Abstract
Previously, the generation of alkyl gold intermediates (B) from nucleophilic addition to inactivated alkenes was limited by the use of trigold oxo complexes (A) with arylphosphine ligands, [(Ar3PAu)3O]BF4, in the presence of amine base. In this mechanism study, we have found that the basicity of the gold complex is key to favoring alkyl gold complex formation. Kinetic and substrate studies have shown that the strongly Bronsted basic IPrAuOH also mediates alkyl gold complex formation. The observation of an intermediate gold amide complex suggests these processes are initiated by deprotonation of the nucleophile. Trigold oxo reactions in the absence of base reveal that the byproduct from the reaction of substrate with A is digold hydroxide complex, [(LAu)2OH]BF4 (C). This complex is catalytically active for urea hydroamination at room temperature to form pyrrolidine D; however, it does not catalyze the hydroalkoxylation of the alcohol substrate. Importantly, catalysis to D occurs faster than can be accounte...
Highlights
The activation of π-bonds is ubiquitous in cationic gold(I)catalyzed reactions
Alkyl Gold Control Experiments Reported by Toste et al.[4] and Typically Proposed Catalytic Cycle for Gold(I) Alkene Hydroamination
Observations on Alkyl Gold Complex Formation of 4 (0.04 M in CDCl3 with 0.33 equiv [(PPh3Au)3O]BF4 and 2 equiv Et3N) to generate alkyl gold complex 5a worked with moderate success at an initial rate similar to that observed with urea 1
Summary
The activation of π-bonds is ubiquitous in cationic gold(I)catalyzed reactions. In a landmark 2008 report, Hammond and et al isolated a vinyl gold intermediate from an intramolecular allenoate cyclization, providing the first proof of the metal’s activating ability.[1]. As many early alkene addition reactions were already suggested to be catalyzed solely by Brønsted acid,[5] the lack of clear evidence for protodeauration has led to alternative proposals for activation, including some that require gold cooperation. Toste and coworkers reported a Brønsted acid-catalyzed diene hydroamination, whereby they proposed that the operative catalyst was alcohol acidified by coordination to gold.[6] a number of mechanistic facets of gold-based alkene activation remain to be clarified, and the general gold-catalyzed mechanism shown in Scheme 1, where protodeauration of a Au−C(sp3) bond follows nucleophilic addition to a gold coordinated alkene, continues to hold sway in the literature. In light of the importance of alkene activation in organic synthesis and in particular, the continuing development of asymmetric methods for gold-catalyzed hydroamination,[7] we sought to further explore the details of this process
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