Phosphabenzenes as Monodentate π-Acceptor Ligands for Rhodium-Catalyzed Hydroformylation

Abstract

A new class of phosphinine/rhodium catalysts for the hydroformylation of terminal and internal alkenes is presented in this study. A series of phosphabenzenes 1-14 has been prepared by condensation of phosphane or tris(trimethylsilyl)phosphane with the corresponding pyrylium salt. Trans-[(phosphabenzene)2RhCl(CO)] complexes 21-25 have been prepared and studied spectroscopically and by X-ray crystal-structure analysis. The hydroformylation of oct-1-ene has been used to identify optimal catalyst preformation and reaction conditions. Hydroformylation studies with 15 monophosphabenzenes have been performed. The catalytic performance is dominated by steric influences, with the phosphabenzene 8/rhodium system being the most active catalyst. Turnover frequencies of up to 45370 h(-1) for the hydroformylation of oct-1-ene have been determined. In further studies, hydroformylation activity toward more highly substituted alkenes was investigated and compared with the standard industrial triphenylphosphane/rhodium catalyst. The reactivity differences between the phosphabenzene and the triphenylphosphane catalyst increase on going to the more highly substituted alkenes. Even tetrasubstituted alkenes reacted with the phosphabenzene catalyst, whereas the triphenylphosphane system failed to give any product. In situ pressure NMR experiments have been performed to identify the resting state of the catalyst. A monophosphabenzene complex [(phosphinine 8)Ir(CO)3H] could be detected as the predominant catalyst resting state.

A theoretical study of the activity in Rh-catalysed hydroformylation: the origin of the enhanced activity of the π-acceptor phosphinine ligand

“…The overall energy barriers reproduce the experimental observations, 13,19 providing a clear picture about the activity differences. The phosphinine system shows a lower value (19.0 kcal.mol -1 ) than the monophosphane complex (22.4 kcal.mol -1 ), while the bis(phosphane)-based complex shows a higher value 10 (25.9 kcal.mol -1 ); see Table 1.…”

“…For a series of phosphinine ligands, the variation in catalytic performance was attributed to the steric properties of the ligand. 13 According to the authors, the bulky phosphinine ligands favour the formation of a monoligand 10 rhodium species, which should have a larger accessible space compared with diligand rhodium species. Similarly, in industrial conditions excess of phosphane ligand is used because the selectivity towards linear aldehyde is improved; however the activity is reduced because the phosphane dissociation 15 equilibrium shifts from the monophosphane [HRh(CO) 3 (PR 3 )] towards the less active HRh(CO)(PPh 3 ) 2 species.…”

“…17 Here, we focus on the study of phosphinine-modified rhodium catalyst [HRh(CO) 3 [HRh(CO) 2 (PR 3 ) 2 ] (Scheme 2). The Rh-phosphinine system showed a much higher activity than classical PPh 3 -based catalysts 13,19 and followed an analogous kinetics. 13 Our aim is to understand at molecular level the factors governing the activity, evaluating them and their interplay.…”

“…The Rh-phosphinine system showed a much higher activity than classical PPh 3 -based catalysts 13,19 and followed an analogous kinetics. 13 Our aim is to understand at molecular level the factors governing the activity, evaluating them and their interplay. In addition, we investigate 35 whether the findings for monophosphane ligands can be extrapolated to diphosphane, re-examining a previous QSAR model 17 by using the easy-to-interpret TMACC descriptors.…”

“…Previous high pressure NMR studies on the analogous and more stable iridium(I) system had indicated that only one phosphinine ligand is coordinated to the transition metal center 60 under hydroformylation conditions. 13 These experiments also suggested an equatorial position for the phosphinine, presumably as two rotamers. The DFT calculated relative energies support the experimental proposal.…”