Suzuki silica-supported catalyst

Scheme 6.30 Suzuki coupling with a silica-supported catalyst in water.

The vapor-phase nitration of benzene with NO2 was investigated by Suzuki et al. using polyorganosiloxanes bearing sulfonic acid groups and silica-supported benzenesulfonic acid catalysts [31]. Phenylsulfonic polysiloxane was the most active among the polysiloxanes tested, its activity was not related to the concentration of acid sites as determined by a titration method, however. From the partial pressure dependence of the reaction rate it was concluded that the formation of NO as the active species was the rate-limiting step [32]. 

The physical nature of the polymer that supports the sulfonic acid functionality appears to play a minor role in these reactions. Suzuki has reported the use of a silica-supported sulfonic acid as an efficient acid catalyst for the vapor-phase nitration of benzene with gaseous NO2 to give nitrobenzene (20) (eq 21). This nitration process is particularly efficient as it gives nitrobenzene in 93% yield. 

With the aim of avoiding the use of phosphorus-based ligands, we had previously developed catalysts based on silica supported bidentate iminopyridine ligand systems. These coordinate Pd well, and the catalysts are active and stable in both the Heck (20,22) and Suzuki (21,22) reactions. Since the catalysts are based on the functionalisation of an aminopropyl groups attached to the silica, these are ideal candidates to translate to a chitosan-based support, and indeed this was achieved in a straightforward manner (Figure 2). 

Heterogeneous catalysts have been applied in the Suzuki-Miyaura reaction in aqueous media. For example, the silica-supported Pd complex 45 was shown to be very active for the Suzuki reaction of 4-chloroacetophenone and phenylboronic add in refluxing water (Scheme 6.30) [129]. The catalyst could be recycled by filtration and washing with ethanol, showing no decrease in activity after eight times of reuse. 

The triethoxysilyl endgroup is a popular functional group to bind the catalyst to a polymeric support [238]. Polymeric supports include silica gel, MCM-41 (mesoporous silica gel) and ITQ-2 (delaminated zeolite) [247]. Corma et al. used this approach to synthesise gold(I) and palladium(II) NHC complexes for Suzuki cross-coupling reactions between iodobenzene and various arylboronic acids (see Figure 4.78) [247]. The results were very modest at 35-80% dependent upon the substitution pattern of the arylboronic acid. Yields with gold(I) catalysts were marginally better than those for palladium(II) complexes. 

Polymer-supported (NHC)-Pd complex 15 was found to be an effective catalyst for the Suzuki reaction of aryl iodide and bromides at room temperature and aryl chlorides at 100°C [20]. More recendy, an IPr-fimctionalized mesoporous ethane-silica was synthesized via the co-condensation of IPr-bridged triethoxysi-lane and bis(triethoxysily)ethane [21]. This new mesoporous material had a high surface area and large pore voliune, and was used to create a library of surface (NHC)-Pd complexes. These materials showed remarkable catalytic activity and could be reused 10 times without a significant decrease in activity. 

Silica is often used as a catalyst support for metal catalysts. Greenway et al. used a 300 pm X 115 pm microchannel for the Suzuki coupling of 4-bromobenzonitrile... 

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