| Substituent groups in aryl- and arylalkylphosphanes: effects on coordination chemistry and catalytic properties | ||
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Besides the reaction conditions, the reactivity of a transition metal center strongly depends on the donor/acceptor properties and steric crowding of the ligands bound to it. Tailoring of the catalyst sphere through sophisticated ligand design allows steering of the selectivity and activity of the catalysts.
Earlier in Rh-catalyzed hydroformylation studies with electronically modified p-CF3-, p-OMe-, and p-NMe2-substituted triphenylphosphanes, the rates of the 1-hexene hydroformylation reaction were found to increase with slightly decreasing i/n ratios as the electron density on the rhodium atom was reduced by electron-withdrawing functionality on the modifying phosphane ligand [95], [96]. Pyridylphosphanes have shown a similar tendency for hydroformylation rates [48], [55]. On the other hand, phosphane ligands such as alkylphosphanes, which have greater σ-electron donor ability (more basic ligands), have in general been found less active since their dissociation requires higher reaction temperatures [37].
Increasing the steric crowding of phosphorus ligands through the addition of o-alkyl substituents of triphenylphosphite [97], [98], [99], [100] or triphenylphosphane [54], [101] or branched/cyclic alkyl groups of alkylphosphane or mixed alkylphenylphosphane [82], [37], [38] has increased iso-selectivities and, in the case of aromatic ligands, also reaction rates in olefin hydroformylation. It has been suggested that bulky ligands favor low coordination numbers and, thus, the coordinative unsaturation of the metal center becomes greater favoring branched products [40], [97], [98], [99], [102], [100].
The phosphane ligands of this work were designed to increase the i/n ratio in propene or 1-hexene hydroformylation. In the 1-hexene process isomerization is unavoidable, yielding the internal olefins 2- and 3-hexene [103]. The internal olefins, which are less reactive, predominantly form branched products, 2-methylhexanal and 2-ethylpentanal [103], [104]. Previous investigations on 1-hexene isomerization have shown that a sterically bulky phosphane ligand or several triphenylphosphane ligands bound to rhodium center hinder the coordination of olefins to a metal center and decrease isomerization [105], [106]. Additionally, donor–acceptor properties of the phosphane ligands affect the electron density of the metal center and thereby the mode and the strength of the coordination of olefins [105], [106].
The 1-hexene hydroformylation tests were carried out at the University of Joensuu and the propene hydroformylation tests at the Helsinki University of Technology. Ms. Merja Harteva has carried out the preliminary propene hydroformylation tests of m-isopropyl-substituted phenylphosphanes (18–20). In both cases the phosphane ligands were combined in situ with the rhodium precursors. The reaction conditions in the 1-hexene hydroformylation were as follows: p = 15 bar [I], [II] or 20 bar [V] or 25 bar [IV], T = 80°C [I], [II], [IV] or 100°C [V] or 120°C [II], precursor = Rh4(CO)12 [I], [II] or Rh(acac)(CO)2 [IV], [V], L/Rh = 5 [V] or 10 [I], [II] or 50 [IV], 1-hexene/Rh = 815 [II], [IV] or 10 000 [I], [V]. Reaction conditions in the propene hydroformylation were: p = 10 bar (CO/H2 = 1), T = 100°C, precursor = Rh(NO3)3, L/Rh = 10, and propene/Rh = 512 [IV]–[V] or 2250 [III] or 3200 [I].
In the 1-hexene hydroformylation tests, the sterically bulky arylphosphane and mixed arylalkylphosphane ligands did not suppress the isomerization activity in the manner earlier observed with bulky tri-n-butylphosphane ligand [106]. The donor–acceptor properties of these phosphanes are dissimilar, which means that the ligands have a different effect on the electron density of the metal center, and probably both the mode and the strength of the coordination of 1-hexene were dissimilar. Optimization of the reaction conditions in 1-hexene hydroformylation was both complicated and time consuming. In part because of that, high isomerization activity was associated with most of the ligands and, apparently, affected the i/n ratios. Even so, in the final stage of the work improved reaction conditions with five of the ligands afforded better chemoselectivity to the aldehydes 2-methylhexanal and 1-heptanal [IV].
The main challenge in propene hydroformylation was to obtain high iso-selectivity with reasonably high initial rate. The properties of the o-alkyl-substituted phosphanes, which produced best iso-selectivity, are discussed in more detail, and, reasons for the catalytic behavior are sought in the structural parameters of the ligands.
The propene hydroformylation reaction studied with o-alkyl-modified phosphanes was sometimes carried out in different propene-to-rhodium ratio, which complicates the comparison of the results. However, the reference ligand (PPh3) was tested under all the reaction conditions and the PPh3 results reveal when the comparison is out of line. Comparison of the catalytic results should give a general indications of the properties of the studied phosphane ligands.
Even though the main purpose of the CF3 substituents was to modify the electronic state of the phosphorus atom, the ortho-substitution in ligands 1 and 2 also affected the cone angles of the ligands. The ligands 1 and 2 behaved alike in the hydroformylation by blocking those reactions that most likely where due to steric reasons.
The Rh-catalysts modified with p-CF3-substituted phosphane 4 showed selectivities for the branched aldehydes at the same levels as PPh3; however, the aldehydes were minor products. The conversions in hydroformylation reactions were for both propene and 1-hexene fairly high. The closely similar ligand 3, in turn, blocked the hydroformylation reactions entirely. The reason for this unexpected behavior is not clear, but handling errors are suggested since ligand 3 oxidized easily in accurate mass peak measurement.
The catalytic behavior of o-SeMe-substituted phenylphosphane (ligand 5) was similar to that of the potentially bidentate o-SMe- and o-OMe-modified ligands: all three blocked the propene hydroformylation reaction. p-SMe-Modification and o-NMe2-modification, on the other hand, produced n-butanal as the main product. Consistent with the potential bidentate coordination mode of o-SMe- and o-NMe2-modified phosphanes [107], the behavior of o-SeMe might also explained in terms of bidentate coordination to the rhodium center.