Tagged organocatalysts

The 1,2,3-triazole-linked fluorous proline organocatalyst 32 was introduced by Pericas and coworkers in 2013 for the asymmetric aldol reactions of acetone with aromatic aldehydes, giving higher enantioselectivities than other proline derivatives (Scheme 11.27). ° The fluorous tag and the use of a per-fluorinated solvent allowed the easy recycling and reuse of 32, for at least six times. 

Chiral 1,2,3-triazolium ionic liquid tethered pyrrolidine catalysts built from (S)-proline and its derivatives have been successfully ap>plied in various catalytic reactions (Khan, Shah et al. 2010 Khan, Shah et al. 2010 Maltsev, Kucherenko et al. 2010 Yacob, Shah et al. 2008). The 1,2,3-triazolium ionic liquid-tagged organocatalysts derived from proline and its derivatives are mostly viscous liquids at room temperature and are completely miscible with polar solvents such as methanol, chloroform, acetonitrile, dimethylsulfoxide, dimethylformamide and water. They are insoluble in less-polar solvents such as n-hexane and diethyl ether. In some cases, the ionic liquid sub-unit serves not only as a phase tag for efficient recycling but also as an effective chiral amplifier through polar interactions and steric shielding. 

Catalytic application of the triazolium ionic liquid tagged organocatalysts 25 and 26 gave very interesting results in the Michael addition of various unmodified aldehydes and ketones to trfl s- -nitrostyrenes. 10 mol % phenyl substituted ionic liquid tagged organocatalyst 25 provided excellent yields (>99%), diastereoselectivities (>99 1) and... 

Scheme 5. Synthesis of (S)-proline derived 1,2,3-triazolium ionic liquid tagged organocatalysts 25 and 26...

Khan, S. S. J. Shah and J. Liebscher (2010). "Synthesis of new ionic-hquid-tagged organocatalysts and their application in stereoselective direct aldol reactions." Tetrahedron, Vol.66, No.27-28, 0ul 3,2010), pp. 5082-5088, ISSN 0040-4020... 

Yacob, Z. (2010). 1,2,3-Triazolium Ionic Liquid Tagged Catalysts in Asymmetric Organocatalysts Department of Chemistry and Bioorganic Chemistry. Berhn, Humboldt University of Berlin. 

Apart from the interaction between catalyst and substrate, halogen bonds can also be used in other aspects of organic synthesis. For example, Legros et al. employed a perfluorinated XB donor to facilitate the recycling of the actual organocatalyst of the reaction. This application was based on the concept to non-covalently tag the catalyst with perfluorinated alkane chains ( pony tailing ) [127], so that the resulting fluorophUic complex readily precipitates from some... 

Figure 22.3 Enamine and iminium intermediates generated by IL-tagged organocatalysts.

Scheme 22.28 Pyrrolidine derivatives tagged to alkylimidazolium cations and their use as organocatalysts in asymmetric Michael reactions of carbonyl compounds with a-nitroalkenes.

Figure 22.6 Organocatalysts bearing a pyrrolidine unit tagged to pyridinium, isoquinolinium, l,2,3 triazolium, benzimidazolium, ortetraalkylammonium cations.

In parallel, Xu and coworkers discovered that in the presence of similar I L-tagged pyrrolidines 84c,d the reaction proceeded nearly as efficiently in the [bmimJIPFs] medium, so there is no need to dehver the acid co-catalyst (TFA) to the system because this role is obviously played by the IL fragment [96]. Catalysts 84a,b were recovered from the reactant medium by precipitation with ether, while the 84c,d/ IL systems were reused after product extraction without further purification. Four reaction cycles of catalysts 84 did not reduce reaction diastereo- and enantioselec-tivities however, the recovered catalysts gradually became less active with each cycle. Surfactant-type IL-supported asymmetric organocatalyst 84e synthesized by Luo and Cheng and coworkers in 2006 catalyzed Michael addition to nitroalkenes with high stereoselectivities in water without any additives [97]. 

In 2011, Hermeke and Toy synthesized phosphonium ion-tagged chiral phosphoric acids 117 and 118 bearing a BINAP core and examined them as recoverable organocatalysts in Friedel-Crafts alkylation of indoles [55], The phosphonium tag position in the catalysts was cmcial for the reaction outcome. Compound 117 with the tags at positions 3 and 3 of phosphoric acid, where they served as spaceblocking groups, failed to catalyze the reactions. However, a shift of the phosphonium ion groups to positions 6 and 6 produced efficient and enantioselective... 

On the other hand, modified-imidazoHdinones endowed with imidazole (125) [149] or a fluorous tag in the side chain (126) [150] proved to be active in the Diels-Alder cycloaddition in the presence of 5 vol.% water (Figure 24.40). The highly efficient organocatalyst 127 promoted, in the presence ofp-(trifluoromethyl) benzoic acid, the asymmetric Diels-Alder reaction of cyclohexenones with aromatic nitroolefins in seawater and brine with excellent chemo-, regjo-, and stereoselectivities [151], It has been postulated that the cychzation was involved in a concerted addition process and that seawater and brine significantly promoted the... 

Supported Organocatalysts in Multiphasic Systems 28.7.3.1 Tagged Organocatalysts... 

Strecker reaction 895 tagged organocatalysts 834 Takemoto s catalyst 317, 318, 330 Tamiflu 1307,1308... 

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