Abstract
Extensive studies of asymmetric intermolecular Heck reaction are described and provide a deeper insight into histidine-catalyzed. In particular, aspects of enantio- as well as diastereoselectivity of these reactions are discussed. As the first report, we synthesized five histidine-based organocatalyts including histidine as a well-defined and biodegradable natural structure alone and in combination with 2,4,6-trichloro-1,3,5-triazine, benzene-1,3-diamine, dimethyl malonate and dimeric structure applied in the asymmetric intermolecular Heck reaction of aryl halides; their efficiency was compared to each other’s. These phosphine-free palladium catalysts were found as efficient catalytic system which provided the superior efficiency with excellent yields, regioselectivity and enantioselectivity. In these among, histidine with 2,4,6-trichloro-1,3,5-triazine which generate the star like molecule palladium catalyst gave the best activity in asymmetric intermolecular Heck reaction with excellent yields and good regio- and enantioselectivity under mild reaction conditions. The asymmetric intermolecular Heck reaction has been limited to aryl and vinyl triflates or aryl iodide in the rare reports of available Pd catalysts. Herein, we extend the reaction to aryl bromides. In addition, the scope of reaction was examined in two different techniques: conventional heating and microwave irradiation and compared. For the first time, microwave irradiation sintering is successfully used for this reaction. Comparison of catalytic activities of our catalyst (Pd/His) with literature examples confirmed our success.
Highlights
In recent years, asymmetric Heck reaction of aryl halides or aryl triflates with 2, 3-dihydrofuran have received considerable attention due to usages of some products in natural synthesis [1,2]
Amino acids are well-known as safe and green ligands for preparing asymmetric catalysts [7,8,9,10], such as proline/cyclobutane which used in asymmetric aldol additions, amino acid-containing peptide organocatalysts applying in asymmetric Michael addition [11], and conjugate addition reactions of aldehydes to maleimide [12,13,14,15,16]
Some developments have been made in palladium catalyzed regioselective intermolecular arylation of dihydrofuran (DHF). [19,20,21,22] the most of these conventional methods were limited to using phosphine ligands. [23,24,25,26,27,28,29,30,31,32,33] As we know, a few reports are available in P-free conditions for this type of reactions that suffer from low selectivity
Summary
Asymmetric Heck reaction of aryl halides or aryl triflates with 2, 3-dihydrofuran have received considerable attention due to usages of some products in natural synthesis [1,2]. Numerous chiral catalytic systems for asymmetric Heck reaction of 2, 3-dihydrofuran with phenyl triflate were examined [3,4,5]; but, application of aryl halides, as more available substrates, in this coupling reaction is rare [6]. The palladium catalyzed bond formation reaction between aryl halides and olefins bearing electron-withdrawing substituents –COOR, –CN) was studied extensively [17, 18], the Heck coupling involving an olefin bearing a hydrogen atom in the allylic position has proved to be a more challenging substrate. Some developments have been made in palladium catalyzed regioselective intermolecular arylation of dihydrofuran (DHF). [19,20,21,22] the most of these conventional methods were limited to using phosphine ligands. [23,24,25,26,27,28,29,30,31,32,33] As we know, a few reports are available in P-free conditions for this type of reactions that suffer from low selectivity
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