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

Abstract

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.