Exploring Electrophilic Hydrophosphination via Metal Phosphenium Intermediates

Two Mo(0) phosphenium complexes containing ancillary secondary phosphine ligands have been investigated with respect to their ability to participate in electrophilic addition at unsaturated substrates and subsequent P-H hydride transfer to "quench" the resulting carbocations. These studies provide stoichiometric "proof of concept" for a proposed new metal-catalyzed electrophilic hydrophosphination mechanism. The more strongly Lewis acidic phosphenium complex, [Mo(CO)4(PR2H)(PR2)]+ (R=Ph, Tolp), cleanly hydrophosphinates 1,1-diphenylethylene, benzophenone, and ethylene, while other substrates react rapidly to give products resulting from competing electrophilic processes. A less Lewis acidic complex, [Mo(CO)3(PR2H)2(PR2)]+, generally reacts more slowly but participates in clean hydrophosphination of a wider range of unsaturated substrates, including styrene, indene, 1-hexene, and cyclohexanone, in addition to 1,1-diphenylethylene, benzophenone, and ethylene. Mechanistic studies are described, including stoichiometric control reactions and computational and kinetic analyses, which probe whether the observed P-H addition actually does occur by the proposed electrophilic mechanism, and whether hydridic P-H transfer in this system is intra- or intermolecular. Preliminary reactivity studies indicate challenges that must be addressed to exploit these promising results in catalysis. Umpolung in P-H activation: Metal-mediated hydrophosphination that relies on electrophilic addition at a phosphenium ligand has been demonstrated for unsaturated substrates including the decidedly unactivated ethylene. This outer-sphere process demonstrates that metal-coordinated secondary phosphines can act as hydride donors to reactive carbocations. The results highlight the tunable Lewis acidity of metal phosphenium complexes.+ image