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Mechanistic Studies of Π-Activation Catalysis by Cationic Gold(I)

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Mechanistic Studies of Π-Activation Catalysis by Cationic Gold(I) Mechanistic Studies of -Activation Catalysis by Cationic Gold(I) and Brønsted-acid by Rachel Elizabeth McKinney Brooner Department of Chemistry Duke University Date:_______________________ Approved: ___________________________ Ross A. Widenhoefer, Supervisor ___________________________ Steven W. Baldwin ___________________________ Katherine J. Franz ___________________________ Qiu Wang Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University 2013 ABSTRACT Mechanistic Studies of -Activation Catalysis by Cationic Gold(I) and Brønsted-acid by Rachel Elizabeth McKinney Brooner Department of Chemistry Duke University Date:_______________________ Approved: ___________________________ Ross A. Widenhoefer, Supervisor ___________________________ Steven W. Baldwin ___________________________ Katherine J. Franz ___________________________ Qiu Wang An abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University 2013 i v Copyright by Rachel Brooner 2013 Abstract 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 -bonds or cycloisomerizations of enynes and related -systems. Mechanisms involving outer-sphere attack of a nucleophile on the electrophilic -ligand of a cationic gold -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) -complexes, a diverse family of 15 gold(I) - complexes in three distinct series are reported herein. First, the synthesis, characterization, and solution behavior of a series of seven gold(I) -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) -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 iv analyses suggest that the environment about one gold center affects the behavior of the proximal gold center through a yet unknown mechanism. Gold(I) -alkene, alkyne, diene, and allene complexes that bear a triphenylphosphine supporting ligand have also been synthesized and characterized in situ. The -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 - (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 intramolecular acid-catalyzed hydrofunctionalizion were studied. In all cases, the transformations were > 95 % selective for anti-addition and displayed rate-laws similar to those expected for metal- v catalyzed variants. A concerted C–H, C–X bond forming mechanism for addition is proposed. vi Contents Abstract ......................................................................................................................................... iv List of Tables ............................................................................................................................. xvii List of Figures ............................................................................................................................. xix List of Schemes ....................................................................................................................... xxvii Acknowledgements ................................................................................................................xxxii 1. Computational and Experimental Studies of the Structure and Binding of Cationic Gold(I) -Complexes of Alkenes and Alkynes ......................................................................... 1 1.1 Introduction: gold(I) -activation catalysis ................................................................... 1 1.2 Binding models for cationic gold(I) -complexes ........................................................ 3 1.2.1 The Dewar-Chatt-Duncanson model for binding in transition metal - complexes .............................................................................................................................. 3 1.2.2 Relativistic effects on the chemistry of gold ............................................................ 4 1.2.3 Theoretical studies of bonding in Au+-ethylene and Au+-acetylene complexes. 5 1.2.4 Theoretical studies of bonding in LAu+-alkene and LAu+-alkyne complexes. ... 8 1.3 Experimental determination of structure and solution behavior in gold(I) - complexes of alkenes and alkynes ..................................................................................... 10 1.3.1 Introduction................................................................................................................ 10 1.3.2 Synthesis, structural characterizations, and solution behavior of cationic gold(I) -alkene complexes ............................................................................................... 12 1.3.2.1 Complexes bearing NHC supporting ligands ............................................... 12 1.3.2.2 Complexes bearing bulky phosphine supporting ligands ........................... 17 vii 1.3.3 Synthesis, structural characterizations, and solution behavior of cationic gold(I) -alkyne complexes ............................................................................................... 21 1.3.3.1 Internal alkyne complexes bearing NHC supporting ligands ..................... 21 1.3.3.2. Complexes bearing a hindered alkyl phosphine ligand .............................. 24 1.4 Summary and discussion .............................................................................................. 27 2. Syntheses, X-ray Crystal Structures, and Solution Behavior of Cationic, Two- Coordinate Gold(I) 2-Diene Complexes ................................................................................. 30 2.1 Introduction ..................................................................................................................... 30 2.1.1 Gold(I) catalyzed hydrofunctionalization of dienes ............................................. 30 2.1.2 Coordination behavior in gold(I) -alkene complexes......................................... 31 2.1.3 Transition metal 2- and 4-(1,3-diene) complexes ............................................... 34 2.1.4 Project goals and scope ............................................................................................. 34 2.2 Results and discussion ................................................................................................... 35 2.2.1 Synthesis and characterization of the gold(I) 2-(trans-1,3-pentadiene) complex 2.6 .......................................................................................................................................... 35 2.2.1.1 Synthesis of [(P1)Au(2-(E)-H2C═C(H)C(H)═C(H)Me)]+ SbF6⁻ (2.6)............ 35 2.2.1.2 X-Ray crystal structure of 2.6 ........................................................................... 36 2.2.1.3 Solution characterization and fluxional behavior of 2.6 ............................... 38 2.2.2 Synthesis and characterization of gold(I) 2-(diene) complexes 2.7 – 2.12 ........ 41 2.2.2.1 Synthesis of gold(I) 2-(diene) complexes 2.7 – 2.12 ..................................... 41 2.2.2.2 X-ray crystal structures of 2.7, 2.9, and 2.12 ................................................... 42 2.2.2.3 Solution characterization of 2.7 - 2.12.............................................................. 47 2.2.3 Fluxional behavior of gold(I) diene complexes ..................................................... 50 viii 2.2.3.1 Fluxional behavior of complexes 2.6 and 2.8 ................................................. 50 2.2.3.2 Fluxional behavior of complexes 2.9-2.12 ....................................................... 54 2.2.4 Intermolecular exchange behavior of gold(I) 2-diene complexes ..................... 63 2.2.4.1 Kinetics of intermolecular exchange of 2,4-dimethyl-1,3-pentadiene
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