Soluble polyisobutylene-supported reusable catalysts for olefin cyclopropanation
Graphical abstract
Introduction
The use of polymers as supports to facilitate the recovery and recycling of both transition metal catalysts and ligands for transition metal catalysts continues to be a current topic of great interest. During the past several decades, most of the research in this area has been focused on the immobilization of ligands and catalysts on insoluble polymeric supports.1 Soluble supports, however, can be used too.2, 3 A solid/liquid separation like that typically effected with a cross-linked polymer can be carried out with soluble polymers using either solvent precipitation or the upper critical solution temperature for specific polymers. However, the higher temperatures, limited solvent choices or the need for excess solvent in a precipitation have hindered the widespread use of such strategies. For example, while most recent work on polymer-bound cyclopropanation catalysts has focused on using insoluble polymer supports,4, 5, 6 we earlier described polyethylene (PE)-bound cyclopropanation catalysts that separate as solids on simple cooling.7 These supports are effective but require elevated temperature—a problem when chiral ligands were appended to the polymer for asymmetric catalysis.8 An alternative approach developed by our group in the last few years that has parallels in the strategy used in fluorous biphasic catalysis is to use phase selectively soluble hydrocarbon-soluble polymers as supports. Such polymers can be used as homogeneous catalysts in a single phase with the substrate and then separated using either thermomorphic or latent liquid–liquid biphasic approaches.9, 10 Either approach can be a convenient and efficient way to effect the separation of a soluble polymer-supported homogeneous catalyst from a product if the polymer selectively dissolves in a phase different than that of the product. Such separations are most practical if the polymer can be isolated as a hydrocarbon (e.g., heptane) solution, where the polymer has a phase selective solubility of 200:1, or more because most organic products of interest are more soluble in polar phases.11 Here, we describe an example of this sort of approach and its application with a low temperature, heptane-soluble polyisobutylene oligomer (PIB), that can be used to support achiral or chiral transition metal cyclopropanation catalysts.
Section snippets
Results and discussion
The dimer of rhodium(II) acetate is an established catalyst for cyclopropanation of alkenes by diazoalkanes. In a previous study, a polyethylene (PE) supported rhodium carboxylate dimer was prepared and successfully used in catalytic alkene cyclopropanation reactions.7 This PE-bound rhodium catalyst was successfully reused 10 times in cyclopropanation of 2,5-dimethyl-2,4-butadiene. Analysis of the product phase showed that less than 1% of the charged metal leached into product phase. However,
Conclusions
In summary, both PIB-bound rhodium carboxylate and bisoxazoline-copper triflate complexes have been synthesized and used in the olefin cyclopropanation reactions. Catalyst reusability was demonstrated in both heptane/EGDA and heptane/acetonitrile solvent systems. Oligomeric PIB-carboxylate ligands can be used to prepare rhodium(II) cyclopropanation catalysts that work at room temperature and that can be recycled and reused in both heptane/EGDA and heptane/acetonitrile solvent systems with
Acknowledgements
The support of this work by the National Science Foundation (CHE-0446107) and the Robert A. Welch Foundation (A-639) is gratefully acknowledged. We also thank Professor Gyula Vigh for his help in GC analysis of cyclopropanation products. Mr. Ye Zhu is acknowledged for his help in performing GC analysis.
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