Effects of Metal Coordination and Transition Metal Properties on Electrophilic Substitution Reactions of Phosphenium Ions

Abstract

An investigation into the electrophilicity of metal free and metal coordinated phosphenium ions for P-C bond formation, particularly phosphine synthesis, has been carried out. Chloride abstraction from metal-free phosphines led to phosphoranylphosphonium ions, which underwent electrophilic addition reactions but were limited by adduct formation with the formed phosphines. Phosphenium ions complexes of chromium, molybdenum, iron, and platinum were investigated and compared to those of tungsten. Group VI bound chlorophosphines [Mo(CO)5(PPh2Cl)] and [Cr(CO)5(PPh2Cl)] were converted into phosphine triflates, via chloride abstraction using AgOSO2CF3, and underwent electrophilic aromatic substitution reactions with pyrrole and indole, but their applicability is limited by difficult isolation of product and resulting low yields. The compounds cis-[Mo(CO)4(PPh2Cl)2] and cis-[Cr(CO)4(PPh2Cl)2] were converted to phosphenium ion complexes via chloride abstraction using AlCl3, but were not reactive toward electrophilic aromatic substitution reactions. Iron bound chlorophosphines [Fe(CO)4(PPh2Cl)] and [Fe(CO)4(PPhCl2)] were used to generate reactive phosphenium ions in situ. These compounds have been shown to undergo electrophilic aromatic substitution reactions with ferrocene, N,N-diethylaniline, pyrrole and indole. Compared to the tungsten analogs, the iron bound chlorophosphines are inexpensive precursors for the functionalization of phosphines, but are useful with a smaller range of substrates than the W complexes. Platinum complexes of the type [PtX2(PPh2Cl)2] (X = Cl, CH3, C6H5) were investigated as phosphenium ion complex precursor via reaction with AlCl3. For X = Cl, chloride abstraction from Pt is preferred. For X = CH3 and C6H5, chloride abstraction from P leads to methyl or phenyl group migration from Pt to P. A library of transition metal catalysts were screened for potential catalytic activity in the electrophilic aromatic substitution method of P-C bond formation, but no catalytic activity was observed.