Polystyrene bound ligands

A remarkable effect of the reaction temperature on the enantioselectivity of the addition of butyllithium to benzaldehyde was found with polystyrene-bound cvs-enofo-S-dimethylamino -(benzyloxy)bornane (8)12. When the soluble monomeric ligand 9 was tested, the enantioselectivity increased with decreasing temperature (53% ee at — 78 C). In contrast, the polymer-bound chiral additive 8 showed an optimum at — 20 C (32% ee). Although the enantioselectivity of this addition reaction is low, an advantage of a polymer-bound chiral auxiliary is that it can be removed by a simple filtration. 

A dry 50 mL Schlenk reaction tube was flushed with nitrogen and charged with a mixture of 7 (527mg, l.Ommol), 1,3-dicyclohexylcarbodiimide (1.04g, 5mmol), pentafluorophenol (920 mg, 5 mmol), 4-(dimethylamino)pyridine as the catalyst and the aminomethylated polystyrene (930 mg, 1.07 mmol g ) in dry dichloromethane (20 mL). The mixture was stirred at room temperature for 24h under a N2 atmosphere. The polymer was filtrated, rinsed sequentially with CH2CI2 and acetone and dried at 50 °C in vacuo to yield the polymer-bound ligand 8 as pale yellow beads. 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

Lohray and coworkers prepared a polymer-bound ligand conceptually similar to polymer 7 (Fig. 3, polymer 9a) [53]. Polymers with 10% of DHQD anchored on the polystyrene backbone were the most efficient catalysts. In contrast to these observations, Song and coworkers reported good yield and high ee with the homopolymer 9b having a 100% DHQD loading (Table 10) [54],... 

V. Mirkhani, M. Moghadam, S. Tangestaninejad, B. Bahramian, Polystyrene-bound imidazole as a heterogeneous axial ligand for Mn(salophen)Cl and its use as biomimetic alkene epoxidation and alkane hydroxylation catalyst with sodium periodate, Appl. Catal. A 311 (2006) 43. 

Immobilized copper-zeolite Y (Cu-HY) bis(oxazolines) were employed as heterogeneous catalysts in carbonyl-ene and imino-ene reactions, allowing the synthesis of a-hydroxy and a-amino carbonyl compounds 163 from 161 and 162 in satisfactory yields and high enantioselection <04AG(E)1685>. The use of a new, insoluble polystyrene-bound Box ligand (IPB-BOX) was also described with good activity (85-95% ee) <04TA3233>. 

Polymerization of ligand monomers is a useful tool for preparing polymer-supported ligands. The cross-linked polystyrene bound ferrocenyl bisphos-phine ligand 21 was prepared by the copolymerization of styrene, divinylben-zene,and l,r-bis(diphenylphosphino)-2-vinylferrocene (20) (Scheme 8) [46]. The loading density of the catalyst on the support was readily controlled by the ratio of the monomers used. 

Immobilization of chiral ligands to effect asymmetric induction in alkylation of aromatic aldehydes by diorganozinc reagents promoted by PEG-im-mobilized ligands 54-57 can also be promoted by soluble polystyrene-bound species. A recent example of this is work where a polystyrene-bound BINOL was prepared [ 105]. This polymer 69 was used to form titanium-BINOLate and AlLibis(binaphthoxide) catalysts for Et2Zn reaction with benzaldehyde and for asymmetric Michael additions of stabilized carbanions to cyclohexenone. While good stereoselectivities were obtained with these catalysts, the synthetic yields were modest. 

During the search for a reasonable enantioselective synthesis route toward the herbicide metolachlor (3), it was tried to immobilize functionalized Josiphos ligands (Scheme 2) [38]. Silica gel- and polystyrene-bound versions... 

Preparation of polymer-supported metal complex. An ethanol solution (20 mL) of palladium dichloride (0.42 mmol) was added to the polymer ligand (chloro-methylated polystyrene-bound porphyrin). The mixture was refluxed and stirred for 15 h. After cooling, the complex was filtered off, washed thoroughly with water and ethanol, and then dried in vacuo. 

The major drawback of phosphine-based ligands is their instability toward air oxidation, which is suspected to be the major cause for catalyst deactivation and metal leaching from the support. For triarylphosphines, such as Ph2PCgH4CH2-PS, the problem is not as pronounced as their alkyldiaryl, dialkylaryl, or trialkyl analogs. Attachment of these hg-ands to a polymeric matrix does seem to attenuate their susceptibility toward air oxidation considerably, and the polymeric ligands can be handled in air briefly for most practical purposes. Moreover, commercial availability of polystyrene-bound triarylphosphines from several sources will undoubtedly stimulate the use of this class of polymeric ligands in the area. 

Asymmetric C—C bond-forming reactions have also been accomplished in flow. Chiral bisoxazolines (Box) are utilized in many asymmetric catalytic reactions as nitrogen-containing bidantate ligands for Lewis acidic metals as well as transition metals. Chiral Cu-Box can be utilized as a Lewis acid catalyst. Salvador et cd. investigated the enantioselective ene reaction using a polystyrene-bound Cu-Box catalyst (33) under flow conditions (Scheme 7.29) [125]. 

Transformations to polymer-bound amino compounds, which are often useful as ligands for metals ions or other free species (67), employ a wide selection of organic reactions. Quaternary ammonium salts result from heating isolated polymer tosylate with tertiary amine they may also be prepared in one step from (hydroxyethyl)polystyrene and toluenesulfonyl chloride and a two-fold excess of amine. 

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