Pd/HOAc-catalyzed Additions Of Alkynes

The study of alkyne compounds is very important in organic chemistry becausealkynes are fundamental building blocks for organic synthesis. Through metal catalysis,alkynes can undergo electrophilic additions as well as nucleophilic addition reactions. As aresult, alkynes can participate in a large number of different reactions and they are used inmany areas. By studying the palladium/acetic catalyzed addition of alkyne compounds, wesuccessfully developed a new way of synthesizing enamides via palladium catalyzedhydroarylations of ynamides with arylboronic acids and allylations of α-nitroacetates withpropynes with high stereoselectivities.Firstly, by studying the hydroarylations of ynamides with phenylboronic acids, wefound the optimized reaction conditions after examining various solvents, acid additives,palladium catalysts and the amount of phenylboronic acids. With the optimized condition a,the substrate scope have been examined and we found that target products is a mixture ofcis/trans isomer with poor stereoselectivity.Secondly, by studying the stereoselectivity of hydroarylations of ynamides withphenylboronic acids, we found the optimized reaction conditions b, which is listed asfollowing: Ynamides (1equiv) and arylboronic acid (3equiv), palladium acetate (20mol%),tri(cyclohexyl)phosphine (40mol%), ethanol (3mL), glacial acetic acid (0.075mmol),stirred at room temperature for10mins, then heated at80oC for12h. The substrate scopehave also been examined and we found that cis addition Z-products can be achieved in highstereoselectivity and regioselectivity, but with lower yield. Low yields were obtained whenother substrates such as1-(phenylethynyl)pyrrolidin-2-one and (4-nitrophenyl)boronic wereinvolved in the reaction.Thirdly, by studying the palladium catalyzed allylations of α-nitroacetates withpropynes, we found the optimized reaction conditions based on previous reports. With theoptimized protocol in hand, we next set out to explore the scope and limitation of thereaction. The reaction worked satisfactorily for various substituted1-phenyl-1-propynesfrom electron-donating groups to electron-withdrawing groups on the benzene ring andfound that trans addition E-products can be achieved in high stereoselectivity with goodyields. In addition, the substituent on the α-carbon of2-nitroacetates can also be replacedby different groups such as methyl, n-butyl and benzyl group, affording the desiredallylated products in good yields. Fourthly, based on literature and results obtained in the laboratory, possiblemechanisms were proposed to explain the two reactions.