Browsing by Subject "Gold"

Purpose

Experimental design

Results

Conclusions

Cationic gold(I) complexes have recently contributed to significant developments in homogenous catalysis. Such complexes have been praised as highly effective catalysts for the functionalization of C-C multiple bonds, leading to research on cationic gold-catalysts developing at an aggressive pace. Despite the progress being made surrounding gold(I)-catalysis, there are still many gaps in our fundamental understanding of the key intermediate complexes and their reactivity in these transformations, exemplified by the often evoked gold alpha-oxo carbene species. While there are existing computational studies suggesting the instability of gold alpha-oxo carbene species, there lacks any experimental evidence to support the stability and reactivity of alternate key intermediate species, such as gold alpha-oxo carbenoid species and gold N-alkenoxypyridinium/sulfonium complexes. Herein, we address the issues surrounding the formation of gold alpha-oxo carbene species in reported literature. We report the synthesis and reactivity of gold pyridinium alpha-oxo carbenoid complexes, gold sulfonium alpha-oxo carbenoid complexes, and gold alpha,alpha-dioxo carbenoid complexes. We then report the direct observation of a gold N-alkenoxysulfonium complex in a gold-catalyzed alkynyl sulfoxide rearrangement reaction and the synthesis of a series of gold-oxide compounds. Together, this research addresses the gaps in knowledge surrounding key intermediate species in gold(I)-catalyzed transformations.

Cationic gold carbene complexes have attracted significant attention, being postulated as intermediates in a range of gold-catalyzed transformations. Regardless of the remarkable progress in the gold (I) catalysis, our fundamental understanding on the key intermediate species and the subsequent reactivity, and mechanistic insight is deficient. This is mainly due to the lack of proper model system with sufficient reactivity, as the majority of known gold carbene complexes are heteroatom stabilized or sterically hindered, and because of a dearth of direct intermediate observations in catalytic systems. Lewis acid mediated leaving group abstraction from a neutral gold precursor provides a convenient method for the generation of rare examples of reactive gold carbene species in high yield and purity, addressing the issue with isolation of such transient species as well as allowing in situ spectroscopic analysis. Subsequent trapping experiment with nucleophiles provides kinetic information about relevant catalytic transformations, and the -ionization strategy is further extended toward generation of transient  -cationic propyl gold species for studying gold to alkene carbene transfer reaction.

Cationic gold(I) complexes containing phosphine and N-heterocyclic carbene based ligands are a powerful catalysts for the hydrofunctionalization of C-C multiple bonds with carbon and heteroatom based nucleophiles as well as the cycloisomerization of enynes and related π-systems. Mechanisms involving outer sphere, nucleophilic attack an activated gold π-complex are typically invoked for both hydrofunctionalizations and cycloisomerizations, however, direct experimental evidence for these mechanisms remain limited.

Gold(I) catalyzed allene racemization is an important background reaction in the hydrofunctionalization of 1,3 disubstituted allenes. It can compromise chirality transfer or be exploited to realized stereoconvergent synthesis of allylic alcohols and amines. The kinetics of the racemization of aromatic 1,3-disubstituted allenes catalyzed by gold(I) phosphine complexes has been investigated. The rate of racemization displayed first order dependence on allene and gold concentration. Kinetic analysis gold(I) catalyzed racemization of allenes as a function of allene and phosphine donor ability established a depletion of electron density on the terminal allene carbons and an accumulation of electron density on the phosphine ligand in the rate-limiting transition state.

Investigation of the mechanism of gold(I) catalyzed hydrofunctionalization of allenes with alcohols, carbamates, and anilines established a variable catalyst resting state depending on the equilibrium binding affinities of the nucleophile and the relative concentrations of allene and nucleophile that are employed. Reversible C-X bond formation may explain the difference in regioselectivity observed for hydroalkoxylation and hydroamination with carbamates. Additionally, in situ analysis of the hydrofunctionalization of enatiopure 1,3-disubstituted allenes for enatiopurity of the allene and product ruled out trapping of an achiral η1-intermediate and established concomitant allene racemization as the cause of loss of enatiopurity.

Finally we report the two gold(I) carbene complexes not stabilized by π-conjugated heteroatoms. First, we report the hydride abstraction from a neutral gold cycloheptatrienyl complex that was isolated and characterized in solution and by single crystal X-ray diffraction. This complex represents the first example of a gold carbenoid complex that lacks conjugated heteroatom stabilization. Second we report the synthesis of the first gold(I) vinylidene via hydride abstraction from a gold (disilyl)ethylacetylide complex to form a cationic β,β-disilacyclopentyl vinylidene complex. The C1 and C2 carbon atoms of the vinylidene complex underwent facile interconversion presumably through the gold π-disilacyclohexyne.

The ability of gold(I) complexes to function as catalysts for myriad organic transformations has led to a dramatic increase in their utilization. Among the homogeneous reactions catalyzed by gold(I), carbon-carbon and carbon-heteroatom bond forming processes are of particular interest for the fields of organic synthesis and pharmaceutical development. Discussed herein are gold(I)-catalyzed methods for the intra- and intermolecular functionalization of alkenes, alkynes, and allenes with nitrogen- and oxygen-based nucleophiles leading to new C‒X bonds (X = N, O).

Approximately 26 cationic gold π-alkene complexes, containing either IPr [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] or P(t-Bu)2o-biphenyl ancillary ligands, were isolated or generated and six complexes were analyzed by X-ray crystallography. Spectroscopy, X-ray crystallography, and alkene binding studies are in accord with a gold−(π-alkene) interaction dominated by σ-donation from the alkene to gold. Kinetic analyses of degenerate isobutylene exchange in both the IPr and phosphine systems are consistent with associative pathways for isobutylene exchange involving cationic bis(alkene) intermediates.

Reaction of a 1:1 mixture of (L)AuCl [L = P(t-Bu)₂o-biphenyl or IPr] and AgSbF₆ with internal alkynes led to isolation of the corresponding cationic, two-coordinate gold π-alkyne complexes in ≥90% yield. Equilibrium binding studies show that the binding affinities of alkynes to gold(I) are strongly affected by the electron density of the alkyne and to a lesser extent on the steric bulk of the alkyne. Treatment of a suspension of (IPr)AuCl and AgSbF₆ with terminal arylacetylenes led to the formation of thermally unstable gold π-alkyne complexes of the form [(IPr)Au(eta2-HC≡CAr)] SbF₆ in ≥86 ± 5% yield, which were characterized by spectroscopy without isolation. Warming these complexes to 0 °C led to C(sp)-H bond cleavage and formation of dinuclear gold(I) σ,π-acetylide complexes of the form {[(IPr)Au]₂(eta1,eta2-C≡CAr)} SbF₆, three of which were isolated in 99% yield and one of which was characterized by X-ray crystallography.

A family of 7 cationic gold(I) π-allene complexes were isolated and fully characterized in solution, and in three cases by X-ray crystallography. Degenerate intermolecular allene exchange kinetic studies for three of the allene complexes are in accord with a two-term rate law of the form rate = k1[complex] + k2[complex][allene] with with Gibb's free energy barriers of 17.4 - 18.8 kcal mol-1 (1) and 15.2 - 17.6 kcal mol-1 (2). Variable temperature NMR analysis of these complexes established facile intramolecular -face exchange through 1-allene intermediates or transition states with barriers of 8.9 - 10.9 kcal mol-1 for phosphine and 9.5 - 12.2 kcal mol-1 for IPr complexes.

Mechanistic investigation of gold(I)-catalyzed intramolecular allene hydroalkoxylation established a mechanism involving rapid and reversible C-O bond formation followed by turnover-limiting protodeauration from a mono(gold) vinyl complex. This on-cycle pathway competes with catalyst aggregation and formation of an off-cycle bis(gold) vinyl complex.