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Synthesis and Reactivity of Xantphos Complexes of Nickel for the Nickel-Catalyzed Azide–Alkyne Cycloaddition

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Title
Synthesis and Reactivity of Xantphos Complexes of Nickel for the Nickel-Catalyzed Azide–Alkyne Cycloaddition
Author
Kim, Jaegwan
Advisor
Rohde, Jan-Uwe
Issue Date
2020-02
Publisher
Graduate School of UNIST
Abstract
The transition-metal-catalyzed cycloaddition of organoazides and alkynes is an attractive method for the synthesis of 1,2,3-triazoles, because it offers high atom economy and high yields. The recently discovered Ni-catalyzed azide–alkyne cycloaddition (NiAAC), which employs [NiCp2] and 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos), allows the synthesis of 1,5-disubstituted 1,2,3-triazoles with high regioselectivity as well as the synthesis of 1,4,5-trisubstituted 1,2,3-triazoles. In comparison with other methods, this reaction can be performed at ambient temperature and not only in organic solvents and under inert conditions but also in water and air. In this work, xantphos complexes of Ni were synthesized and their reactivity was investigated to gain insights into the identity and formation of the catalytically active species as well as the possible mechanism of this reaction. The air-sensitive alkyne complexes [Ni(xantphos)(PhCCR)] (where R = H or Ph) were synthesized by reaction of [Ni(cod)2] (where cod = 1,5-cyclooctadiene) with xantphos and the respective alkyne. The complexes were characterized by 1H, 13C and 31P NMR spectroscopy and elemental analysis. The complex [Ni(xantphos)Cl2] was synthesized by reaction of NiCl2·6H2O with xantphos and characterized by ESI mass spectrometry and elemental analysis. The catalytic activity of these complexes was explored in reactions of benzylazide with the terminal alkyne phenylethyne and the internal alkyne diphenylethyne in toluene under inert conditions. In case of the terminal alkyne, the Ni0 complexes [Ni(xantphos)2] and [Ni(xantphos)(PhCCR)] produced isolated yields of over 85 % of 1,5-disubstituted 1,2,3-triazole and ≤6 % of 1,4-disubstituted 1,2,3-triazole, similar to yields obtained with the [NiCp2]/xantphos system. When the internal alkyne was used, yields of over 85 % of 1,4,5-trisubstituted 1,2,3-triazole were obtained. In contrast, [Ni(cod)2] and [Ni(xantphos)Cl2] gave at best poor yields (<20 %) but in most cases only trace amounts of the 1,2,3-triazoles. The catalytic activity of [Ni(xantphos)2] was preserved in air with either toluene or H2O as the solvent in the presence of the additive Cs2CO3, which is also required in case of the [NiCp2]/xantphos system. Furthermore, the catalytic reactions of [Ni(xantphos)2] and [Ni(xantphos)(PhCCH)] were investigated by NMR spectroscopy. Both complexes exhibit high catalytic activity even with a lower catalyst loading of only 1 mol %. These studies showed that [Ni(xantphos)(PhCCH)] is present during the catalytic reaction in both cases. Reactions of these complexes with individual substrates revealed that [Ni(xantphos)2] reacts with phenylethyne to afford [Ni(xantphos)(PhCCH)] under release of one of the two xantphos ligands and that the alkyne complex rapidly reacts with benzylazide to form 1,5-disubstituted 1,2,3-triazole. Moreover, [Ni(xantphos)2] also reacts with benzylazide under release of xantphos, but this reaction is slower than that with phenylethyne. A systematic study of reactions of [NiCp2] with xantphos, phenylethyne and benzylazide in varying stoichiometric ratios indicated the formation of [Ni(xantphos)2] and [Ni(xantphos)(PhCCH)]. These results suggest that the alkyne complex may be formed in the [NiCp2]/xantphos system and may act as the catalytically active species.
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Department of Chemistry
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