Page 125 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

Catalysis, Homogeneous 473

FIGURE 40 Preferred coordination modes of Xantphos (left) and
dppe.

thought that also in this instance this coordination mode
FIGURE 39 Tolman’s cone angle.
gave rise to the desired selectivity. The two structures are
shown in Fig. 40.
qualitative sense. Many attempts have been undertaken to Bidentate ligands have been made and used exten-
define a reliable steric parameter complementary to the sively in many studies on coordination compounds and
electronic parameter. Most often used is Tolman’s param- organometallic compounds. The majority of bidentate lig-
eter θ (theta). Tolman proposed to measure the steric bulk ands, however, lead to “bite” angle of the bidentate on the
of a phosphine ligand from CPK models in the following metal between 75 and 95 . If a bidentate is to coordinate
◦
way. From the metal center, located at a distance of 2.28 in a bis-equatorial fashion, it should have a bite angle of
˚
A from the phosphorus atom in the appropriate direction, ∼120 . Only very few ligands are available. Three of them
◦
a cone is constructed which embraces all the atoms of the are shown in Fig. 41. Molecular modeling and crystallo-
substituents on the phosphorus atom (see Fig. 39). graphic studies by Casey and van Leeuwen have learned
The cone angle is measured, and these cone angles θ that indeed the “natural” bite angle of these ligands is
(simply in degrees) are the desired steric parameters. Crys- 110–120 . They give linear to branched product ratios up
◦
tal structure determinations have shown that in practice
to 100.
the angles realized in the structures are smaller than the
θ-values would suggest. In reality, intermingling of the
R-substituents leads to smaller effective cone angles. 7. Two-Phase Hydroformylation: Water-Soluble
Some typical values are: Catalysts
Ligand PR 3 ,R=: θ-value: In the 1980s a new process has come on stream employing
H 87 a two-phase system with rhodium in a water phase and the
CH 3 O 107 substrate and the product in an organic phase. The catalyst
n-Bu 132 used is a rhodium complex trisulfonated triphenylphos-
PhO 128 phine (tppts) (Fig. 42) which is highly water-soluble (in
Ph 145 the order of 1 kg of the ligand “dissolves” in1kgofwa-
i-Pr 160 ter). The ligand forms complexes with rhodium that are
170 very similar to the ordinary triphenylphosphine complexes
C 6 H 11
t-Bu 182 (i.e., RhH(CO)(PPh 3 ) 3 ). The rhodium complex resides in
the aqueous phase. The substrates, propene/CO/H 2 , are
very slightly water-soluble. Upon reaction a second phase
5. Ligand Effects in Hydroformylation
forms, the organic phase, consisting of the product bu-
Ligand effects on the selectivity of the rhodium-catalyzed tanal and dissolved propene and syn-gas. The two phases
hydroformylation has been extensively studied. The rate are intensely stirred to ensure a fast transport of the gases
may vary several orders of magnitude and also the se- from the gas phase to the organic and water phase.
lectivity to either the branched or the linear product may The two phases are removed from the reactor and sepa-
vary dramatically with minor changes in the electronic and rated in a settler tank after the heat and the gases have been
steric properties of the ligand. removed. Often in organic synthesis the separation of two
layers is not such an easy process step, but in this particular
instance a very clean separation of the two layers occurs.
6. Bidentates with Constrained Bite Angles
Most importantly, the rhodium/ligand components remain
A few classes of new ligands have been introduced that completely in the aqueous phase. This type of separation
give very high selectivity to linear products. All use the is very attractive in a large-scale, continuous process. For
concept that a bis-equatorial coordination of the phospho- the flow-scheme of the process, see Fig. 43.
rus ligands is needed to raise the selectivity to linear prod- Ruhrchemie has commercialized the process, after the
uct. As mentioned above for triphenylphosphine, it was initial work had been done by workers at Rhone-Poulenc,