title: Mechanism of the N-protecting group dependent annulations of 3-aryloxy alkynyl indoles under gold catalysis: a computational study.
abstract: The mechanism of the gold-catalyzed annulations of 3-aryloxy alkynyl indoles developed by Tu et al. was studied by DFT calculations. It was found that both indole derivatives of electron-donating and electron-withdrawing protective groups would first undergo the 5-exo-dig cyclization simultaneously upon activation by cationic [PR(3)Au(+)] species. However, divergent reactivity of the resulting spirocyclic intermediate in competitive 1,2-alkenyl migration and nucleophilic water addition reactions towards C3 was predicted. When protected by electron-donating group, the 1,2-alkenyl migration occurs to generate a tricyclic intermediate, from which an aromatic Claisen rearrangement/nucleophilic addition sequence results in the observed 1,2-phenoxy migration. In case of electron-withdrawing group, the 1,2-alkenyl migration would be unfavorable. Instead, the nucleophilic addition of water oxygen to C3 is more facile, and leads to the hemiketal intermediate. The possible roles of water-cluster and OTf anion as proton shuttles in both reactions were also evaluated.
authors.affiliation: College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, PR China.
authors.forename: Bing
authors.initials: B
authors.lastname: Cheng
authors.forename: Genping
authors.initials: G
authors.lastname: Huang
authors.forename: Liang
authors.initials: L
authors.lastname: Xu
authors.forename: Yuanzhi
authors.initials: Y
authors.lastname: Xia
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D002384:Catalysis; D006046:Gold / Q000737:chemistry*; D007211:Indoles / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure
chemical_list: D000480:Alkynes; D007211:Indoles; D006046:Gold