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Abstract

The development of new phosphines and their application in catalysis has been a major research topic during the last decades. Despite the structural variety of the known and available phosphines, nowadays, most of these ligands can be classified as moderate π-acceptors at best. As long as such electronic properties are sufficient for the derived applications, more than enough examples depicting different steric parameters are available.
However, some catalytic reactions require the presence of ancillary ligands with very strong π-acceptor character. In these cases only few commercially available neutral P-centered ligands exist (PF₃, P(CF₃)₃ and PCl₃). Unforunately, these compounds are usually both toxic and moisture sensitive, which represents a strong limitation for their application in catalysis.
Therefore, the design of new π‐acceptor ligands is still a challenge. In our approach two cyclopropenium substituents were attached to the phosphorus atom of the phosphine in order to introduce enhanced π-acceptor character. These biscyclopropenium phosphines, unlike their polyhalogenated analogues, were air stable. While the variation of the third (non-cationic) substituent attached to the phosphorus of the dicationic phosphine allowed further modification of its electronic properties, the decoration of one cyclopropenium unit with longer aliphatic chains facilitated the solution of the charged species in non-polar solvents like toluene. Furthermore, the BF₄^- counter-ion of some of the prepared phosphines was exchanged for SbF₆^- in order to investigate its influence on the reactivity in catalysis later on.
Subsequent coordination of these dicationic phosphines to various transition metals afforded the corresponding Au(I), Pt(II), Ir(I) and Ag(I) complexes.
The reactivity of the prepared dicationic Au(I)-precatalysts was investigated in the 6-endo-dig cycolisomerization of biphenyl-alkynes with two substituents in internal positions to their corresponding phenanthrene derivatives. After successful optimization, this methodology was applied in the synthesis Calanquinone C.
Finally, the substrate scope of the utilized Au(I)-precatalysts was expanded from the synthesis of phenanthrene derivatives to the preparation of naphto-furans.