Phosphabenzenes as monodentate pi-acceptor ligands for rhodium-catalyzed hydroformylation

A new class of phosphinine/rhodium catalysts for the hydroformylation of te rminal and internal alkenes is presented in this study. A series of phospha benzenes 1-14 has been prepared by condensation of phosphane or tris(trimet hylsilyl)phosphane with the corresponding pyrylium salt. Trans-[(phosphaben zene)(2)RhCl(CO)] complexes 21-25 have been prepared and studied spectrosco pically and by X-ray crystal-structure analysis. The hydroformylation of oc t-l-ene has been used to identify optimal catalyst preformation and reactio n conditions. Hydroformylation studies with 15 monophosphabenzenes have bee n performed. The catalytic performance is dominated by steric influences, w ith the phosphabenzene 8/ rhodium system being the most active catalyst. Tu rnover frequencies of up to 45370 h(-1) for the hydroformylation of oct-l-e ne have been determined. Tn further studies, hydroformylation activity towa rd more highly substituted alkenes was investigated and compared with the s tandard industrial triphenylphosphane/rhodium catalyst. The reactivity diff erences between the phosphabenzene and the triphenylphosphane catalyst incr ease on going to the more highly substituted alkenes. Even tetra-substitute d alkenes reacted with the phosphabenzene catalyst, whereas the triphenylph osphane system failed to give any product In situ pressure NMR experiments have been performed to identify the resting state of the catalyst. A monoph osphabenzene complex [(phosphinine 8)Ir(CO)(3)H] could be detected as the p redominant catalyst resting state.