CAC - Anchored Precursors

Ligands

Substrates

Solvents

Anchored
Precursors

Large Scale
Continuous Use

Since a large number of chiral ligands are either proprietary or have been produced for a specific substrate, the idea of having to produce anchored homogeneous catalysts from these species individually was not very appealing, particularly from a marketing standpoint. However, it is possible to anchor a catalyst precursor to a support and then treat this anchored species with a ligand to produce the desired anchored catalyst. As depicted by Fig. 1 the anchored homogeneous catalyst can be prepared in two ways. The homogeneous catalyst can be made first by reaction of the ligand with an appropriate precursor, such as the
Rh(COD)₂⁺ species and the resulting complex then reacted with the HPA/support material. Another method is to first anchor the
Rh(COD)₂⁺onto the support through an HPA and then to treat this solid precursor with the ligand to give the anchored catalyst.

Figure 1

The anchored precursor, Rh(COD)₂⁺ BF₄^- or RuCl₂ (p-cymene)₂
(20 µmoles in 1 mL of alcohol), is added to the heteropoly acid modified alumina under an inert atmosphere, as described above. This material can be used directly or dried for future use. Stirring an alcoholic suspension of the anchored precursor with 1.1 equivalents of the ligand in an inert atmosphere for 8 to 12 hours completes the ligand exchange and gives the anchored homogeneous catalyst. Ligand exchange on the anchored Ru(p-cymene) is best accomplished by heating the suspension at 50°C for two hours.
The Table lists the comparative reaction data for the hydrogenation of methyl 2-acetamidoacrylate (1) over tethered Rh(DiPamp) catalysts prepared either by the anchoring of the preformed catalyst or by reaction the tethered Rh(COD)2 precursor with the DiPamp ligand.

Reaction rates and product ee's obtained on hydrogenation of 1 over alumina anchored Rh(DiPamp) catalysts prepared from a supported precursor or from the complex.^a

From Precursor

From Complex

Use Number	Rate^b	ee%	Rate^b	ee%
1	0.28	79	0.39	84
2	0.48	85	0.49	87
3	0.49	87	0.54	87
4	0.58	87
5	0.55	85

^a Hydrogenations run at 25°C under 1 atm H2 using 20 moles of supported Rh(DiPamp) to saturate 0.35 mmole of 1 for each run.

^b moles H2/mole Rh/min.

This procedure was used to prepare a number of different anchored complexes. An anchored Wilkinson's catalyst was prepared by treating an anchored Rh(COD)₂ with 2 equivalents of triphenylphosphine. This and the standard anchored Wilkinson was used to hydrogenate
1-hexene. Both catalysts gave nearly identical reaction rates. Anchored Rh(BPPM) catalysts prepared in both ways were used to hydrogenate 2-acetamidocinnammic acid in essentially identical rates and product enantioselectivity over four re-uses of each catalyst.

To test the stability of the anchored Rh(COD)₂ the
2-acetamidocinnamic acid hydrogenation was run over anchored Rh(BPPM) catalysts prepared from both a freshly made anchored precursor and one which had been sitting on the shelf in air for two years. After the first use of the catalyst made from the old precursor, the rates and selectivities were very similar.

Anchored ruthenium precursors were also investigated. It was found that a Ru(p-cymene) precursor complex could be anchored to alumina by PTA. The hydrogen uptake curves for the hydrogenation of DMIT using an anchored Rh(TMBTP) catalyst prepared by treating the anchored Ru(p-cymene) with the TMBTP ligand are very similar to those obtained using the complex to prepare the catalyst.



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