Improving the Catalytic Activity in the Rhodium-Mediated Hydroformylation of Styrene by a Bis(N-heterocyclic silylene) Ligand.

For the first time, a significant boost in catalytic activity in the rhodium-catalysed hydroformylation of an alkene by using a bidentate bis(N-heterocyclic silylene) ligand is reported. This is shown by the hydroformylation of styrene at 30 bar CO/H₂ pressure in the presence of [HRh(CO)(PPh₃)₃] with an excess of the ferrocenediyl-based bis-NHSi ligand 4, [({η⁵-C₅H₄{PhC(NtBu)₂}Si})²Fe], which results in superior catalytic activity, compared with the bidentate diphosphines DPPF (3a) and xantphos (3b). In contrast, the hydroformylation of styrene in the presence of [HRh(CO)(PPh₃)₃] with excesses of the monodentate NHSi ligands [{PhC(NtBu)₂}SiNMe₂] (1) and [{C₂H₂(NtBu)₂}Si:] (2) at 30 bar CO/H₂ pressure revealed considerably slower conversion to the aldehyde products than [HRh(CO)(PPh₃)₃], with or without an excess of PPh₃, showing catalyst deactivation. Surprisingly, the germanium analogue of 4 is shown to be virtually catalytically inactive. The superior activity of 4, compared with the xantphos-containing benchmark system, is rationalized on the basis of solution NMR spectroscopic studies, and the comparative catalyst cycles are elucidated using density functional theory (DFT) methods. The latter quantum-chemical studies explain very well the favourable energy pathway for the hydroformylation of styrene using 4 versus xantphos. [ABSTRACT FROM AUTHOR]