Computational Insights Into The Mechanisms And The Origins Of Selectivities Of Pd(0) And Au(?)catalyzed Hydroamination Of Alkynes

Recently,transition metal catalyzed hydroamination of alkynes has obtained great attention.Comparing to other traditional protocols in constructing the carbon-nitrogen bonds,hydroamination offers several advantages in terms of its easy accessibility of raw material,atom-economy,simple operation,mild reaction conditions and efficiency.Large amounts of nitrogen compounds could be synthesized via hydroamination,which has important scientific significance and application value.However,the mechanism of hydroamination remains unclear.In addition,the incentives for generation of chemo-,regio-and enantio-selevtivities in hydroamination is still worth of further research.DFT studies were carried out to investigate the detailed mechanism of Pd(0)and benzoic acidco-catalyzed hydroamination of internal alkynes.In the presence of the benzoic acid,the formation of a hydridopalladiumintermediate via the oxidative addition(OA)of the O-H bond of benzoic acid into the Pd(0)complex center might not be afavorable reaction pathway to start the reaction.Instead,after the coordination of benzoic acid with the Pd(0)-alkyne complex,a proton transfer process from the acid to carbon of alkyne is found to be a favorable pathway,leading to thealkenyl(PhCOO)Pd(II)intermediate.Next,the resulting alkenyl(PhCOO)Pd(II)species would produce phenylalleneintermediate via a ?-H elimination step assisted by the formed benzoate anion.Subsequently,the benzoic acid might undergo asecond proton step to the phenylallene intermediate to produce the ?-allylpalladium species.Finally,the amine substrate couldundergo a nucleophilic addition to the terminal carbon of the ?-allylpalladium species to produce the hydroamination product.Gold(?)-catalyzed intramolecular cycloisomerizations of propargylic ureas can yield ultimate chemo-and regioselective heterocycle products.When the substituent on the group of ureas is phenyl,N-cyclization took place via 6-endo-N attack to generate imidazol-2-ones.When the substituent is p-toluene sulfonyl,O-cyclization via 5-exo-O attack is rationalized to be the favorable pathway yielding oxazolidin-2-imines.With regard to the region-and chemo-selectvities generated by the reaction remaining unclear,computational calculations are applied to study the mechanismand the orgins of the selectivies.For two selective protocols,chemo-and regioselectivities attribute to three factors:(1)the substituent effect of urea contributing to nucleophilicity of nitrogen atom;(2)the substituent effect of terminal position in alkynl group contributing to the different electrophilicity of carbon atom of alkyne;(3)the formation of hydrogen-bond interaction between the anion and substrates in the system.