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Ruthenium-catalyzed azide-alkyne cycloaddition, and cyclometallation of 2-vinylpyridine with MCl{208}(PPh{209}){209} and MHCl(PPh{209}){209} (M=Ru, Os)

Authors Zhang, Li
Issue Date 2008
Summary Substantial progress in ruthenium-catalyzed cycloadditions of organic azides and alkynes are reported. A series of ruthenium(II) complexes have been evaluated as the catalytic precursors for cycloaddition reactions of azides with alkynes. The catalytic activity of the ruthenium complexes and regioselectivity of the catalytic reactions were found to be dependent on the ligand environment around ruthenium. The Cp*RuCl complexes such as Cp*RuCl(PPh3)2, Cp*RuCl(COD) and Cp*RuCl(NBD) are effective at promoting the [3+2] cycloaddition. Under the influence of Cp*RuCl(PPh3)2 and Cp*RuCl(COD), alkylazides readily react with terminal alkynes to give selectively 1,5-disubstituted triazoles. The catalytic reactions proceed easily at room temperature. The present Cp*RuCl system can also catalyze the reactions of alkylazides with internal alkynes to provide 1,4,5-trisubstituted triazoles. However, the catalytic activity could be deactivated due to the formation of catalytically inactive tetrazene complexes. DFT calculations suggest that the Cp*Ru(II)-catalyzed cycloaddition reactions of azides with alkynes involves oxidative coupling of azides and alkynes to give metallacycles, followed by a rate-determining reductive elimination. The proposed mechanism is approved by the isolation of the complexes Cp*RuCl{η3-NH(R2)NNCH=C(OR)(R1)}(R = Me or Et; R1 = Bu, Ph or –C10H7; R2 = -C6H4Me or -CH2Ph). In the second part of this thesis, we studied the cyclometallation of 2-vinylpyridine with MCl2(PPh3)3 and MHCl(PPh3)3 (M = Ru, Os). Treatment of RuCl2(PPh3)3 in benzene at room temperature with 2-vinylpyridine produces the vinylpyridine complex RuCl2(2-CH2=CHC5H3N)(PPh3)2. In the presence of Cs2CO3, RuCl2(PPh3)3 reacts with 2-vinylpyridine at room temperature to give the cyclometallated complex RuCl(CH=CHC5H4N)(CH2=CHC5H4N)(PPh3), which is also produced from the reaction of RuHCl(PPh3)3 with 2-vinylpyridine. OsCl2(PPh3)3 reacts with 2-vinylpyridine/Cs2CO3 at room temperature to give the analogous cyclometallated complex OsCl(CH=CHC5H4N)(CH2=CHC5H4N)(PPh3). In the presence of NaBF4, the reaction produces [Os(CH=CHC5H4N)(CH2=CHC5H4N)(PPh3)2]BF4. The dihydrogen complex Os(H2)Cl(CH=CHC5H4N)(PPh3)2 is produced from the reaction of OsHCl(PPh3)3 with 2-vinylpyridine.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2008
Language English
Format Thesis
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