptane product which can further isomerize in the presence of Ag+. In a further effort to understand the reactive species in catalytic cycloisomerizations, the first example of a gold cyclopropyl carbene was synthesized and fully characterized. </p><p>Finally, in an effort understand the mechanistic distinctions between electrophilic metal-catalyzed hydrofunctionalization and similar Brønsted acid-catalyzed additions, the kinetics and stereochemistry of intramolucar acid-catalyzed hydrofunctionalizion were studied. In all cases, the transformations were > 95 % selective for <italic>anti</italic>-addition and displayed rate laws similar to those expected for metal-catalyzed variants. A concerted C-H, C-X bond forming mechanism for addition is proposed.</p>Dissertatio