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Staudinger Ph0H Mitsunobu D.~D
R—N3 -----------> R-NH2 + --------> PhOR
5.7.3. Polymeric Oxidizing Reagents
Swern oxidations have been performed using the PEG2000 bound sulfoxide 34 as a dimethylsulfoxide (DMSO) substitute (reaction 13).49,50 Several alcohols were efficiently oxidized to their aldehydes or ketones using this reagent, oxalyl chloride, and triethylamine. Precipitation of the polymer with cold diethyl ether and filtration through a pad of silica afforded the desired oxidized products in very good yields and purities. The reduced sulfide polymer could be reoxidized to sulfoxide 34 with sodium metaperiodate and used again in reactions with no appreciable loss in oxidation capacity.
(i) 34, (COCI)2, CH2CI2
(ii) Et20 (polymer ppt)
Several other polymer-bound oxidizing reagents have recently been reported in the literature (reaction 14). A polyethyleneimine-supported silver dichromate 35 has been shown to be a stable, mild, and efficient oxidizing agent for the conversion of alcohols to carbonyl compounds.51 A
168 POLYMER-ASSISTED SOLUTION-PHASE METHODS
polymer-supported isoxazolinium permanganate 36 was shown to oxidize alcohols to carbonyl compounds in nearly quantitative yields.52 It was also reported that the polymer 36 could be regenerated and reused several times. Similarly, pyrazolinium permanganate and pyrazolinium chromium (VI) reagents have also been reported as efficient oxidizing agents.53,54 A polymer-supported perruthenate 37 has also been recently reported for the oxidation of alcohols to their aldehydes or ketones.55 Polymer 37 can be used either in stoichiometric amounts or as a catalyst in combination with a co-oxidant (/V-methylmorpholine oxide or trimethylamine oxide). The desired oxidized products were obtained in high yields and purities after filtration and evaporation. It was also shown that the catalyst could be reactivated and reused in further oxidation reactions.
5.7.4. Polymeric Reagents for Oxirane to Thiirane Conversion
A series of epoxides has been efficiently converted to the corresponding thiiranes using a polymer-bound quaternary ammonium thiocyanate38. The polymeric byproduct of this transformation is the supported cyanate.56
5.8. POLYMER-SUPPORTED CATALYSTS
5.8.1. Polymer-Supported Palladium Catalysts
A polymer-bound Pd(0)-phosphine catalyst 39 has been reported that is soluble in water or mixed aqueous/organic media.57 This catalyst was used
5.8. POLYMER-SUPPORTED CATALYSTS 169
to mediate allylic substitutions and cross-coupling reactions of aryl iodides with terminal alkynes. The catalyst was removed by ether-induced precipitation or thermal-induced precipitation. The latter operation takes advantage of the inverse temperature solubility dependence of the polymer, that is, increased temperature results in a decreased solubility of 39.
Ri-^^\/OCOi^ + NuH ---------------> Ri^\/Nu
(Nu is benzenesuifinate or secondary amine)
r1—³ + hoc-r2 ................-39 »- Ri~^3 = Rz
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
170 POLYMER-ASSISTED SOLUTION-PHASE METHODS
5.8.2. Polymer-Supported Catalysts for Epoxidation and Dihydroxylation
Polyaniline-supported Co(II) catalyst 41 was used to catalyze the epoxidation of various alkenes under oxygen atmosphere at ambient temperature.61 ~63 One report61 described a synthesis of oc-hydroxy-(3-aminopyrrolidine amides as potential HIV protease inhibitors (Scheme 1). In this synthesis, catalyst 41 also mediated epoxide ring opening of the epoxide intermediate by an aniline to afford the desired product.
Polymer-bound trifluoromethyl aryl ketone 42 was prepared by attaching 4-(trifluoroacetyl)benzoic acid to a suitably functionalized resin and used as a catalyst in Oxone-mediated epoxidations.64 The reactions proceed by in situ generation of the polymer-supported (trifluoromethyl)-dioxirane. A series of epoxides was formed in good to excellent yield.