Studies On Palladium(â…¡)-Catalyzed Tandem Reactions

Based on the palladium(II) catalyzed reactions explored in our group, the thesis is focused on the development of several palladium(II)-catalyzed tandem reactions to synthesize indole and coumarin derivatives expediently, both of the two heterocyclic compounds are useful chemicals and medical intermediates.The thesis mainly contains three parts:(1) Pd(OAc)₂-catalyzed acyloxylation/cyclization of 2-(3'-alkenyl)indolesIn the presence of benzoquinone (oxidant), a new and mild Pd(OAc)₂-catalyzed reaction for the intramolecular acyloxylation/cyclization of 2-(3'-alkenyl)indoles is developed. Acyloxylpalladation of C=C double bond and C-H activation of indoles are the two key steps in our reactions. The newly formed cycles contain some oxygen-containing functionalized groups, which might be transferred further to other indole derivatives. Moreover, benzoquinone was found to be crucial for the success of this reaction. Besides its role as an oxidant and a ligand, it can also transfer the reaction pathway from the carbopalladation to acyloxypalladation.(2) Cationic palladium complexes catalyzed tandem cyclic reactions of arylboronic acids and 2-formylphenyl but-2-ynoatesWe develope a new cationic palladium complexes catalyzed tandem reaction involving cis-cabopalladation of carbon-carbon triple bond and 1,2-addition to aldehydes initiated by transmetalation of arylboronic acids. Two kinds of coumarins deravitives can be obtainded selectively by utilizing catalysts with different counteranions under very mild conditions, one is substituted coumarins and the other is dihydrocoumarins. The asymmetric version of the reaction using (S,S)-BDPP as the ligand to yield 3-alkylidenedihydrocoumarins was successful with high enantioselectivity. It is worth noting that counteranions in the catalysts have great influence to the reactivity and chemoselectivity. (3) Cationic palladium complexes catalyzed tandem reaction of 2-alkynylanilines and aldehydesWe disclose a novel cationic palladium complexes catalyzed tandem reaction of 2-alkynylanilines and aldehydes, which is the first example of a tandem reaction initiated by an aminopalladation of an alkyne and an addition to the carbonyl group to quench the carbon-palladium bond. This method offords a very easy way to synthesize multiple substituted 3-hydroxyalkyl indoles only by one step. In addition, this strategy may find further application in the future for rapidly constructing other useful heterocyclic compounds. The main disadvange for this reaction is that the substituent on the nitrgen in indoles must be a strong electron-withdrawing group (tosyl), and only activated aldehyde can be used in this reaction.