Oxidizing agents polymer-bound

The six-position may be functionalized by electrophilic aromatic substitution. Either bromination (Br2/CH2Cl2/-5°) acetylation (acetyl chloride, aluminum chloride, nitrobenzene) " or chloromethylation (chloromethyl methyl ether, stannic chloride, -60°) " affords the 6,6 -disubstituted product. It should also be noted that treatment of the acetyl derivative with KOBr in THF affords the carboxylic acid in 84% yield. The brominated crown may then be metallated (n-BuLi) and treated with an electrophile to form a chain-extender. To this end, Cram has utilized both ethylene oxide " and dichlorodimethyl-silane in the conversion of bis-binaphthyl crowns into polymer-bound resolving agents. The acetylation/oxidation sequence is illustrated in Eq. (3.54). 

A final class of multifunctional initiators is based on the use a (muUi)functional polymer and a low molecular weight redox agent. Radicals on the polymer chain arc generated from the polymer bound functionality by a redox reaction. Ideally, no free initiating species are formed. The best known of this class are the polyol-redox and related systems. Polymers containing hydroxy or glycol and related functionality are subject to one electron oxidation by species such as ceric ions or periodate (Scheme 7.23).266,267 Substrates such as cellulose,... 

The catalytic system has been successfully extended to polymer-bound lithium amide co-bases of type 65 (see Table 4) which, like C—Li bases of type 63 and 64, are efficient regenerating agents of HCLA and poorly reactive toward oxiranes. For instance, the isomerization of cyclohexene oxide by 0.05 equiv of HCLA 55 in the presence of 1.45 equiv of 65 affords ( l-cyclohexenol in 92% ee (entry 15). It is of interest to note that, similarly to co-bases 63 and 64, the use of 65 leads to an increase of selectivity compared to the stoichiometric reaction at room temperature (Table 2, entry. ... 

Taylor, R. T., Polymer-Bound Oxidizing Agents, Chap. 7 in Polymeric Reagents and Catalysts, W. T. 

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... 

It is possible to achieve selective esterification at OH-6 in good yield even with acetyl chloride by employing low temperature in the presence of a controlled amount of acylating agent.13,93 Some tin derivatives (such as dibutyltin oxide) also show significant selectivity towards position 6 (Scheme 7).94,95 A polymer-bound stannyl reagent was also applied for the selective C-6 acetylation of... 

The transition states of the latter are therefore more sensitive to stereochemical and electronic influences, which also leads to a higher selectivity than in analogous electrochemical conversions. At some oxide electrodes, such as the Ni(OH)2 electrode [13], the oxidation occurs as inner sphere electron transfer by hydrogen atom transfer. Also at doped titanium anodes this seems to be partially the case [14]. It cannot be definitely excluded that also in some oxidations at platinum anodes, higher valency oxides at the surface act as inner sphere electron transfer agents. Electrochemical" inner sphere electron transfers are intentionally used in indirect electrochemical conversions where selective chemical oxidants or reductants are regenerated by electron transfer from the electrode [15]. They are also immobilized by attaching polymer-bound electrocatalysts as mediators to the electrode surface [16]. 

Taylor, R. T., Polymer-Bound Oxidizing Agents, Chap. 7 in Polymeric Reagents and Catalysts, W. T. Ford, ed.. Am. Chem. Soc. Symp. Ser., Vol. 308, American Chemical Society, Washington, D. C., 1986. 

This present work examines the present development of a variety of polymer-bound oxidizing agents. Since the review is not meant to be exhaustive, it focuses on reagents which are chemically bound to a synthetic polymer. Reagents supported on inorganic supports such as cellte, silica, graphite, clay, etc. are not included (6,7), nor are enzymes. 

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