Insights into Alkene Activation by Gold: Nucleophile Activation with Base as a Trigger for Generation of Lewis Acidic Gold

• Published in: ACS catalysis 2016-11, Vol.6 (11), p.7357-7362
• Main Authors: Zhu, Yuyang, Zhou, Wentong, Petryna, Ellen M, Rogers, Brock R, Day, Cynthia S, Jones, Amanda C
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.6b01674

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 Brønsted 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 accounted for by alkyl gold protodeauration, and the alkyl gold does not react significantly on the catalyzed time scales observed. Preformed gold triethylamine complexes are not reactive toward alkenyl alcohols; however, when alkenyl urea was treated with gold triethylamine complex [Ph3PAu­(NEt3)]­SbF6, a similar case was observed, in which alkyl gold formation triggered the initiation of a pathway to form D. These results show that Brønsted basic conditions highly favor the formation of alkyl gold intermediates by a process that incorporates both nucleophile and π-activation. Additional experiments support the conclusion that the alkyl gold intermediates are not productive structures en route to pyrrolidine D. Work by Nolan et al. suggests that C is not simply a Brønsted Acid, but rather a source of cationic gold and gold hydroxide (L-Au-OH). Alkene gold activation modes that do not involve protodeauration of discrete alkyl gold intermediates must be considered more carefully in future studies.