Polyether dendritic catalysts

Portnoy and coworkers immobilized chiral hydroxyproline derivatives on polystyrene support functionalized with polyether dendrons (Scheme 15.45).These catalysts promoted the aldol addition of acetone to aromatic aldehydes with excellent enantioselec-tivities, significantly superior to those achieved in the same reaction with analogous catalyst lacking the dendritic interface. The same group prepared polymer-supported chiral bifunctional aminocarbamate and aminourea catalysts for nitro-Michael reaction (Scheme 15.46). However, in this case the dendritic catalysts were inferior to their simpler dendron-lacking analogues. 

Dendritic molecules with a single triethylene amine core surrounded by hyperbranched polyether sectors catalyze the nitro-aldol reaction between aromatic aldehydes and nitroalkanes (Eq. 3.5).15 The activity of the catalysts decreases when the generation number increases. No significant changes in stereo-control are observed on passing from lower- to higher-generation dendrimers. 

The advantages that heterogeneous catalysts have is that they are easily separable from the product, and can be recycled. A number of studies have been conducted in which ligands have been attached or bound to polymeric material to provide an immobilized ligand, and these include polyacrylate and silica [27], polyurea [28], polythiourea [29], polyether [30, 31] and dendritic [32] systems. Upon metal coordination, the immobilized catalysts have retained most of the activity and selectivity, but they now provide the advantage of simple separation and recycling. For exam-... 

For a recent report on the application of this phenyl transfer protocol using a proline-derived catalyst with dendritic polyether wedges, see X. y. Liu, C. y. 

Hyperbranched polyethers can be synthesized via the A2+B3 approach, when diepoxides (3-24) are reacted with triols, such as TMP. Emrick et al. used 1,2,7,8-diepoxyoctane as the A2 monomer and TMP as the B3 monomer with tetra-n-butylam-monium chloride as the nucleophilic catalyst. Nucleophilic attack of the chloride ion on an epoxide at the less-hindered terminal carbon led to the formation of secondary alkoxide. Due to the equilibrium between primary and secondary alk-oxides via proton exchange, nucleophilic attack of primary alkoxides on the epoxide rings resulted in the formation of aliphatic hyperbranched polyether. As the feed ratio of the diepoxide and TMP was varied from 1.5 to 3, the resulting hb polyether contained two types of terminal units (T), one type of dendritic unit (D), and linear units (L) as shown in Scheme 4. The polydispersity index (PDI) of the polyether increased with the increase of molecular weight (from 1.5-1.8 at M = 1000 up to 5.0 at Mw = 7000). The products are viscous liquids with glass transition temperatures below room temperature. 

Portnoy and Goren immobilized M-alkylated imidazoles on polystyrene-bound polyether dendrons via three different synthetic routes (Scheme 15.43). All these systems catalyzed the Baylis-Hillman addition of methyl vinyl ketones to aromatic aldehydes, displaying a very strong positive dendritic effect and a significant enhancement effect of water as a cosolvent. Remarkably, substrates that did not undergo the reaction with the nondendritic immobilized or soluble M-alkyl imidazole catalysts underwent a smooth reaction with one of the catalytic second-generation dendrons. 

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