Silylium ion Migration Dominated Hydroamidation of Siloxy-alkynes

ABSTRACT: Intermolecular hydroamidation of internal alkynes has been a long-standing challenge, one of the most successful examples so far is silver-catalyzed hydroamidation of siloxy-alkynes for which meet the standards of atom economic and mild reaction conditions of ideal hydroamidation. However, the mechanism of this reaction remains controversy. Using density function theory (DFT), we revealed that the reaction takes place through a silylium ion migration mediated hydroamidation (SMH) pathway. The SMH pathway goes through two steps, the first step is Ag+ promoted proton and silylium ion exchange between siloxy-alkynes and amide, leading to ketene and silyl-imines, the second step is Ag+ catalyzed nucleophilic addition between ketene and silyl-imines, following with a silylium ion migration afford the final product. In this reaction, Ag+ activate the siloxy-alkyne into silylium ion and silver-ketene through p-π conjugate effect, the silylium ion then act as catalyst. According to our calculation, the scopes of alkynes in this reaction may be extend to silyl-substituted ynamines or silyl-substituted ynamides, and the scopes of amide maybe expanded into p-π conjugate system such as diazoles, diazepines and so on. Our calculations also reveal a concise way to construct substituted enamides through Ag+ catalyzed nucleophilic addition between ketenes (or ketene imines) and silyl-substituted p-π conjugate system such as silyl-imines, silyl-diazoles.