Maurizio Benaglia

Recoverable and recyclable catalysts

Preface. Acknowledgements. Contributors. 1 The Experimental Assay of Catalyst Recovery: General Concepts ( John A. Gladysz ). 1.1 Introduction. 1.2 Catalyst Precursor vs Catalyst. 1.3 Catalyst vs Catalyst Resting State. 1.4 Catalyst Inventory: Loss Mechanisms. 1.5 Evaluation of Catalyst Recovery. 1.6 Prospective. References. 2 Surface-functionalized Nanoporous Catalysts for Renewable Chemistry ( Brian G. Trewyn, Hung-Ting Chen and Victor S.-Y. Lin ). 2.1 Introduction. 2.2 Immobilization Strategies of Heterogeneous Catalysts. 2.3 Efficient Heterogeneous Catalysts with Enhanced Reactivity and Selectivity with Functionality. 2.4 Other Heterogeneous Catalyst Systems on Nonsilica Supports. 2.5 Conclusion. References. 3 Insoluble Resin-supported Catalysts ( Gang Zhao and Zhuo Chai ). 3.1 Introduction. 3.2 Transition Metal Catalyzed C-C Bond Formation Reactions. 3.3 Oxidation. 3.4 Reduction. 3.5 Organocatalyzed Reactions. 3.6 Annulation Reactions. 3.7 Miscellaneous. 3.8 Conclusion. References. 4 Catalysts Bound to Soluble Polymers ( Tamilselvi Chinnusamy, Petra Hilgers and Oliver Reiser ). 4.1 Introduction. 4.2 Soluble Supports - General Considerations. 4.3 Recent Developments of Soluble Polymer-supported Catalysts. 4.4 Recent Examples for Reactions Promoted by Catalysts Bound to Soluble Polymers. 4.5 Conclusion. List of Abbreviations. References. 5 Polymeric, Recoverable Catalytic Systems ( Qiao-Sheng Hu ). 5.1 Introduction. 5.2 Polymeric Catalyst Systems. 5.3 Summary. Acknowledgements. References. 6 Thermomorphic Catalysts ( David E. Bergbreiter ). 6.1 Introduction. 6.2 Thermomorphic Catalyst Separation Strategies. 6.3 Hydrogenation Reactions Under Thermomorphic Conditions. 6.4 Hydroformylation Reactions Under Thermomorphic Conditions. 6.5 Hydroaminations Under Thermomorphic Conditions. 6.6 Pd-catalyzed Reactions Under Thermomorphic Conditions. 6.7 Polymerization Reactions Under Thermomorphic Conditions. 6.8 Organocatalysis Under Thermomorphic Conditions. 6.9 Cu(I)-catalyzed 1,3-Dipolar Cycloadditions Under Thermomorphic Conditions. 6.10 Thermomorphic Hydrosilylation Catalysts. 6.11 Thermomorphic Catalytic Oxidations. 6.12 Conclusions. References. 7 Self-supported Asymmetric Catalysts ( Wenbin Lin and David J. Mihalcik ). 7.1 Introduction. 7.2 Self-supported Asymmetric Catalysts Formed by Linking Catalytically Active Subunits via Metal-Ligand Coordination. 7.3 Self-supported Asymmetric Catalysts Formed by Post-synthetic Modifications of Coordination Polymers. 7.4 Self-supported Asymmetric Catalysts Formed by Linking Multitopic Chiral Ligands with Catalytic Metal Centers. 7.5 Conclusions and Outlook. Acknowledgments. References. 8 Fluorous Chiral Catalyst Immobilization ( Tibor Soos ). 8.1 Introduction. 8.2 Fluorous Chemistry and its Basic Recovery Concepts. 8.3 Application of Fluorous Chiral Catalysts. 8.4 Summary. References. 9 Biphasic Catalysis: Catalysis in Supercritical CO2 and in Water ( Simon L. Desset and David J. Cole-Hamilton ). 9.1 Introduction. 9.2 Biphasic Catalysis. 9.3 Aqueous Biphasic Catalysis. 9.4 Supercritical Carbon Dioxide. 9.5 Conclusion. References. 10 Asymmetric Catalysis in Ionic Liquids ( Lijin Xu and Jianliang Xiao ). 10.1 Introduction. 10.2 Metal-catalyzed Asymmetric Reactions in ILs. 10.3 Asymmetric Organocatalytic Reactions in ILs. 10.4 Concluding Remarks. References. 11 Recoverable Organic Catalysts ( Maurizio Benaglia ). 11.1 Introduction. 11.2 Achiral Organic Catalysts. 11.3 Chiral Organic Catalysts. 11.4 Catalysts Derived from Amino Acids. 11.5 General Considerations on Recyclable Organocatalysts. 11.6 Outlook and Perspectives. References. 12 Organic Polymer-microencapsulated Metal Catalysts ( Jun Ou and Patrick H. Toy ). 12.1 Introduction. 12.2 Non-cross-linked Polymer-microencapsulated Catalysts. 12.3 Cross-linked Polymer-microencapsulated Catalysts. 12.4 Summary Table. 12.5 Conclusions. References. 13 Organic Synthesis with Mini Flow Reactors Using Immobilised Catalysts ( Sascha Ceylan and Andreas Kirschning ). 13.1 Introduction. 13.2 Catalysis in Mini Flow Reactors with Immobilised Catalysts. 13.3 Miscellaneous Enabling Techniques for Mini Flow Systems. 13.4 Perspectives and Outlook. References. 14 Homogeneous Catalysis Using Microreactor Technology ( Johan C. Brandt and Thomas Wirth ). 14.1 Introduction. 14.2 Acid-catalysed Reactions. 14.3 Liquid-liquid Biphasic Systems. 14.4 Photocatalysis. 14.5 Asymmetric Catalytic Reactions. 14.6 Unusual Reaction Conditions. References. 15 Catalyst Immobilization Strategy: Some General Considerations and a Comparison of the Main Features of Different Supports ( Franco Cozzi ). 15.1 Introduction. 15.2 General Considerations on Catalyst Immobilization. 15.3 Comparison of Different Supports Employed for the Immobilization of Proline. 15.4 Comparison of Different Supports Employed for the Immobilization of Bis(oxazolines). 15.5 Conclusions. References. Index.

Poly(Ethylene Glycol)-Supported Proline: A Versatile Catalyst for the Enantioselective Aldol and Iminoaldol Reactions

(2S,4R)-4-Hydroxyproline has been anchored to the monomethyl ether of poly(ethylene glycol), MW 5000, by means of a succinate spacer to afford a soluble, polymer-supported catalyst (PEG-Pro) for enantioselective aldol and iminoaldol condensation reactions. This organic catalyst can be considered as a minimalistic version of a type I aldolase enzyme, with the polymer chain replacing the enzymes peptide backbone, and the proline residue acting as the enzymes active site. In the presence of PEG-Pro (0.25–0.35 mol equiv.), acetone reacted with enolizable and non-enolizable aldehydes and imines to afford β-ketols and β-aminoketones in good yield and high enantiomeric excess (ee), comparable to those obtained using non-supported proline derivatives as the catalysts. Extension of the PEG-Pro-promoted condensation to hydroxyacetone as the aldol donor opened an access to synthetically relevant anti-α,β-dihydroxyketones and syn-α-hydroxy-β-aminoketones, that were obtained in moderate to good yields, and good to high diastereo- and enantioselectivity. Exploiting its solubility properties, the PEG-Pro catalyst was easily recovered and recycled to promote all of the above-mentioned reactions, that occurred in slowly diminishing yields but virtually unchanged ees.

Enantioselective aldol condensations catalyzed by poly(ethylene glycol)-supported proline

Keywords: aldol reactions; asymmetric catalysis; catalyst immobilization; chiral bases; soluble polymers

Recoverable and recyclable chiral organic catalysts

In the present perspective article a few selected, significant works reported in the field of supported chiral organic catalysts will be presented, with a special attention to the papers appeared that after 2002. A few considerations on the methodologies, the future and the problems related to the immobilization of chiral organic catalysts will also be briefly discussed.

Through-space interactions between face-to-face, center-to-edge oriented arenes: importance of polar-π effects

Two series of conformationally restricted polycyclic compounds (1-3 and 4-7) have been synthesized as model systems for studying the through-space interactions between face-to-face, center-to-edge (parallel-offset) oriented arenes. These compounds feature different X substituents on one of the interacting rings. By monitoring the variation of the delta Gz for the rotation around the aryl-aryl bond in 1-7 as a function of X by 2D [1H,1H] EXSY NMR spectroscopy, it was found that the barriers increase on passing from electron-donating to electron-withdrawing substituted derivatives. Quantum mechanical calculations [MP2/DVZ (2d,p)//B3LYP/DVZ(2d,p)] gave barrier values and variations in agreement with the experimental data. The results are consistent with a repulsive arene-arene interaction dominated by electrostatic effects.

Stereoselective organic reactions promoted by immobilized chiral catalysts in continuous flow systems

The immobilization of the catalyst on a support with the aim of facilitating the separation of the product from the catalyst, and thus the recovery and recycling of the latter, can be regarded as an important improvement for a catalytic process. However, a system where a catalyst must not be removed from the reaction vessel is even more attractive: in continuous flow methods the immobilized catalyst permanently resides in the reactor where it transforms the entering starting materials into the desired products. The retention of the catalytic species inside the reaction vessel can be achieved by different techniques ranging from ultrafiltration through a MW-selective membrane to immobilization on different supports. In this review we will discuss the most significant examples of stereoselective reactions promoted by immobilized chiral catalysts and performed under continuous flow conditions, with particular attention to the more recent contributions of the last few years.

Poly(ethylene glycol)‐Supported Chiral Imidazolidin‐4‐one: An Efficient Organic Catalyst for the Enantioselective Diels–Alder Cycloaddition

A tyrosine-derived imidazolidin-4-one was immobilized on a modified poly(ethylene glycol) and converted in situ into a soluble polymer-supported catalyst for the enantioselective Diels–Alder cycloaddition of acrolein to 1,3-cyclohexadiene (up to 92% ee) and 2,3-dimethyl-1,3-butadiene (73% ee). Catalyst recycling (up to four cycles) was accompanied by some loss of the chemical efficiency and marginal erosion of the enantioselectivity.

The S‐Thioester Enolate/Imine Condensation: A Shortcut to β‐Lactams

The condensation between imines and S-thioester metal enolates provides a mild, efficient, and straightforward route to the preparation of β-lactams. The synthetic applications and the stereochemical aspects of this reaction are discussed.

Immobilization of catalysts derived from Cinchona alkaloids on modified poly(ethylene glycol)

Abstract The straightforward immobilization of some derivatives of Cinchona alkaloids on modified poly(ethylene glycol)s is reported. The compounds, obtained by simple reactions exploiting different sites for the attachment of the alkaloids to the polymer, were tested as catalysts in the enantioselective benzylation of the benzophenone imine of glycine t -butyl ester (ee up to 64%) and in the conjugate addition of thiophenol to cyclohexenone (ee 22%). The observed stereoselectivities were compared to those obtained either with the unsupported catalysts or with the catalysts immobilized on different polymeric matrixes. The influence of the poly(ethylene glycol) moieties on the catalytic activity is discussed.

Highly Stereoselective Metal-Free Catalytic Reduction of Imines: An Easy Entry to Enantiomerically Pure Amines and Natural and Unnatural α-Amino Esters

A highly efficient catalytic stereoselective ketimine reduction is described. The combination of an inexpensive chiral organocatalyst, easily prepared in a single step, and of a very cheap removable chiral auxiliary allowed us to obtain enantiomerically pure amino compounds. The methodology allowed synthesis of chiral secondary and primary amines and natural and unnatural amino esters in high yields often with total control of the absolute stereochemistry.