5.2. Alkyl-substituted arylphosphanes and arylalkylphosphanes (II–V)

The selectivity to isobutanal was increased at the cost of activity with all the o-alkyl-substituted aryl- and arylalkylphosphanes as well as with (o-phenylphenyl)­diphenyl­phosphane (14).

The iso-selectivities (S_i) of Rh-catalysts modified with meta-isopropyl-substituted phenylphosphanes (18–20) in propene hydroformylation were comparable with the selectivity of the catalyst modified with PPh₃ (S_i = 36%). The catalysts modified with o-alkyl-substituted phosphanes and the o-phenyl-substituted phenylphosphane (14) showed highest selectivity to branched aldehyde isobutanal (S_i = 42–53%), except for the (2-methyl­naphthyl)­dialkylphosphanes (29, 30: S_i = 35% and 33%, respectively). In the improved reaction conditions for 1-hexene hydroformylation, the selectivity to branched 2-methylhexanal was higher for Rh-catalysts modified with the o-substituted arylphosphanes (12–14, 16–17, S_i = 26–32%) than for the PPh₃-modified catalyst (S_i = 24%) [IV]. However, the effect of the modified ligands on branching was less than for propene. Polyaromatic anthryl­phosphanes 6–7 without o-alkyl-substituent did not promote the formation of branched aldehydes [I]. Evidently the o-substituents of arylphosphanes near the coordination sphere of rhodium have a significant steering effect in the propene hydroformylation, the electronically different, bulky o-alkyl-substituted polyaromatic phosphane ligands, such as the the (2-methyl­naphthyl)­­dialkyl­phosphanes (29 and 30), interfere with the formation of branched isobutanal.

The initial rates and conversions for propene hydroformylation were more unpredictable than the selectivities. Initial rates and conversions were highest when the Rh-catalyst was modified with PPh₃ (53 and 45 mol mol_Rh^–1 s^–1 when the propene/Rh ratio was 2250 and 512, respectively, conversion 98–99%). Moreover, in preliminary tests m-isopropyl-substituted phenyl­phosphanes (18–20) increased the initial rate of propene hydroformylation even up to the level of the reference ligand PPh₃ and also showed good conversions. The Rh-catalysts modified with (o-alkylphenyl)­diphenyl­phosphanes (8, 10, 12–13, 15–16) gave higher initial rates (20–35 mol mol_Rh^–1 s^–1) than the mixed (o-alkyl­phenyl)­­dialkyl­phosphanes (24–25, 27: 12–17 mol mol_Rh^–1 s^–1). Likewise, the conversions of the (o-alkylphenyl)­diphenyl­phosphanes (63–70%) were higher than the mixed (o-alkyl­phenyl)­­­dialkyl­phosphanes (35–52%). The o-alkyl-substituent containing pyridyl­phosphanes 21–23 [III], [V] and the o-phenyl-substituted phenylphosphane 14 gave lower initial rates in propene hydroformylation than the o-alkyl-substituted phenylphosphanes. These initial rates of the pyridylphosphanes were in contrast to earlier observations in 1-octene hydroformylation [48], [55]. With the improved reaction conditions, the conversion in 1-hexene hydroformylation was as much as 90 to 99% when the catalyst was modified with o-substituted arylphosphanes 13, 14, or 17 compared with 82% for PPh₃ [IV]. The steric crowding of bis(o-alkylphenyl)diphenyl­phosphanes (9, 11) and the greater σ-donor ability of mixed bis(o-methylphenyl)­isopropyl­phosphane (28) and (2-methyl­naphthyl)­­dialkyl­phosphanes (29–30) almost blocked the propene hydroformylation reaction and the initial rates were only about 2 to 5 mol mol_Rh^–1 s^–1). The bulky o-substituent reduces the initial rate of propene hydroformylation, but altering of the electronic effects of the o-substituents and changing the two unsubstituted phenyl groups to the alkyl groups directly bonded to phosphorus—isopropyls or cyclohexyls—have more unpredictable suppressing effects. Relative to o-alkyl-substituted phenyl­phosphanes the m-isopropyl-substituted phenylphenylphosphanes allow more space for the substrate in the coordination sphere of rhodium, but they also have less ability to release electron density to the phosphorus atom [64]. For these reasons the m-isopropyl-substituted phenyl­phosphanes have higher initial rates than o-alkyl-substituted phenylphosphanes in propene hydroformylation but are less capable of steering the reaction toward branched isobutanal.

The parameters of o-substituted arylphosphane ligands investigated for the description of catalytic behavior in propene hydroformylation were the cone angle, the ³¹P NMR shift and the ¹³C NMR shifts. Calculated cone angles are a measure of the steric properties of the ligands and do not characterize any electronic effects variations of phosphane ligands. Note, however, that the cone angles of closely related phosphanes may correlate with the ¹³C NMR shifts (Scheme 12 in section 4.2.2.1). Schemes 16 and 17 show that the selectivity increases and the initial rate decreases as the cone angle of o-substituted arylphosphane increases. However, even minor electronic changes such as 2,5-dimethyl-substitution of the phenyl ring (ligand 16) can cause a deviation in the selectivity pattern (Scheme 16). Electronic modifications of the ligands have a still more unpredictable effect on the initial rate (Sheme 17) and a trend is seen only for closely related ligands whose steric bulk has increased step by step (8, 12, 13, 17).

Besides the electronic propertis of the ligand, the ³¹P NMR shift reflects the spatial surrounding of the phosphorus atom. Schemes 18 and 19 show that the selectivity increases and the initial rate decreases as the ³¹P NMR shift of o-alkyl-substituted arylphosphanes decreases. o-Phenyl-substituted ligand 14 was off the chart of selectivity (Scheme 18), apparently, because the out of cone oriented phenyl substituent altered the electronic effect (such as gave stronger ring current effect), which affected the ³¹P NMR shifts differently than did the o-alkyl substitutent. As in the case of cone angles, (Scheme 17), a correlation between the ³¹P NMR shifts and initial rates is observed for the closely related phosphanes. Even the electronically altered o-methyl-substituted naphthyl­phosphane 17 seems to fit the trend (Scheme 19).

The ¹³C NMR shift of C⁷ (see Scheme 11 for numbering of carbons) is an indirect measure of the stereoelectric state of the phosphane ligand and, simultaneously, it seems to correlate linearly with the cone angles of the closely related o-substituted phenylphosphane ligands (8, 10, 12, 13, and 14; Scheme 12). The ring current effect is less important in ¹³C NMR spectroscopy [108] and the extra ring current effect created by polyaromatic groups could thus be neglected in the case of the C⁷ shift. Furthermore, the carbon C⁷ may be less affected by the steric effects of bulky o-substituents than is the phosphorus atom. The shift of the polyaromatic ligand 14 appears in both correlations (Schemes 20 and 21). However, the initial rate of the polyaromatic ligand 17 does not fit in the initial rate trend of o-substituted phenylphosphanes. Evidently, the bulkier naphthyl group reduces the initial rate more drastically than the steric bulk of the o-substituents. Analogously, the anthryl groups of ligand 7 weakened the conversion in propene hydroformylation [I].

All the parameters discussed in this section could be expected to correlate more or less linearly with the catalytic results of catalyst systems modified with a still wider group of closely related o-alkyl-substituted phenylphosphanes. In the case of the parameter δ_C7 even the electronically dissimilar ligand 14 fit in the correlations with initial rate and selectivity to isobutanal. At the same time, the fit of the dissimilar ligands 15, 16, 17, and 21 in some of the correlations must be considered coincidence at this time given the limited results for these kinds of ligands.

The parameters discussed above allow a rough prediction of the catalytic properties of sterically and electronically closely related phosphane ligands. The findings also show that separation of the steric and electronic effects of phosphane ligands is hard indeed. It is also important to bear in mind that use of any of these parameters alone could lead to erroneous conclusions.