Alcohols polymer bound catalyst

The polymer-bound catalysts A-C. (Table 31) are prepared by reaction of the corresponding amino alcohols with partially chloromethylated 1 -2% cross-linked polystyrene. In the case of A, the enantioselectivity of the addition of dialkylzincs to aldehydes is higher than with the corresponding monomeric ephedrine derivatives (vide supra). Interesting insights into the mechanism of the alkylation of aldehydes by dialkylzinc reagents can be obtained from the experi-... 

In reactions with polymer-bound catalysts, a mass-transfer limitation often results in slowing down the rate of the reaction. To avoid this disadvantage, homogenous organic-soluble polymers have been utilized as catalyst supports. Oxazaborolidine 5, supported on linear polystyrene, was used as a soluble immobilized catalyst for the hydroboration of aromatic ketones in THF to afford chiral alcohols with an ee of up to 99% [40]. The catalyst was separated from the products with a nanofiltration membrane and then was used repeatedly. The total turnover number of the catalyst reached as high as 560. An intramolecularly cross-linked polymer molecule (microgel) was also applicable as a soluble support [41]. 

Oxidation of alcohols [52] The appropriate alcohol (3 mmol), tBuOH (70% solution in water, 6 mmol), and 64 (25.6 mg, 1 mol% Co complex) in dichloromethane (20 mL) were refluxed for 4 h, the reaction mixture being agitated by means of slow nitrogen bubbling. After coohng, the polymer-bound catalyst was removed by filtration and the product mixture was quenched with sodium sulfite. The purity of the crude product mixture was found to be high, and final purification was achieved by recrystaUization in the case of soHds and by flash chromatography in the case of Hquids. 

All of the polymers swell in both tetrahydrofuran and ethanol. Since ethanol would be expected to compete with the polymer bound alcohols for sites at the catalyst, tetrahydrofuran was chosen as the reaction solvent. The polymer bound catalysts were prepared by stirring the polymer and /x-dichlorobis(l,5-cyclooctadiene) dirhodium (I) in tetrahydrofuran for several hours, and after filtration the yellow catalyst was then transferred under argon to the reaction vessel containing the substrate. Solvent was added, and the reaction vessel was pressurized with hydrogen. At the end of the reaction, the pressure was released and the product was iso-... 

Metathesis reactions have been known for many years and can be carried out using both homogeneous and heterogeneous341 catalysts. Although other catalysts have been used, 42 ruthenium complexes s j ve been the most common. With the development of the Grubbs catalyst (428) the reaction has taken on increa-ed synthetic utility. The Grubbs catalyst is also stable to Lewis acidic conditions.Catalysts have been developed that allow the reaction to take place in aqueous and alcohol solution and polymer-bound catalysts have been used.346... 

Atactic poly(methallyl)alcohol reacts with PClPha to give a polyphosphite Pn, and on treatment with [Rh2(Cl)2(CO)4] or [RhaCCOaCnorbornadienela] polymer-bound catalysts are obtained which are active in the hydrogenation of a variety of olefins. The species derived from the norbornadiene complex catalyses oct-l-ene hydrogenation at a rate dependent on catalyst concentration with solutions 0.125 mol 1 in olefin and at a rate oc [olefin] / up to a concentration of 0.3 mol 1. No attempt is made to rationalize these results in mechanistic terms. 

Various polymer-bound (polystyrene-bound) oxazaboroHdine catalysts for the reduction of secondary alcohols were reported [128]. These can simply be prepared by condensation of the resin-bound boronic acid with chiral 1,2-amino alcohols. The best results as far as enatioselectivity is concerned were obtained with oxaza-borohdine (59) (Scheme 4.36). 

We synthesized 8 by the one-step reaction of [Ph4(Tl -C4CO)]Ru(CO)3 with benzyl chloride. In contrast to previous alcohol racemization catalysts, 8 was stable in the air during racemization [30]. The racemization was performed even under 1 atm of molecular oxygen. Thus, alcohol DKR was for the first time possible with 8 in the air at room temperature (R)-l-phenylethyl acetate (99% yield, greater than 99%e.e.) was obtained from 1-phenylethanol by using 4mol% of 8, CALB and isopropenyl acetate in the presence of potassium phosphate (Scheme 1.22). This catalyst system was effective for both benzylic and aliphatic alcohols. The synthetic method for 8 was applied to the preparation of a polymer-bound derivative (9). Hydroxymethyl polystyrene was reacted with 4-(chloromethyl)benzoyl chloride to... 

Schreiber and co-workers (436) prepared a library calculated to contain 2.18 million polycyclic compounds through the 1,3-dipolar cycloaddition of a number of nitrones with alkenes supported on TentaGel S NH2 resin (Scheme 1.83). (—)-Shikimic acid was converted into the polymer bound epoxycyclohexenol carboxylic acid 376 (or its enantiomer), coupled to the resin via a photolabile linker developed by Geysen and co-workers (437) to allow release of the products from the resin in the presence of live cells by ultraviolet (UV)-irradiation. A range of iodoaromatic nitrones (377) was then reacted with the ot,p-unsaturation of the polymer-bound amide in the presence of an organotin catalyst, using the tandem esterification/ dipolar cycloaddition methodology developed by Tamura et al. (84,85) Simultaneous cyclization by PyBrop-mediated condensation of the acid with the alcohol... 

The direct electrochemical oxidation of aliphatic alcohols occurs at potentials which are much more positive than 2.0 V w. SCE. Therefore, the indirect electrolysis plays a very important role in this case. Using KI or NaBr as redox catalysts those oxidations can be performed already at 0.6 V vs. SCE. Primary alcohols are transformed to esters while secondary alcohols yield ketones In the case of KI, the iodo cation is supposed to be the active species. Using the polymer bound mediator poly-4-vinyl-pyridine hydrobromide, it is possible to oxidize secondary hydroxyl groups selectively in the presence of primary ones (Table 4, No. 40) The double mediator system RuOJCU, already mentioned above (Eq. (29)), can also be used effectively Another double mediator system... 

A polyethylene glycol-polystyrene graft copolymer palladium catalyst has been used in allylic substitution reactions of allyl acetates with various nucleophiles in aqueous media.58 Another polymer-bound palladium catalyst 40 was developed and used in a Heck coupling of allylic alcohols with hypervalent iodonium salts to afford the substituted allylic alcohols as the sole products under mild conditions with high catalytic efficiency.59 The same polymer-bound palladium catalyst has also been used for Suzuki cross-coupling reactions.60... 

A polymer-bound 2-pyrrolidinemethanol 47 was prepared and used as a chiral ligand in the reaction of diethylzinc with benzaldehyde. The alcohols were formed with 100% conversion and 89% enantiomeric excess (equation 19).69 The same results were obtained using 2 or 5 mol % of the catalyst. 

A material with nitrogen-coordinated Ru was obtained from a silica-linked 2-(phenylazo)pyridine ligand. Results for cyclobutanol oxidation with 02 and the sacrificial oxidant isobutyraldehyde indicate that one- and two-electron oxidations occur simultaneously. The stability of the catalyst is not always guaranteed, probably because acids may be formed in oxidations of alcohols (284). Leaching problems are also encountered with a polymer-bound Ru Schiff base complex, used in oxidation of benzyl alcohol (285). 

The same system was employed in the reduction of 4-phenyl-2-butanone to (S)-4-phenyl-2-butanol using HLADH as well as 5-ADH from Rhodococcus sp. with high enantioselectivity [113]. With pentamethylcyclopentadienyl-4-ethoxy-methyl-2,2 -bipyridinechloro-rhodium(III) as mediator and HLADH as catalyst, after 5 h 70% of 4-phenyl-2-butanone was reduced to (S)-4-phenyl-2-butanol with 65% ee. Using 5-ADH. 76% of the ketone was converted to the (S)-alcohol after 5 h with 77% ee. Furthermore, this system has been applied in an electrochemical EMR with a polymer bound rhodium complex as mediator. 

Furthermore, it is reported that metal catalysts usually require a basic pH for sufficient activity for the aerobic oxidation of alcohols in water [85]. Thus, Biffis found that the productivity of the Au nanoclusters can be enhanced by running the reaction at basic pH (pH 9.9) in their system. However, further enhancement of the pH of the reaction solution was not possible, due to the precipitation of catalyst caused by the increased hydrophobicity of the microgel through deprotonation of the polymer-bound amino groups [84]. In our work, we tried to carry out the catalytic reaction... 

The homogeneous chiral phosphine/DPEN-Ru catalyst can be immobilized by use of polymer-bound phosphines such as polystyrene-anchored BINAP (APB-BINAP) [57, 98], Poly-Nap [99], and poly(BINOL-BINAP) [100], poly(BINAP) [101]. These complexes hydrogenate T-acetonaphthone and acetophenone with S/C of 1000-10 000 under 8 10 atm H2 to give the corresponding secondary alcohols in 84-98% e.e. The recovered complexes are repeatedly used without significant loss of reactivity and enantioselectivity. Immobilization allows the easy separation of catalyst from reaction mixture, recovery, and reuse. These advantages attract much attention in combinatorial synthesis. 

The reaction of an alcohol and a carboxylate anion with diethyl azodicarbox-ylate EtOOCN=NCOOEt and PhsP is called the Mitsunobu esterification reac-tion. This reaction can also be considered as an 8 2. Other Mitsunobu catalysts are available, " and a polymer-bound phosphine has been used. A renewable phosphine ligand has been developed. Note that other functional groups, including azides and thiocyanates can be generated from alcohols using Mitsunobu conditions. 

Phosphonates have also been prepared from alcohols and (Ar0)2P(=0)Cl, NEts and a TiCl4 catalyst. ° The reaction of (R0)2P(=0)H and aryl iodides with a Cul catalyst leads to aryl phosphonates. " Polymer-bound phosphonate esters have been used for olefination. Dienes are produced when allylic phosphonate esters react with aldehydes. 

Seebach s group demonstrated the utility of polymer-bound, chiral titanium TADDOLates in preparing chiral secondary alcohols (Figure 3.24).The polymeric catalyst 37 was contained inside a mesh tea bag, and was reused by simply charging fresh reagents and solvent. The ruggedness of the system was shown when the product enantioselectivity dropped from 96% (S) to only 92% (S) over 20 successive runs, and the average yield was 90% [50]. 

Gancet, C Preparation of Esters of Carboxylic Acids Directly From Carboxylic Acids and Alcohols Using a Catalyst System Comprising a Sulfonic Acid and a Polymer-Bound Tertiary Amine. European Patent 1,167,337, Jan 2, 2002. 

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