Xantphos ligands tuneable bite angles

Hydroformylation. The hydroformylation results for the series 15-24 are shown in Table 8.2 [60], From the table one can see that the selectivity for linear aldehyde increases with wider bite angles. For ligands 15-19 the l b ratio increases, but at higher bite angles the effect levels off. 

In an effort to assign the bands to ee and ae isomers the (thixantphos)Rh(CO)2D complex was measured for comparison. Upon H/D exchange, only Vi and v3 shift to lower wavenumbers (respectively 18 and 14 cm-1), and therefore, these two bands are assigned to the carbonyl frequencies of the ee complex. The two bands that do not shift, v2 and v4, belong to the ae complex. From the disappearance of a low-frequency shoulder upon H/D exchange, it can be concluded that one of the rhodium hydride vibrations is partly hidden under v4. 

During hydroformylation the IR spectra of the rhodium species (diphosphine)Rh(CO)2H for ligands 26, 18, and 30 do not change. The respective ee ae equilibria are not influenced, and no other carbonyl complexes are observed thus these are the catalyst s resting state. The coupling constants in the NMR spectra reflect the equilibria between ae and ee species. 

Hydroformylation results in Table 8.3 show that, with the exception of ligands 28 and 30, the rate of the reaction increases with decreasing phosphine basicity. An explanation for the deviant behaviour of 28 and 29 can be that catalyst formation is incomplete or deactivation of the catalyst occurs. Decreasing phosphine basicity facilitates CO dissociation from the (diphosphine)Rh(CO)2H complex and enhances alkene coordination to form the (diphosphine)Rh(CO)H(alkene) complex, and therefore, the reaction rate increases. 

While the l b ratio increases with the %-vaIuc. no electronic effect of the ligands on the selectivity for linear aldehyde is observed. The selectivities for linear aldehyde are all between 92 and 93%. The increase in l b ratio with decreasing phosphine basicity can be attributed completely to an increased tendency of the branched alkyl rhodium species to form 2-octene instead of 

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