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Pd-and Rh-catalyzed Oxidative Coupling Of Activated C-H Bonds And Unsaturated Moleculars

Posted on:2012-07-06Degree:MasterType:Thesis
Country:ChinaCandidate:J L ChenFull Text:PDF
GTID:2131330332999449Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Transition metal-catalyzed direct functionalization of C-H bonds via C-H activation pathway represents an important and atom-economic strategy to construct complex structures. Activation of C-H bond, particularly under chelation-assistance, followed by oxidative coupling of unsaturated molecules has attracted considerable attention because condensed (hetero)cycles are generated and these structural motifs are widely present in natural products and pharmaceuticals.Rhodium catalysts are well known for C-C coupling reactions that proceed via a C-H activation pathway owing to a wide range of synthetic utility. And, palladium catalysts are notable as well for its outstanding capacity to mediate oxidative C-H activation via C-H activation pathway with high functional group compatibility and low cost. At the first proportion, in chapter 1, a review of oxidative coupling reaction has shown to introduce the background of synthesis chemistry about oxidative coupling between arenes and alkenes or alkynes. At the end of this part, an protocol of my own experiments has made including significance, evidence, feasibility, and so on to grasp the whole article.Over the past several years simple rhodium(III) complexes, particularly [RhCp*Cl2]2, have stood out as a highly efficient catalyst to mediate oxidative coupling between arenes and alkynes or alkenes. Therefore, rhodium(III) complexes were chosed as initial catalysts to fulfill the desired prospects. The author reasoned that N-aryl-2-aminopyridines are suitable substrates for C-H activation in the N-aryl ring with the pyridyl being a directing group. Furthermore, the proximal NH functional group may act as a nucleophile to undergo further transformations. The optimized conditions about coupling with alkynes then worked out after an array of complicated screening (illustrate in chapter 2). After that, plenty of substrates were ready to demonstrate the vast applicability of this catalytic system. Although high yield and great efficiency had given out of this work, an issue that the unsymmetrically substituted alkynes such as PhC≡CMe or PhC≡CnPr are not fit for this condition. In addition, the high cost of the rhodium catalyst makes it less competitive.To better define the scope of N-aryl-2-aminopyridine in oxidative coupling, the author further applied acylates as the coupling partners for oxidative Heck-type reactions. The coupled product was identified as an interesting quinolone on the basis of IR, NMR spectroscopy, and X-ray crystallography. Substrates with both electron-rich and electron–poor N-aryl groups reacted in high isolated yield although relatively lower yields were obtained for those bearing electron-withdrawing groups. A plausible mechanism for the novel oxidative coupling between N-aryl-2-aminopyridine and acrylate is suggested in this section. Cyclometallation gives a six-membered Rh(III) intermediate with a loss of an acid, which can undergo insertion of an incoming acylate. Subsequentβ-hydride elimination takes place to afford a thermodynamically stable trans-olefin, which can be in equilibrium with the cis one. Attack of the NH on the carbonyl group of the cis intermediate generates the final product, quinolone.To solve the problems left behind by former works, a further study had continued. Pd(MeCN)2Cl2 could act as an efficient catalysts for oxidative coupling with alkynes but not fit for coupling with alkenes. The wonderful job had submitted to supplement the Rh(III) catalytic system. It is an inexpensive and efficient method to synthesis derivative indoles by using Pd(MeCN)2Cl2 catalysts.
Keywords/Search Tags:C-H activation, oxidative coupling, palladium, rhodium, indoles, quinolones
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