Quantitative Evaluation of the Catalytic Activity of Dendrimers with Only One Active Center at the Core: Application to the Nitroaldol (Henry) Reaction

Carbonylative Cyclization of 2-Iodofluorobenzenes and 2-Aminophenols with Recyclable Palladium-Complexed Dendrimers on SBA-15: One-Pot Synthesis of Dibenzoxazepinones

A novel, efficient, and practical route to dibenzoxazepinones has been developed through a one-pot heterogeneous palladium-catalyzed aminocarbonylation/aromatic nucleophilic substitution (SNAr) sequence starting from readily available 2-iodofluorobenzenes and 2-aminophenols. The carbonylative cyclization reaction proceeds smoothly in dimethyl sulfoxide (DMSO) at 120 °C with 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) as the base by using a polyamidoamine (PAMAM)-dendronized SBA-15-supported bidentate phosphine-palladium complex [G(1)-2P-Pd(OAc)2-SBA-15] as the catalyst under 10 bar of CO, yielding a wide variety of dibenzo[b,e][1,4]oxazepin-11(5H)-one derivatives in good to excellent yields. Moreover, this new heterogenized dendritic palladium catalyst has competitive advantages in that it can be facilely recovered by simple filtration in air and recycled more than eight times without any significant loss of activity. The broad substrate scope, high functional group tolerance, and excellent palladium catalyst recyclability of the reaction make this approach a general, efficient, and economical method for the construction of valuable dibenzoxazepinone derivatives.

Organocatalysis with dendrimers

This review gives an overview of the use of dendrimers and dendrons as organocatalysts, i.e. as catalysts in the absence of any metal. A large variety of dendrimeric structures have already been used for such a purpose, varying in size (generation), type and location (core or surface) of the organocatalytic entities, and overall chemical composition. The main types of reactions catalyzed concern bond formation (in particular C-C bonds), bond cleavage (in particular of esters), reductions and oxidations. In many cases, good to excellent enantioselectivities have been observed, in some cases associated with a positive dendritic effect (better properties when the generation of the dendrimer increases). Due to their large size compared to products, the dendrimeric organocatalysts can be often recovered and reused several times.

Diphenylphosphine containing macromolecules in the methoxycarbonylation of ethene: the effect of macromolecular architecture on the selectivity of the reaction

Various polyhedral oligomeric silsesquioxanes containing diphenylphosphine moieties at their periphery have been used in the methoxycarbonylation of ethene. Those with a -CH(2)CH(2)- spacer between the silicon and the phosphorus atoms (G0-8ethylPPh(2) and G1-16ethylPPh(2)) only produce methyl propanoate whilst a similar macromolecule with a -CH(2)- spacer between Si and P (G1-16methylPPh(2)) gives only copolymer. The effect of the molecular architecture is discussed in comparison with the selectivities observed when using small molecule analogues.

Synthesis of large ring macrocycles (12-18) by recyclable palladium-complexed dendrimers on silica gel catalyzed intramolecular cyclocarbonylation reactions

Intramolecular cyclocarbonylation reactions with palladium-complexed dendrimers on silica gel as catalysts are very effective for the synthesis of twelve- to eighteen-membered ring macrocycles. This process can tolerate a wide variety of functional groups, including halide, ether, ketone, and ester. The heterogeneous dendritic catalysts facilitate excellent substrate reactivity, affording oxygen-, nitrogen-, or sulfur-containing tricyclic heterocycles in 70-92 % yields. Importantly, these systems are easily recovered by simple filtration and reused several times with only a slight loss of activity.

Polymer-supported proline-decorated dendrons: dendritic effect in asymmetric aldol reaction

The yield and enantioselectivity of an asymmetric aldol reaction, catalyzed by a proline derivative immobilized on polystyrene via dipolar cycloaddition, are remarkably improved by the dendronization of the support.

Intramolecular carbonylation reactions with recyclable palladium-complexed dendrimers on silica: synthesis of oxygen, nitrogen, or sulfur-containing medium ring fused heterocycles

Palladium-complexed dendrimers supported on silica were evaluated as catalysts for intramolecular carbonylation reactions. The results showed that dendritic catalysts display high activity, affording oxygen, nitrogen, or sulfur-containing seven- or eight-membered ring fused heterocycles in excellent yields. Moreover, these catalysts have competitive advantages in that they can be easily recovered by simple filtration in air and reused for up to eight cycles with only a slight loss of activity.

Dendrimers as artificial enzymes

Dendrimers are regular tree-like macromolecules accessible by chemical synthesis from a variety of building blocks. Their topology enforces a globular shape that offers a unique opportunity to design artificial enzymes. Catalytic groups such as metal complexes and cofactors can be placed at the dendrimer core to exploit microenvironment and selectivity effects of the dendritic shell. In a second approach, attaching catalytic groups in multiple copies at the end of the dendritic branches may lead to cooperativity effects. Finally, exploration of dendritic structural space by screening combinatorial libraries of peptide dendrimers for catalytic activity can lead to discovery of functional dendrimers with enzyme-like properties, in a process mimicking natural selection.