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A polymer-supported Sharpless epoxidation catalyst was prepared using linear poly(tartrate ester) catalyst ligands 43.65 This catalyst system was used in the reaction of trans-hex-2-en- 1-î¿ with titanium te/ra-isopropoxide and tert-butyl hydroperoxide to afford the desired epoxide in high chemical yield and moderate enantiomeric excess.
?1 q2 MfiHN FHTA Oxnrm NaHCO, R1 Î R2
Ti(0-/-Pr)4 ’BuOOH, 4A sieves CH2CI2, -20 °Ñ
5.8. POLYMER-SUPPORTED CATALYSTS 171
0 COoMe N' "
(polyaniline supported Ñî (II) salen)
Ph Y N OH
X = H or OAc
A polymeric cinchona alkaloid-derived ligand 44 was prepared and used to catalyze the asymmetric dihydroxylation of olefins (see the diagram below).66 Both aliphatic and aromatic olefins afforded diols with good enantioselectivities.
44, 0.5 mol%
0s04, 0.25 mol% stoichiometric oxidant
POLYMER-ASSISTED SOLUTION-PHASE METHODS
5.8.3. Polymer-Supported Catalysts in Carbon-Carbon Bond-Forming Reactions
A partially soluble polyallylscandium triflamide ditriflate 45 was prepared and used to catalyze a three-component coupling reaction.67 An aldehyde, an aromatic amine, and an alkene were mixed in the presence of the catalyst to afford tetrahydroquinolines (equation 17). The catalyst was recovered from the reaction mixtures by precipitation with hexane and could be recycled without loss of activity. Another polymer-supported scandium catalyst was prepared by treating Nafion with scandium chloride to afford the Nafion-scandium catalyst 46.68 This catalyst was used in allylation reactions of carbonyl compounds by tetraallyltin (equation 18). It could be easily recovered by filtration and reused without appreciable loss of activity.
H2CyTHF, 25 °Ñ
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) © jjjg same results were obtained using 2 or 5 mol % of the catalyst.
5.9. POLYMERS FOR REACTION QUENCHING/WORKUP 173
PhCHO + Et2Zn
PhMe, 0 °Ñ, 48 h
A naphthalene supported polymer was prepared by radical copolymerization of 2-vinylnaphthalene, styrene, and divinylbenzene.70 This catalyst was used to mediate metallation of alkyl chlorides by lithium. The reaction was done in the presence of electrophiles to afford, after quenching, the desired addition products.
5.9. POLYMERS FOR REACTION QUENCHING/WORKUP
Chemically functionalized polymers have also been used in polymer-assisted solution-phase synthesis to perform reaction-quenching functions. These polymers often are used in operations that substitute for traditional liquid-phase extractions in classical synthesis.
Resin quenching is often preferable to traditional liquid-phase extractions in automated processes. The advantages of this technique in robotic syntheses are
• avoidance of emulsions;
• circumvention of unreliable meniscus-reading or volume transfer robotic-software commands; and
• simultaneous extractions by use of multiple-quenching resins (mixed-resin beds).20
A + Â
Scheme 2 depicts an application of the carboxyl-functionalized IRC-50S resin 48 to quench parallel-array organometallic reactions.20 Nonisolable alkoxide salts formed from the reaction of aldehydes with organolithiums
174 POLYMER-ASSISTED SOLUTION-PHASE METHODS
R1CHO + or R2MgX
-> R2H +
M = Li or MgX
or Grignard reagents were quenched by resin 48 to give the corresponding alcohol products after simple filtration. Excess organometallic reactants were simultaneously quenched to give volatile hydrocarbon byproducts. Mixed-resin beds were also highlighted in this report: The polyamine-func-tionalized resin 2 was simultaneously used to sequester unreacted aldehydes from the solution phase.
Diatomacious earth has recently been reported as an insoluble polymer support for accomplishing the equivalent of liquid-phase extraction. Preadsorption of water, aqueous base, or aqueous acid onto diatomacious earth provided an insoluble, flow-through material for parallel, high-throughput purification of solution-phase reactions.71 Scheme 3 illustrates this tech-
diatomacious earth pretreated with dilute acid
(removes p2N'Pr.HCI and R3NH2)