Catalysts amino

During the 1990s concern increased about the odour and volatility of amino catalysts, particularly in enclosed spaces such as automobiles. Odourless low volatility (and hence low-fogging) catalysts based on salt-like or ionic carboxylates containing active amine centres became available. Another approach was to incorporate amine groups into the polymer to provide a built-in rather than a free-standing catalyst. 

Bis-phenoxo hydrocarbyl complexes, with Ti(IV), 4, 331 Bisphenoxo ligands, on mono-Cp Ti(IV) complexes, 4, 485 Bis(phenoxy-aldehyde) dichlorides, with Zr(IV), 4, 787 Bis(phenoxy-amino) catalysts, in propylene polymerization,... 

Scheme 5.33 Comparison of proline-derived amino-catalysts for asymmetric a-chlorination of aldehydes.

After the rediscovery of (S )-proline-catalysed reactions in 2000 and the discovery of the outstandingly efficient family of Jorgensen-Hayashi s catalysts in 2005, many research groups around the world have struggled in the search for different and more efficient chiral amino-catalysts, based on the pyrrolidine scaffold. A huge number of steric and electronic structural variations have been proposed along the way, sometimes resulting in the... 

Frechet s group reported a similar cascade reaction but using two immobilized amino catalyst derivatives (Scheme 6.14) [40]. They found that the combination of two catalysts, the salt of unidazolidinone with polystyrene sulfonate 114 and polymer-supported diphenylprolinol methyl ether 115, efficiently catalyzed a MCR of two consecutive... 

It clearly emerges that the addition of equimolar amounts of TEA to the SA groups causes a more significant increase in the degree of reaction. Such a result is consistent with the proposed model, in which the amino catalyst remains coordinated to... 

Comparing yields, enantioselectivity, and diastereoselectivity of different amino catalysts in the aldolization of cyclic ketones with p-nitrobenzaldehyde. 

Amino Catalyst-Based Cooperative Catalysis with Multifarious Co-Catalysts 11343... 

Amino catalysts are one of the most dominant organocatalysts in the activation and transformation of aldehydes or ketones through enamine or iminium intermediates. Initially, the presence of an acid co-catalyst is crucial for the catalytic activation. Later it was reaUzed that the introduction of a chiral counteranion to the catalytic system enables the reactions proceeding through cationic intermediates to be conducted in a highly enantioselective manner, and thus stereochemical control could be effectively induced by the chiral counteranion. This strategy has been defined as asymmetric counterion-directed catalysis (ACDC) [25]. 

An important early example of the combinational use of a chiral amino catalyst and chiral Bronsted acid was presented by list et al. in 2006. They found that the valine ester phosphate salt formed by chiral BlNOL-derived Br0nsted acid 77 and chiral amino acid-derived primary amine 78 was effective for the transfer hydrogenation of various a,p-imsaturated ketones 74 with commercially available Hantzsch ester 75 to give saturated ketones 76 in excellent enantioselectivities (Scheme 43.15) [26]. 

Scheme 43.15 Combining a chiral amino catalyst and a chiral Bronsted acid for the transfer...

Cascade reactions triggered by the combination of chiral amines and achiral Brpnsted acid were well documented on the basis of enamine and iminium ion formation, while examples with the combination of a chiral amine catalyst and a chiral Brpnsted acid were rare, hi 2(X)7, Zhou and List reported an elegant cascade intramolecular aldol-reduction process to prepare chiral 3-substituted cyclohexyl-amines by combining achiral enamine catalysis and chiral phosphoric acid catalysis [38]. Unusually, achiral aryl primary amine was exploited as an amino catalyst to generate a transient enamine intermediate to facilitate an intramolecular aldolization-dehydration process, while chiral phosphoric acid was harnessed to accelerate the following conjugate reduction-reductive amination cascade. Starting from readily available 2,6-diketones and aryl amines, pharmaceutically relevant 3-substituted cyclohexyamine derivatives were readily produced in satisfactory yield and excellent enantioselectivity (Scheme 9.42). 

Jprgensen et al. and Macmillan et al. independently reported on the successful enantioselective a-chlorination and a-bromination of carbonyl groups using amino catalysts. a-lodination remained unsolved until the recent discovery of the binol-derived catalysts by Maruoka et A general catalytic cycle, involving an enamine... 

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