Mechanistic Studies of pi-Activation Catalysis by Cationic Gold(I) and Brønsted-acid
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 π-bonds or cycloisomerizations of enynes and related π-systems. Mechanisms involving outer-sphere attack of a nucleophile on the electrophilic π-ligand of a cationic gold π-complex are typically invoked for gold(I)-catalyzed hydrofunctionalization and cycloisomerization processes, but direct experimental evidence for this mechanism is limited.
As an extension of the pioneering research in the Widenhoefer lab on the synthesis and characterization of gold(I) π-complexes, a diverse family of 15 gold(I) π-complexes in three distinct series are reported herein. First, the synthesis, characterization, and solution behavior of a series of seven gold(I) π-diene complexes is reported. In each case, gold binds preferentially to the less substituted C═C bond of the diene, but intermolecular exchange of the complexed and uncomplexed C═C is facile. The gold-alkene interaction is stabilized via substitution-dependant donation of electron density from the uncomplexed C═C bond to the complexed C═C bond of the diene.
In addition, a pair of axially chiral dicationic, bis(gold) π-alkene complexes that contain a 2,2′-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) π-alkene, alkyne, diene, and allene complexes that bear a triphenylphosphine supporting ligand have also been synthesized and characterized in situ. The π-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.
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 π-(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.
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.
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