Origin of the Bite Angle Effect on Rhodium Diphosphine Catalyzed Hydroformylation

Abstract

The bite angle effect on the rhodium diphosphine catalyzed hydroformylation was investigated in detail. A series of xantphos-type ligands with natural bite angles ranging from 102°to 121°was synthesized, and the effect of the natural bite angle on coordination chemistry and catalytic performance was studied. X-ray crystal structure determinations of the complexes (nixantphos)Rh(CO)H(PPh 3 ) and (benzoxantphos)Rh(CO)H(PPh 3 ) were obtained. In contrast to the natural bite angle calculations, approximately the same diphosphine bite angles were observed in both crystal structures. The solution structures of the (diphosphine)Rh(CO)H(PPh 3 ) and (diphosphine)Rh(CO) 2 H complexes were studied by IR and NMR spectroscopy. The spectroscopic studies showed that all (diphosphine)Rh(CO) 2 H complexes exhibit dynamic equilibria between diequatorial (ee) and equatorial-apical (ea) isomers. The equilibrium compositions could not be correlated with the calculated natural bite angles. In the hydroformylation of 1-octene an increase in selectivity for linear aldehyde formation and activity was observed with increasing natural bite angle. For styrene the same trend in selectivity for the linear aldehyde was found. For the first time CO dissociation rates of (diphosphine)Rh(CO) 2 H complexes were determined using 13 CO labeling in rapidscan high-pressure (HP) IR experiments. The observed CO dissociation rates for three complexes proved to be orders of magnitude higher than the hydroformylation rates and, contrary to the hydroformylation activity, did not reveal a correlation with the natural bite angle. These findings indicate that the bite angle effect on hydroformylation activity is dominated by the rates of reaction of the reactive, unsaturated (diphosphine)Rh(CO)H intermediates with CO and alkene. The bite angle affects the selectivity in the steps of alkene coordination and hydride migration; the structure of the saturated (diphosphine)Rh(CO) 2 H complex has only some circumstantial relevance to the selectivity.

Phenoxaphosphino‐Modified Xantphos‐Type Ligands in the Rhodium‐Catalysed Hydroformylation of Internal and Terminal Alkenes

“…(2a) and (3)]. [29] The rate of CO dissociation does not play a role as this reaction is 10 ± 100 times faster than the rate of hydroformylation as was found by van der Veen et al [12,30] ð7Þ…”

A Microfluidic Approach to the Rapid Screening of Palladium‐Catalysed Aminocarbonylation Reactions

“…Ligands with wider bite angles, such as Xantphos, have been generally recognised as having a positive effect on hydroformylation, hydrocyanation, CO/alkene copolymerisation, cross-coupling and carbonylation reactions. [24,50,51] At higher temperatures (150 8C) all the palladium diphosphine catalysts gave good yields (> 75%, Figure 3 and Figure 4) of N-benzylbenzamide, the only exception being the [PdCl 2 A C H T U N G T R E N N U N G (binap)] complex which gave a moderate yield of 45%. This was surprising considering its structural similarity to the biphenphos and synphos ligands, and previous reports of highly successful methoxycarbonylation reactions of heteroaryl chlorides.…”

Bis-(thiosemicarbazonato) Zn(ii) complexes as building blocks for construction of supramolecular catalysts

“…6) in approximately equimolar amounts. 59 The shoulder at about 1920 cm −1 might be the rhodiumhydride (Rh-H) stretch, which is very rarely observed due to its weak intensity. 60 For comparison, application of ZnTPP instead of Zn(btsc) leads exclusively to the formation of the mono-ligated rhodium complex, since the assembly L4·(ZnTPP) 3 is so large that only one phosphorus can coordinate to rhodium.…”