Resins resin-bound catalysts

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... 

To recycle a valuable amine acylation catalyst, Janda and co-workers10 attached a proline-based catalyst to a polymeric support for the enan-tioselective kinetic resolution of alcohols (entry 6). The resin-bound catalyst behaves similarly to the soluble catalyst, providing good yields of secondary alcohols and their corresponding esters with good to excellent enantioselectivities for various substrates. 

Barrett et al. [150] have used ROM-CM to attach ROMP norbornene polymers to vinyl-PS via resin-bound catalyst 77 (Scheme 25). For further examples of ROM-CM of norbornene derivatives on solid supports see Ref. [151]. The... 

At a loading as low as 1 mol%, 10b promoted the hydrocyanation of A-allyl or -benzyl imines derived from aromatic and aliphatic aldehydes and of some ketones in very high yield and almost complete stereoselectivity (see Scheme 2). It is interesting to note that the soluble and the resin-bound catalysts performed equally well. Recovery and recycling of the supported catalyst was shown to occur without any erosion of chemical or stereochemical efficiency over ten reaction cycles. 

Further simplification was attained by immobilization of the catalyst on a solid polymer (49), so that its separation from the product was reduced to mere mechanical filtration the resin bound catalyst could then be used for another run. Ofthe number of polymeric supports investigated, Merrifield (49) and Wang resins were identified as most suitable, exhibiting the same behavior and efficiency [18]. However, owing to the heterogeneous nature of the system, the enantioselectivity of the reaction decreased by 10 15% ee (Table 4.11, entries 6 12). In view ofthe swelling properties, toluene was found to be less suitable than chloroform (compare entries 6 and 7), which makes the method less environment friendly. Furthermore, a conditioning effect was observed for these systems the second run was always found to be more enantioselective than the first one by 10% ee and this level was maintained in the subsequent runs (compare entries 7 with 8 12). The latter effect stems from the... 

Smith [14] has recently reported the successful application of a basic resin-bound catalyst. 

During the last few years, several resin-bound catalysts have been developed for the enantioselective addition of diethylzinc to aldehydes. The related results have been summarised by Shuttleworth in a recent review. 5... 

The synthesis of a triphenylphosphine functionalised TentaGel was reported by Uozumi Hayashi and coworkers.The resin was loaded with Pd and used as a resin-bound catalyst in allylic substitution reactions as shown in Scheme 3.6.12. 

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... 

In another approach, a chiral auxiliary can be fixed to solid-supported substrate and help direct the stereoselectivity of the asymmetric reactions to provide enantiopure molecules. In this chapter, we present some representative examples of resin-bound catalysts and chiral auxiliaries in asymmetric synthesis. For detailed reviews about the polymer-bound chiral ligands used in conjunction with metals and metalloids in asymmetric catalysis, chiral organic catalysts attached to polymer supports, and chiral auxiliaries on solid support, readers are referred to Refs 6 and 7. 

Until now, the most efficient approach to synthesize Freidinger lactams 147 started from a resin-bound cinnamylamine 144. A Fukuyama-Mitsunobu reaction to 145 followed by sulfonamide cleavage and a consecutive appropriate acylation built up the diene 146, which underwent ring-closing metathesis involving Grubb s catalyst 123 to generate the desired lactams 147 (Scheme 27, Table 5) [35d]. 

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]. 

Several palladium catalysts for formation of aryl sulfides from aryl halides have been investigated more recently. A combination of Pd2(dba)3 and DPEphos catalyzed the formation of a broad range of diaryl sulfides in the presence of 1 mol.% palladium and NaO-t-Bu base in toluene solvent.12,rThe highest yields of alkyl aryl sulfides were obtained from aryl triflates and n-butyl thiol catalyzed by a combination of palladium acetate and BINAP. However, these reactions contained 10 mol.% catalyst, were long, and required deactivated aryl triflates. A combination of Pd2(dba)3 and DPPF catalyzed the coupling of thiols with resin-bound aryl halides.121... 

Several combinatorial approaches to the discovery of transition metal based catalysts for olefin polymerization have been described. In one study Brookhart-type polymer-bound Ni- and Pd-(l,2-diimine) complexes were prepared and used in ethylene polymerization (Scheme 3).60,61 A resin-bound diketone was condensed with 48 commercially available aminoarenes having different steric properties. The library was then split into 48 nickel and 48 palladium complexes by reaction with [NiBr2(dme)] and [PdClMe(COD)], respectively, all 96 pre-catalysts being spatially addressable. 

One obvious synthetic route to isoxazoles and dihydroisoxazoles is by [3+2] cycloadditions of nitrile oxides with alkynes and alkenes, respectively. In the example elaborated by Giacomelli and coworkers shown in Scheme 6.206, nitroalkanes were converted in situ to nitrile oxides with 1.25 equivalents of the reagent 4-(4,6-di-methoxy[l,3,5]triazin-2-yl)-4-methylmorpholinium chloride (DMTMM) and 10 mol% of N,N-dimethylaminopyridine (DMAP) as catalyst [373], In the presence of an alkene or alkyne dipolarophile (5.0 equivalents), the generated nitrile oxide 1,3-dipoles undergo cycloaddition with the double or triple bond, respectively, thereby furnishing 4,5-dihydroisoxazoles or isoxazoles. For these reactions, open-vessel microwave conditions were chosen and full conversion with very high isolated yields of products was achieved within 3 min at 80 °C. The reactions could also be carried out utilizing a resin-bound alkyne [373]. For a related example, see [477]. 

An inverted system based on the C02-soluble catalyst Pd(OAc)2/PtBu3 has been utilized for Suzuki-coupling of resin-bound substrates [33], The use of scC02 with polymer-supported substrates seems highly attractive owing to the known plasticizing... 

---