Resin-bound catalysts, activity

The Jacobsen group has also shown that the recycling of the resin-bounded catalyst can be successfully performed [152,154]. Moreover, they have developed an efficient method for the hydrolysis of the aminonitrile into the corresponding amino acid. This method was apphed for the commercial production of optically active K-amino acids at Rhodia ChiRex (e.g. tert-leucine) the catalyst was immobihsed on a resin support (4 mol %, 10 cycles) and the intermediate hydrocyanation adduct was trapped by simply replacing TFAA with HCOOH/AC2O, for example. Highly crystalhne formamide derivatives were thus obtained in excellent yields (97-98% per cycle) with very high enantioselectivities (92-93% per cycle) [158]. 

The possibility of solving the catalyst recovery problem by attaching active catalyst centers to insoluble pol)rmeric substrates was recognized early(26), as was the possible use of chiral PTC catalysts to introduce chirality in products(1). Much work in both these areas has been partially successful(27). However, the results have not been completely satisfactory in that resin bound catalysts have shown much lower catalytic activity than soluble catalysts and they frequently lose their activity with repeated use. Chiral... 

Employing a resin-bound catalyst 92 for enol ester synthesis is an important extension of this strategy [48]. It is noteworthy that the solid-supported catalyst 92 displays the same activity as its homogeneous analog 107 and is amenable for recycling. 

One of the earliest examples of polystyrene-bound palladium was reported by Tera-sawa and co-workers in 1975, when such a system was used for hydrogenation of olefins and acetylenes and isomerization of double bonds. However, it is not clear if the high activity exhibited by resin-bound catalysts toward hydrogenation was due to the reduced heterogeneous paUadium deposited on the resin. Pittman et al. in 1976 prepared a series of diphenylphosphinated PS-based palladium catalysts and studied their behavior... 

These catalysts were first tested as resin-bound derivatives via HTS, first with metals and then without. Three libraries of chiral molecules, based on three different enantiomerically pure diamines, bulky salicylidene moities and optically active ii-amino acids were used for structure optimisation (Scheme 37 TBSCN = fBuMe2SiCN) [152]. 

Buchwald has shown that, in combination with palladium(II) acetate or Pd2(dba)3 [tris(dibenzylideneacetone)dipalladium], the Merrifield resin-bound electron-rich dialkylphosphinobiphenyl ligand (45) (Scheme 4.29) forms the active polymer-supported catalysts for amination and Suzuki reactions [121]. Inactivated aryl iodides, bromides, or even chlorides can be employed as substrates in these reactions. The catalyst derived from ligand (45) and a palladium source can be recycled for both amination and Suzuki reactions without addition of palladium. 

The resultant fabrics are unique in that they have many functional property improvements thermal adaptability due to the phase change nature of the bound polyol, durable press or resiliency, soil release, reduction of static charge, antimicrobial activity, enhanced hydrophilicity and improved flex life, and resistance to pilling. Because of the different molecular weights of polyols, resins, acid catalysts, and fabric constructions, there are numerous modified fabrics that can be produced with sets of improved attributes. Each fabric must be carefully evaluated for optimum curing conditions and formulations to produce the desired product. Several licenses have been granted for this process. Various types of apparel, healthcare items, and industrial fabrics are currently evaluated for commercial production [381,382]. 

The microwave-induced reaction with a polymer-supported bis-pyridyl ligand (Scheme 16.87) was also slow, presumably because the insoluble resin-bound ligand makes the catalyst heterogeneous. After microwave irradiation at 160 °C for 30 min, however, the reaction was complete and the branch-to-linear ratio of the product was 35 1, an enantiomeric excess of 97%. The polymer-supported ligand has obvious advantages over its unsupported analogue because it could be reused at least seven times without any loss of activity [134]. 

In a related study, polymer-supported triphenyl phosphine was used in palladium-catalyzed cyanations [136]. Commercially available resin-bound triphe-nylphosphine was mixed with palladium(II) acetate in N,N-dimethylformamide to generate the heterogeneous catalytic system under a nitrogen atmosphere. The reagents were then added to the activated catalyst and the mixture was irradiated at 140 °C for 30-50 min (Scheme 16.89). Finally, the resin was removed by filtration and evaporation of the solvent furnished the desired benzonitriles in high yields and excellent purity. 

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